Bsi bs en 50162 2004 (2005)

BRITISH STANDARD BS EN 50162 2004 Protection against corrosion by stray current from direct current systems The European Standard EN 50162 2004 has the status of a British Standard ICS 29 020; 77 060[.]

Identification

When assessing potential corrosion risks from direct current (d.c.) interference, it is essential to analyze the electrical properties and the location of the interference source, along with any anomalies noted during routine cathodic protection measurements.

There are four principal ways to identify stray-current interference These are to measure one or more of the following:

– structure to electrolyte potential fluctuations;

– deviations from normal structure to electrolyte potentials;

– voltage gradients in the electrolyte;

– line currents in pipelines coupons or metallic cable sheaths

NOTE The measurement of current fluctuations and current polarity changes is particularly useful for identifying interference in complex networks

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After stray current interference has been identified further measurements must be carried out to assess the risk of corrosion.

Measurement

To evaluate the corrosion risk of metal structures due to stray current, it is essential to consider the positive potential shift of the affected structure If there is a possibility of cathodic corrosion, as outlined in Annex A and EN 12954, the corrosion risks must also be assessed by examining the negative potential shift Measurements of the structure-to-soil potential should be taken using a reference electrode positioned directly above the impacted structure.

To determine the polarity and magnitude of stray current, potential gradient measurements can be conducted using two reference electrodes One electrode should be positioned directly above the structure affected by interference, while the second electrode should be placed at a distance of at least a specified minimum.

Measuring the magnitude and direction of current flow and/or the potential shift at coupons or test probes will help to assess a possible corrosion risk

To obtain representative data, it is essential to carefully choose measurement techniques, sample periods, and the number of readings Accurate measurements depend on selecting appropriate voltage recording equipment, while also considering factors such as input impedance, sample period (or chart speed), and the necessary signal conditioning and filtering.

Measurement techniques are described in EN 13509

To assess the impact of stray currents on interference structures, it is essential to measure the stable voltage gradients or potentials between the structure and the electrolyte both when the stray current source is active and inactive A comparison of these measurements will provide insights into the interference effects If it is not feasible to turn off the stray current source temporarily, extrapolation from tests conducted under varying operational conditions of the stray current source should be utilized to estimate the interference.

To accurately measure fluctuating voltage drops caused by interference from a d.c traction system, it is essential to use a continuous chart recorder or digital data logger Recordings should capture both the peak interference periods and, if feasible, times of minimal interference Notably, many interference sources display their maximum and minimum levels within a 24-hour cycle.

To effectively associate stray current with its source, it is recommended to simultaneously record the measured values of the affected system along with an operating parameter of the stray current source.

Values recorded during the non operational period of the interfering system shall be considered as the normal or unaffected potentials

NOTE A judgment should be made where the interfering system is not de-energised during non operational periods

6 Criteria for stray-current interference

Anodic interference

A positive shift in potential on the structure constitutes anodic interference (see Annex A)

Anodic interference (see Annex B) on structures without cathodic protection is acceptable if the positive potential shift ∆U is lower than the criterion given in Table 1

The acceptable positive potential shift, denoted as ∆U, which includes the ohmic voltage drop (IR-drop), is influenced by the resistivity of the electrolyte As the resistivity increases, the IR-drop component of the measured potential shift also rises.

Assessing whether anodic interference meets the acceptance criteria outlined in Table 1 can be challenging when potentials fluctuate rapidly A decision must be made regarding the duration and extent of potential excursions beyond the criteria to determine their acceptability This evaluation may consider the duration and frequency of the excursions or the average potential shift If the judgment remains inconclusive, IR-free potential measurements should be conducted, and the criteria in column three of Table 1 (∆U/mV excluding IR drop) should be applied.

Table 1 – Acceptable positive potential shifts ∆ U for buried or immersed metal structures which are not cathodically protected

Structure metal Resistivity of the electrolyte ρ (Ωm)

Maximum positive potential shift ∆ U (mV)

Steel in buried concrete structures

Structures protected against corrosion by cathodic protection shall be deemed to be exposed to unacceptable stray current interference if the IR free potential is outside the protective potential range

To evaluate the acceptability of stray current interference the installation of test probes and coupons should be considered

In situations with fluctuating interference current probe measurements as described in Annex D can also be used to evaluate the acceptability of interference

In specific scenarios, such as when influenced by direct current (d.c.) traction, the reliability of the measurement method may be questioned In such cases, alternative techniques, like weight loss coupons, can be employed to verify that the structure is effectively cathodically protected.

Measurements should be carried out during a period of normal operation of the interfering system.

Cathodic interference

Cathodic interference from stray currents is considered excessively high when it results in the IR-free potential becoming more negative than the established limiting IR-free potential.

