Bsi bs en 50122 2 2010 (2011)

Unknown raising standards worldwide™ NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW BSI Standards Publication BS EN 50122 2 2010 Incorporating February 2011 and March 2011 corr[.]

General

The impact of stray currents is influenced by the design of the traction power supply system These stray currents can exit the return circuit, potentially affecting both the return circuit and nearby installations, as outlined in Clause 4.

Beside to the operating currents, the most important parameters for the amount of stray current are:

– the conductance per length of the tracks and the other parts of the return circuit,

– the distance of the substations,

– the longitudinal resistance of the running rails,

If the railway system meets the requirements and measures of this European Standard, the system is assumed to be acceptable from the stray current point of view.

Criteria for the protection of the tracks

The primary factor affecting stray currents departing from the tracks is the conductance per unit length between the track and the earth Additionally, the corrosion rate plays a crucial role in evaluating the associated risks.

The rail potential is crucial for understanding the parameters associated with stray currents, including traction currents, the longitudinal resistance of running rails, resistance to earth, and the length of feeding sections It is essential that there is no direct electrical connection, whether accidental or intentional, to earthing installations for this analysis to be valid.

Experience proves that there is no damage in the tracks over a period of 25 years, if the average stray current per unit length does not exceed the following value:

(average stray current per length of a single track line)

For double track lines, the maximum average stray current value should be doubled, and for more than two tracks, the value increases proportionally The averaging process takes into account only the total positive portions of the stray current over a 24-hour period or its multiples.

To avoid the need for additional investigations as outlined in section 5.4, it is essential that the conductance per length, G’RE, and the average rail potential, U RE, remain within specified limits throughout the system's lifetime.

– G’RE ≤ 0,5 S/km per track and U RE ≤ + 5 V for open formation (1)

– G’RE ≤ 2,5 S/km per track and U RE ≤ + 1 V for closed formation (2)

For the average rail potential shift U RE only positive values of the rail potential are considered

The averaging period shall be 24 h or multiples

NOTE 2 A guide value for the sampling rate is 2 per second

Licensed Copy: Wang Bin, ISO/EXCHANGE CHINA STANDARDS, 30/03/2011 09:13, Uncontrolled Copy, (c) BSI

If the requirements in Equations (1) and (2) are not met, an alternative value for G’ RE shall be calculated and used for the design, applying Equation (3)

I’ = 2,5 mA/m per track or the value coming from the investigation in 5.4

NOTE 3 For a double track line the value for the maximum conductance per length should be multiplied by two For more than two tracks the values increase accordingly

NOTE 4 As it is not easy to measure the stray currents directly, the measurement of the rail potential is a convenient method

According to Equation (3), the acceptable conductance per length can be calculated for a single track line

The simulation of the traction power supply for scheduled train operations yields valuable data on stray current per length, which is essential for design purposes Clause C.1 outlines a method for calculating dead-end tracks, which is considered conservative as the actual values tend to be lower.

Upon completion of the construction phase, it is essential to verify that the permissible conductance per length, as outlined in Equations (1), (2), or (3), is satisfied Proven measurement methods are detailed in Annex A.

During operation, compliance with the limits of conductance per length according to Equations (1), (2) or (3) shall be maintained.

Criteria for systems with metal reinforced concrete or metallic structures

In systems with metal reinforced concrete or metallic structures, like:

– viaducts, the impact on the structures shall be considered

The voltage shift of the structure versus earth is an additional criterion for assessment

Research indicates that there is no need for alarm if the average potential shift between a structure and the earth during peak traffic hours remains below +200 mV for steel in concrete structures For buried metal constructions, the acceptable values vary based on soil resistivity and material type For detailed requirements, consult EN 50162:2004, Table 1.

To prevent unacceptable stray current effects on both the tunnel structure and external structures, it is essential to calculate the longitudinal voltage between any two points of the interconnected metal-reinforced tunnel The maximum longitudinal voltage must remain below the permissible potential shift This conservative approach guarantees that the actual tunnel potential relative to the earth will be minimized.

Specific investigations and measures

If the criteria outlined in sections 5.2 and 5.3 are not met, or if alternative construction methods are proposed, an early-stage study is required This study is also essential for significant modifications to existing lines, particularly when there is a potential for increased stray current issues.

