EN 50122-2:2010

SLOVENSKI STANDARD
01-december-2010
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SIST EN 50122-2:1998
SIST EN 50122-2:1998/A1:2003
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Railway applications - Fixed installations - Electrical safety, earthing and bonding - Part
2: Provisions against the effects of stray currents caused by d.c. traction systems
Bahnanwendungen - Ortsfeste Anlagen - Elektrische Sicherheit, Erdung und
Rückstromführung - Teil 2: Schutzmaßnahmen in Bezug auf elektrische Sicherheit und
Erdung
Applications ferroviaires - Installations fixes - Sécurité électrique, dispositions pour les
courants de retour et mise à la terre - Partie 2: Mesures de protection contre les effets
des courants vagabonds issus de la traction électrique à courant continu
Ta slovenski standard je istoveten z: EN 50122-2:2010
ICS:
13.260 9DUVWYRSUHGHOHNWULþQLP Protection against electric
XGDURP'HORSRGQDSHWRVWMR shock. Live working
29.280 (OHNWULþQDYOHþQDRSUHPD Electric traction equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 50122-2
NORME EUROPÉENNE
October 2010
EUROPÄISCHE NORM
ICS 29.120.50; 29.280 Supersedes EN 50122-2:1998 + corr. Aug.2001 + A1:2002

English version
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
Applications ferroviaires -  Bahnanwendungen -
Installations fixes - Ortsfeste Anlagen -
Sécurité électrique, mise à la terre et Elektrische Sicherheit, Erdung und
circuit de retour - Rückleitung -
Partie 2: Mesures de protection contre les Teil 2: Schutzmaßnahmen gegen
effets des courants vagabonds issus de la Streustromwirkungen durch Gleichstrom-
traction électrique à courant continu Zugförderungssysteme

This European Standard was approved by CENELEC on 2010-10-01. CENELEC members are bound to comply
with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard
the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and notified
to the Central Secretariat has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus,
the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia,
Spain, Sweden, Switzerland and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Management Centre: Avenue Marnix 17, B - 1000 Brussels

© 2010 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 50122-2:2010 E
Foreword
This European Standard was prepared by SC 9XC, Electric supply and earthing systems for public
transport equipment and ancillary apparatus (Fixed installations), of Technical Committee CENELEC
TC 9X, Electrical and electronic applications for railways. It was submitted to the formal vote and was
approved by CENELEC as EN 50122-2 on 2010-10-01.
This document supersedes EN 50122-2:1998 + A1:2002.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN and CENELEC shall not be held responsible for identifying any or all such patent
rights.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2011-10-01

– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2013-10-01
This draft European Standard has been prepared under a mandate given to CENELEC by the
European Commission and the European Free Trade Association and covers essential requirements
of EC Directives 96/48/EC (HSR), 2001/16/EC (CONRAIL) and 2008/57/EC (RAIL). See Annex ZZ.
______________
– 3 – EN 50122-2:2010
Contents
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 6
4 Identification of hazards and risks . 6
5 Criteria for stray current assessment and acceptance . 7
5.1 General . 7
5.2 Criteria for the protection of the tracks . 7
5.3 Criteria for systems with metal reinforced concrete or metallic structures . 8
5.4 Specific investigations and measures . 8
6 Design provisions . 9
6.1 General . 9
6.2 Return circuit . 9
6.3 Non-traction related electrical equipment .10
6.4 Tracks of other traction systems .11
6.5 Return busbar in the substation .11
6.6 Level crossings .11
6.7 Common power supply for tram and trolleybus .11
6.8 Changeover from the mainline to depot and workshop areas .11
7 Provisions for influenced metallic structures .11
7.1 General .11
7.2 Tunnels, bridges, viaducts and reinforced concrete slab track .12
7.3 Adjacent pipes or cables .13
7.4 Voltage limiting devices .13
8 Protection provisions applied to metallic structures.13
9 Depots and workshops .14
10 Tests and measurements .14
10.1 Principles .14
10.2 Supervision of the rail insulation .14
Annex A (informative) Measurement of track characteristics .16
A.1 Rail resistance .16
A.2 Conductance per length between running rails and metal reinforced structures .17
A.3 Conductance per length for track sections without civil structure .18
A.4 Local conductance per length for track sections without civil structure .19
A.5 Insulating rail joints .21
A.6 Insulating joints between metal reinforced structures .21
Annex B (informative) Stray current assessment – Rail insulation assessment using rail
potential .23
B.1 Continuous monitoring of the rail potential .23
B.2 Repetitive measurements of the rail potential to monitor the conductance .24

