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testing and measurement cover 5/15/07 2:47 PM Page 5

Electrical Testing

and Measurement Handbook Vol 7

Published by The Electricity Forum

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Tel: (905) 686-1040 Fax: (905) 686 1078

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The Electricity Forum

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All rights reserved No part of this book may be reproduced without

the written permission of the publisher.

ISBN-978-0-9782763-2-4 The Electricity Forum

215 - 1885 Clements Road, Pickering, ON L1W 3V4

© The Electricity Forum 2007

ELECTRICAL TESTING

AND MEASUREMENT HANDBOOK

TABLE OF CONTENTS

ELECTRICAL MEASUREMENT AND TESTING CONTACT-LESS SENSING AND

THE AUTO-DETECT INFRASTRUCTURE

Forward - Khaled Nigim 5DON’T RISK IT: USE CORRECT ELECTRICAL MEASUREMENT TOOLS AND PROCEDURES TO

MINIMIZE RISK AND LIABILITY

Larry Eccleston 7 ISOLATION TECHNOLOGIES FOR RELIABLE INDUSTRIAL MEASUREMENTS

National Instruments 11RESISTANCE MEASUREMENTS, THREE- AND FOUR-POINT METHOD

15CLAMP-ON GROUND RESISTANCE TESTER, MODELS 3711 & 3731 STEP-BY-STEP USAGE

Chauvin Arnoux, Inc and AEMC® Instruments 21MEASURING MAGNETIC FIELDS, ELECTRIC AND |MAGNETIC FIELDS

Australian Radiation Protection and Nuclear Agency 23ELECTRIC AND MAGNETIC FIELDS, MEASUREMENTS AND POSSIBLE EFFECT ON HUMAN HEALTH,

WHAT WE KNOW AND WHAT WE DON’T KNOW IN 2000

California Department of Health Services and the Public Health Institute

California Electric and Magnetic Fields Program 25

A NEW APPROACH TO QUICK, ACCURATE, AFFORDABLE FLOATING MEASUREMENTS

Tektronix IsolatedChannel Technology 31HIGH-VOLTAGE MEASUREMENTS AND ISOLATION -GENERAL ANALOG CONCEPTS

NI Analog Resource Center. 35STANDARD MEASUREMENTS: ELECTRIC FIELDS DUE TO HIGH VOLTAGE EQUIPMENT

Ralf Müller and Hans-Joachim Förster 39IDENTIFICATION OF CLOSED LOOP SYSTEMS

NI Analog Resource Center 43SELECTING AND USING TRANSDUCERS FOR TRANSFORMERS FOR ELECTRICAL MEASUREMENTS

William D Walden 45HOW TO TROUBLESHOOT LIKE AN EXPERT, A SYSTEMATIC APPROACH

Warren Rhude, Simutech Multimedia Inc. 53ELECTRICAL INDUSTRIAL TROUBLESHOOTING

Larry Bush 55THE ART OF MEASURING, LOW RESISTANCE

Tee Sheffer and Paul Lantz, Signametrics 59STANDARDS FOR SUPERCONDUCTOR AND MAGNETIC MEASUREMENTS

National Institute of Standards and Technology 63MULTI CHANNEL CURRENT TRANSDUCER SYSTEMS

DANFYSIK 67FALL-OF-POTENTIAL GROUND TESTING, CLAMP-ON GROUND TESTING COMPARISON

Chauvin Arnoux, Inc 69

AN INTRODUCTION TO ANTENNA TEST RANGES, MEASUREMENTS AND INSTRUMENTATION

Jeffrey A Fordham Microwave Instrumentation Technologies, LLC 71

44 Electrical Testing and Measurement Handbook – Vol 7

DERIVING MODEL PARAMETERS FROM FIELD TEST MEASUREMENTS

J.W Feltes, S Orero, B Fardanesh,E Uzunovic, S Zelingher, N Abi-Samra 79TESTING ELECTRIC STREETLIGHT COMPONENTS WITH LABVIEW-CONTROLLED

ni.com 93MAGNETO-MECHANICAL MEASUREMENTS FOR HIGH CURRENT APPLICATIONS

Jack Ekin, NIST- Electromagnetic Division 101

A BASIC GUIDE TO THERMOGRAPHY

Land Instruments International Infrared Temperature Measurement 105

Maintaining a highly functional electric system is

depend-ent on the operational and maintenance level of the integrated

components that are geared together to serve the customer An

effective preventive maintenance setup is dependent on the

relia-bility of the sensing devices and relaying instrumentation as well

as on the operator’s understanding of the process functionality

Early measuring devices were designed and based on

electromechanical indicating instrumentation Their solo

oper-ability necessitated around the clock operator attention Such

devices were accurate but provided limited adaptability for

inter-facing with today’s centralized centers

As the semi-conducting integrated circuits devices start to

invade the market, many instruments are now inter-actable with

each other and some can be used to sense and record data from

various sensing elements in a sequential manner and generate

their own diagnostic reports within a very brief time Today’s

sensors are built around plug-and-play infrastructure which is

based on the IEEE 1451.4 standard that brings plug-and-play

capabilities to the world of transducers With plug-and-play

tech-nology, the operator stores a Transducer Electronic Datasheet

(TEDS) directly on a sensor The sensor identifies itself with all

needed information once and is hooked to a data bus

TEDS-compatible measurement systems can auto-detect and

automati-cally configure these “smart sensors” for measurement, reducing

setup time and eliminating transcription errors that commonly

occur during sensor configuration This enables the operator to

focus on overall system operation rather than on individual

com-ponent operation

Furthermore, measuring relaying units and associated

sensing elements technologies has advanced rapidly over the

past 20 years A particular advancement is noted in the

contact-less measuring sensors and measured data handling capability

This progression in the testing and measurement field provides a

wider scope of applications and shorter time for interrupting

early failure signals As an example, the cases where infra-red

imaging techniques are used are now part of the routine

mainte-nance of distribution transformers The infrared image indicates

the hottest spot and temperature distribution inside a large

distri-bution transformer without the need of embedding sensors

Earlier techniques for measuring temperature were based on

col-lecting data from various temperature sensors entrenched inside

the transformer windings If one or more sensors were faulty, the

gathered data would be incomplete and the transformer has to be

taken out of service Replacing the sensors is a timely and

cost-ly procedure Today’s data handling and processors that either

control the data flow from one or more sensors or part of the

human machine interface supervisory system, have the

capabili-ty to run self-diagnostics routines to alert the operator to any

abnormal behavior from the various sensing elements, and

gen-erate a check list to help figure out any culprits

This edition of the Electrical Testing and Measurement

Handbook introduces the fundamental applications of electricaltesting and instrumentation and guidelines on the correct proce-dures, and how to interpret and diagnose measured reports thatenable the operator to maintain a high degree of functionality ofthe system with minimum interruption

This handbook addresses various practical aspects oftoday’s electrical engineering infrastructure through selectedarticles available for scientific sharing

The articles are grouped into 4 sections Section 1

address-es the basics and fundamentals of electric taddress-esting techniquaddress-esusing various measuring sensors normally incorporated in many

of today measuring instruments Section 2 addresses safe tion, procedures and handling of instruments Section 3 intro-duces various sensing and measuring devices that can be used in

opera-a wide opera-areopera-a of opera-applicopera-ation And finopera-ally, section 4 showcopera-ases fieldapplications of instrumentation in various parts of the electricalengineering industry

The Electricity Forum endeavors to provide correct andtimely information for their readers in their handbook series Wewelcome readers’ suggestions and constructive feedback, andcontributions Please submit your technical articles that showcase your experience in testing and measurement tools and sys-tems directly to the handbook editor’s desk (HB2007@electrci-tyforum.com)

ELECTRICAL MEASUREMENT AND TESTING CONTACT-LESS SENSING AND THE AUTO-DETECT INFRASTRUCTURE

Forward by Khaled Nigim

6 Electrical Testing and Measurement Handbook – Vol 7

1 EXECUTIVE SUMMARY

Between five and ten times on any given day, arc flash

explosions sufficient to send a burn victim to a special burn

cen-ter take place in the U.S These incidents and other less serious

electrical accidents result in injury – sometimes death – lost

work time, medical costs and insurance claims, downtime, the

list goes on The cost to both the victim, the victim’s family and

the company involved, are high Yet many of these accidents can

be prevented The combination of training, good measurement

technique, and the use of proper tools can significantly reduce

the chance of an accident occurring

IS YOUR COMPANY AT RISK? HOW WOULD YOU ANSWER

THE FOLLOWING QUESTIONS?

1 Do you have a documented electrical measurement

safety program?

2 Do you regularly inspect your electrical measurement

equipment for damage that could imperil safety?

3 Do your workers involved in taking electrical

measure-ments receive annual, intensive training on how to work

safely?

4 Does your organization insure that only properly rated

test instruments are used in your facility?

If you answered yes to three of the questions above,

con-gratulations – you’re doing a better job than most employers to

reduce the chance of accidents associated with taking electrical

measurements But there’s still room to do more This resource

kit was designed to help you develop an electrical measurement

safety program that significantly reduces your risk

The high-energy electrical systems common in today’s

workplace bring not only increased efficiency, but increased

lev-els of hazard and risk for electrical workers and their employers

Workers taking electrical measurements on high-energy

systems frequently work close to potentially lethal electrical

cur-rents This danger can significantly increase due to the presence

of transient voltage spikes Transient spikes riding on these powerful

industrial currents can produce the conditions that cause the extremely

hazardous phenomenon of arc flash

To help manage the risks inherent in high-energy

electri-cal systems, national and international standards bodies have

developed rules that categorize electrical environments according

to their potential danger Personal protective equipment, including

test instruments, is categorized according to the NFPA-70E

Standard for Electrical Safety Requirements for Employee

Work-places, related to the incident energy levels and arc flash

bound-ary distances

To help ensure safety in today’s high-energy, high-hazard

environments, leading manufacturers have re-engineered their

test instruments to enhance both reliability and safety Such tools

can help companies avoid the many perils caused by high-energy

electrical accidents: disruption of operations, higher insurancecosts, litigation and, most importantly, human suffering

In today’s society, where medical costs are escalating andlawsuits are common, wise managers will take every step toreduce the level of risk, help increase employee safety and mini-mize the organization’s operational and financial exposure Thismeans that management must ensure that employees use appropri-ate personal protective equipment, including new-generation testtools independently tested to help ensure that they perform up tospecification And employees must use that equipment correctly,and receive training in safe electrical measurement procedures

2 INTRODUCTION: MANAGING HAZARDS IN THE ELECTRICAL ENVIRONMENT

Today’s industrial and business electrical supply systemsdeliver high levels of electrical energy – up to 480 volts in theUnited States, and up to 600 volts in Canada Such high-energycircuits can create significant hazard and risk

Another characteristic of most high-energy electrical supplysystems is the presence of short-duration voltage kickback spikes,called transients

When such spikes occur while measurements are beingmade, they can cause a plasma arc to form – inside the measurementtool, or outside The high fault current available in 480-volt and 600-volt systems can make the resulting arc flash extremely hazardous Mitigating such risks requires the use of Personal ProtectiveEquipment (PPE) including test instruments engineered and tested

to meet appropriate standards, adherence to safe measurement cedures, and proper inspection and maintenance of test instruments

pro-In this paper we will cover:

• Understanding the High-Energy Environment

• Voltage Transients

• The Danger of Arc Flash

• Measurement Categories CAT I, CAT II, CAT III andCAT IV

• Measurement Tools as Part of Personal Protective Equipment

• Safety Requirements for Measurement Tools

• Test Tool Inspection and Maintenance

• Safe Measurement Processes and Procedures

• Conclusions and Recommendations

3 UNDERSTANDING THE HIGH-ENERGY ENVIRONMENT

Businesses simply could not survive without largeamounts of electrical power Manufacturing operations and officeheating, ventilation and air conditioning systems require largeamounts of power, and computer systems have now becomemajor power users

The need to supply large amounts of power in the mostcost-effective way has led firms to choose higher-energy, higher-voltage supply systems, which cost less to install

DON’T RISK IT: USE CORRECT ELECTRICAL MEASUREMENT TOOLS AND PROCEDURES TO MINIMIZE RISK AND LIABILITY

Larry Eccleston, Product Testing Manager, Fluke Corporation, Member, IEC Standards Committee