The possible impact of stray current corrosion shall be investigated, where the following aspects are included, such as:

– insulation from earth of the rails and connected metallic structures,

– humidity of the track bed,

– longitudinal resistance of the running rails,

– number of and distance between the substations,

– effects of inequalities in the no load voltages of substations,

– substation no-load voltage and source impedance,

Clause 6 and Clause 7 show suitable corrective provisions

General

Any provisions employed to control the effects of stray currents shall be checked, verified and validated according to this European Standard

The system design must be finalized early enough to incorporate the findings into key system parameters that affect stray current effects, such as the spacing of substations and the design of civil structures.

Return circuit

In order to minimise stray current caused by a d.c traction system, the traction return current shall be confined to the intended return circuit as far as possible

As the return circuit in case of d.c traction systems usually is not connected to earth, safety requirements for the rail potential according to EN 50122-1:2010, 6.2.2 and Clause 9, shall be fulfilled

To maintain low longitudinal resistance in running rails, it is essential that rail joints are either welded or connected using low-resistance rail joint bonds This ensures that the increase in longitudinal resistance does not exceed 5%.

To minimize longitudinal resistance, it is effective to utilize rails with a larger cross section and implement cross bonding of the running rails and tracks, where signaling considerations permit.

Effective insulation of the running rails and the entire return circuit is essential when the running rails serve as part of the return circuit.

The design of the track must ensure that the insulation quality of the rails remains largely unaffected by water To meet the specified values in Equations (1), (2), and (3) of section 5.2, effective water drainage from the substructure of the running rails is crucial.

NOTE 1 The values given for the conductance per length apply to a track consisting of two running rails with tie bars as well as the attached system parts

NOTE 2 The following provisions can be made to achieve the required values of the conductance G’ RE for rails laid in an open formation:

– wooden sleepers or reinforced-concrete sleepers with insulating fastening;

– distance between running rails and ballast

NOTE 3 The following provisions can be made to achieve the required values of the conductance G’ RE for rails laid in a closed formation:

– fitting of the running rails in an insulating resin bed;

– provision of insulating intermediate layers between the tracks and the bearing systems;

Return conductors, if required, are laid in parallel to the running rails and shall be connected to them at regular intervals They shall be insulated from earth

Return cables connect the running rails with the substation They shall have an insulating outer sheath, so that no stray currents can leave or enter

NOTE Where mechanical damage is likely, return cables should have an additional protection

6.2.6 Electrical separation between the return circuit and system parts with earth-electrode effect

In order to reduce stray currents, no part of the return circuit shall have a direct conductive connection to installations, components or metallic structures which are not insulated from earth

In cases where there is a direct conductive connection to installations, components, or metallic structures that are not insulated from the earth, it is essential to adhere to the values specified in Equations (1), (2), and (3) of section 5.2 for the return circuit and its connected parts.

To mitigate the effects of stray currents when a connection to the return circuit is necessary for electric shock protection, appropriate measures must be implemented.

– open connection with the return circuit, in this case the voltage-limiting device shall satisfy the requirements given in EN 50122–1:2010, Annex F;

– insulation of the equipment or components that are connected to the running rails, from foundations or components that are earthed;

– insulation of the metal reinforcement of the structure from earth

For exceptions regarding workshops and similar locations see Clause 9

The insulated conductor rail, known as the "fourth rail," serves as a return path for traction current When this rail is live and not linked to the running rails, it typically prevents stray currents For systems utilizing both third and fourth rails, each conductor rail must be insulated from the earth in accordance with the nominal voltage requirements outlined in EN 50163.

6.2.7 Rail-to-rail and track-to-track cross bonds

Rail-to-rail cross bonds, tie bars, track-to-track cross bonds and other bonds which can come in contact with earth shall be insulated.

Non-traction related electrical equipment

Non-traction related electrical equipment shall be installed according to EN 50122-1:2010, Clause 7

Tracks of other traction systems

Generally, the tracks of other traction systems shall not have any direct conductive connection to tracks of d.c traction systems

Tracks without contact line may be connected to the return circuit in special cases if they fulfil the requirements given in 6.2.3

If running rails are used by d.c and a.c traction systems, additional provisions shall be made against the stray current hazard and against impermissible touch voltages, see EN 50122-3

Additional provisions will not impact other safety criteria, especially those aimed at reducing touch voltage and ensuring the proper functioning of power supply, track circuits, and communication systems.