Negative potential shifts from cathodic interference in a structure often indicate the presence of anodic interference in other areas When significant negative potential shifts (e.g., ∆U > 500 mV, including IR-drop) are detected, it is advisable to locate areas with anodic potential shifts to ensure compliance with the relevant standards.

Values recorded during the non operational period of the interfering system shall be considered as the normal or unaffected potentials

7 Reduction of stray current interference – Modifications to current source

General

To effectively minimize stray current interference, it is essential to first address the source of the interference If this approach proves impractical or ineffective, focus should shift to the affected structure In certain situations, implementing mitigation measures at both the source and the structure may be necessary to achieve an acceptable level of interference.

Secondary interference occurs when the source of disruption is affected by another structure To address this issue effectively, it is recommended to first modify the original source of interference If altering the original source is not feasible, adjustments to the secondary interference source may be necessary.

Principles

Under normal operating conditions the earth shall not be used to carry any direct currents For exceptions to this principle see 7.5, 7.7, 7.8

Structures which are a source of interference shall not be connected to foreign buried or immersed metal structures unless it is necessary for safety or stray current corrosion protection reasons.

Direct current systems at industrial sites

All conductors in direct current systems, including power systems and welding equipment, must be insulated from the ground If earthing or equipotential bonding is required for safety reasons, it is crucial to implement measures that prevent stray currents, such as ensuring earthing occurs at a single point only.

To ensure safety and efficiency, the weld current circuit should be minimized in length It is important to avoid using earthed metal structures, including railroad tracks, crane tracks, overhead pipe crossings, or buried pipelines, for conducting current.

Direct current systems at ports

New crane installations at ports must be designed for alternating current operation, with any necessary direct current generated locally at the point of use Additionally, each conductor carrying direct current should be properly insulated from the earth.

In cases where a direct current crane system requires an earth connection for operation, it is essential to implement special measures to prevent stray currents, such as installing an insulated return conductor Additionally, a stray current drainage system should be established if the interference from stray currents to buried metal structures exceeds acceptable levels.

7.4.2 Quayside direct current welding stations

Each ship must be equipped with one or more independent quayside welding stations Utilizing a single direct current welding system for multiple ships can lead to stray currents, which may result in significant corrosion damage to dolphins or fenders This issue arises because equipotential bonds between ships do not effectively mitigate stray current interference.

Connections for the operation of welding equipment shall be directly bonded to the ships' hulls, e.g by welding

NOTE Such problems can be overcome by placing the welding station(s) on board

7.4.3 Direct current power supply to ships

Direct-current power systems on ships featuring complete earth insulation and earth protective relays may be supplied with d.c electric power from shore

In a ship's direct-current power system with single phase earthing, alternating current power is supplied and then rectified onboard for utilization in the direct-current systems.

Direct current communication systems

All communication systems must be designed to prevent normal direct current from flowing through the earth While direct-current pulses, such as those used for dialing or earthing, may pass through the earth, they must not cause stray-current interference with adjacent pipelines or cables Additionally, pipelines and cables should not serve as earthing connections.

Traffic signals shall be designed such that direct currents do not normally flow through the earth.

Direct current traction systems

To minimize stray currents affecting foreign structures, the traction system must be designed accordingly Typically, direct current traction systems operate with the negative pole connected to the rails, although there are rare instances where the positive pole is used Modern d.c railways implement a current feedback system during braking Compliance with the requirements outlined in EN 50122-2 is essential, and the methods to achieve this are primarily focused on effective current management.

– adjustment of the power supply system,

– improvement of the return circuit,

– isolation of the return circuit from ground, grounded metallic structures (pipelines, cables, bridges and tunnels) and other rail systems

When planning a d.c railway project, it is essential to address the requirements and methods for suppressing stray current from the outset This proactive approach ensures that stray current suppression is factored into the decision-making process regarding the locations and sizes of substations.

High-voltage direct current transmission systems

High voltage direct current (HVDC) transmission systems primarily come in two configurations: monopolar and bipolar Bipolar HVDC systems are preferred to prevent stray current interference Additionally, the earthing design of HVDC systems must ensure that current does not flow through the earth during normal operations and that earth current is minimized during fault or unbalanced load conditions.

The system design must account for the potential high levels of stray currents that buried or immersed metal structures could encounter, even when located far from the converter stations' terminal earths.

Terminal earth electrodes shall be designed for installation in low-resistivity soil or seawater to minimize the total earthing resistance and near-surface voltage gradients around the earth electrodes

The location of terminal earth electrodes can significantly affect the stray current interference on buried or submerged structures and must be carefully considered

Calculated near surface voltage gradients should be checked by current tests with a test electrode prior to final decision on the location of the permanent earth electrodes

7.7.3 Interference measurements prior to commissioning

Before commissioning, it is essential to identify stray current exposure areas, where potential gradients may interfere with other structures, through calculations and preferably tests at reduced current Additionally, metallic structures within these areas should be located and tested for stray current interference to estimate the degree of interference during final commissioning.