8 Electrical Testing and Measurement Handbook – Vol 7

As a result of these trends, industrial and business

opera-tions today incorporate higher levels of electrical energy, which

can lead to increased hazard and risk for those who build and

maintain these systems It is common for industrial and

commer-cial maintenance workers and electricians to work with high levels

of energy In the U.S., 480-volt, three-phase electrical supply

systems are commonplace In Canada, systems use up to 600 volts

Although classified as “low voltage”, both 480-volt and 600-volt

systems can easily deliver potentially lethal amounts of current

sufficient to fuel an arc flash – an extremely hazardous occurrence

4 VOLTAGE TRANSIENTS: DANGER IN A MICROSECOND

The presence of voltage kickback spikes, called

tran-sients, is another characteristic of electrical supply systems that

adds to the potential danger encountered when taking electrical

measurements

Transients are present in almost every electrical supply

system In industrial settings, they may be caused by the switching

of inductive loads, and by lightning strikes Though such transients

may last only tens of microseconds, they may carry thousands of

amps of energy from the installation For anyone taking

measure-ments on electrical equipment, the consequences can be devastating

When such spikes occur while measurements are being

made, they can cause a plasma arc to form – inside the

measure-ment tool, or outside The high fault current available in 480-volt

and 600-volt systems can generate an extremely hazardous

con-dition called arc flash

5 UNDERSTANDING ARC FLASH

How can such a problem develop? A transient of

suffi-cient magnitude can cause an arc to form between conductors

within an instrument, or across test leads Once an arc occurs, the

total available fault current similar to the bolted current can feed

the arc and cause an explosion

The result may be an arc flash, which can cause a plasma

fireball fueled by the energy in the electrical system Temperatures

can reach about 6,000 degrees Celsius, or 10,000 degrees

Fahrenheit

Transients are not the only source of arc-flash hazard A

very common misuse of handheld multimeter can trigger a

sim-ilar chain of events

If the multimeter user leaves the test leads in the amps

input terminals and connects the meter leads across a voltage

source, that user has just created a short through the meter While

the voltage terminals have a high impedance, the amps terminals

have a very low impedance This is why a meter’s amps circuit

must be protected with fuses

Another common and dangerous misuse of test equipment

is measuring ohms or continuity on a live circuit These

measure-ments should be made only on circuits that are not energized

6 ARC FLASH AS A SAFETY ISSUE

Detailed information on the frequency and cost of arc flash

accidents is difficult to find Accident reports may not distinguish

arc flash from electric shock In addition, employers may be

reluctant to discuss or report incidents that can be so dangerous

and costly

Dr Mary Capelli-Schellpfeffer of the University of

Chicago provides the most authoritative estimates of arc flash

fre-quency Her firm, CapSchell, Inc., a Chicago-based research and

consulting firm, estimates that between five and ten times a day,

arc flash explosions sufficient to send a burn victim to a special

burn center take place in the U.S

7 MEASUREMENT CATEGORIES: CAT I, CAT II, CAT III AND CAT IV

To provide improved protection for users, industry dards organizations have taken steps to clarify the hazards pres-ent in electrical supply environments The American NationalStandards Institute (ANSI), the Canadian Standards Association(CSA), and the International Electro-Technical Commission(IEC) have created more stringent standards for voltage testequipment used in environments of up to 1000 volts

stan-The pertinent standards include ANSI S82.02, CSA 1010.1 and IEC 61010 These standards cover systems of 1000volts or less, including 480-volt and 600-volt, three-phase cir-cuits For the first time, these standards differentiate the transienthazard by location and potential for harm, as well as the voltagelevel

22.2-ANSI, CSA and IEC define four measurement categories

of over-voltage transient impulses The rule of thumb is that thecloser the technician is working to the power source, the greaterthe danger and the higher the measurement category number.Lower category installations usually have greater impedance,which dampens transients and helps limit the fault current thatcan feed an arc

• CAT (Category) IV is associated with the origin ofinstallation This refers to power lines at the utility con-nection, but also includes any overhead and under-ground outside cable runs, since both may be affected bylightning

• CAT III covers distribution level wiring This includes480-volt and 600-volt circuits such as 3-phase bus andfeeder circuits, motor control centers, load centers anddistribution panels Permanently installed loads are alsoclassed as CAT III CAT III includes large loads that cangenerate their own transients At this level, the trend tousing higher voltage levels in modern buildings haschanged the picture and increased the potential hazards

• CAT II covers the receptacle circuit level and plug-inloads

• CAT I refers to protected electronic circuits

Some installed equipment may include multiple categories

A motor drive panel, for example, may be CAT III on the 480-voltpower side, and CAT I on the control side

8 MEASUREMENT TOOLS PART OF PERSONAL PROTECTIVE EQUIPMENT

Another organization playing an important role in lishing safety standards for electrical workers is the National FireProtection Association (NFPA) NFPA establishes guidelines forelectrical measurement tools in its standard 70E, “Standard forElectrical Safety Requirements for Employee Workplaces, 2004Edition”

estab-Standard 70E also includes important requirementsregarding the use of other Personal Protective Equipment (PPE)

in various environments and installation/maintenance activities The NFPA standard makes it clear that test instrumentsand accessories must be matched to the environment where theywill be used These are the pertinent sections:

• “Test instruments, equipment, and their accessories shall

be rated for circuits and equipment to which they will beconnected.” (Part II, Chapter 3, Paragraph 3-4.10.1)

• “Test instruments, equipment, and their accessories shall

be designed for the environment to which they will be

exposed, and for the manner in which they will be used.”

(Part II, Chapter 3, Paragraph 3-4.10.2)

A table included in NFPA Standard 70E, Table 3-3.9,

“Hazard Risk Category Classifications,” provides additional

guidance regarding the personal protective equipment

recom-mended for use in work on a variety of equipment types at

vari-ous voltage levels.i

9 SAFETY REQUIREMENTS FOR MEASUREMENT TOOLS

Management must ensure that, in compliance with NFPA

70E, test tools meet the standards for the environment where

they are used The entire testing ‘system’, including the meter

and its internal fusing system, as well as the test leads and

attach-ments, must comply with regulations for measurement

environ-ment and hazard level

In addition, tools must be included as an integral part of

the Personal Protective Equipment that technicians are required

to use when working on high-energy systems

Beyond these requirements, however, management must

ensure that the measurement tools in use are designed, certified

and maintained so that they will meet the more advanced and

stringent safety requirements of today Management must account

for three factors when assessing test tool safety: Category rating

(older, unrated tools were not made for today’s electrical

environ-ment), independent testing and certification, and regular inspection

and maintenance It is important to note that the category rating for

personnel protective equipment has no relationship to the CAT

ratings identified as part of the markings of test and

measure-ment equipmeasure-ment

the electrical environment in which they will be used For example,

a 220-volt, three-phase system requires a tester rated CAT III or

IV Old, unrated test instruments do not meet IEC guidelines for

required PPE While they may be perfectly accurate and appear

to perform well, even the best meters of yesterday were designed

for a world where working conditions and safety standards were

far different Such test tools may not meet contemporary standards

vital area of safety, some tools may not perform as promised by

the manufacturer Measuring devices rated for a high-energy

environment may not actually deliver the safety protections,

such as adequate fusing, claimed on their specification sheets

THE CRUCIAL DIFFERENCE BETWEEN ‘DESIGNED’ AND ‘TESTED’

It is important to understand that standards bodies such as

ANSI, CSA and IEC are not responsible for enforcing their

stan-dards This means that a meter designed to a standard may not

actually have been tested and proven to meet that standard It is

not uncommon for meters under test to fail before achieving the

performance their manufacturers claim for them

The best assurance for users and their employers is to

select test instruments that have been tested and certified to

per-form up to specification by independent testing laboratories To

provide an extra measure of confidence, select test tools labeled

to show that they have been certified to meet the appropriate

contemporary standards by two or more independent labs This

ensures that test instruments have passed the most rigorous tests

and meet every applicable standard Such independent testing

labs include Underwriters Laboratories (UL) in the United

States, Canadian Standards Association (CSA) in Canada and

TUV Product Service in Europe.ii

10 TEST TOOL INSPECTION AND MAINTENANCE

accurately and safely, test tools must be regularly inspected andmaintained The need for inspection is clearly recognized by theNational Fire Protection Association NFPA Standard 70E laysout the requirement that test tools must be visually inspected fre-quently to help detect damage and ensure proper operation Part

II, Chapter 4, Paragraph 4-1.1 provides the details:

all associated test leads, cables, power cords, probes, and nectors shall be visually inspected for external defects and dam-age before the equipment is used on any shift If there is a defect

con-or evidence of damage that might expose an employee to injury,the defective or damaged item shall be removed from service,and no employee shall use it until repairs and tests necessary torender the equipment safe have been made.iii

Visual inspection alone, however, may not detect all sible test instrument problems To help ensure the highest level

pos-of safety and performance, additional inspection and testingshould be conducted:

checked for the following points:

• Look for the 1000-volt, CAT III or 600-volt, CAT IV ing on the front of meters and testers, and a “doubleinsulated” symbol on the back

rat-• Look for approval symbols from two or more independenttesting agencies, such as UL, CSA, CE, TUV or CTICK

• Make sure that the amperage and voltage of meter fuses

is correct Fuse voltage must be as high or higher thanthe meter’s voltage rating The second edition ofIEC/ANSI/CSA standards states that test equipmentmust perform properly in the presence of impulses onvolts and amps measurement functions Ohms and con-tinuity functions are required to handle the full metervoltage rating without becoming a hazard

• Check the instrument’s manual to determine whether theohms and continuity circuits are protected to the samelevel as the voltage test circuit If the manual does notindicate, your supplier should be able to determinewhether the meter passed the second edition of IEC61010

Typically a fuse in good condition should showmvalue of close to zero, but you should alwayscheck your meter owner’s manual for the speci-fied reading

compo-nents of the test tool system, test leads, probes and attachmentsmust meet the requirements of the testing environment and bedesigned to minimize hazard Test leads must be certified to acategory that equals or exceeds that of the meter or tester

• Examine test leads for such features as shrouded nectors, finger guards, CAT ratings that equal or exceedthose of the meter, and double insulation

con-10 Electrical Testing and Measurement Handbook – Vol 7

• Visually inspect for frayed or broken wires The length

of exposed metal on test probe tips should be minimal

• Test leads can fail internally, creating a hazard that

can-not be detected through visual inspection But it is

pos-sible to use the meter’s own continuity testing function

to check for internal breaks

0.3 Ω

11 SAFE MEASUREMENT PROCESSES AND PROCEDURES

In addition to the consistent use of safe, correctly rated

and inspected test tools discussed in the preceding sections, safe

electrical measurement requires adherence to correct

measure-ment procedures Safety training programs should incorporate

both elements of safe measurement – equipment and procedures

In addition to equipment inspection (detailed in Section

10 above), safe measurement procedures include:

• Lockout/Tagout procedures – NFPA provides

exten-sive information and guidance on lockout/tagout

prac-tices and devices in Part II, Chapter 5 of NFPA 70E.iv

• Three-step test procedure – Before making the

determi-nation that a measured circuit is dead, it is important to

verify that test instruments are operating correctly To do

so, the technician should use a three-step test procedure

First, check for correct test tool operation by using the

tool to test a circuit known to be live Then, test the target circuit

Finally, as a double check on test tool operation, test the original

known circuit once again This procedure provides the user a

strong measure of confidence that the test tool is operating

cor-rectly, and that the target circuit is performing as measured

• Neutral first and last – The user should attach the test

lead to a neutral contact first, then attach a lead to a hot

contact to conduct the test In detaching test leads, first

remove the hot contact, then remove the neutral test lead

• One hand only – When possible, it is good practice to

follow the old electrician’s advice and keep one hand in

a pocket when testing But common sense must rule

Conditions at the test location may make it impractical

to use this technique

12 CONCLUSIONS AND RECOMMENDATIONS

Unlike some other important safety initiatives, the measuresrequired to bolster the safety of electrical measurement tools andprocedures are not difficult or costly Yet these steps can provideimportant benefits by improving worker safety, avoiding the dis-ruption of business operations, reducing risk and avoiding possibleincreases in insurance costs

Employers should begin by ensuring that technicians arefully trained in correct use of all personal protective equipment,including test instruments

As a companion measure, make sure the required PPE isreadily available, meets today’s standards, and is inspected toensure it is in optimum condition

Test instruments are an essential component of PPE.Employers should inspect all test instruments to ensure they arerated, tested and certified by independent testing agencies tomeet safety requirements for the environments where they areused Replace test instruments that do not meet current stan-dards, because they may create extra hazard, risk and liability Finally, personnel should be trained in the correct proce-dures for taking safe measurements, including methods for per-sonally inspecting and testing their instruments to ensure theyare in good condition and function correctly

i NFPA 70E Standard for Electrical Safety Requirements forEmployee Workplaces, 2000 Edition, pages 55 through 58 ©

iii NFPA 70E Standard for Electrical Safety Requirements forEmployee Workplaces, 2000 Edition, page 63 © 2000 NFPA

ivIbid, pp 64-66

OVERVIEW

Voltage, current, temperature, pressure, strain, and flow

measurements are an integral part of industrial and process

con-trol applications Often these applications involve environments

with hazardous voltages, transient signals, common-mode

volt-ages, and fluctuating ground potentials capable of damaging

measurement systems and ruining measurement accuracy To

overcome these challenges, measurement systems designed for

industrial applications make use of electrical isolation This

white paper focuses on isolation for analog measurements,

provides answers to common isolation questions, and includes

information on different isolation implementation technologies

UNDERSTANDING ISOLATION

Isolation electrically separates the sensor signals, which

can be exposed to hazardous voltages1, from the measurement

system’s low-voltage backplane Isolation offers many benefits

including:

• Protection for expensive equipment, the user, and data

from transient voltages

• Improved noise immunity

• Ground loop removal

• Increased common-mode voltage rejection

Isolated measurement systems provide separate ground

planes for the analog front end and the system backplane to

sep-arate the sensor measurements from the rest of the system The

ground connection of the isolated front end is a floating pin that

can operate at a different potential than the earth ground Figure 1

represents an analog voltage measurement device Any

common-mode voltage that exists between the sensor ground and the

meas-urement system ground is rejected This prevents ground loops

from forming and removes any noise on the sensor lines

NEED FOR ISOLATION

Consider isolation for measurement systems that involveany of the following:

• Vicinity to hazardous voltages

• Industrial environments with possibility of transientvoltages

• Environments with common mode voltage or ing ground potentials

fluctuat-• Electrically noisy environments such as those withindustrial motors

• Transient sensitive applications where it is imperative

to prevent voltage spikes from being transmitted throughthe measurement system

Industrial measurement, process control, and automotivetest are examples of applications where common-mode voltages,high-voltage transients, and electrical noise are common.Measurement equipment with isolation can offer reliable measure-ments in these harsh environments For medical equipment indirect contact with patients, isolation is useful in preventing powerline transients from being transmitted through the equipment Based on your voltage and data rate requirements, youhave several options for making isolated measurements Youcan use plug-in boards for laptops, desktop PCs, industrial PCs,PXI, Panel PCs, and Compact PCI with the option of built-inisolation or external signal conditioning Isolated measurementscan also be made using programmable automation controllers(PACs) and measurement systems for USB

ISOLATION TECHNOLOGIES FOR RELIABLE

INDUSTRIAL MEASUREMENTS

National Instruments

Figure 1 Bank Isolated Analog Input Circuitry

Hazardous Voltages are greater than 30 Vrms, 42.4 Vpk or 60 VDC Figure 2 Isolated Data Acquisition Systems

12 Electrical Testing and Measurement Handbook – Vol 7

METHODS OF IMPLEMENTING ISOLATION

Isolation requires signals to be transmitted across an

isola-tion barrier without any direct electrical contact Light emitting

diodes (LEDs), capacitors, and inductors are three commonly

available components that allow electrical signal transmission

without any direct contact The principles on which these devices

are based form the core of the three most common technologies

for isolation – optical, capacitive, and inductive coupling

OPTICAL COUPLING

LEDs produce light when a voltage is applied across

them Optical isolation uses an LED along with a photo-detector

device to transmit signals across an isolation barrier using light

as the method of data translation A photo-detector receives the light

transmitted by the LED and converts it back to the original signal

Optical isolation is one of the most commonly used methods

for isolation One benefit of using optical isolation is its immunity

to electrical and magnetic noise Some of the disadvantages

include transmission speed, which is restricted by the LED

switching speed, high-power dissipation, and LED wear

CAPACITIVE COUPLING

Capacitive isolation is based on an electric field that

changes based on the level of charge on a capacitor plate This

charge is detected across an isolation barrier and is proportional

to the level of the measured signal

One advantage of capacitive isolation is its immunity to

magnetic noise Compared to optical isolation, capacitive

isola-tion can support faster data transmission rates because there are

no LEDs that need to be switched Since capacitive coupling

involves the use of electric fields for data transmission, it can be

susceptible to interference from external electric fields

INDUCTIVE COUPLING

In the early 1800s, Hans Oersted, a Danish physicist,

dis-covered that current through a coil of wire produces a magnetic

field It was later discovered that current can be induced in a

second coil by placing it in close vicinity of the changing netic field from the first coil The voltage and current induced inthe second coil depend on the rate of current change through thefirst This principle is called mutual induction and forms thebasis of inductive isolation

mag-Inductive isolation uses a pair of coils separated by alayer of insulation Insulation prevents any physical signaltransmission Signals can be transmitted by varying currentflowing through one of the coils, which causes a similar current

to be induced in the second coil across the insulation barrier.Inductive isolation can provide high-speed transmission similar

to capacitive techniques Because inductive coupling involvesthe use of magnetic fields for data transmission, it can be sus-ceptible to interference from external magnetic fields

ANALOG ISOLATION AND DIGITAL ISOLATION

Several commercial off-the-shelf (COTS) componentsare available today, many of which incorporate one of the abovetechnologies to provide isolation For analog input/output chan-nels, isolation can be implemented either in the analog section

of the board, before the analog-to-digital converter (ADC) hasdigitized the signal (analog isolation) or after the ADC has digitized the signal (digital isolation) Different circuitry needs

to be designed around one of these techniques based on the tion in the circuit where isolation is being implementing You canchoose analog or digital isolation based on your data acquisitionsystem performance, cost, and physical requirements Figure 6shows the different stages of implementing isolation

loca-Figure 3 Optical Coupling

Figure 4 Capacitive Isolation

Figure 5 Inductive Coupling

Figure 6a Analog Isolation

Figure 6b Digital Isolation

The following sections cover analog and digital isolation

in more detail and explore the different techniques for

imple-menting each

ANALOG ISOLATION

The isolation amplifier is generally used to provide isolation

in the analog front end of data acquisition devices “ISO Amp”

in Figure 6a represents an isolation amplifier The isolation

amplifier in most circuits is one of the first components of the

analog circuitry The analog signal from a sensor is passed to the

isolation amplifier which provides isolation and passes the signal

to the analog-to-digital conversion circuitry Figure 7 represents

the general layout of an isolation amplifier

In an ideal isolation amplifier, the analog output signal is

the same as the analog input signal The section labeled “isolation”

in Figure 7 uses one of the techniques discussed in the previous

section (optical, capacitive, or inductive coupling) to pass the

signal across the isolation barrier The modulator circuit

pre-pares the signal for the isolation circuitry For optical methods,

this signal needs to be digitized or translated into varying light

intensities For capacitive and inductive methods, the signal is

translated into varying electric or magnetic fields The

demodu-lator circuit then reads the isolation circuit output and converts

it back into the original analog signal

Because analog isolation is performed before the signal is

digitized, it is the best method to apply when designing external

signal conditioning for use with existing non-isolated data

acquisi-tion devices In this case, the data acquisiacquisi-tion device performs

the analog-to-digital conversion and the external circuitry provides

isolation With the data acquisition device and external signal

con-ditioning combination, measurement system vendors can develop

general-purpose data acquisition devices and sensor-specific signal

conditioning Figure 8 shows analog isolation being implemented

with flexible signal conditioning that uses isolation amplifiers

Another benefit to isolation in the analog front end is protection for

the ADC and other analog circuitry from voltage spikes

There are several options available on the market for

measurement products that use a general-purpose data

acquisi-tion device and external signal condiacquisi-tioning For example, the

National Instruments M Series includes several non-isolated,

gen-eral-purpose multifunction data acquisition devices that provide

high-performance analog I/O and digital I/O For applications

that need isolation, you can use the NI M Series devices with

external signal conditioning, such as the National Instruments

SCXI or SCC modules These signal conditioning platforms

deliver the isolation and specialized signal conditioning needed

for direct connection to industrial sensors such as load cells, strain

gages, pH sensors, and others

DIGITAL ISOLATION Analog-to-digital converters are one of the key compo-nents of any analog input data acquisition device For best performance, the input signal to the analog-to-digital convertershould be as close to the original analog signal as possible.Analog isolation can add errors such as gain, non-linearity andoffset before the signal reaches the ADC Placing the ADC clos-

er to the signal source can lead to better performance Analogisolation components are also costly and can suffer from longsettling times Despite better performance of digital isolation,one of the reasons for using analog isolation in the past was toprovide protection for the expensive analog-to-digital convert-ers As the ADCs prices have significantly declined, measure-ment equipment vendors are choosing to trade ADC protectionfor better performance and lower cost offered by digital isola-tors (see Figure 9)

Compared to isolation amplifiers, digital isolation nents are lower in cost and offer higher data transfer speeds Digitalisolation techniques also give analog designers more flexibility tochoose components and develop optimal analog front ends formeasurement devices Products with digital isolation use current-and voltage-limiting circuits to provide ADC protection Digitalisolation components follow the same fundamental principles ofoptical, capacitive, and inductive coupling that form the basis ofanalog isolation

compo-Figure 7 Isolation Amplifier

Figure 8 Use of Isolation Amplifiers in Flexible Signal Conditioning Hardware

Figure 9 Declining Price of 16-Bit Analog-to-Digital Converters Graph Source: National Instruments and a Leading ADC Supplier

14 Electrical Testing and Measurement Handbook – Vol 7

Leading digital isolation component vendors such as

Avago Technologies (www.avagotech.com), Texas Instruments

(www.ti.com), and Analog Devices (www.analog.com) have

developed their isolation technologies around one of these basic

principles Avago Technologies offers digital isolators based on

optical coupling, Texas instruments bases its isolators on

capac-itive coupling, and Analog Devices isolators use inductive coupling

OPTOCOUPLERS

Optocouplers, digital isolators based on the optical

cou-pling principles, are one of the oldest and most commonly used

methods for digital isolation They can withstand high voltages

and offer high immunity to electrical and magnetic noise

Optocouplers are often used on industrial digital I/O products,

such as the National Instruments PXI-6514 isolated digital

input/output board (see Figure 10) and National Instruments

PCI-7390 industrial motion controller

For high-speed analog measurements, optocouplers,

however, suffer from speed, power dissipation, and LED ware

limitations associated with optical coupling Digital isolators

based on capacitive and inductive coupling can alleviate many

optocoupler limitations

CAPACITIVE ISOLATION

Texas Instruments offers digital isolation components

based on capacitive coupling These isolators provide high data

transfer rates and high transient immunity Compared to

capac-itive and optical isolation methods inductive isolation offers

lower power consumption

INDUCTIVE ISOLATION

iCoupler®technology, introduced by Analog Devices in

2001 (www.analog.com/iCoupler), uses inductive coupling to

offer digital isolation for high-speed and high-channel-count

applications iCouplers can provide 100 Mb/s data transfer rates

with 2,500 V isolation withstand; for a 16-bit analog

measure-ment system that implies sampling rates in the mega hertz

range Compared to optocouplers, iCouplers offer other benefits

such as reduced power consumption, high operating temperature

range up to 125 °C, and high transient immunity up to 25 kV/ms

iCoupler technology is based on small, chip-scale

formers An iCoupler has three main parts – a transmitter,

trans-formers, and a receiver The transmitter circuit uses edge trigger

encoding and converts rising and falling edges on the digitallines to 1 ns pulses These pulses are transmitted across the iso-lation barrier using the transformer and decoded on the otherside by the receiver circuitry (see Figure 11) The small size ofthe transformers, about three-tenths of a millimeter, makes them

practically impervious to external magnetic noise iCouplers

can also lower measurement hardware cost by integrating up tofour isolated channels per integrated circuit (IC) and, compared

to optocouplers, they require fewer external components

Measurement hardware vendors are using iCouplers

to offer high-performance data acquisition systems at lowercosts National Instruments industrial data acquisitiondevices intended for high-speed measurements, such as theisolated M Series multifunction data acquisition devices,

use iCoupler digital isolators (see Figure 12) These devices

provide 60 VDC continuous isolation and 1,400 Vrms/1,900VDC channel-to-bus isolation withstand for 5 s on multipleanalog and digital channels and support sampling rates up to

250 kS/s National Instruments C Series modules used in the NIPAC platform, NI CompactRIO, NI CompactDAQ, and other

high-speed NI USB devices also use the iCoupler technology

SUMMARY

Isolated data acquisition systems can provide reliablemeasurements for harsh industrial environments with hazardousvoltages and transients Your need for isolation is based on yourmeasurement application and surrounding environments.Applications that require connectivity to different specialty sen-sors using a single, general-purpose data acquisition device canbenefit from external signal conditioning with analog isolation.Where as applications needing lower-cost, high-performanceanalog inputs benefit from measurement systems with digitalisolation technologies

Figure 10 Industrial Digital I/O Products Optpcouplers

Figure 11 Introduction Coupling-Based iCoupler Technology from Analog Devices Source: Analog Devices (www.analog.com/iCoupler)

Figure 12 National Instruments Isolated M Series Multifuntion DAQ Uses

FOUR-POINT RESISTANCE MEASUREMENTS

Ohmmeter measurements are normally made with just a

two-point measurement method However, when measuring very

low values of ohms, in the milli- or micro-ohm range, the two-point

method is not satisfactory because test lead resistance becomes a

significant factor

A similar problem occurs when making ground mat

resist-ance tests, because long lead lengths of up to 1000 feet are used

Here also, the lead resistance, due to long lead length, will affect

the measurement results

The four-point resistance measurement method eliminates

lead resistance Instruments based on the four-point

measure-ment work on the following principle:

• Two current leads, C1 and C2, comprise a two-wire

cur-rent source that circulates curcur-rent through the resistance

under test

• Two potential leads, P1 and P2, provide a two-wire

volt-age measurement circuit that measures the voltvolt-age drop

across the resistance under test

• The instrument computes the value of resistance from

the measured values of current and voltage

THREE-POINT RESISTANCE MEASUREMENTS

The three-point method, a variation of the four-point

method, is usually used when making ground (earth) resistance

measurements With the three-point method, the C1 and P1 terminals

are tied together at the instrument and connected with a short

lead to the ground system being tested This simplifies the test in

that only three leads are required instead of four Because this

common lead is kept short, when compared to the length of the

C2 and P2 leads, its effect is negligible Some ground testers are

only capable of the three-point method, so are equipped with

only three test terminals The three-point method for ground tem testing is considered adequate by most individuals in theelectrical industry and is employed on the TPI MFT5010 and theTPI ERT1500

sys-The four-point method is required to measure soil resistivity.This process requires a soil cup of specific dimensions into which

a representative sample of earth is placed This process is not oftenemployed in testing electrical ground systems although it may bepart of an initial engineering study

PURPOSE/TPI INSTRUMENT FEATURES

PURPOSEThe purpose of electrical ground testing is to determinethe effectiveness of the grounding medium with respect to trueearth Most electrical systems do not rely on the earth to carryload current (this is done by the system conductors) but the earthmay provide the return path for fault currents, and for safety, allelectrical equipment frames are connected to ground

The resistivity of the earth is usually negligible becausethere so much of it available to carry current The limiting factor

in electrical grounding systems is how well the grounding

elec-trodes contact the earth, which is known as thesoil/ground rod interface This interface resistance com-ponent, along with the resistance of the grounding con-ductors and the connections, must be measured by theground test

In general, the lower the ground resistance, thesafer the system is considered to be There are differentregulations which set forth the maximum allowableground resistance, for example: the National ElectricalCode specifies 25 ohms or less; MSHA is more strin-gent, requiring the ground to be 4 ohms or better; electricutilities construct their ground systems so that theresistance at a large station will be no more than a fewtenths of one ohm

TPI GROUND TEST INSTRUMENT CHARACTERISTICS

• To avoid errors due to galvanic currents in the earth, TPIground test instruments use an AC current source

• A frequency other than 60 hertz is used to eliminate thepossibility of interference with stray 60 hertz currentsflowing through the earth

• The three-point measurement technique is utilized toeliminate the effect of lead length

• The test procedure, known as the Fall-of-Potential

Method, is described on the following page.

RESISTANCE MEASUREMENTS THREE- AND FOUR-POINT METHOD

Figure 1

16 Electrical Testing and Measurement Handbook – Vol 7

THREE-POINT FALL-OF-POTENTIAL TEST PROCEDURE

GROUND TEST PROCEDURE

In the Fall-of-Potential Method, two small ground rods –

often referred to as ground spikes or probes – about 12" long are

utilized These probes are pushed or driven into the earth far

enough to make good contact with the earth (8" – 10" is usually

adequate) One of these probes, referred to as the remote current

probe, is used to inject the test current into the earth and is placed

some distance (often 100') away from the grounding medium

being tested The second probe, known as the potential probe, is

inserted at intervals within the current path and measures the

voltage drop produced by the test current flowing through the

resistance of the earth

In the example shown on the following page, the remote

current probe C2 is located at a distance of 100 feet from the

ground system being tested The P2 potential probe is taken out

toward the remote current probe C2 and driven into the earth at

ten-foot increments

Based on empirical data (data determined by experiment and

observation rather than being scientifically derived), the ohmic value

measured at 62% of the distance from the ground-under-test to the

remote current probe, is taken as the system ground resistance

The remote current probe must be placed out of the

influ-ence of the field of the ground system under test With all but the

largest ground systems, a spacing of 100 feet between the

ground-under-test and the remote

current electrode is adequate

When adequate spacing

between electrodes exists, a

plateau will be developed on

the test graph Note: A remote

current probe distance of less

than 100 feet may be

ade-quate on small ground

sys-tems.

When making a test where sufficient spacing exists, theinstrument will read zero or very near zero when the P2 poten-tial probe is placed near the ground-under-test As the electrode

is moved out toward the remote electrode, a plateau will bereached where a number of readings is approximately the samevalue (the actual ground resistance is that which is measured at62% of the distance between the ground mat being tested and theremote current electrode) Finally, as the potential probeapproaches the remote current electrode, the resistance reading willrise dramatically

It is not absolutely necessary to make a number of ments as described above and to construct a graph of the readings.However, we recommend this as it provides valuable data for futurereference and, once you are setup, it takes only a few minutes totake a series of readings

measure-The electrical fields associated with the ground grid andthe remote electrodes are illustrated on AN0009-5 An actualground test is detailed on AN0009-6, and a sample Ground TestForm is provided on AN0009-7 See AN0009-8 for a simpleshop-built wire reel assembly for testing large ground systems

SHORT-CUT METHOD

The short cut method described here determines theground resistance value and verifies sufficient electrode spacing –and it does save time This procedure uses the 65' leads suppliedwith the TPI instruments

• Connect the T1 instrument jack with the 15' green lead

to the ground system being tested

• Connect the T3 instrument jack with the red lead to theremote current electrode (spike) placed at distance of 65'(full length of conductor) from the ground grid beingtested

• Connect the T2 instrument jack with the black lead tothe potential probe placed at 40 feet (62% of the 65' dis-tance) from the ground grid being tested and measurethe ground resistance

• Move the P2 potential probe 6' (10% of the total tance) to either side of the 40' point and take readings ateach of these points If the readings at these two pointsare essentially the same as that taken at the 40' point, ameasurement plateau exists and the 40' reading is valid

dis-A substantial variation between readings indicates ficient spacing

insuf-THREE-POINT FALL-OF-POTENTIAL METHOD

INSTRUMENT SET-UP

Figure 2

Figure 3

A NOTE ON INSTRUMENT LABELING CONVENTIONS

The TPI MFT5010 and TPI ERT1500 use

the terminal designations T1 (C1/P1), T2 (P2), and

T3 (C2)

The corresponding lead designations on the

MFT5010 are E (Earth), S & H

The corresponding lead designations on the

ERT1500 are E (Earth), P (Potential), C (Current)

TEST CURRENT PATH

• Test Current (AC ) flows from instrument

T3 to remote current probe C2 on the red

lead

• Test Current flows from remote current

probe C2 back through the earth to the

ground being tested as shown by dashed

blue line

• Test current flows out of ground grid back

to instrument T1 on the short green lead

• Black potential lead P1 is connected to instrument

T2 and is taken out at 10' increments It measures

voltage drop produced by the test current flowing

through the earth (P1 to P2 potential)

EQUAL-POTENTIAL PLANES

THE EXISTENCE OF EQUAL-POTENTIAL PLANES

• When current flows through the earth from a remote test

electrode (in the case of a ground test) or remote fault, the

volt-age drop which results from the flow of current through the

resistance of the earth can be illustrated by equal-potential

planes The equal-potential planes are represented in the dashed

lines in drawings below where the spacing between concentric

lines represents some fixed value of voltage

• The concentration of the voltage surrounding a

ground-ing element is greatest immediately adjacent to that ground This

is shown by the close proximity of lines at the point where the

current enters the earth and again at the point where the current

leaves the earth and returns to the station ground mat

• In order to achieve a proper test using the Fall-of-PotentialGround Test Method, sufficient spacing must exist between thestation ground mat being tested and the remote current electrodesuch that the equal-potential lines do not overlap As shown by theblack line in the Sample Plot, adequate electrode spacing willresult in the occurrence of a plateau on the resistance plot Thisplateau must exist at 62% of the distance between the ground matand the remote electrode for the test to be valid Insufficient spac-ing results in an overlap of these equal-potential planes, as illus-trated at the bottom of this page and by the red line on the SamplePlot

• See the Safety Note on AN0009-6 for information on thehazards of Step and Touch-Potentials

Figure 4

Figure 5

18 Electrical Testing and Measurement Handbook – Vol 7

ACTUAL FIELD TEST

This actual ground test was conducted on a pad-mount

transformer in a rural mountain area The single-phase

trans-former is supplied by a 12470/7200 volt grounded wye primary

and the transformer is grounded by its own ground rod as well as

being tied to the system neutral which is grounded at multiple

points along the line The distribution line is overhead with just

the “dip” to the transformer being underground

Ground Test Data Remote Current Probe C2 @ 100 Feet P2 Distance from Transformer in Feet Instrument Reading in Ohms

Terminal T1 of the TPI MFT5010 tester was connected to

the transformer case ground with the short green lead The

remote Current Probe C2 was driven in the ground at a location

100 feet from the transformer and connected to Terminal T3 of

the instrument with the red test lead

Terminal T2 of the tester was connected, using the 100'black lead, to the P2 potential probe This ground stake was insertedinto the ground at 10' intervals and a resistance measurement wasmade at each location and recorded in the table above

The relatively constant readings in the 4 ohm range between

40 and 70 feet are a definite plateau that indicates sufficient lead

spacing The initial readings close to the transformer are lower, and

there is a pronounced “tip-up” as the P2 probe approaches the

remote current electrode C2

The measured ground resistance at 62 feet (62% of thedistance) was 4.3 ohms and is taken as the system ground resist-ance This is an excellent value for this type of an installation

20 Electrical Testing and Measurement Handbook – Vol 7

SAFETY NOTE – POSSIBLE EXISTENCE OF HAZARDOUS

STEP AND TOUCH POTENTIALS

It is recommended that rubber gloves be worn when driving

the ground rods and connecting the instrument leads

The possibility of a system fault occurring at the time the

ground test is being conducted is extremely remote

However, such a fault could result in enough current flow

through the earth to cause a possible hazardous step potential

between a probe and where the electrician is standing, or hazardous

touch potential between the probes and the system ground The

larger the system, in terms of available fault current, the greater the

possible risk

REEL ASSEMBLY

A SHOP-BUILT GROUND TEST WIRE REEL ASSEMBLY

This simple, low-cost, and easy-to-build wire reel assembly

is handy for making Ground (Earth) Resistance measurements on

large ground systems The unit shown below has 500 feet of wire

for testing medium-to-large ground fields typical of those found in

industrial plants and substations For testing even larger systems,

such as those installed for power generating plants, wire lengths of

1000 feet can be used Wrap-on wire markers are installed every

ten feet on the current lead to simplify placement of the remote

current and potential probes Your electrical distributor will

prob-ably have empty surplus reels available for the asking – the ones

shown below are about 12 inches in diameter The conductor is

standard #12 THHN Even though the TPI ERT1500 and the

MFT5010 use an AC test signal, the test results are unaffected by

the inductance of any wire left on the reels

1 Turn instrument on by pressing the green “ON/OFF”

button (far right) Continue holding the green button

down until the battery life indicator comes on

2 Check battery life indicator – make sure at least 20

percent remains

3 Check calibration – locate the 25W calibration gauge

supplied with the tester and clamp the meter around

any leg of the gauge

4 Observe instrument reading – the reading should bewithin 1.0W of gauge specification (25W) If reading

is correct, proceed to step 5 If not, clean instrumentand repeat steps 3 and 4 If you are not able to get theinstrument to read within 1.0W after cleaning instru-ment, do not proceed Have the instrument repaired

5 Remove instrument from gauge Observe instrumentreading with nothing in the clamps The readingshould be greater than 1000W OR read If either ofthese conditions is observed, continue to step 6 If not,clean instrument (see instructions below) and repeatsteps 3 through 5 If, after cleaning instrument, you arestill unable to get the instrument to perform asdescribed in steps 4 and 5, open the jaws approximate-

ly 1/2 inch and let them snap shut Make sure that thejaws close properly If the unit still does not performproperly, do not proceed Have the instrumentrepaired

6 Switch instrument to Current Mode (Press buttonlabeled “A” for Amps)

7 Clamp instrument around the ground wire or rod

8 Observe reading – if less than 1.0A, proceed to step 9

If between 1.0 and 5.0A, make note of reading andcontinue to step 9 If greater than 5A, terminate testand remove instrument from the ground wire or rodand correct the problem before re testing

9 Switch instrument to Resistance (W) Mode (Pressbutton labeled with Ohm (W) symbol)

10 Wait for reading to stabilize and record reading Lockreading by pressing “HOLD”

11 Remove instrument from ground wire or rod andreclamp to gauge

12 Observe reading – the reading should be within 1.0W

of gauge value If reading is OK – measurement isvalid If reading is wrong, clean instrument (seeinstructions below) and repeat from step 4

CLEANING THE HEADS

To ensure optimum performance, it is important to keepthe probe jaw mating surfaces clean at all times Failure to do somay result in erroneous readings To clean the probe jaws, use avery fine sandpaper (600 grit) to avoid scratching the surface,then gently clean with a soft cloth Make sure that the instru-

CLAMP-ON GROUND RESISTANCE TESTER

MODELS 3711 & 3731 STEP-BY-STEP USAGE

Chauvin Arnoux, Inc AEMC Instruments

step 2

step 3

22 Electrical Testing and Measurement Handbook – Vol 7

ment is oriented such that no debris or filings will fall into the

unit while cleaning Check with your finger afterwards to be

sure that no foreign material remains on the jaw surfaces (both

top and bottom)