Return busbar in the substation

The substation shall be arranged so that direct current does not flow in the substation structure earth

When performing maintenance work, it is essential to consider the risks associated with stray currents in relation to equipment earthing In substations and similar facilities, return busbars must be insulated from the earth For safety purposes, a voltage-limiting device, at least of type O, should be installed to connect the return busbar to the earth as necessary.

EN 50122-1:2010, Annex F For substations in depots and workshops see Clause 9.

Level crossings

At level crossings, it is essential to ensure that the conductance per length of the running rails in a closed formation does not surpass that of the adjacent tracks.

Common power supply for tram and trolleybus

When trolleybuses and tramways share a substation for traction power, one of the trolley contact wires may connect to the track return system as per EN 50122-1 It is essential to verify that the protective measures for both systems remain adequate to minimize stray current effects.

The insulation of running rails must be aligned with other measures to ensure that touch voltages remain within acceptable limits as specified by EN 50122-1 during operation, including scenarios involving short-circuits and earth faults.

Changeover from the mainline to depot and workshop areas

7 Provisions for influenced metallic structures

General

The resistance between uninsulated conductive structures and the track return system must be high Direct connections between earth and track return systems are generally prohibited, except in depots and workshops However, in specific industrial systems with direct current (d.c.) traction, such as open cast coal mines, a direct connection to earth may be permitted, depending on the surrounding conditions.

Tunnels, bridges, viaducts and reinforced concrete slab track

In tunnel structures with conductive components, it is essential to implement measures to mitigate the effects of stray currents Protection against electric shock must be considered, and these requirements also extend to viaducts, bridges, and reinforced concrete slab tracks.

NOTE The provisions to reduce the stray current effects in tunnel structures with conductive components can depend on:

– whether the predominant source of the stray current is internal or external to the tunnel,

– whether the main priority is to protect the tunnel metallic structures, or to protect other metallic structures external to the tunnel and the railway

Stray currents can flow into metal reinforced concrete tunnels and other conductive structures, potentially affecting external conductive structures To mitigate this influence, equipotential bonding must be implemented in the lower sections of the tunnels or other conductive structures to meet the voltage requirements specified in section 5.3 This bonding is essential for ensuring safety and compliance.

– a sufficient number of reinforcing bars,

– if necessary, additional conductors of appropriate cross section laid within the tunnel

In certain situations, specific tunnel sections may be exempt from the equipotential bonding required for the rest of the tunnel To ensure equipotential bonding in other sections, an insulated cable can be used to extend over the segregated tunnel area.

NOTE For stray current protection purposes only, it is possible to achieve adequate electrical conductivity of reinforcing bars within a structure section by means of conventional steel wire wrapping

In regions where stray currents have minimal impact on external structures and achieving a high rail-to-earth resistance is challenging due to factors like humidity or unclean ballast, the primary concern should be the corrosion of metallic structures within the tunnel.

Reinforced concrete tunnel structures must incorporate insulating joints to create longitudinal sections, preventing stray currents from adjacent systems from flowing through the tunnel and causing unwanted electrical connections between different areas.

If the resistance between tunnel structures and the earth is high, such as in rock tunnels, reinforced concrete tunnel structures may be segmented into longitudinal sections using insulating joints.

If there is any risk of an impermissible voltage between simultaneously accessible parts, refer to

At ring joints between each section, terminals shall be provided for test purposes A reliable electrical connection shall be made between these terminals and the longitudinal reinforcing bars

NOTE 2 Normally, no connection will be made between the terminals of adjacent sections

Steel-reinforced concrete tunnels and their components made of conductive materials must remain electrically isolated from external pipes, cables, and adjacent systems that are not insulated from the ground If maintaining this electrical separation is unfeasible due to varying earthing systems within a building, there is a risk of stray current exchange, leading to potential stray current corrosion In such scenarios, continuous monitoring is essential, and rail-to-structure earth connections should be promptly eliminated.

Connections of the tunnel reinforcement to additional earthing systems are permissible in order to satisfy earthing requirements for protective provisions

7.2.5 Cables, pipework and power supply from outside

To prevent stray current exchange between structural earth and remote non-railway installations, it is essential to implement precautions At the entry points of metal-reinforced concrete or metallic railway structures, including tunnels, viaducts, depots, and workshops, all external metallic pipework, hydraulic lines, cable sheaths, and earth connections must be electrically isolated from the structure's earth to eliminate any conductive links to external earth electrodes.

