Field Testing of Overcurrent Trip Units for Low Voltage Circuit Breakers Used in DC Applications

PLEASE READ IT CAREFULLY BEFORE REMOVING THE WRAPPING MATERIAL THIS AGREEMENT CONTINUES ON THE BACK COVER.Field Testing of Overcurrent Trip Units for Low Voltage Circuit Breakers Used in

ELECTRIC POWER RESEARCH INSTITUTE (EPRI) PLEASE READ IT CAREFULLY BEFORE REMOVING THE WRAPPING MATERIAL THIS AGREEMENT CONTINUES ON THE BACK COVER.

Field Testing of Overcurrent Trip Units for Low Voltage Circuit Breakers

Used in DC Applications

August, 1994

Prepared byEddie L Davis, Edan Engineering Corp

Daniel L Funk, Edan Engineering Corp

Prepared for

Nuclear Maintenance Applications Center

1300 Harris BoulevardCharlotte, North Carolina 28262

Operated by

Electric Power Research Institute

3412 Hillview AvenuePalo Alto, California 94304EPRI Project Manager

Effective October 1, 2008, this report has been made publicly available in accordance with Section 734.3(b)(3) and

published in accordance with Section 734.7 of the U.S Export Administration Regulations As a result of this publication,

this report is subject to only copyright protection and does not require any license agreement from EPRI This notice

supersedes the export control restrictions and any proprietary licensed material notices embedded in the document

prior to publication

TR-104513

NMAC provides a conduit

for the ongoing exchange

of information among

utilities and industry

maintenance personnel.

maintenance activities, and have proven both extremely successful and cost-effective in improving maintenance practices.

The NMAC approach helps individual nuclear facilities incorporate the collective wisdom of the industry into their own maintenance and operating plans.

operated by EPRI

This Tech Note investigates and

provides recommendations for

field testing the overcurrent trip

units of low voltage circuit

breakers used in direct current

(DC) applications Although

industry guidance is available for

field testing low voltage circuit

breakers in alternating current (AC)

applications, guidance for testing

breakers used in DC circuits is

virtually nonexistent.

Fault theory and breaker operating

principles are discussed at a depth

necessary to technically

substantiate recommended

practices contained in this Tech

Note The response of low voltage

circuit breaker overcurrent trip

units to AC and DC current is

compared to facilitate an

understanding of the issues and

concerns surrounding overcurrent

test methods for low voltage circuit

breakers used in dc applications.

The applicability of this information

to a test program for DC system breakers is described in detail.

This Tech Note addresses whether

or not overcurrent test results obtained using AC current are representative of a breaker’s performance under DC conditions.

This document demonstrates that technically valid test results can be obtained using either AC or DC test methods.

The final recommendations presented favor AC testing over DC testing based on familiarity with the test method and economic considerations; however, it is stressed that either test method can yield technically acceptable results The potential benefits and Iimitations of each test method, AC

or DC, should be understood thoroughly before selecting a test method or interpreting test results.

Field Testing of Low Voltage Circuit Breakers

1 0● Scope

The scope of this document is limited to field testing issues associated with

low voltage circuit breaker used in DC applications Low voltage circuit

breakers are generally defined as breakers rated for service in systems up to

600 VAC or 250 VDC There are two main categories - molded case circuit

breakers (MCCBs) and low voltage power circuit breakers (LVPCBs) Both

breaker types are evaluated within the scope of this Tech Note Breakers

equipped with solid-state trip units are not normally used in DC circuits and

are not discussed

This Tech Note focuses on overcurrent testing Accordingly, discussions are

primarily concerned with characterizing the response of low voltage circuit

breaker overcurrent trip units to different levels of DC overcurrent and

explaining how the expected response potentially affects field test methods

Although other field inspections and tests are recommended for breaker

maintenance programs in addition to overcurrent testing, existing industry

guidance is considered adequate for these other inspections and tests, as

discussed in the “Background” section

The discussion in this document is geared toward nuclear power plants

Nuclear plants operate within strict safety and regulatory criteria and are

required to periodically demonstrate the operability of plant safety

equipment The technical information presented, however, is applicable to

all low voltage circuit breakers used in DC applications

An overcurrent test program for DC circuit breakers should consider the

following

Available time-current characteristic curves available from the

manufacturer

Available test equipment - AC or DC test equipment

Type of test - instantaneous or thermal trip unit

Acceptance criteria for the test

This Tech Note addresses the above program elements for the overload trip

test and the instantaneous trip test It also provides a technical overview of

the differences between AC and DC current effects on MCCBs and LVPCBs

The emphasis is on MCCBs because of their greater industry use in DC

systems

2 0● Background

A straightforward solution to the issue regarding prudent methods of field

testing low voltage circuit breakers used in DC applications at nuclear power

plants has been elusive, primarily due to three factors:

1 A lack of technical information regarding breaker performance

Billions of MCCBs have been installed for AC applications MCCBs for AC

applications are well understood and field test equipment is readily available

to verify that these breakers are functional The National Electrical

Manufacture Association (NEMA) has issued AB4-1991, Guidelines for

Inspection and Preventive Maintenance of Molded Case Circuit Breakers Used

in Commercial and Industrial Applications, to assist users with field testing

methods Although not explicitly stated in AB4- 1991, the test methods

described apply to AC MCCBs; DC applications are not addressed

Virtually every building or facility contains MCCBs for AC circuit isolation

and system protection; however, few, if any, breakers may be installed for DC

applications For example, a nuclear plant may have anywhere from

hundreds to thousands of MCCBs installed in AC systems, but only a few

dozen installed in DC systems And, the number of DC LVPCBs will be

substantially less than the number of DC MCCBs Frequently, the total

number of LVPCBs installed in the plant’s DC system is as few as four, if

any These larger, more expensive breakers are typically used only as

battery output breakers

NEMA AB4-1991 and EPRI NP-7410, Volume 3, Molded-Case Circuit

Breaker Maintenance, provide detailed information regarding maintenance,

inspection, and testing of MCCBs Although these documents do not

explicitly address the unique characteristics of MCCBS used in DC

applications, the information in these documents is directly applicable to

MCCBs in DC applications for the following inspections or tests:

Overheating inspection

Enclosure inspection

Mechanical operation inspection Insulation resistance test

Insulated pole resistance test Rated hold test

Auxiliary device tests

Additional guidance beyond that provided in existing industry documents ormanufacturers’ literature is needed for the overload trip test and the

instantaneous trip test for DC MCCBs The purpose of this Tech Note is toprovide this additional information

3.0 Comparison of DC to AC Characteristics in Overcurrent

Trip Units

The tripping characteristics of low voltage circuit breakers are generally

provided by the manufacturers in the form of time-current characteristic

curves A typical time-current curve is shown in Figure 1 The overcurrent

devices that provide the tripping function for each region of the curve are

discussed in the following sections

3.1 MCCB Time Delay Trip Units

Time delay trip units protect against sustained overloads and incorporate an

intentional time delay in the tripping function These units control the

breaker’s trip characteristics in the time delay trip region (see Figure 1) The

time delay trip function is normally achieved by a thermal trip unit, a

bimetal element consisting of two bonded strips of metal having different

rates of thermal expansion (see Figure 2) Line current passing through the

bimetal element, which is often part of the current carrying path, causes the

element to heat and deflect If the bimetal element deflects sufficiently, it

trips the latch on the breaker trip mechanism

The time needed for the bimetal to deflect and trip the circuit breaker varies

inversely with current A longer time delay is allowed before tripping when a

light overload occurs and a quicker response occurs for heavy overloads This

inverse-time delay response typically starts at about 125% of rated current

and extends to the instantaneous trip region

For many MCCBs, thermal trip units are calibrated by the manufacturer and

are not adjustable In some newer breakers, the time-current response is

adjustable

The thermal response of the bimetal strip is directly related to the heat

energy dissipated in the bimetal element This energy comes from resistive

heating of the element and is therefore a function of power, which in turn is a

function of the current flowing through the bimetal strip Average power is

related to current by

The current in the above expression represents the effective current In a DC

system, the effective current is the steady-state DC current In an AC

system, current varies sinusoidally about the zero axis and alternately

reaches a positive and negative peak value (see Figure 3)

Figure 2 - Typical Time-Current Characteristic Curve

Figure 1 - MCCB Thermal Trip Unit

Figure 3 - Typical Alternating Current

The effective current for an AC signal is defined as the root-mean-square

(RMS) current and is related to the peak value by

The above relationship applies only to a single frequency sinusoidal AC

signal AC current is normally defined in terms of RMS current rather than

its peak value (Appendix A contains a more detailed derivation of the

expression for RMS current) The same concept applies to voltage also For

example, a 120 VAC power source has an RMS value of 120 VAC and a peak

value of about 170 VAC

The RMS value of AC current is effectively equal to a DC current of the same

magnitude in terms of its power transmission and heating capability, hence

the term effective current Based on this relationship, the bimetal strip will

respond in virtually the same manner regardless of whether the applied

current is AC or DC, provided that the AC RMS value equals the DC value

Since the thermal response (deflection) of the bimetal strip is effectively the

same for AC RMS and DC current, the time delay trip characteristics

depicted by the breaker’s time-current characteristic curve represent the

breakers’s performance in either AC or DC applications Thus, test results

obtained using AC or DC test methods should also be equivalent

Some fully magnetic MCCBs are also capable of providing a time delay tripfunction, although this type of design is seldom used for DC applications.Magnetic forces generated in the breaker’s coil are concentrated in a movableiron core In response to overcurrent, the core is drawn closer to the tripmechanism armature until the magnetic force on the armature trips thebreaker The trip time delay is obtained by use of a damping fluid whichslows movement of the iron core AC and DC current can cause a differentresponse in this type of trip unit The response of magnetic trip units isdiscussed in the next section.