Annex C (informative) Estimation of stray current and impact on metal structures .25
C.1 Estimation of the stray currents passing from the running rails to the earth .25
C.2 Estimation of the longitudinal voltage in metal reinforced structures .26
Annex ZZ (informative) Coverage of Essential Requirements of EC Directives .28
Bibliography .29

Figures
Figure A.1 ― Measurement of the rail resistance for a rail of 10 m length .16
Figure A.2 ― Measuring arrangement for the conductance per length G´ between rails and
RS
metal reinforced structure .17
Figure A.3 ― Determination of the conductance per length G´ for track sections without civil
RE
structures .18
Figure A.4 ― Measuring arrangement for the local conductance per length .19
Figure A.5 ― Test of insulating rail joints .21
Figure A.6 ― Test of insulating joints in metal reinforced structures .22
Figure B.1 ― Continuous monitoring of the rail potential .23

– 5 – EN 50122-2:2010
1 Scope
This European Standard specifies requirements for protective provisions against the effects of stray
currents, which result from the operation of d.c. traction systems.
As experience for several decades has not shown evident corrosion effects from a.c. traction systems
and actual investigations are not completed, this European Standard only deals with stray currents
flowing from a d.c. traction system.
This European Standard applies to all metallic fixed installations which form part of the traction
system, and also to any other metallic components located in any position in the earth, which can
carry stray currents resulting from the operation of the railway system.
This European Standard applies to all new d.c. lines and to all major revisions to existing d.c. lines.
The principles may also be applied to existing electrified transportation systems where it is necessary
to consider the effects of stray currents.
It provides design requirements to allow maintenance.
The range of application includes:
a) railways,
b) guided mass transport systems such as:
1) tramways,
2) elevated and underground railways,
3) mountain railways,
4) trolleybus systems, and
5) magnetically levitated systems, which use a contact line system,
c) material transportation systems.
This European Standard does not apply to:
d) mine traction systems in underground mines,
e) cranes, transportable platforms and similar transportation equipment on rails, temporary
structures (e.g. exhibition structures) in so far as these are not supplied directly from the contact
line system and are not endangered by the traction power supply system,
f) suspended cable cars,
g) funicular railways.
This European Standard does not specify working rules for maintenance.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
EN 50122-1:2010, Railway applications – Fixed installations – Electrical safety, earthing and the return
circuit – Part 1: Protective provisions against electric shock
EN 50122-3:2010, Railway applications – Fixed installations – Electrical safety, earthing and the return
circuit – Part 3: Mutual interaction of a.c. and d.c. traction systems
EN 50162:2004, Protection against corrosion by stray current from direct current systems
EN 50163, Railway applications – Supply voltages of traction systems