Further measurements of buried or immersed metallic structures within the stray current exposure area will be conducted For bipolar systems with an earthing system, testing will occur during monopolar operation, with each electrode functioning alternately as both an anode and a cathode.

If interference is deemed unacceptable, appropriate protective measures must be implemented These measures are necessary even in cases where interference arises solely during fault or unbalanced conditions in a bipolar system.

Parties involved can establish an agreement to define the limits for faults and unbalanced operations in a bipolar system, including the maximum current levels and operational duration.

Cathodic protection systems

Cathodic protection systems may cause cathodic interference on structures buried in the vicinity of impressed current anodes

Structures buried in the vicinity of a cathodically protected structure may experience positive potential shifts resulting from the potential gradients around large coating defects on the protected structure

Measures to reduce or eliminate interference (see Clause 6) are described in the following paragraphs

7.8.2 Adjustment of transformer rectifier output

To ensure effective cathodic protection, the output of the rectifier on an interfering structure must be set to the minimum required level In specific situations, it may be beneficial to distribute the total current using additional rectifiers and ground beds.

High-quality coatings on structures reduce the need for cathodic protection current, thereby minimizing stray current interference If a cathodically protected structure has coating defects, it is essential to locate and repair them to decrease interference levels affecting nearby structures.

The interference from impressed current anodes depends on the current output, distance to neighbouring structures, and the resistivity of the surrounding medium

To minimize interference, it is essential to keep neighboring structures outside the anode field area, where the potential gradient may cause shifts beyond the specified limits in sections 6.1 and 6.2.

– increasing the distance from the anode to neighbouring structures (either horizontally or vertically) This is the most effective method;

– reducing the voltage gradient around the groundbed by enlarging the groundbed geometry or by reducing the current output (see 7.8.2);

– locating distributed anodes close to the structure to be protected

To mitigate anodic interference, a drainage bond between structures can be implemented to maintain the positive potential shift within the specified limits of Clause 6 Additionally, a resistor may be incorporated to control the current flow, as referenced in Clause 8.

Interference caused by electrical drainage (secondary interference)

Drainage from a direct current (d.c.) source to a structure can lead to significant d.c currents flowing back to the current source through the structure, potentially causing the structure to act as a source of interference.

The drainage current shall be minimized The use of automatically controlled drainage may be considered

The principles mentioned in 7.8.3 apply

The principles mentioned in 7.8.5 apply

8 Reduction of stray current interference – Modifications to the interfered structure

General

Modifications to the interfered structure can include several approaches: installing mitigation devices, bonding the interfering and interfered structures, altering the electrical continuity of the interfered structure, and increasing the distance from the interfering structure.

The choice of remedial measures that may be applied to the interfered structure is dictated by criteria relating to both the interfering and the interfered structure such as:

– the location of the interfering source, which can be important in finding a solution which is both technically and economically satisfactory;

– the electrical status of the interfered structure, e.g the nature of its insulation, its electrical continuity and whether cathodic protection is applied;

– the characteristics of the environment between the interfered structure and the interfering structure (soil conductivity and presence of nearby metallic structures);

– the level of stray current effects Currents can vary from a fraction of an ampere to tens of amperes.

Design prerequisites

Applying coatings to interfered structures enhances their resistance to soil, which effectively lowers stray current levels This improvement simplifies the design and operation of necessary countermeasures.

To prevent unintended metal contact with stray current sources, it is crucial that reinforced concrete structures and other metal components, such as casings, do not come into direct contact with these sources.

Since the interference level decreases with distance, new structures should be located as far as possible from known stray current sources.

Installation of mitigation devices

The purpose of installing mitigation devices is to minimize or completely stop stray current from the affected structure from flowing into the environment, ensuring compliance with the specifications outlined in Clause 6 (refer to Annex E).

This can be achieved by

– returning the stray current via a metallic bond (known as drainage) from the interfered structure back to the d.c current source (see 8.3.2, 8.3.3, 8.3.4),

– returning the stray current through the ground by the use of earthing electrodes from the interfered structure back to the d.c current source (see 8.3.5),

– applying a direct current through the ground or water to the interfered system (see 8.3.6)

In all cases the devices shall be adjusted so that the minimum current is used to achieve the desired objective

The application of these measures requires the co-operation and approval of the concerned parties (see Clause 4)

Connecting a drainage bond between a long-buried structure, such as a pipeline or cable, and a more negative structure like a substation's negative busbar can significantly amplify stray current, heightening the risk of interference with other buried structures This connection may also accelerate corrosion in nearby structures, such as running rails in direct current traction systems Consequently, it is essential to conduct investigations on the source and any nearby or intersecting foreign structures after such a connection is made If required, additional countermeasures should be implemented to mitigate these risks.