NOTE 1 This can be achieved by:

– insulating parts in the pipes or alternatively complete insulation of the pipes from the structure earth,

– installation of transformers with separate windings or by the use of the TT system in accordance with EN 50122-1

NOTE 2 When necessary for safety reasons, every section of metal pipe may be connected to the structure earth.

Adjacent pipes or cables

To minimize stray current interference, it is essential to keep the conductive parts of d.c traction systems as far away as possible from buried pipes or cables.

For the assessment of possible stray current impact the measurement method given in A.4 can be used

NOTE Experience has shown that for crossings of tracks with pipes or cables a minimum distance of 1 m is adequate for stray current protection purposes.

Voltage limiting devices

To ensure protection against impermissible voltages, it is essential to install voltage-limiting devices (VLD) between the return circuit and the metallic components of the structure Compliance with protective measures against stray current effects is required, as outlined in EN 50122-1:2010, Annex F.

8 Protective provisions applied to metallic structures

This European Standard aims to minimize stray currents to mitigate stray current corrosion Conventional protective measures for non-railway installations may be employed if deemed necessary Any additional protective methods must be coordinated with relevant stakeholders and adhere to applicable standards regarding stray current corrosion.

Connecting any metallic structure to the return bus bar in a substation, even through a polarized electric drainage device, can elevate the overall stray current Consequently, such connections should be made with careful consideration of their impact on running rails and other potentially affected structures.

Polarised electric drainage is generally applicable only when the structure to be protected is remote from other structures

In depots and workshops, the close proximity of tracks minimizes voltage drop, allowing for a direct connection between the structure earth and the return circuit for safety through equipotential bonding, contrary to the general requirement in section 7.1 Consequently, running rails in these areas must be insulated from the main line using rail joints, with traction power supplied by separate transformer-rectifier units Exceptions may be permitted if a stray current study demonstrates no adverse effects For information on touch voltage across rail joints, refer to EN 50122-1:2010, Table 6.

In cases where the running rails of the depot and workshop area are linked to the main line, it is essential that the insulation of the return circuit against earth matches that of the main line.

To meet safety standards, it is essential to install voltage-limiting devices (VLD-O) when required These devices must be configured to comply with the specifications outlined for depots and workshops in EN 50122-1:2010, section 9.3.2.3.

Principles

To prevent corrosion damage in the return circuit and protect adjacent metallic structures, it is essential to evaluate the stray current conditions during commissioning and monitor them throughout operation Necessary corrective actions should be implemented based on these assessments.

Direct measurement of stray currents is difficult, alternative methods have been proven to be practical

The measurement of the resistance in the return circuit to earth or the voltage against earth during train operation is crucial It is essential to investigate changes in conductance per length, and if necessary, implement countermeasures to address any issues related to stray current.

Unintended return circuit to earth connections shall be removed sufficiently early so that significant damage will not be caused by stray current corrosion

For stray current assessments a recognised method shall be used in certain intervals Annex A lists appropriate procedures.

Supervision of the rail insulation

10.2.1 Continuous monitoring of the rail potential

The rail potential along the line changes, when the conductance per length changes significantly, e.g in case of low-resistive electrical connections between the return circuit and earth

During operation the rail potential is compared with a reference situation The reference situation is the situation, in which the system meets the requirements according to this European Standard

Variations in rail potential along the line highlight deficiencies in conductance per length, including individual rail-to-earth connections in the return circuit, which can significantly impact the stray current situation.

Continuous rail potential monitoring is essential for overseeing the integrity of the return circuit, identifying connections between the return circuit and the ground, and detecting contamination of rail fastenings A suggested method for this process is outlined in section B.1.

If a continuous monitoring is not applied, repetitive monitoring shall be carried out

The time interval for the repetitive inspections shall be fixed depending on the specific risk situation

NOTE For the repetitive monitoring a period of five years is recommended

10.2.2.2 Repetitive measurement of the conductance per length of the running rails

The requirements for conductance per length according to 5.2 shall be fulfilled Proven methods for measurement are given in A.1 to A.5

10.2.2.3 Repetitive measurement of the rail potential

The rail potential along the line varies due to electrical connections between the return circuit and the earth, or changes in the conductance per length of the running rails These variations can be detected through measurements, provided that other influencing factors, such as operational currents and train schedules, remain relatively stable.