CLAMP-ON GROUND RESISTANCE TESTING

The clamp-on ground resistance testing technique offers

the ability to measure the resistance without disconnecting the

ground This type of measurement also offers the advantage of

including the bonding to ground and the overall grounding

con-nection resistances

PRINCIPLES OF OPERATION

Usually, a common distribution line grounded system can

be simulated as a simple basic circuit as shown in Figure A or

an equivalent circuit, shown in Figure B If voltage E is applied

to any measured grounding system Rx through a special

trans-former (used in Models 3711 and 3731), current I flows through

the circuit, thereby establishing the following equation

Therefore, E/I = Rx is established If it is detected with E

kept constant, measured grounding resistance can be obtained

Refer again to Figures A and B Current is fed to a special

trans-former via a power amplifier from a 2.3 kHz constant voltage

oscillator This current is detected by a detection CT Only the 2.3

kHz signal frequency is amplified by a filter amplifier This occurs

before the A/D conversion and after synchronous rectification It

is then displayed on the LCD of the Model 3711/3731 meter

The filter amplifier is used to cut off both earth current at

commercial frequency and high-frequency noise Voltage is

detected by coils wound around the injection CT, which is then

amplified, rectified, and compared by a level comparator If the

clamp is not closed properly, an “open jaw” annunciator appears

2 That the earth is the return path to the point where theclamp-on meter is connected and not wire or othermetal structures (see Figure C)

3 If the measurement point is not connected to a parallel low resistance network (such as the case with

series-a single rod), series-a temporseries-ary pseries-ath mseries-ay be creseries-ated by necting a jumper cable from the measurement point to

con-a low resistcon-ance like con-a pole ground (see Figure D)

Every thing electrical from a toaster to a high-voltage

power line produces electric and magnetic fields Both the electric

and magnetic fields are strong close to an operating source The

strength of the electric field depends on the voltage and is present

in any live wire whether an electrical appliance is being used or not

Magnetic fields, on the other hand, are produced by electric

cur-rents and are only present when an appliance is operating i.e there

is no magnetic field when an electrical appliance is turned off

HEALTH EFFECTS

Currently there is no evidence that exposure to electric

fields is a health hazard (excluding electric shock) Whether

exposure to magnetic fields is equally harmless remains an open

question A large number of scientific studies performed on

ani-mals and cells have not found a health risk Some epidemiological

studies, however, have suggested a weak link between intense and

prolonged exposure to magnetic fields and childhood leukaemia

MAGNETIC FIELD UNITS

The strength of the magnetic field is expressed in units of

Tesla (T) or microtesla (µT) Another unit, which is commonly

used is the Gauss (G) or milligauss (mG), where 1 G is

equiva-lent to 10-4T (or 1 mG = 0.1 µT)

THE GAUSS METER

There is a range of different instruments that can measure

the magnetic field strength The gauss meter is a hand-held

device that provides a simple way of performing such

measure-ments ARPANSA has two different gauss meter models available

for hire, which are a Teslatronics Model 70 and a Sypris Model

4080 Both these instruments operate in a similar manner and they

are shown in the figure below

Both gauss meters measure alternating fields from 25 Hz

to 1000 Hz in units of mG They do not measure and will givefalse readings from mobile phones Readings taken very close (afew cm) to other electronic devices (as distinct from electricaldevices such as heaters, washing machines etc) may also givefalse readings Shaking or vibrating either unit may also givefalse readings Since the meters only measure varying magneticfields, they will not measure the earth’s magnetic field which isstatic and has a value of approximately 500 mG

When either meter is turned on, it will perform an initialself-diagnostic test by showing all available readouts on its digitaldisplay Following the initial test, the meter will display the mag-netic field intensity at the location where it is held or placed andthe intensity will change if moved accordingly If the negative sign

is still showing after the initial test, that indicates that the meter isrunning low on power and the battery needs to be replaced

PERFORMING MEASUREMENTS

Measurements of the magnetic field in the home are

general-ly taken in the middle of the room at about one metre from the ground

or in locations where people spend a significant amount of time, forexample, the bed Measurements should also be performed severaltimes over the course of a day This is to allow for possible variations

to electricity demand which presumably would peak during theevening at about 7.00 pm Measurements can also be made at anyother locations of interest

It is important to remember that, as mentioned earlier,research suggests that if any health effects exist, they are associat-

ed with prolonged magnetic field exposure Measurements takenwith the gauss meter are instantaneous (i.e measured at one point

in time) and do not accurately reflect prolonged exposure levels

TYPICAL MAGNETIC FIELD STRENGTHS

Magnetic fields within homes can vary at different locationsand also over time The actual strength of the field at a given loca-tion depends upon the number and kinds of sources and their dis-tance from the location of measurement Typical values measured

in areas away from electrical appliances are of the order of 2 mG.Magnetic fields from individual appliances can vary con-siderably as well, depending on the way they were designed andmanufactured One brand of hair dryer, for example, may gener-ate a stronger magnetic field than another In general, appliances,which use a high current (such as those which have an electricmotor) will lead to relatively high readings It should also benoted that different body parts will be exposed to different mag-netic field levels from the same appliance, depending on how farthat part of the body is from the appliance when in use Typicalvalues of magnetic fields measured at normal user distance fromsome common domestic electrical appliances are listed in thefollowing table

MEASURING MAGNETIC FIELDS ELECTRIC AND MAGNETIC FIELDS

Australian Radiation Protection and Nuclear Agency

24 Electrical Testing and Measurement Handbook – Vol 7

HOMES NEAR POWER LINES

The power lines that are present in typical

neighbour-hoods are called “distribution” lines and they usually carry less

voltage than “transmission” lines, which carry very high voltages

As stated earlier, however, it is the current and not the voltage that

is associated with the strength of the magnetic field Therefore,

proximity to high voltage lines will not necessarily give a high

reading unless those lines are also carrying a large current

Typical values of magnetic fields measured near power lines and

substations are listed in the table below

INTRODUCTION

Our daily use of electricity is taken for granted, yet

scien-tific and public concern has arisen about possible health effects

from electric and magnetic fields (EMF) that are created by the

use of electricity Because of this concern, the California Public

Utilities Commission authorized a statewide research, education

and technical assistance program on the health aspects of

expo-sure to magnetic fields and asked the Department of Health

Services to manage it Even though both electric and magnetic

fields are present with the use of electrical power, interest and

research have focused on the effects of 50 and 60 Hertz (Hz)

magnetic fields, called “power frequency” fields, from sources

such as power lines, appliances and wiring in buildings This is

because it is known that magnetic fields are difficult to shield

and because early scientific studies showed a possible relationship

between human exposure to certain magnetic field sources and

increased rates of cancer

Even now, scientists are not sure if there are health risks

from exposure to 50 and 60 Hz magnetic fields, or if so what is

a “safe” or “unsafe” level of exposure People frequently ask about

EMF risk when they are choosing where to live This choice

should include consideration of proven risks of the location, such

as the possibility of earthquake, flooding, or fire, or the presence

of traffic, radon, or air pollution To some people even limited

evidence for a possible EMF risk weighs heavily in their

deci-sions For others, different considerations take precedence There

really is no one right answer to these questions because each

sit-uation is unique

The California EMF Program developed this fact sheet to

give an overview of the present state of knowledge and provide

a basis for understanding the current limitations on the ability of

science to resolve questions about the possible health risks of

magnetic field exposure This paper describes electric and magnetic

fields, high field sources and how to interpret field measurements

once they are made It includes discussions of the controversy about

possible health effects, as well as current California state policy and

what the government is doing to address public concern

WHAT ARE ELECTRIC AND MAGNETIC FIELDS OR “EMF”

Before man-made electricity, humans were exposed only

to the magnetic field of the earth, electric fields caused by

charges in the clouds or by the static electricity of two objects

rubbing together, or the sudden electric and magnetic fields

caused by lightning Since the advent of commercial electricity

in the last century we have been increasingly surrounded by

man-made EMF generated by our power grid (composed of

pow-erlines, other electrical equipment, electrical wiring in buildings,

power tools, and appliances) as well as by higher frequency

sources such as radio and television waves and, more recently,

cellular telephone antennas

EMF: INVISIBLE LINES OF FORCE Wherever there is electricity, there are also electric andmagnetic fields, invisible lines of force created by the electriccharges Electric fields result from the strength of the chargewhile magnetic fields result from the motion of the charge, or thecurrent Electric fields are easily shielded: they may be weak-ened, distorted or blocked by conducting objects such as earth,trees, and buildings, but magnetic fields are not as readilyblocked Electric charges with opposite signs (positive and neg-ative) attract each other, while charges with the same sign repeleach other The forces of attraction and repulsion create electricfields whose strength is related to “voltage” (electrical pressure).These forces of attraction or repulsion are carried through spacefrom charge to charge by the electric field The electric field ismeasured in volts per meter (V/m) or in kilovolts per meter(kV/m) A group of charges moving in the same direction iscalled an “electric current.” When charges move they createadditional forces known as a “magnetic field.” The strength of amagnetic field is measured in “gauss” (G) or “tesla” (T), whilethe electric current is measured in “amperes” (amps) Thestrength of both electric and magnetic fields decrease as onemoves away from the source of these fields

FIELDS VARY IN TIME

An important feature of electric and magnetic fields is theway they vary in time Fields that are steady with respect todirection, rate of flow, and strength are called “direct current”(DC) fields Others, called “alternating current” (AC) fields,change their direction, rate of flow, and strength regularly overtime The magnetic field of the earth is DC because it changes solittle in one year that it can be considered constant However, themost commonly used type of electricity found in power lines and

in our homes and work places is the AC field AC current doesnot flow steadily in one direction, but moves back and forth Inthe U.S electrical distribution system it reverses direction 120times per second or “cycles” 60 times per second (the directionreverses twice in one complete cycle) The rate at which the ACcurrent flow changes direction is expressed in “cycles per sec-ond” or “Hertz” (Hz) The power systems in the Untied Statesoperate at 60 Hz, while 50 Hz is commonplace elsewhere Thisfact sheet focuses on “power frequency” 60 Hz fields and not thehigher frequency fields generated by sources such as cellularphone antennas

DESCRIBING MAGNETIC FIELDS The concentration of a chemical in water can be described

by citing a single number Unlike chemicals, alternating electricand magnetic fields have wave-like properties and can bedescribed in several different ways, like sound A sound can beloud or soft (strength), high or low-pitched (frequency), have

ELECTRIC AND MAGNETIC FIELDS

MEASUREMENTS AND POSSIBLE EFFECT ON HUMAN HEALTH

— WHAT WE KNOW AND WHAT WE DON’T KNOW

California Department of Health Services and the Public Health Institute

California Electric and Magnetic Fields Program

26 Electricity Testing and Measurement Handbook – Vol 7

periods of sudden loudness or a constant tone, and can be pure or

jarring Similarly, magnetic fields can be strong or weak, be of

high frequency (radio waves) or low frequency (powerline waves),

have sudden increases (“transients”) or a constant strength, consist

of one pure frequency or a single dominant frequency with some

distortion of other higher frequencies (“harmonics”) It is also

important to describe the direction of magnetic fields in relation to

the flow of current For instance, if a magnetic field oscillates back

and forth in a line it is “linearly polarized.” It may also be

impor-tant to describe how a field’s direction relates to other physical

conditions such as the earth’s static magnetic fields

MEASURING MAGNETIC FIELDS AND IDENTIFYING THE

SOURCES OF ELEVATED FIELDS

MEASURING MAGNETIC FIELD STRENGTH

The strength or intensity of magnetic fields is commonly

measured in a unit called a Gauss or Tesla by magnetic field

meters called “gaussmeters.” A milligauss (mG) is a thousandth

of a gauss, and a microtesla (uT) is a millionth of a tesla (one

milligauss is the same as 0.1 microtesla) The magnetic field

strength in the middle of a typical living room measures about

0.7 milligauss or 0.07 microtesla As noted above, the strength of

the magnetic field is only one component of the mixture that

characterizes the field in a particular area Measuring only

mag-netic field strength may not capture all the relevant information

any more than the decibel volume of the music you are playing

captures the music’s full impact The main health studies to date

have only measured magnetic field strength directly or

indirect-ly and assessed its association with disease Some scientists

won-der if the weak association between measured magnetic fields

and cancer in these studies might appear stronger if we knew

which aspect of the EMF mixture to measure Other scientists

wonder if any such aspect exists

WHERE ARE WE EXPOSED TO 60 HZ EMF?