3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 50122-1:2010 apply.
4 Identification of hazards and risks
D.C. traction systems can cause stray currents which could adversely affect both the railway
concerned and/or outside installations, when the return circuit is not sufficiently insulated versus earth.
The major effects of stray currents can be corrosion and subsequent damage of metallic structures,
where stray currents leave the metallic structures. There is also the risk of overheating, arcing and fire
and subsequent danger to persons and equipment both inside and outside the railway or trolley bus
system.
The following systems, which can produce stray currents, shall be considered:
– d.c. railways using running rails carrying the traction return current including track sections of
other traction systems bonded to the tracks of d.c. railways;
– d.c. trolleybus systems which share the same power supply with a system using the running rails
carrying the traction return current;
– d.c. railways not using running rails carrying the traction return current, where d.c. currents can
flow to earth or earthing installations.
All components and systems which can be affected by stray currents shall be considered such as:
– running rails,
– metallic pipe work,
– cables with metal armour and/or metal shield,
– metallic tanks and vessels,
– earthing installations,
– reinforced concrete structures,
– buried metallic structures,
– signalling and telecommunication installations,
– non-traction a.c. and d.c. power supply systems,
– cathodic protection installations.
Any provisions employed to control the effects of stray currents shall be checked, verified and
validated according to this European Standard.
The system design shall be completed sufficiently early that the results can be taken into account in
the essential system parameters, which influence the stray current effects, like the spacing of the
substations and in the design of the civil structures, see also 5.4.
The entity responsible for the design and erection of the railway infrastructure shall make sure that
electrical requirements for railway related civil structures are met.
In case of major revisions of existing lines the effects on the stray current situation shall be assessed
by calculation and/or by measurements.
If stray current provisions affect electrical safety, protective provisions against electric shock according
to EN 50122-1 shall take precedence over provisions against the effects of stray currents.

– 7 – EN 50122-2:2010
5 Criteria for stray current assessment and acceptance
5.1 General
The amount of stray currents and their effects depend on the overall system design of the traction
power supply. Stray currents leaving the return circuit can affect the return circuit itself and
neighbouring installations, see 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,
– spacing of cross bonds.
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.
5.2 Criteria for the protection of the tracks
The most important influencing variable for stray currents leaving the tracks is the conductance per
unit length between track and earth. The corrosion rate is the main aspect for the assessment of risk.
The rail potential provides the main information about the relevant parameters, which represent the
stray currents. These parameters are the traction currents, the longitudinal resistance of the running
rails, the resistance to earth and the length of the feeding sections. The precondition for this
proceeding is that there is no direct electrical connection either accidental or intended to earthing
installations.
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:
I’ = 2,5 mA/m
max
(average stray current per length of a single track line).
NOTE 1 For a double track line the value for the maximum average stray current should be multiplied by two. For more than
two tracks the value increases accordingly. For the averaging process, only the total positive parts of the stray current over 24 h
or multiples are considered.
If the following values for the conductance per length G’ and average rail potential U are not
RE RE
exceeded during the system life-time, further investigations according to 5.4 need not be performed.
– G’ ≤ 0,5 S/km per track and U ≤ + 5 V for open formation (1)
RE RE
– G’ ≤ 2,5 S/km per track and U ≤ + 1 V for closed formation (2)
RE RE
For the average rail potential shift U only positive values of the rail potential are considered.
RE
The averaging period shall be 24 h or multiples.
NOTE 2 A guide value for the sampling rate is 2 per second.

If the requirements in Equations (1) and (2) are not met, an alternative value for G’ shall be
RE
calculated and used for the design, applying Equation (3).
I
RE
′
G =
(3)
RE
U
where
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.
NOTE 5 Simulation of the traction power supply for scheduled train operation can provide values for the stray current per
length for design purposes. A method of calculating dead-end tracks is given in Clause C.1. This is a conservative method,
because the actual values are lower.
When the construction phase has been completed, it shall be proven that the permissible conductance
per length according to Equations (1), (2) or (3) is fulfilled. Annex A indicates proven methods for the
measurement.
During operation, compliance with the limits of conductance per length according to Equations (1), (2)
or (3) shall be maintained.
5.3 Criteria for systems with metal reinforced concrete or metallic structures
In systems with metal reinforced concrete or metallic structures, like:
– reinforced track bed,
– tunnels, or
– viaducts,
the impact on the structures shall be considered.
The voltage shift of the structure versus earth is an additional criterion for assessment.
Experience has shown, that there is no cause for concern, if the average value of the potential shift
between the structure and earth in the hour of highest traffic does not exceed + 200 mV for steel in
concrete structures. For buried metal constructions the values depend on soil resistivity and the
material. For both requirements refer to EN 50162:2004, Table 1.
In order to avoid inadmissible stray current effects at the tunnel structure and at structures outside of
the tunnel, the longitudinal voltage between any two points of the through connected metal reinforced
tunnel structure should be calculated. The maximal longitudinal voltage shall be smaller than the
permissible potential shift. As an example for calculation see C.2. This is a conservative procedure
which ensures that the actual values for the tunnel potential against earth will be lower.
5.4 Specific investigations and measures
If the requirements stated in 5.2 and 5.3 are not achieved, or if other methods of construction are
planned, a study shall be carried out at an early planning stage. The study becomes also necessary in
case of major revisions of existing lines, when the stray current situation is likely to become worse.