It is important to recognize that in certain countries, National Electrical and/or Safety Regulations may prohibit the use of specific mitigation techniques In all cases, the prevailing National Regulations take priority over stray current mitigation requirements.

In a direct drainage bond, current can flow in both directions; however, it is essential that the potential at the bond's connection to the d.c current source remains consistently more negative than that of the affected structure This ensures that the current flow in the bond does not reverse.

Since the rails and the interfered structure may temporarily reverse polarity, direct drainage bonds shall not be installed on d.c traction systems

Structure-to-electrolyte potential shifts and current flow can be limited by the inclusion of a resistor in the bond A fuse may also be integrated as a protection against overload

This method is not intended to provide cathodic protection to the interfered structure

In cases where a ship is docked for an extended duration at a port with cathodic protection for steel sheet piling or structures like dolphins and fenders, it is essential to bond the ship to these protected structures This bonding helps prevent stray-current corrosion of the hull, ensuring the vessel's integrity and longevity.

The International Marine Organisation (IMO) no longer recommends this practice, emphasizing the need for special precautions in hazardous areas, particularly during the loading and offloading of flammable materials For additional guidance, refer to the "International Safety Guide for Oil Tankers and Terminals (ISGOTT)" 4th edition, published by the Oil Companies International Marine Forum in 1998.

Unidirectional drainage bonds, or polarized electrical bonds, allow drainage current to flow in a single direction These bonds are particularly useful in scenarios where the potential of the affected structure is not consistently more positive than that of the direct current (d.c.) source, such as in d.c traction systems.

To ensure safe operation in direct drainage bonds, integrating a resistor and fuse is essential to limit current flow Additionally, the drainage current can be automatically regulated using a permanently installed sensing electrode.

This method is not intended to provide cathodic protection to the interfered structure

Forced drainage bond, or forced electric drainage, is employed when direct or unidirectional drainage fails to adequately remove stray currents from an affected structure due to its insufficient negative potential This technique is particularly relevant in situations where stray currents are generated by a direct current (d.c.) traction system.

A forced drainage bond incorporates a transformer rectifier in the bond between the interfered structure and the source of interference

When applied on pipelines and cables, a forced drainage bond can protect longer sections against stray current corrosion than when a unidirectional drainage bond is used

Frequent and significant voltage fluctuations between running rails and nearby structures can lead to considerable variations in drainage current and structural potential To keep the potential of the affected structure consistently below a predetermined threshold, an automatically controlled forced drainage bond can be employed Careful selection of an appropriate location for the permanent sensing electrode is crucial when implementing this technique.

An earthing electrode system establishes a low-resistance metallic connection to the soil, effectively minimizing the current flow from affected structures into the ground This approach is suitable for situations with low interference levels and when the structures, such as long pipelines or cables, are well insulated from electrical sources, like stray currents from HVDC transmission systems or cathodic protection systems.

Earthing electrode systems using galvanic anodes generally do not protect a structure exposed to stray currents from a d.c traction system

An impressed current system can effectively reduce stray current effects when interference levels are minimal, focusing on mitigating these effects rather than providing cathodic protection Such systems are primarily considered under specific conditions.

– the interfered structure is well coated;

– the distance between the d.c current source and the interfered structure is too great for a drainage bond;

– electric drainage between the interfering and the interfered structure is not appropriate, for safety reasons or to limit interference on neighbouring foreign structures;

– the interfered structure has to be permanently cathodically protected for other reasons

As in the case of forced electric drainage an automatically controlled transformer rectifier may be used

8.3.7 Modifying the electrical continuity of the interfered structure

To mitigate stray current interference on long structures like steel pipelines or reinforced concrete, it is effective to electrically isolate sections of the structure This approach reduces the potential difference between the structure and the electrolyte Installing isolating joints is a recommended method for achieving electrical isolation, as detailed in EN 12954.

When using isolating joints, it is essential to take precautions to prevent corrosion caused by current flow across the joint through the ground Compliance with the criteria outlined in Clause 6 is necessary.

For pipes carrying a conductive electrolyte precautions should be taken against possible internal corrosion on the pipe wall at the anodic side of the isolating joint (see EN 12954)

Interference on coated cast iron pipes must be considered in areas with steep potential gradients, particularly those exceeding 200 mV per individual pipe length Such conditions can arise within 10 meters of d.c operated traction systems and near ground beds If interference is deemed unacceptable, it is essential to short-circuit the pipe joints using cables and implement the measures outlined in sections 8.3.2, 8.3.3, and 8.3.4.

Tiêu đề	Protection Against Corrosion By Stray Current From Direct Current Systems
Trường học	British Standards Institution
Chuyên ngành	Electrotechnical Standards
Thể loại	British Standard
Năm xuất bản	2005
Thành phố	Brussels

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Số trang	34
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