There are “power frequency” electric and magnetic fields

almost everywhere we go because 60 Hz electric power is so

widely used Exposure to magnetic fields comes from many

sources, like high voltage “transmission” lines (usually on metal

towers) carrying electricity from generating plants to communities

and “distribution” lines (usually on wooden poles) bringing

elec-tricity to our homes, schools, and work places Other sources of

exposure are internal wiring in buildings, currents in grounding

paths (where low voltage electricity returns to the system in

plumbing pipes), and electric appliances such as TV monitors,

radios, hair dryers and electric blankets Sources with high voltage

produce strong electric fields, while sources with strong currents

produce strong magnetic fields The strength of both electric and

magnetic fields weakens with increasing distance from the source

(table 1) Magnetic field strength falls off more rapidly with distance

from “point” sources such as appliances than from “line” sources

(power lines) The magnetic field is down to “background” level

(supposed to be no greater than that found in nature) 3-4 feet from

an appliance, while it reaches background level around 60-200 feet

from a distribution line and 300-1000 feet from a transmission line

Fields and currents that occur at the same place can interact to

strengthen or weaken the total effect Hence, the strength of the

fields depends not only on the distance of the source but also the

dis-tance and location of other nearby sources

IDENTIFYING SOURCES OF ELEVATED MAGNETIC FIELDS Sometimes fairly simple measurements can identify theexternal or internal sources creating elevated magnetic fields.For example, turning off the main power switch of the house canrule out sources from use of power indoors Magnetic field meas-urements made at different distances from power lines can helppinpoint them as sources of elevated residential magnetic fields.Often, however, it takes some detective work to find the majorsources of elevated magnetic fields in or near a home Currents

in grounding paths (where low voltage electricity returns to thesystem in plumbing pipes) and some common wiring errors canlead to situations in which source identification is difficult andrequires a trained technician It is almost always possible to findand correct the sources of elevated magnetic fields when they aredue to faulty electrical wiring, grounding problems, or appli-ances such as lighting fixtures

60 HZ MAGNETIC FIELD EXPOSURE DURING A TYPICAL DAY Exposure assessment studies of adults who wore meas-urement meters for a 24- to 48-hour period suggest that the aver-age magnetic field level encountered during a typical 24 hours isabout 1 mG About 40% of magnetic field exposures found inhomes come from nearby power lines, while 60% come fromother sources such as stray currents running back to the electri-cal system through the grounding on plumbing and cables, cur-rent “loops” due to incorrect internal wiring in the home, andbrief exposure to appliances and electrical tools

Table 1 Examples of magnetic field strengths at particular distances from appliance surfaces

MAGNETIC FIELD SURVEY OF HOMES IN THE SAN FRANCISCO BAY AREA

The California Department of Health Services surveyed

homes in the San Francisco Bay Area in the mid-1990s In this

study, magnetic field measurements were taken in the middle of

the bedroom, family room and kitchen and at the front door of

these homes under normal power conditions (any appliances or

electrical devices turned on at the onset of the measurement period

were left on) As shown in table 2, about half the houses in the

Bay Area had an average level below 0.71 mG and 90 percent

had average levels below 1.58 mG

MAGNETIC FIELDS GENERATED BY CURRENT FLOWING THROUGH WIRES CAN

BE REDUCED

Two wires with current flowing in opposite directions create

magnetic fields going in opposite directions If the wires are

placed close together and have currents of similar magnitude the

magnetic fields cancel each other This principle is often used to

lower magnetic fields For example, an underground distribution

cable has a “hot” line (carrying current to the user) and a “neutral”

line (carrying it away) that generate low magnetic fields when they

are placed close together The underground cables can be placed

close together because it is possible to insulate them heavily to

pre-vent arcing Overhead power lines cannot be placed this close

together because of the weight of the needed insulation and the

need for worker safety For most distribution and transmission

lines, however, California utilities use three-wire or four-wire

sys-tems The current in these lines alternates in strength and direction

in slightly different phases (not alternating completely together) It

is sometimes possible to optimize these phase differences so that

the magnetic fields from the wires cancel each other

WHAT CAN WE SAY ABOUT A MEASUREMENT ONCE WE HAVE IT?

A concerned person would like to know if the

measure-ments found in his or her home are “safe” or “unsafe.” Right now,

most scientists do not feel that the data are solid enough to make

predictions about the health risks of magnetic field strength

When magnetic field exposure (or its estimate) increases there is

no evident orderly increase of a health risk The highest level of

magnetic field strength measured in homes is below the intensity

found in almost all the cellular and animal experiments that have

produced subtle biological effects This makes scientists and

pol-icy makers reluctant to set health-based standards for magnetic

field exposures However, it is possible to find out how

measure-ments in your home compare to other homes and if these

meas-urements are “typical” or not The information in tables 1 and 2

may be helpful in deciding if your home is typical

DOSE-RESPONSE RELATIONSHIP

A special problem in the study of health effects of ronmental factors is how to measure exposure in a way that ade-quately reflects the true amount of the person’s exposure to thesubstance being studied This true amount is called the “dose.”With cigarette smoke and toxic chemicals, there is a positiverelationship between the size (or strength) of the dose and theadverse health effect it produces: the higher the dose, the greaterthe effect With magnetic fields, however, some laboratory evi-dence suggests that this is not always the case, and very confus-ing relationships have been seen Biological effects or changesappear at strengths of certain levels, disappear at higher levels,only to appear again at still higher levels Varying the frequency(speed of alternation), for example from 60 Hz to 120 Hz, showssimilar “effect windows” of magnetic fields To complicatethings further, some laboratory experiments have shown aneffect with intermittent (“pulsed”) exposures, others with

envi-“spikes” or transients, and still others with continuous exposure.There is some evidence that the orientation of alternating fields

in relation to the direction of the earth’s static magnetic field isalso important in making a biological effect Generally, theeffects observed are only biological changes that may or may nottranslate into true health effects

LIMITATIONS OF DIRECT MAGNETIC FIELD MEASUREMENTS Those human health studies investigating the relationship

of magnetic field exposure and cancer measured magnetic fieldsusing one-time, short-term measures (i.e., for 24 hours) of onearea such as the bedroom, or one-time spot measurements (i.e.,for one minute) in several different rooms of the participants’homes It was assumed that these home measurements adequate-

ly estimate a person’s total exposure However, these measurescan not be used to assess the biological importance of the length

of exposure, the number of times there are high exposures, or thepresence of other components of the field such as harmonics.Also, field intensity (strength) varies at different times of dayand different seasons, depending on electricity use Dinnertimereadings are often higher than readings in the middle of the night

In addition, an area measure may not reflect a personal exposurethat is dependent on the amount of time a person spends in thearea measured

CONTROVERSY ABOUT POSSIBLE HEALTH EFFECTS

The controversy about EMF health effects derives from:1) the fact that many scientists believe power line magneticfields emit little energy and are therefore too weak to have anyeffect on cells; 2) the inconclusive nature of laboratory experi-ments; and 3) the fact that epidemiological studies of peopleexposed to high EMF are inconclusive

1 WEAK FIELDS MAY HAVE TOO LITTLE ENERGY TO CAUSE BIOLOGICAL EFFECTSThe electromagnetic spectrum covers a large range of fre-quencies (expressed in cycles per second or Hertz) The higherthe frequency, the greater the amount of energy in the field X-rays have very high frequencies, and are able to ionize moleculesand break chemical bonds, which damages genetic material andcan eventually result in cancer and other health disorders Highfrequency microwave fields have less energy than x-rays, butstill enough to be absorbed by water in body tissues, heatingthem and possibly resulting in burns Radio frequency fieldsfrom radio and TV transmitters are another step weaker than micro-waves Although they alternate millions of times per second, they

Table 2 Distribution of average magnetic field strength of San Francisco Bay Area homes.

28 Electricity Testing and Measurement Handbook – Vol 7

can’t ionize molecules and can only heat tissues close to the

transmitter Electric power fields (50 and 60 Hz) have much

lower frequencies than even radio waves and hence emit very

low energy levels that do not cause heating or breakage of bonds

They do create electrical currents in the body, but in most cases

these currents are much weaker than those normally existing in

living organisms For these reasons, many scientists argue that it

is unlikely that 60 Hz power frequency magnetic fields at the

strengths commonly found in the environment have any physical

or biological effects on the body

2 INCONSISTENT LABORATORY RESULTS

As stated above, 60 Hz power frequency magnetic fields do

create weak electric currents in the bodies of people and animals

In the mid-1970s a variety of laboratory studies in cell cultures and

whole animals demonstrated that these fields produce biological

changes when applied in intensities of hundreds or thousands of

milligauss Some scientists observed effects at lower strengths, but

average daily personal exposure is only about 1 mG Biological

effects that seem to be attributable to magnetic fields are subtle and

difficult to reproduce These studies are continuing in an effort to

understand how magnetic fields affect living tissue Some

labora-tory scientists have found that magnetic fields can produce

changes in the levels of specific chemicals the human body makes

(such as the hormone melatonin), as well as changes in the

func-tioning of nerve cells and nervous systems of other animals

However, the jury is still out as to whether this type of change can

lead to any increased risk to human health

In the mid-1990s, scientists conducted a series of EMf

animal studies Most of these studies showed little or no

associ-ation between EMF and cancer or adverse reproductive effects

This convinced some scientists that EMF’s were harmless

However, others pointed out that the animals’ EMF exposures in

these studies might not adequately capture some aspect of EMF

exposure that could have biological effects on humans

3 INCONCLUSIVE EPIDEMIOLOGICAL STUDIES

Epidemiology examines the health of groups of people,

and epidemiological studies make statistical comparisons about

how often diseases occur in “exposed” and “nonexposed”

groups Studies in which the disease rate is higher for the

exposed group than nonexposed (said to have “positive” results)

do not necessarily show a direct cause for disease, but rather

indicate that there is some sort of relationship between exposure

and disease Most epidemiological studies of magnetic fields

have been of two types One kind focused on children with cancer

to see whether their home magnetic field measurements were

higher or if they were more likely to live in homes with overhead

powerlines carrying high current than a comparable group of

children without cancer The other type of study looked at rates

of death and disease of adults assumed to be heavily exposed to

magnetic fields at work, with exposure often indirectly assessed

by using job titles, to determine if their rates were higher than

adults assumed to be working in low magnetic field environments

CHILDHOOD CANCER STUDIES

Public concern has arisen because of media reports about

epidemiological studies that showed an association between

childhood cancer and proximity to high current-carrying

over-head power lines In 1996, a special committee of the National

Research Council (NRC) made a careful review of 11

epidemio-logical studies examining the relationship between childhood

leukemia and residential proximity to this type of power lines.1

For these studies, a child’s exposure to magnetic fields was mated three ways First, the type and proximity of power lines(“wire codes”) near the child’s home was assessed Those houseswith lines nearby with the potential to carry high current wereclassified as “high current configuration” and were assumed tohave higher magnetic field levels (due to higher current) thanhouses near lower current configuration power lines (figure 1).Second, exposure was estimated by measurements of magneticfields taken in the child’s home at the time of the study – oftenmany years after diagnosis of their cancer And third, exposurewas approximated by estimating what the home magnetic fieldlevels were right after the children were diagnosed, using linedistance from the house and past utility records of current flow

esti-in the lesti-ines duresti-ing the appropriate time period

The NRC made a statistical summary and comparison ofthese eleven studies They concluded that children living in highcurrent configuration houses are 1.5 times as likely to developchildhood leukemia than children in other homes Despite thisconclusion, the NRC was a unable to explain this elevated riskand recommended that more research be done to help clarify theissue One reason for this uncertainty is that wire-code classifi-cation assumes that houses with high wire-codes have highermagnetic field levels than low wire-code houses, but high wire-codes may also be a proxy for some type of exposure besidesmagnetic fields that is not yet understood For example, highwire-code houses tend to have higher traffic density nearby,resulting in higher air pollution levels However, traffic densityseems to be an unlikely explanation for the wire-code associationfound in these studies

In 1997, the NRC statement seemed to be contradicted bythe findings of Dr M S Linet of the National Cancer Institute in

a large epidemiological study1i Her researchers estimated sure to magnetic fields in two ways, wire-codes as defined above(based on distance of different types of power lines near thehome) and home area measurements The study found no associ-ation between living in high wire-code houses and childhoodleukemia On the other hand, the study found that children living

expo-in houses with high average magnetic field levels did have higherrates of cancer in general

Figure 1 Summary of results of power line distance(“wire code”) and childhood leukemia studies.

THE EMF RAPID PROGRAM WORKING GROUP STATEMENT ON CHILDHOOD LEUKEMIA

In 1998, a working group of experts gathered by the

feder-al EMF RAPID program (see “Governmentfeder-al Regulation,” below)

reviewed the research on the possible health risks associated with

EMF A majority felt that the epidemiology studies of childhood

leukemia provide enough evidence to classify EMF as a “possible

human carcinogen,” meaning they think it might cause cancer

This does not mean that it definitely causes cancer, however The

working group’s findings are published in a report posted on the

program’s Web site (see address below)

IF REAL, HOW IMPORTANT WOULD THIS RISK OF CHILDHOOD LEUKEMIA BE?