– 9 – EN 50122-2:2010
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,
– timetable and vehicles,
– neighbouring metallic structures.
Clause 6 and Clause 7 show suitable corrective provisions.
6 Design provisions
6.1 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 shall be completed sufficiently early that the results can be taken into account in
the essential system parameters which influence the stray current effects, like the spacing of the
substations and the design of the civil structures.
6.2 Return circuit
6.2.1 General
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.
6.2.2 Resistance of running rails
The longitudinal resistance of the running rails shall be low. Therefore, rail joints shall be welded or
connected by rail joint bonds of low resistance such that the longitudinal resistance of the rails is not
increased by more than 5 %.
The longitudinal resistance can be reduced by the use of rails with greater cross section and/or cross
bonding of the running rails and/or the tracks where signalling considerations allow.
6.2.3 Track system
A high level of insulation from earth of the running rails and of the whole return circuit is required,
when the running rails are used as part of the return circuit.
The track shall be so designed that the insulation quality of the rails toward earth will not be
diminished substantially by water. In order to fulfil the values given in Equations (1), (2) and (3) of 5.2
the water drainage of the substructure of the running rails is essential.
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’ for rails laid in an
RE
open formation:
– clean ballast;
– 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’ for rails laid in a
RE
closed formation:
– fitting of the running rails in an insulating resin bed;
– provision of insulating intermediate layers between the tracks and the bearing systems;
– effective drainage.
6.2.4 Return conductors
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.
6.2.5 Return cables
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.
NOTE In case of direct conductive connection to installations, components or metallic structures which are not insulated
from earth, the values given in Equations (1), (2) and (3) of 5.2 should be fulfilled for the return circuit and parts connected to it.
If a connection to the return circuit is unavoidable for reasons of protection against electric shock,
provisions shall be taken to reduce the stray current effects. These can be for example:
– 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.
A conductor rail insulated from earth, the so-called "fourth rail", can be used for the traction return
current. If this is a live part and not connected to the running rails, usually no stray currents occur. In
the case of conductor rail systems with third and fourth rails, each conductor rail shall be insulated
from earth depending on the nominal voltage of the system according to 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.
6.3 Non-traction related electrical equipment
Non-traction related electrical equipment shall be installed according to EN 50122-1:2010, Clause 7.

– 11 – EN 50122-2:2010
6.4 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.
Any additional provisions shall not affect other safety criteria, particularly such that are made to
minimise the touch voltage as well as such that are made to operate the power supply, track circuits
and communication systems.
6.5 Return busbar in the substation
The substation shall be arranged so that direct current does not flow in the substation structure earth.
Risks from stray current relating to the earthing of equipment due to maintenance work shall be taken
into account. The return busbars in substations and similar installations shall be operated so that they
are insulated from earth. Where required for safety reasons a voltage-limiting device (minimum
type O) to connect between the return busbar and earth shall be provided in accordance with
EN 50122-1:2010, Annex F. For substations in depots and workshops see Clause 9.
6.6 Level crossings
At level crossings, where the running rails are laid in a closed formation, care shall be taken that the
value of the conductance per length does not exceed the value of the neighbouring tracks.
6.7 Common power supply for tram and trolleybus
If trolleybuses and tramways receive their traction power from the same substation, one of the trolley
contact wires may be connected with the track return system referred to EN 50122-1. In this case it
shall be checked to determine whether the protective provisions for both systems to minimise stray
current effects are still sufficient.
The insulation of the running rails shall be coordinated with other provisions ensuring that acceptable
touch voltages according to EN 50122-1 are not exceeded during operation, in case of a short-circuit
and in case of an earth fault.
6.8 Changeover from the mainline to depot and workshop areas
See Clause 9.
7 Provisions for influenced metallic structures
7.1 General
The resistance between conductive structures which are not insulated from earth and the track return
system shall be high. Except in the case of depots and workshops (see Clause 9) earth and track
return systems shall not be connected directly. A direct connection to earth may also be allowed in
certain industrial systems with d.c. traction taking into consideration the particular surrounding
conditions, e.g. open cast coal mines.