NOTE Successive measurements should preferably use the same locations and methods

Regular measurement of rail potential at consistent intervals is essential for identifying insulation deficiencies Upon detection of such issues, immediate countermeasures should be implemented to rectify the situation For effective measurement techniques, refer to the guidelines provided in section B.2.

Rail resistance

Measuring rail resistance is essential for establishing the relationship between rail current and the resulting voltage, which is crucial for determining conductance per length, particularly in tunnel applications.

Figure A.1 shows the recommended measuring arrangement

Figure A.1 ― Measurement of the rail resistance for a rail of 10 m length

The measuring d.c current I is periodically to be switched on and off in order to check other effects during the off-period

Variation of reading values should be taken into account by several measurements Significant differences resulting from changed polarity of the measuring circuit should be investigated

The method is only valid when measurements are taken without any traction current If this is not feasible, simultaneous measurements should be conducted to negate the influence of currents other than the measuring current.

The measuring points at the running rails should be at least 1 m away from the injection points

The longitudinal voltage drop U A and U B is measured for each of the two adjacent sections of rail

The rail resistance is calculated according to the Equation (A.1)

I is the injected current, in amperes (A);

R R10m is the longitudinal resistance of a rail section of 10 m of rail 1 in ohms (Ω);

U on, off is the voltage drop in rail 1 in volts (V), with and without injected current

The measurement assumes that no rail-to-rail crossbonds or track-to-track crossbonds, e.g

Conductance per length between running rails and metal reinforced structures

During measurement, it is essential to electrically isolate the running rails from those outside the metal-reinforced structures using insulating rail joints or rail cuts In tunnels, it is practical to position this separation at the transition ramp leading to the at-grade tracks.

A specialized measuring setup and sequence enable the measurement of conductance per length of running rails against metal-reinforced structures without requiring additional insulating rail joints in the track area The conductance per length, denoted as \$G'_{RS}\$, is measured as illustrated in Figure A.2 and calculated using Equation (A.2), which involves averaging three measured voltage values.

Figure A.2 ― Measuring arrangement for the conductance per length G ´ RS between rails and metal reinforced structure

The measuring d.c current I, injected between the rails and the structure is to be switched on and off periodically

G’ RS is the conductance per length between rails and structure, in Siemens per kilometre (S/km, with 1 S/km = 1/Ωkm);

I RA, I RB is the current flowing beyond the ends A, B of the measured section, in amperes (A);

U RS is the voltage between the rail and the structure at the point of injection, in volts (V);

U RSA, U RSB is the voltage between the rail and the structure at the point A, B of the structure, in volts (V);

L is the length of the section to be measured, in kilometres (km)

The currents I RA and I RB can be gained with help of the voltage drop measurement of the rail described in A.1

It should be ensured that the measurement are not influenced by rail to earth connections or activated voltage-limiting devices (VLD).

Conductance per length for track sections without civil structure

The track section to be examined is separated from the continuing lines by insulated rail joints or rail cuts The length of the track section should not exceed 2 km

If the track section exceeds 2 km, the A.4 method should be applied at key locations, or insulated rail joints must be installed beforehand.

The conductance per length of the separated track section is determined in accordance with the method shown in Figure A.3 and Equation (A.3)

Figure A.3 ― Determination of the conductance per length G ´ RE for track sections without civil structures o f f

G’ RE is the conductance per length between track and earth in Siemens per kilometre with 1 S = 1/Ω;

U RE is the voltage between the rail and earth;

L is the length of the section to be measured in kilometres (km)

NOTE 2 Preferably copper/copper sulphate electrodes should be used

A direct current (d.c.) measuring current, denoted as I, is introduced into the rails at both ends of the insulated rail joints This current must be periodically activated and deactivated The measuring current I travels from the rails of the section under examination into the ground and subsequently returns to the rails of the adjacent track section.

The voltage between the rail and earth (U RE) and the measuring current are crucial for determining the conductance per length It is essential to measure the voltage with a reference electrode.