Each year an average of six cases of leukemia are

diag-nosed per 100,000 children Six percent of American houses are

near high-current-carrying power lines.2If the epidemiological

association is correct that means that in such houses there would

be three additional cases of leukemia among 100,000 children

due to the effects of EMF from the nearby power lines (This is

almost the increased risk of lung cancer of an adult nonsmoker

who lives in a smoking household.) Among the 500,000 children

in California who live nearest high-current-carrying power lines

there could be a theoretical 15 extra cases of leukemia each year

compared to the number of cases if they lived further away In

California, we regulate chemicals whose typical exposures

gen-erate a theoretical life-time risk of one per 100,000 An added

risk of three sick children per 100,000 per year is larger than this

From an individual’s point of view, this risk, if real, would be

small: 99,991 out of 100,000 children would not get leukemia

each year

OCCUPATIONAL STUDIES

The occupational studies looking at magnetic field

expo-sure and various health outcomes show mixed results

Occupations assumed to have higher than normal magnetic field

levels included electricians, telephone linemen, electric welders,

electronic technicians, utility workers, electrical engineers and

sewing machine operators In general, but not always, workers of

these occupations were more likely to have higher rates of brain

tumors, leukemia, testicular tumors and male breast cancer than

expected A particular brain tumor (astrocytoma) occurred more

often among men who worked for many years in jobs with high

estimated exposure levels such as electricians, linemen, and

elec-trical engineers.3A large study of Canadian and French utility

workers found an association between estimated high magnetic

field exposures based on area measures of certain occupations

and myeloid leukemia, a rare type of blood cancer.4On the other

hand, another large study found no increase in mortality from

brain tumors, leukemia or other cancers among electrical

work-ers with estimated high magnetic field exposure over many

years.5 Differences among study results may exist simply

because the studies used different study populations and methods

for estimating high occupational magnetic field exposure Also,

these surrogate measures estimating high occupational magnetic

field levels could be proxies for other types of exposure at work

besides magnetic fields

COMPARING THE SCIENTIFIC EVIDENCE ON MAGNETIC FIELDS TO THAT OF

ENVIRONMENTAL TOBACCO SMOKE

There are regulations in place protecting us from

environ-mental tobacco smoke They are based on the strength of its

asso-ciation with disease and the consistent epidemiological evidence

for it What’s the difference between this evidence and that for

magnetic fields? First, no magnetic field epidemiological studyhas found an association with disease that is as strong as thatimplicating a two-pack-a-day smoking habit The strength of theassociation found for leukemia in electric train engineers, whoare exposed to magnetic fields of hundreds of milligauss all daylong, is no stronger than the strength of the association relatingresidential magnetic field levels (generally less than 10 mG) tochildhood leukemia Second, there is no laboratory evidenceabout magnetic field exposure that is as convincing as that forlung cancer and smoking— magnetic field animal studies havebeen inconsistent These differences make scientists much morecautious about interpreting the magnetic field epidemiology asdangerous than the environmental tobacco smoke epidemiology

GOVERNMENTAL REGULATION

STATE REGULATIONS Lack of understanding has kept scientists from recom-mending any health-based regulations Despite this, several stateshave adopted regulations governing transmission line-generatedmagnetic fields at the edge of the “right-of-way” (“ROW,” the areaimmediately surrounding power lines left clear for access formaintenance and repairs) because of concern about the risk ofelectric shock from strong electric fields present in these areas(table 3) All current regulations relate to transmission lines; nonegovern distribution lines, substations, appliances or other sources

of electric and magnetic fields

The California Department of Education requires minimumdistances between new schools and the edge of transmission linerights-of-way The setback guidelines are: 100 feet for 50-133 kVlines, 150 feet for 220-230 kV lines, and 350 feet for 500-550 kVlines Once again, these were not based on specific biologicalevidence, but on the rationale that the electric field drops tobackground levels at the specified distances

The California Public Utilities Commission (CPUC),upon the recommendation of a Consensus Group composed ofcitizens, utility representatives, union representatives, and publicofficials, recommended that the state’s investor-owned utilitiescarry out “no and low cost EMF avoidance measures” in con-struction of new and upgraded utility projects This means that4% of the total project cost is allocated to mitigation measures ifthese measures will reduce magnetic field strength by at least15% The strategy is to address public concern and cope with

Table 3 Transmission line EMF standards and guide-lines adopted by certain states for utilities’ rights-of-way (ROW).

30 Electricity Testing and Measurement Handbook – Vol 7

potential but uncertain risks until a policy based on scientific fact

can be developed The CPUC also followed the Consensus

Group’s recommendation to establish the research, education and

technical assistance programs of the California EMF Program

under the guidance of the California Department of Health

Services It is expected to provide information that will be useful

to those responsible for making public policy in the future

FEDERAL EFFORTS

At the Federal level, the Federal Energy Policy Act of 1992

included a five-year program of electric and magnetic field (EMF)

Research and Public Information Dissemination (EMF-RAPID)

The EMF-RAPID Program asked these questions: Does exposure

to EMF produced by power generation, transmission, and use of

electric energy pose a risk to human health? If so, how significant

is the risk, who is at risk, and how can the risk be reduced?

In 1998, a working group of experts gathered by the

EMF-RAPID Program met to review the research that has been

done on the possible health risks associated with EMF This

group reviewed all of the studies that have been done on the

sub-ject, and then voted on whether they believed that exposure to

EMF might be a health risk They then published a report

describing their findings A majority of the scientists on this

working group voted that the epidemiology studies of childhood

leukemia and residential EMF exposures provide enough

evi-dence to classify EMF as a “possible human carcinogen.”6This

means that, based on the evidence, these researchers believe that

it is possible that EMF causes childhood leukemia, but they are

not sure About half of the group’s members thought that there is

also some evidence that workplace exposure to EMF is

associat-ed with chronic lymphocytic leukemia in adults The group also

concluded that there was not enough evidence to determine

whether EMF exposure might cause other diseases.6

The EMF-RAPID Program released its final report to

Congress in 1999 This report explains the program’s findings,

including the results of its working group and many research

projects The final report states that “the NIEHS believes that

there is weak evidence for possible health effects from [power

frequency] ELF-EMF exposures, and until stronger evidence

changes this opinion, inexpensive and safe reductions should be

encouraged.”7(page 38) The report specifically suggests

educat-ing power companies and individuals about ways to reduce EMF

exposure, and encouraging companies to reduce the fields

creat-ed by appliances that they make, when they can do so

inexpen-sively7 (page 38) For more information on the EMF-RAPID

program or to look at these reports, contact the EMF-RAPID

Program, National Institute of Environmental Health Sciences,

National Institutes of Health, P.O Box 12233, Research Triangle

Park, North Carolina27709, or visit their Web site at

http://www.niehs.nih.gov/ emfrapid When ordering a copy of

the final report, refer to NIH publication number 99-4493

CONCLUSION

Public concern about possible health hazards from the

delivery and use of electric power is based on data that give

cause for concern, but which are still incomplete and

inconclu-sive and in some cases contradictory A good deal of research is

underway to resolve these questions and uncertainties Until we

have more information, you can use “no and low cost avoidance”

by limiting exposure when this can be done at reasonable cost

and with reasonable effort, like moving an electric clock a few

feet away from a bedside table or sitting further away from the

computer monitor Table 1 shows how quickly fields fall off asone moves away from appliances – they virtually disappear at 3-

5 feet You might stop using an electric appliance you do not

real-ly need You may also consider home testing, which can identifyfaulty electrical wiring that can produce shock hazards and cur-rent code violations as well as elevated magnetic fields InCalifornia, the investor-owned utilities are required by the CPUC

to provide magnetic field measurement at no charge to their tomers So far, in the absence of conclusive scientific evidence,there is no sufficient basis for enacting laws or regulations to limitpeople’s exposure to EMF, so it is up to individuals to decide whatavoidance measures to take, based on the information available REFERENCES

cus-1 a) Wertheimer N et al Electrical wiring configurations andchildhood cancer American Journal of Epidemiology.1979; 109:273-84

b) Fulton JP et al Electrical wiring configurations and hood leukemia in Rhode Island American Journal ofEpidemiology 1979; 111:292-96

child-c) Savitz DA et al Case control study of childhood cancer andexposure to 60-Hz magnetic fields American Journal ofEpidemiology 1988; 128:21-38

d) Coleman M et al Leukaemia and residence near electricitytransmission equipment: A case-control study BritishJournal of Cancer 1989; 60:793-98

e) London SJ et al Exposure to residential electric and netic fields and risk of childhood leukemia AmericanJournal of Epidemiology 1991; 134:923-37

mag-f) Feychting M et al Magnetic fields and cancer in childrenresiding near Swedish high-voltage power lines AmericanJournal of Epidemiology 1993; 138:467-81

g) Fajardo-Gutierrez AJ et al Residence close to high-tensionelectric power lines and its association with leukemia in chil-dren (Spanish) Biol Med Hosp Infant Mex 1993; 50:32-38 h) Petridou ED et al Age of exposure to infections and risk ofchildhood leukaemia British Medical Journal 1993; 307:774 i) Linet MS et al Residential exposure to magnetic fields andacute lymphoblastic leukemia in children New EnglandJournal of Medicine 1997; 337:1-7

2 Zaffanella L Survey of residential magnetic sources EPRIFinal Report 1993; No TR 102759-v1 No TR 102759-v2

3 Savitz DA et al Magnetic field exposure in relation toleukemiaand brain cancer mortality and electric utility workers.American Journal of Epidemiology 1995; 141: 1-12

4 Theriault G et al Cancer risk associated with posure to magnetic fields among utility workers in Ontario andQuebec, Canada and France American Journal of Epidemiology.1994; 139: 550-572

occupationalex-5 Sahl JD et al Cohort and nested case-control studies ofhematopoietic cancers and brain cancer among electric utilityworkers Epidemiology 1993; 4: 104-114

6 National Institute of Environmental Health Sciences.Assessment of health effects from exposure to power-line fre-quency electric and magnetic fields NIEH Working GroupReport 1998

7 National Institute of Environmental Health Sciences Healtheffects from exposure to power-line frequency electric and mag-netic fields NIEH Final Report ot Congress 1998

Engineers and technicians often need to make “floating”

measurements where neither point of the measurement is at

ground (earth) potential This measurement is often referred to as

a differential measurement “Signal common” may be elevated to

hundreds of volts from earth

In addition, many of these differential measurements

require the rejection of high common-mode signals*1in order to

evaluate low-level differential signals Unwanted ground currents

can also add bothersome hum and ground loops Too often, users

resort to the use of potentially dangerous measurement techniques

to overcome these problems

The TPS2000 Series oscilloscopes use innovative Isolated

Channel technology to deliver the world’s first 4-isolated-channel,

battery-operated oscilloscope to allow engineers and technicians

to make multi-channel isolated measurements quickly, accurately

and affordably – all designed with your safety in mind

FLOATING AN OSCILLOSCOPE: A DEFINITION

“Floating” a ground-referenced oscilloscope is the

tech-nique of defeating the oscilloscope’s protective grounding system –

disconnecting “signal common” from earth, by either defeating the

grounding system or using an isolation transformer This technique

allows accessible parts of the instrument such as chassis, cabinet,

and connectors to assume the potential of the probe ground lead

connection point This technique is dangerous, not only from the

standpoint of elevated voltages present on the oscilloscope (a

shock hazard to the operator), but also due to cumulative

stress-es on the oscilloscope’s power transformer insulation This strstress-ess

may not cause immediate failure, but may lead to future

danger-ous failures (a shock and fire hazard), even after returning the

oscilloscope to properly grounded operation

Not only is floating a ground-referenced oscilloscope

dan-gerous, but the measurements are often inaccurate This potential

inaccuracy results from the total capacitance of the oscilloscope

chassis being directly connected to the circuit-under-test at the

point where the ground lead is connected

A GUIDE TO MAKING QUICK, ACCURATE AND

AFFORDABLE FLOATING MEASUREMENTS

There are several products that enable you to make

float-ing measurements, but they may lack the versatility, accuracy or

affordability that you need In addition, there are four key

meas-urement considerations that a user needs to take into account

when selecting the right product to make an accurate floating or

differential measurement:

1 – What is the differential measurement range?

2 – What is the common mode measurement range?

3 – What are the loading characteristics of the probe? Are they balanced or unbalanced?

4 – What is the Common Mode Rejection Ratio (CMRR)over the measurement frequency range?

A common, but risky, practice is to disconnect the scope’s AC main power cord ground and attach the probe groundlead to one of the test points Tektronix strongly recommendsagainst this unsafe measurement practice Unfortunately, thispractice puts the instrument chassis, which is no longer grounded

oscillo-to earth, at the same voltage as the test point that the probeground lead is connected to The user touching the instrument

A NEW APPROACH TO QUICK, ACCURATE,

AFFORDABLE FLOATING MEASUREMENTS

Tektronix IsolatedChannel Technology

*1 A “common-mode signal” is defined as a signal that is present at both points

in a circuit Typically referenced to ground, it is identical in amplitude, frequency,

and phase Making a floating measurement between two points requires rejecting

the “common-mode signal” so the difference signal can be displayed.

Management and Safety in the Workplace

While the subject of this technical note is floating measurements, some tions of terms and general precautions must be understood before proceeding Historically, floating measurements have been made by knowingly defeating the built-in safety ground features of oscilloscopes or measurement instruments in various manners.

defini-THIS IS AN UNSAFE AND DANGEROUS PRACTICE AND SHOULD NEVER BE DONE!

Instead, this technical note describes instruments, accessories, and practices that can make these measurements safely as long as standard safety practices and precautions are observed.