7.2 Tunnels, bridges, viaducts and reinforced concrete slab track
7.2.1 Basic proceeding
In a tunnel structure which incorporates conductive components it can be necessary to make
provisions to limit possible effects of stray currents. The requirements for protection against electric
shock shall be taken into account. This is also applicable 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.
7.2.2 Longitudinal interconnection
In the case of tunnels with metal reinforced concrete structures or other conductive structures it is
possible that stray currents can flow into such structures and from there cause influence to other
conductive structures outside the tunnel. In this case the effect of such influence shall be reduced by
means of equipotential bonding in the lower part of the individual tunnel sections or other conductive
structures to achieve the voltage requirements according to 5.3. This equipotential bonding shall be
achieved by:
– a sufficient number of reinforcing bars,
– mats connected together,
– other conductive structural parts,
– if necessary, additional conductors of appropriate cross section laid within the tunnel.
Individual tunnel sections in particular cases may be excepted from the equipotential bonding of the
rest of the tunnel. The equipotential bonding of the other tunnel sections can be achieved by means of
an insulated cable extending over the segregated tunnel section.
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.
7.2.3 Sectionalized reinforcement
In areas where the influence of stray currents on structures outside the tunnel is not significant and
when a sufficiently high value of rail to earth resistance cannot be achieved, e.g. due to humidity or
ballast which is not sufficiently clean, the main consideration shall be the corrosion of the tunnel
metallic structures.
Reinforced concrete tunnel structures shall be divided into longitudinal sections by insulating joints in
the case where stray currents from other adjacent systems can flow along the tunnel structure, thus
causing an undesirable electrical connection between different areas.
NOTE 1 If the resistance between these structures and earth is relatively high, for example in rock tunnels, the reinforced
concrete tunnel structures may also be divided into longitudinal sections by insulating joints.
If there is any risk of an impermissible voltage between simultaneously accessible parts, refer to
EN 50122-1.
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.

– 13 – EN 50122-2:2010
7.2.4 External conductive parts
The reinforcement of steel-reinforced concrete tunnels and tunnel components consisting of
conductive materials shall not have any electrical connection to pipes and cables located outside the
tunnel or to the return circuit or to any adjacent systems which are not insulated from earth. If this
electrical separation is not possible, for instance due to different earthing systems being present in
one building danger of stray current exchange and in consequence danger of stray current corrosion
exists. In this case continuous monitoring, shall be provided, see 10.2.1, and rail to structure earth
connections shall be removed promptly.
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
Precautions are necessary in order to avoid possible stray current exchange between structure earth
and remote non-railway installations. At the entrance to metal reinforced concrete or metallic railway
structures, such as tunnels and viaducts, depots and workshops, all metallic pipe work, hydraulic lines,
cable sheaths, and connections to earth, coming from outside shall be separated electrically from the
structure earth in order to avoid any conductive connection 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.
7.3 Adjacent pipes or cables
Where d.c. traction systems are close to buried pipes or cables, efforts shall be made to ensure that
the conductive parts are kept as far away as practicable to minimise stray current interference.
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.
7.4 Voltage limiting devices
If voltage-limiting devices (VLD) between the return circuit and the metallic components of the
structure are provided as a protective provision to prevent impermissible voltages the protective
provisions against the effects of stray current shall be satisfied; see EN 50122-1:2010, Annex F.
8 Protective provisions applied to metallic structures
The provisions in this European Standard are intended to reduce stray currents in order to reduce
stray current corrosion. Conventional protective provisions against natural corrosion for non-railway
installations can be used if they are considered to be necessary. If additional protective methods are
taken into account, the overall protection concept shall be agreed with affected parties and comply
with the relevant standards concerning stray current corrosion.
NOTE The connection of any metallic structure to return bus bar in a substation even via a polarised electric drainage
device will increase the overall stray current. Therefore, the connection of any metallic structure to the return bus bar should be
made only with due consideration given to the overall effect on the running rails and other structures which could be influenced.
Polarised electric drainage is generally applicable only when the structure to be protected is remote from other structures.