3.2 MCCB Instantaneous (Magnetic) Trip Units

Magnetic trip units protect against short circuits They are often calledinstantaneous trip units since they are actuated without any intentional timedelay A magnetic trip unit consists of an electromagnet connected to thebreaker trip bar (see Figure 4) Line current passing through the magneticelement results in an attractive force on the armature (a movable iron slug).When a short circuit occurs, the magnetic force generated by the short circuitcurrent is strong enough to overcome internal spring tension The armaturethen moves and actuates the latch on the breaker trip mechanism Thistripping action has no intentional built-in time delay; trip actuation is afunction of the short circuit current magnitude

Figure 4 - Magnetic Trip Unit

Magnetic trip units may be either adjustable or nonadjustable MCCBs rated

below 150 amperes generally have nonadjustable magnetic trip units

Breakers rated 225 amperes and above typically have adjustable magnetic

trip units If an adjustment is possible, the magnetic pickup setting is

usually adjusted by a linkage that varies the spring tension on the magnet

armature

The instantaneous trip function shown on time-current characteristic curves

starts with the vertical band that intersects the time delay region (see Figure

1) Trip times inside this vertical portion of the time-current curves are not

precisely defined because this region defines the transition from thermal

tripping to magnetic tripping This transition region represents the setpoint

tolerance of the magnetic trip unit Depending on the exact trip point of the

instantaneous unit, tripping within this transition region can be thermal,

with a short time delay, or magnetic, with no intentional delay Beyond the

transition region is the instantaneous region In this region, the magnetic

trip unit alone should cause the breaker to trip

The instantaneous trip is distinctly different in nature than the thermal trip

The instantaneous trip occurs in response to a magnetic force generated by

the current Current flowing through the magnetic trip unit generates a

magnetic force on the trip armature according to the following expression for

a simple armature design:

Henries/meter)Effective air gap distance in armature

Cross-sectional area of armature

Combined reluctance (resistance to magnetic force of all parts

of the armature and depends primarily on the armature geometricarrangement)

As shown in the above expression for a simple armature arrangement, theforce is a function of (1) the instantaneous current magnitude, (2) theeffective number of turns in the current coil, (3) the air gap distance betweenthe armature contacts, (4) the cross-sectional area of the armature, and (5)the reluctance of the iron path in the armature For a given MCCB design,the only variable is current; the other parameters are design dependent andare constant for a given breaker.

As shown, the magnetic force is proportional to the square of theinstantaneous current With AC current, the magnetic force becomes larger

as the sinusoidal current varies from zero to its peak value, with themaximum force created at the peak current value Since the instantaneoustrip point depends on the magnetic force exerted on the trip armature of thebreaker, the trip point is dependent on the peak current and not the RMScurrent over a finite period Note that instantaneous DC current has no suchdistinction between RMS and peak values (neglecting rise time and ripple)

Even though magnetic trip units react to the instantaneous value of ACcurrent, the trip characteristics are generally expressed in terms of RMScurrent on MCCB time-current characteristic curves Because the magnetictrip unit will not respond the same to AC RMS and DC current, the breaker’sperformance under DC current is expected to vary by some amount from the

AC trip characteristics depicted by the time-current curve

Remember that the peak and RMS value of AC current are related by thefactor Based on the foregoing discussion, one might expect that thedifference in instantaneous trip response between AC and DC current would

be about i.e., a DC current might be about 1.41 times larger than an ACRMS current to initiate an equivalent instantaneous trip However, thismuch of a difference will not generally be observed because:

1) The AC peak current is not a sustained value; it is only the peak of avarying sinusoidal current; and

2) The DC current is a sustained value even though it is smaller than the ACpeak (assuming the DC current is equal in magnitude to the AC RMSvalue)

For these reasons, the effective difference between AC and DC currents onthe instantaneous trip response can vary from 10% to 40%, i.e., the value of