Local conductance per length for track sections without civil structure

The study examines the average conductance per length of a line extending up to 2 km above the earth, utilizing the procedure outlined in A.3 Additionally, the method illustrated in Figure A.4 and described in Equations (A.4) to (A.6) allows for the assessment of local conductance per length and stray current effects at the measurement site Notably, the measurement can be conducted without the need to cut the rails.

The measurement is performed during operation The rail potential u RE is registered between the running rails and a reference electrode E2 placed on the earth at a distance b from the running rail R2

The rail potential gradient u 1-2 is registered by way of a second electrode E1, which is placed at the distance a from the running rail R2, and the remote electrode E2

NOTE 1 Preferably copper/copper sulphate electrodes should be used

A data logger records both voltages, allowing for the plotting of the rail potential gradient $\Delta U_{1-2}$ as a function of the rail potential $\Delta U_{RE}$ The slope of the linear regression of this relationship represents the stray current transfer ratio $m_{sr}$.

Moreover, the soil resistivity ρE near the electrode E1 is to be determined a b s tg

E1 electrode 1 (close to the rail)

E2 electrode 2 (remote electrode) u RE (t) rail potential u 1–2 (t) voltage between electrodes E1 und E2 a distance between the rail R2 and electrode E1 b distance between the rail R2 and electrode E2 s tg track gauge

Figure A.4 ― Measuring arrangement for the local conductance per length

For single-track lines the local conductance per length is calculated as follows:

To double-track lines the following applies:

G’RE represents the local conductance per length between the running rails and the earth, measured in Siemens per kilometre (S/km) The stray current transfer ratio is denoted as m Sr, while soil resistivity is indicated by ρE, measured in ohm-metres (Ωm) The distance from the outer running rail to the nearby electrode is represented by 'a' in metres (m), and 'b' denotes the distance to the remote electrode Additionally, 's tg' refers to the track gauge in metres (m), and 's td' indicates the track distance between centres, also in metres (m).

Components like water drainage boxes and rail tie bars can distort equipotential lines near running rails, necessitating a minimum distance of 1 meter (a) Additionally, the electrode must be placed outside the voltage gradient, typically requiring a distance of 30 meters (b) in urban areas It's important to verify if this distance is adequate during measurements To evaluate the insulation of running rails, measurements should be conducted at multiple locations, particularly at crossings with other buried installations.

The stray current transfer ratio, denoted as \$m_{sr}\$, is effective for regularly evaluating the insulation of running rails in relation to the ground Any alterations in the insulation of the running rails will correspondingly affect the stray current that escapes into the earth, reflecting changes in the rail potential gradient.

In case that the two preconditions are fulfilled:

Electrodes positioned near the rail must maintain a distance that aligns with the spacing of parallel or intersecting metallic installations The measurement period should be a multiple of the timetable cycle, as the root-mean-square value of the rail potential gradient changes reflects the maximum potential shift of these metallic installations Additionally, this root-mean-square value is directly proportional to the stray current activity (U SCA).

U SCA is the stray current activity of the potential gradient in volts (V); n is the number of the measured data;

U 1–2,i is the instantaneous value of the potential gradient in volts (V);

U is the average of the potential gradient in volts (V)

The assessment follows the guidelines outlined in EN 50162:2004, Table 1 If stray current activity surpasses the permissible limits specified in EN 50162, it may be essential to conduct long-term measurements over a 24-hour period to evaluate the positive potential shift of the metallic structure.

Insulating rail joints

Figure A.5 illustrates the setup used for testing an insulated rail joint It is crucial to ensure that there are no rail-to-rail or track-to-track cross bonds present within the entire measuring arrangement.

U 1,on/off U 2,on/off off on

Figure A.5 ― Test of insulating rail joints

In on-status, a constant current of a few Ampere is fed into the rail across the rail joint to be tested

The functionality of a rail joint is determined by the flow of current towards and away from it Voltage drops, specifically U₁,on and U₂,on, are measured along 10-meter sections of the rail.

In off-status, directly after the switching off of the current the voltages U 1,off and U 2,off are registered

The functionality of the insulating rail joint F J is stated in percent:

F J is the functionality of the insulating rail joint in percent;

U 1, on/off is the voltage drop in rail measuring section 1 in volts (V);

U 2, on/off is the voltage drop in rail measuring section 2 in volts (V)

Measured values of F J ≤ 95 % indicate that a galvanic connection across the joint exists or that the insulating rail joint is faulty.