9 Depots and workshops
As the tracks in depots and workshops are concentrated on a small area, no major voltage drop arises
in this track area. Therefore, in deviation from the generic requirement in 7.1 a direct connection
between the structure earth and the return circuit is tolerable for equipotential bonding for safety
reasons. As a consequence of that, the running rails in depots or workshops and similar locations shall
be separated from the main line by insulating rail joints and the traction power shall be supplied by
separate transformer-rectifier units. Exceptions are allowed provided a stray current study proves no
negative effects can occur. For touch voltage across the rail joints see EN 50122-1:2010, Table 6.
If the running rails of the depot and workshop area are connected to the main line, the insulation of the
return circuit versus earth shall be the same as on the main line.
NOTE In order to fulfil the safety requirements, voltage-limiting devices (VLD-O) should be installed if necessary. The
setting of them shall fulfil the values for depots and workshops according EN 50122-1:2010, 9.3.2.3.
10 Tests and measurements
10.1 Principles
In order to avoid corrosion damage in the return circuit according to 5.2 and damage in neighbouring
metallic structures according to 5.3, the stray current situation shall be assessed during
commissioning and checked during operation. Corrective measures can then be taken where
necessary.
Direct measurement of stray currents is difficult, alternative methods have been proven to be practical.
They are based on the measurement of the resistance of the return circuit to earth or the voltage
against earth resulting from train operation. Changes in the conductance per length need preferably
be investigated and countermeasures shall be started, if required by the stray current situation.
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.
10.2 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.
Changes in the rail potential along the line indicate deficiencies in the conductance per length
including individual rail-to-earth connections of the return circuit, which can affect the stray current
situation.
Continuous monitoring of the rail potential supports the supervision of the continuity of the return
circuit, detects connections between the return circuit and earth and pollution of rail fastenings. A
recommended method is given in B.1.

– 15 – EN 50122-2:2010
10.2.2 Repetitive monitoring
10.2.2.1 General
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 changes when electrical connections between the return circuit and
earth arise or when the conductance per length of the running rails changes. Such situations can be
detected by measurements, if other influencing parameters, e.g. operation currents and train schedule
do not change significantly.
NOTE Successive measurements should preferably use the same locations and methods.
If the repetitive measurement of the rail potential along the line in regular intervals detects deficiencies
in the rail insulation, countermeasures shall be started to improve the situation. A recommended
method for measurement is given in B.2.

Annex A
(informative)
Measurement of track characteristics
A.1 Rail resistance
The measurement of the rail resistance is necessary to obtain the relation between the rail current and
the resulting voltage for the subsequent determination of the conductance per length e.g. in a tunnel.
Figure A.1 shows the recommended measuring arrangement.
I
U U
A,on/off B,on/off
off on
I I
A B
10 m 10 m
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 valid only if these measurements are performed without any traction current. Where this
is not practical, the measurements should be made at the same time in order to eliminate the effect 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 and U is measured for each of the two adjacent sections of rail.
A B
The rail resistance is calculated according to the Equation (A.1).
(U − U )+ (U − U )
A, on A, off B, on B, off
R = (A.1)
R10m
I
where
I is the injected current, in amperes (A);
R is the longitudinal resistance of a rail section of 10
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