When making measurements on instruments or circuits that are capable of ering dangerously high-voltage, high-current power, measurement technicians should always treat exposed circuits, bus-bars, etc., as being potentially “live,” even when circuits have been shut off or disconnected This is particularly true when connecting or disconnecting probes or test leads.

deliv-32 Electricity Testing and Measurement Handbook – Vol 7

becomes the shortest path to earth ground Figure 1 illustrates

this dangerous situation V1 is the “offset” voltage above true

ground, and VMeas is the voltage to be measured

Depending upon the unit-under-test (UUT), V1 may be

hundreds of volts, while VMeas might be a fraction of a volt

Floating the chassis ground in this manner threatens the

user, the UUT, and the instrument In addition, it violates

indus-trial health and safety regulations, and yields poor measurement

results Moreover, line-powered instruments exhibit a large

par-asitic capacitance when floated above earth ground As a result,

floating measurements will be corrupted by ringing, as shown in

Figure 2

Battery-operated oscilloscopes, such as the TDS3000B

Series oscilloscopes, when operated from AC line power using a

standard power cord, exhibit the same limitations as traditional

oscilloscopes However, AC power is not always available where

you want to make oscilloscope measurements In the case of the

TDS3000B Series oscilloscopes, the optional battery pack

(TDS3BATB) allows you to operate the oscilloscope without the

need for AC power However, it can only make safe floating

measurements up to 30 VRMS

Traditional oscilloscopes emphasize performance (bandwidth,

versatility), trading off the ability to make floating measurements

DIFFERENTIAL OR ISOLATED PROBES

Differential or isolated probes offer a safe and reliable way

to adapt a grounded oscilloscope to make floating measurements

Neither of the two probe contacts need be at earth ground and the

probe system as a whole is isolated from the oscilloscope’s chassis

ground

Differential probes offer a balanced impedance load to the

device-under-test (DUT) However, they add a layer of cost and

complexity to the measurement apparatus They may require anindependent power supply, and their gain and offset characteristicsmust be factored into every measurement Differential probe-equipped oscilloscopes emphasize performance and safety (band-width, isolation), trading off form-factor benefits such as portabilityand cost

SIGNAL FIDELITY BEGINS AT THE PROBE TIP

An oscilloscope is actually a measurement system ing of preamplifiers, acquisition/measurement circuits, displays,and probes The role of the probe is sometimes overlooked.Nevertheless, improper probes or probing techniques can affectthe measurement outcome Obviously, it’s essential to use compat-ible probes that match the instrument’s bandwidth and impedance.Less understood is the effect of ground-lead inductance Aslead length increases, parasitic inductance increases (Lparasitic inFigure A) Lparasitic is in the signal path and forms a resonant LCcircuit with the inherent parasitic capacitance of the oscilloscope(Cparasitic) As Lparasitic increases, the resonant frequencydecreases, causing “ringing” (see Figure 2) that visibly interfereswith the measured signal Simply stated, the common lead must be

consist-as short consist-as physical constraints of the circuit-under-test will allow

In regard to capacitance, even isolated, battery-poweredoscilloscopes exhibit capacitance with respect to earth ground InFigure A, Cparasitic describes the oscilloscope’s parasitic capaci-tance from its ground reference (through the isolated housing) toearth ground Like parasitic inductance, Cparasitic must be kept to aminimum in order to force the resonant frequency of the LC circuit

as high as possible If Cparasitic is large, ringing may occur withinthe test frequency range, hampering the measurement

An instrument’s parasitic capacitance to ground is

dictat-ed by its internal design The physical environment can alsoprompt ringing Holding the instrument or placing it on a largeconductive surface during measurements can actually increaseCparasitic and lead to ringing For extremely sensitive measure-ments, it might even be necessary to suspend the oscilloscope inmid-air!

A NEW APPROACH TO QUICK, ACCURATE, AFFORDABLE FLOATING MEASUREMENTS

The most common method of isolation in a wide bandwidthoscilloscope system in use today is a two-path approach in whichthe input signal is broken up into two signals: low frequency andhigh frequency This approach requires expensive optocouplersand wideband linear transformers for each input channel The TPS2000 Series uses an innovative approach,Isolated Channel technology, which eliminates the two-pathmethod and uses only one wideband signal path for each inputchannel – from DC to the bandwidth of the oscilloscope This

Figure 1: A floating measurement in which dangerous voltages occur on the oscilloscope

chassis V1 may be hundreds of volts.

Figure 2: Ringing caused by parasitic inductance and capacitance distorts the signal and

invalidates measurements

Figure 2: Parasitic inductance and capacitance can affect measurement quality

patent-pending technology enables Tektronix to offer the world’s

first four-input Isolated Channel, low-cost, battery-operated

oscilloscope, featuring eight hours of continuous battery

opera-tion The TPS2000 Series oscilloscopes are ideal for engineers

and technicians who need to make four-channel isolated

measure-ments and need the performance and ease-of- use of a low-cost,

battery operated oscilloscope

The TPS2000 Series’ four Isolated Channel input

architec-ture provides true and complete channel-to-channel isolation for

both the “positive” input and the “negative reference” leads,

including the external trigger input Figure 3 illustrates the Isolated

Channel concept

The most demanding floating measurement requirements

are found in power control circuits, such as motor controllers and

uninterruptible power supplies, and industrial equipment In such

application areas, voltages and currents may be large enough to

present a threat to users and test equipment

Isolated Channel technology is the preferred solution for

measurement quality and is designed with your safety in mind.*2

The TPS2000 oscilloscopes offer an ideal solution when a large

common mode signal is present True channel-to-channel isolation

minimizes parasitic effects; the smaller mass of the measurement

system is less prone to interaction with the environment

A properly isolated battery-powered instrument doesn’t

concern itself with earth ground Each of its probes has a “Negative

Reference” lead that is isolated from the instrument’s chassis, rather

than a fixed ground lead Moreover, the “Negative Reference” lead

of each input channel is isolated from that of all other channels

This is the best insurance against dangerous short circuits It also

minimizes the signal degrading impedance that hampers

measure-ment quality in single-point grounded instrumeasure-ments

The TPS2000 Series oscilloscope inputs are always

float-ing whether operated from battery power or connected to AC

power through an AC power adapter Thus, these oscilloscopes

do not exhibit the same limitations as traditional oscilloscopes

SPEED DEBUG AND CHARACTERIZATION WITH DRT

SAMPLING TECHNOLOGY (TIP)

The TPS2000 Series oscilloscopes offer digital real-time

(DRT) acquisition technology that allows you to characterize a

wide range of signal types on up to four channels

simultaneous-ly Up to 2 GS/s real-time sample rate is the key to the

extraordi-nary bandwidth – 200 MHz in the TPS2024 This bandwidth/

sample rate combination makes it easy to capture the

high-fre-quency information, such as glitches and edge anomalies thateludes other oscilloscopes in its class, so that you can be sure to get

a complete view of your signal to speed debug and characterization

MAKING QUICK, ACCURATE FLOATING MEASUREMENTS WITH TPS2000 SERIES OSCILLOSCOPES

POWER CONTROL CIRCUITS:

Power control technologies use both high-power siliconcomponents and low-power logic circuits The switching transis-tors at the heart of most power control circuits require measure-ments not referenced to ground Moreover, the power circuit mayhave a different ground point (and therefore a different groundlevel) than the logic circuit, yet the two often must be measuredsimultaneously

The channel-to-channel isolation of the TPS2000 Seriesprovides a real-world measurement advantage in addition to itsobvious safety benefits Figure 4 is a screen image depictingwaveforms taken at two different points in a power control cir-cuit Notice that the lower waveforms are about 200 A p-p, whilethe upper trace is about 5 V p-p Because each of the TPS chan-nels is fully isolated from the other (including the negative refer-ence leads), and equipped with its own uncompromised DigitalReal Time digitizer, there’s no cross-talk between the two sig-nals Were the oscilloscope channels not adequately isolated,there might be misleading artifacts coupled from the 200 A sig-nal to the smaller waveform; these might be misinterpreted as acircuit problem when in reality it’s an instrument problem Theability of the TPS Series to discretely capture two waveforms ofvastly differing amplitudes reduces guesswork and improvesproductivity

HARMONICS MEASUREMENTS REVEAL UNSEEN POWER PROBLEMS

An understanding of the harmonics within a power grid isessential to the safe and cost-effective use of electrical power.Line harmonics are a growing problem in a world movingincreasingly toward nonlinear power supplies for most types ofelectronic equipment Nonlinear loads, such as switching powersupplies, tend to draw non-sinusoidal currents Their impedancevaries over the course of each cycle, creating sharp positive andnegative current peaks rather than the steady curve of a sine

Figure 3: TPS2000 Series oscilloscope’s Isolated Channel architecture provides complete

isolation from dangerous voltages

*2 Do not float the P2220 probe common lead to > 30 VRMS Use the P5120 probe (floatable to 600 VRMS CAT II or 300 VRMS CAT III) or a similarly rated passive high-voltage probe, or an appropriately rated high-voltage differential probe when floating the common lead above 30 VRMS, subject to the ratings of such high-voltage probe.

Figure 4: The 4-channel TPS2024oscilloscope’s channel-to-channel isolation eliminates cross-talk effects when large and small signals are captured simultaneously

34 Electricity Testing and Measurement Handbook – Vol 7

wave The rapid changes in impedance and current in turn affect

the voltage waveform on the power grid As a result, the line

voltage is corrupted by harmonics; the normally sinusoidal shape

of the voltage waveform may be flattened or distorted

There’s a limit to the amount of harmonic distortion that

equipment can tolerate Load-induced harmonics can cause motor

and transformer overheating, mechanical resonances, and

danger-ously high currents in the neutral wires of three phase equipment

In addition, line distortions may violate regulatory standards in

some countries

The TPS2024’s comprehensive, four-channel capability,

along with its optional power analysis software, enables

connec-tion to all three conductors of a three-phase system to measure

and analyze line harmonics Its “Harmonics” mode – invoked

with a single button–captures the fundamental frequency plus

harmonics 2 through 50 Using only the oscilloscope’s standard

voltage probe, it’s possible to execute a harmonic voltage

meas-urement An optional current probe acquires current harmonics

with the same ease

Figure 5 illustrates a current harmonic measurement The

amplitudes are computed by the instrument’s internal DFT

(Discrete Fourier Transform) algorithm In this case the bar graph

reveals a very strong fifth harmonic level Excessive fifth harmonic

levels (along with certain other odd harmonics) are a classic cause

of neutral-wire currents in three-phase systems

POWER READINGS – MORE THAN JUST WATTS

Voltage and current measurements are by nature

straight-forward and absolute A test point has only one voltage and one

current value at a given instant in time In contrast, power

meas-urements are voltage-, current-, time-, and phase-dependent

Terms like “reactive power” and “power factor,” which were

devised to characterize this complex interaction, are not so much

measurements as computations

The power factor is of particular interest in these

compu-tations This is because many electrical power providers charge

a premium to users whose power factor is not sufficiently close

to 1.0, the ideal value At a power factor of 1.0, voltage and

cur-rent are in phase Inductive loads – especially large electric

motors and transformers – cause voltage and current to shift phase

relative to each other, reducing the power factor Some utility

com-panies apply a surcharge in such cases because the inefficiency

causes energy loss in the form of heat in the power lines There are

procedures to remedy power factor problems, but first the power

characteristics must be quantified

The TPS Series embraces a full suite of power

measure-ments Among these are true power, reactive power, crest factor,

phase relationships, di/dt and dv/dt, and of course power factor

Figures 6, 7 and 8 show TPS Series screen images summarizing

these and other power measurements All of the measurements,

with the exception of waveform analysis and phase relationships,

require a current probe (or itsequivalent) and a voltage probeworking in tandem All of thesemeasurements employ the instru-ment’s one-button applicationfunction

MEASURING SWITCHING LOSS TO IMPROVE PRODUCT EFFICIENCY

Today’s power designers face increasing pressure toimprove the efficiency of their power designs A major factoraffecting the efficiency is the power loss occurring in the switchingsection of the design Optimizing this factor can prove complex.The TPS Series allows the designer to look at switchinglosses in their design through the instrument’s one-button appli-cation function The switching loss will be characterized as turn-

on loss, turn-off loss, conduction loss and total device loss.Figure 9 is a TPS Series screen image showing the switching lossmeasurements

CONCLUSION

Engineers and technicians confront high voltages and rents and must often make potentially hazardous floating measure-ments Where other alternatives may lack the versatility, accuracy oraffordability to make floating measurements, the TPS2000 Seriesemploys unique IsolatedChannel technology to allow engineers andtechnicians to make these measurements quickly, accurately andaffordably

cur-Figure 5: Harmonic distortion measurements

Figure 6: TPS Series’ instantaneous power analysis

Figure 7: TPS Series’ waveform analysis

Figure 8: TPS Series’ dv/dt and di/dt cursors (dv/dt cursors shown)

Figure 9: TPS Series’ switching loss display showing turn-on, turn-off and conduction losses