Insulating joints between metal reinforced structures

Figure A.6 illustrates the setup for testing the insulating joint between structures A and B During the measurement process, a second insulating measurement joint must be opened, while it remains closed during normal operation.

When joint 2 is not opened, the resistance of insulating joint 1 is approximately equal to the combined earth resistances of structures A and B, considering their mutual influence Due to the large surface areas in contact with the ground, this resistance is minimal and insufficient for evaluating the effectiveness of insulating joint 1 However, when insulating joint 2 is opened, part of structure A is disconnected, making its earth resistance the primary factor in the measured resistance of insulating joint 1.

U ba,U on,U off off on

1 insulating joint to be measured

2 joint to be opened to perform measurement

Figure A.6 ― Test of insulating joints in metal reinforced structures

The resistance test utilizes the four-pole measurement method, beginning with an initial basic voltage measurement (U ba) at the insulating joint before applying current A current of approximately 10 A is introduced for a few minutes, after which the current (I) and voltage (U on) at the insulating joint are recorded just before the current is turned off Finally, the voltage (U off) is measured about one second after the current interruption.

The joint resistance R joint is calculated according to Equation (A.8):

R joint is the resistance of the joint in ohms (Ω);

U on is the voltage during on-status in volts (V);

U off is the voltage during off-status after interruption of the current in volts (V);

I is the injected current in amperes (A)

If R joint ≥ 0,5 Ω and the polarisation voltage U off – U ba ≥ 0,1 V the resistance of the joint is high enough

Stray current assessment – Rail insulation assessment using rail potential

Continuous monitoring of the rail potential

Continuous monitoring of rail potential is conducted at specific locations along the line, such as substations and passenger stations To account for fluctuations in traffic throughout the day, a 24-hour averaging process is recommended This method operates continuously without disrupting train traffic.

A change in the average rail potential may indicate a variation in rail-to-earth conductance, raising concerns about increased stray currents Consequently, the average value is assessed against a reference situation that complies with the relevant European Standard.

In systems with unclear stray current situations, obtaining a reference condition is challenging The distance between measurement points is determined by the desired resolution for identifying changes The rail potential shape along the line is influenced by substation locations, timetables, and train characteristics during traction and braking To effectively register the typical rail potential shape, it is practical to position registration points at substations and passenger stations This approach allows for the identification of deficiencies in the electrical separation of the return circuit from installations that contact the earth, such as tunnels, bridges, or other civil structures adjacent to the track bed.

Using a centralized data acquisition system, an automatic indication of the situation and location of a rail to earth connection can be provided for maintenance purposes, see Figure B.1

1 remote evaluation via modem or internet

6 measuring sensors and data transmission network

Figure B.1 ― Continuous monitoring of the rail potential

Repetitive measurements of the rail potential to monitor the conductance

For accurate measurements, it is essential to maintain the same preconditions as those used in previous reference measurements Multiple measuring points are established along the railway line, strategically located near substations, passenger stations, technical buildings, or other installations where stray currents may lead to specific issues.

Earth electrodes, whether existing or newly installed, serve as reference points for measurements The setup mirrors Figure B.1, excluding centralized evaluation Rail potential is recorded at measurement points using a data logger or voltmeter equipped with minimum, maximum, and mean storage functions To enhance assessment, it is beneficial to graphically plot these three values against the route Measurements should be taken over an extended period to ensure a reliable average value is obtained.

The arithmetic mean values, along with the maximum and minimum values, are calculated from each location to establish a voltage characteristic along the line This characteristic is influenced by factors such as conductance per unit length, rail resistance, and traffic Variations in the voltage characteristic can signal changes in rail insulation, necessitating further investigations with more detailed measurements when such changes are detected.