DC current that will initiate a trip will typically be 10% to 40% higher thanthe AC RMS current that will trip the breaker For example, a breaker thattrips at 1,000 amps AC RMS current should trip at about 1,200 amps DCcurrent if the conversion factor is 20% The conversion factor between ACand DC current for each to cause an equivalent instantaneous trip is not a

constant in which a single conversion factor can be applied to all MCCB sizes

and ratings The difference between the two varies with frame size as well as

the minimum to maximum ratings within the frame size

Design features that cause these variations include:

Assembly variations within the magnetic circuit

Another factor differentiating AC from DC current is the cycles of forceobtained from AC current When exposed to AC overcurrents, the triparmature may “chatter”, knocking the latch partially off with each electricalcycle Eventually with enough cycles, the latch is moved sufficiently to tripthe breaker With DC current, there are no cycles; the magnetic forcegenerated by DC current must be sufficient to trip the armature latch withits single forced motion.

3.3 Low Voltage Power Circuit Breaker Overcurrent Trip Units

LVPCBs typically utilize a magnetic electro-mechanical overcurrent tripunit The trip assembly usually consists of a magnet, a series coil thatcarries the breaker primary current, and a moving armature Depending onthe type of time-delay mechanism used by the device, the movement of thearmature may be delayed by either an oil dashpot (long-time delay) or aratchet gear or mechanical timer (short-time delay) Instantaneous tripsusually rely on a calibration spring to restrain the armature Even thoughthe operating requirements are similar, the design of the overcurrent tripunit varies widely among manufacturers

The overcurrent trip unit function is similar to MCCBs, operating on aninverse-time current delay principle; the higher the primary current throughthe series coil, the faster the device trips the breaker The instantaneous tripoccurs with no intentional time delay

LVPCBs often allow a long-time delay and a short-time delay trip Thelong-time delay trip device typically consists of a trip armature, an oildashpot, and a common series coil The series coil is common to the long-timedelay trip device, the short-time delay trip device, and the instantaneous tripdevice The armature is restrained by a calibration spring When the

magnetic force produced by an overcurrent condition in the series coilovercomes the restraining force of the calibration spring, the armaturemoves, but is slowed by the flow of oil through an orifice in the oil dashpotplunger The time required to displace the plunger is inversely proportional

to the force attempting to move the armature The armature then moves alever on the trip shaft which trips the breaker Some designs use an airdiaphragm rather than an oil dashpot, but the end result is essentially thesame The pickup point for this device can usually be varied from 80% to160% of rated current

The short-time trip delay device often consists of a trip armature, a ratchetgear, and a common series coil The trip armature is restrained by acalibrating spring When the magnetic force produced by an overcurrentcondition in the series coil overcomes the calibration spring force, thearmature is slowed by a ratchet gear, which produces an additional time

delay After this short time delay, the armature moves a lever on the trip

shaft which trips the breaker Some designs use a mechanical timer or a

secondary armature interlocked with the long-time delay trip unit rather

than a ratchet gear arrangement The pickup point of this device can usually

be varied from 200% to 1000% of rated current

The instantaneous overcurrent trip device consists of a trip armature

restrained by a calibration spring and a common series coil When the

magnetic force produced by an overcurrent condition in the series coil

overcomes the calibration spring force, the armature operates a lever on the

trip shaft which trips the breaker

A comparison of AC to DC overcurrent response is similar to that described

for MCCB magnetic trip units because the trip elements of LVPCBs are

actuated by magnetic force Although a time-delay function is included in the

design for LVPCB overcurrent trip units, the sensing element is unlike the

thermal trip unit in an MCCB For this reason, a LVPCB’s response may be

different for AC and DC overcurrents in both the time delay and the

instantaneous trip regions Manufacturers usually performed extensive

testing of LVPCBs and the associated trip units during their development to

characterize the response to both AC and DC currents This information is

reflected on the manufacturer’s time-current curves; they should be reviewed

to determine any variations in response between AC and DC

LVPCB overcurrent trip units used in DC applications may have design

differences from those used in AC applications, even if the model number for

a given trip unit is the same for either application In particular, the trip

unit calibration spring and the armature air gap may be adjusted for a DC

application The net result of this design modification is that the DC

time-current curves may be almost identical in shape to the AC curves A

review of the AC and DC time-current curves might prompt an interpretation

that the trip response to AC and DC currents is identical However, the

previous discussion regarding the different response between AC and DC

currents in a magnetic trip unit still applies; in this case, the identical

characteristics possibly shown on the time-current curves was achieved by

varying the design depending on whether the application was AC or DC The

manufacturer should be contacted to determine any design differences for

overcurrent trip units in AC or DC applications that might affect an

interpretation of the time-current curves Also, the manufacturer should be

able to provide a DC to AC conversion factor to account for the different trip

response obtained with an AC test current on a DC trip unit