Countermeasures can be necessary depending on the results

NOTE 1 The period could be the hour of the highest traffic or alternatively approximately three times the time table cycle

NOTE 2 Care should be taken in interpretation of the results, when the preconditions have been changed, like traffic or line configuration

Estimation of stray current and impact on metal structures

Estimation of the stray currents passing from the running rails to the earth

The traction return current flowing in the running rails causes changes in the rail potential leading to stray current flow into the earth via the track insulation

Considering the risk of corrosion, the stray current per length is decisive for the loss of material

The most severe case is a dead-end line or line extension, which is connected with an existing rail network

For the calculation method the following assumptions are used:

– the existing railway is modelled as a Norton equivalent circuit The source admittance is estimated by way of the inverse value of the characteristic resistance of the system;

– the traction current is fed in the return circuit at the end of this section

The change in rail potential is calculated as follows:

U RE is the rail potential;

I is the average value of the traction return current in the considered section in the hour of the highest load;

R C is the characteristic resistance of the system running rails/structure;

L C is the characteristic length of the system running rails/structure;

L is the length of the considered line section;

R’ R is the longitudinal resistance of the running rails including parallel return conductors per length;

G’ RE is the conductance of the running rails versus earth per length

Using the rail potential and the conductance of the running rails versus earth the stray current per length is calculated according to Equation (C.4):

I’ is the stray current leaking from the rails relative to the length

Equations (C.1) and (C.4) result in Equation (C.5):

If the stray current per length, divided by the number of parallel tracks is less than 2,5 mA/m, the conditions according to 5.2 are fulfilled

The results derived from Equation (C.5) are conservative, as they account for a dead-end track and do not consider train movements in adjacent line sections, potentially leading to calculated values that are higher than actual conditions If the calculated value surpasses 2.5 mA/m for a single track, a more detailed calculation is recommended.

Estimation of the longitudinal voltage in metal reinforced structures

In reinforced concrete structures such as tunnels, viaducts, and reinforced track beds, stray currents can leak from the running rails into the reinforcement If the reinforcement lacks longitudinal interconnection, these stray currents may flow into the earth through the outer reinforcement, potentially causing corrosion in areas with non-homogeneous conductance To mitigate this risk, a low-resistive longitudinal interconnection of the reinforcement is essential, as it reduces the longitudinal voltage drop When the voltage remains below the thresholds specified in EN 50162:2004, Table 1, the risk of stray current corrosion is minimal Therefore, it is crucial to calculate the longitudinal voltage \$U_S\$ for metal-reinforced structures alongside stray current considerations.

The longitudinal voltage drop in metal reinforced structures, caused by train operation, depends on the following affecting parameters:

– length of the considered line section;

– length of the adjacent line sections;

– conductance between the running rails and the structure;

– conductance between the structure and the earth;

– longitudinal resistance of the running rails;

– longitudinal resistance of the interconnected structure;

– traction return current in the considered line section;

– traction return currents of adjacent lines

For a return circuit section in an infinitely long metal reinforced structure the longitudinal voltage is calculated as follows:

U S is the longitudinal voltage in the metal reinforced structure;

G’ RE is the conductance of the running rails versus earth per length;

I is the average value of the traction return current in the considered section in the hour of

L is the length of the considered line section;

L C is the characteristic length of the system running rails/structure;

R’ R is the longitudinal resistance of the running rails including parallel return conductors per length;

R’ S is the resistance of the structure per length

The results derived from Equation (C.6) are conservative, based on the assumption that the structures on either side of the analyzed section are infinitely long Additionally, the analysis does not account for train movements in adjacent sections or the conductance per length between the structure and the earth, which may lead to calculated values that are higher than actual conditions If the calculated results surpass the values specified in EN 50162:2004, Table 1, it is advisable to employ a more detailed calculation method.

Coverage of Essential Requirements of EC Directives

This European Standard has been prepared under a mandate given to CENELEC by the European

The standard established by the Commission and the European Free Trade Association encompasses all essential requirements outlined in Annex III of the EC Directive 96/48/EC (HSR), Annex III of the EC Directive 2001/16/EC (CONRAIL), and Annex III of the EC Directive 2008/57/EC (RAIL).

Compliance with this standard provides one means of conformity with the specified essential requirements of the Directives concerned

WARNING: Other requirements and other EC Directives may be applicable to the products falling within the scope of this standard

IEC 60050-811:1991, International Electrotechnical Vocabulary – Chapter 811: Electric traction

Tiêu đề	Railway Applications — Fixed Installations — Electrical Safety, Earthing And The Return Circuit Part 2: Provisions Against The Effects Of Stray Currents Caused By D.C. Traction Systems
Tác giả	Wang Bin
Trường học	BSI Standards
Chuyên ngành	Railway Applications
Thể loại	Standard
Năm xuất bản	2010
Thành phố	Brussels

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