IEC 62128-3:2013

Railway applications - Fixed installations - Electrical safety, earthing and the return circuit - Part 3: Mutual interaction of a.c. and d.c. traction systems

IEC 62128-3:2013 specifies requirements for the protective provisions relating to electrical safety in fixed installations, when it is reasonably likely that hazardous voltages or currents will arise for people or equipment, as a result of the mutual interaction of a.c. and d.c. electric traction systems. It also applies to all aspects of fixed installations that are necessary to ensure electrical safety during maintenance work within electric traction systems. This standard applies to all new lines, extensions and to all major revisions to existing lines.

Applications ferroviaires - Installations fixes - Sécurité électrique, mise à la terre et circuit de retour - Partie 3: Interactions mutuelles entre systèmes de traction en courant alternatif et en courant continu

La CEI 62128-3:2013 spécifie des exigences relatives aux mesures de protection liées à la sécurité électrique des installations fixes, lorsqu'il existe un risque raisonnable de présence de tensions ou de courants dangereux pour les personnes ou les équipements, dû aux interactions entre systèmes de traction à courant alternatif et à courant continu. La présente norme s'applique également à tous les aspects des installations fixes, nécessaires pour garantir la sécurité électrique pendant les travaux de maintenance réalisés au sein des systèmes de traction électrique. La présente norme s'applique à l'ensemble des nouvelles lignes, extensions et révisions majeures apportées aux lignes existantes.

General Information

• Status: Published
• Publication Date: 11-Sep-2013
• ICS: 45.060.01 - Railway rolling stock in general
• Technical Committee: TC 9 - Electrical equipment and systems for railways
• Drafting Committee: WG 49 - TC 9/WG 49
• Current Stage: PPUB - Publication issued
• Start Date: 12-Sep-2013
• Completion Date: 15-Sep-2013

Overview

The IEC 62128-3:2013 is an international standard published by the International Electrotechnical Commission (IEC) focused on the electrical safety aspects in fixed railway installations affected by the mutual interaction of alternating current (a.c.) and direct current (d.c.) traction systems. It covers protective provisions necessary to mitigate the risks of hazardous voltages or currents that may arise due to the coexistence or proximity of a.c. and d.c. electric traction infrastructure.

This standard is highly relevant to electrical engineers, railway system designers, and maintenance teams involved in railway electrification projects, especially those dealing with mixed a.c./d.c. environments. It applies to new installations, extensions, and major revisions of existing rail networks, including urban transit systems like tramways, metro lines, and trolleybus systems.

Key Topics

• Mutual Interaction of a.c. and d.c. Systems
  The core focus of IEC 62128-3 is on understanding and controlling the interactions that occur between a.c. and d.c. traction systems where they run in parallel, cross, share infrastructure, or meet at system separation sections. These interactions can cause hazardous potentials and stray currents impacting safety.

• Types of Coupling
  The standard distinguishes between galvanic (direct electrical connection) and non-galvanic (inductive and capacitive) coupling. Each coupling type poses different safety risks, such as unwanted circulating currents or induced voltages, which must be carefully managed.

• Zones of Mutual Interaction
  To protect personnel and equipment, the standard defines zones where interaction effects must be assessed and where special technical measures are implemented. These zones exist where the a.c. and d.c. systems influence each other electrically through return circuits or proximity.

• Touch Voltage Limits
  IEC 62128-3 establishes threshold limits for touch voltages considering both a.c. and d.c. components, addressing long-term and short-term conditions to reduce the risk of electric shock to maintenance workers and the public.

• Technical Safety Requirements
  It provides comprehensive guidance on earthing, return circuits, insulation coordination, and the design of overhead contact lines (OCL) to prevent hazardous situations. Special considerations are given for shared buildings, common structures, and workshops where combined voltages can be more pronounced.

Applications

• Railway Electrification Projects
  Engineers designing new railway lines or upgrading existing infrastructure can apply IEC 62128-3 to ensure safe integration of diverse traction power systems.

• Maintenance and Safety Procedures
  The standard guides the safe operation and maintenance of mixed a.c. and d.c. traction systems, specifying protective measures to avoid electrical hazards during work on or near traction equipment.

• Urban Transit Systems
  Applications include tramways, underground trains, and trolleybus networks where a.c. and d.c. systems coexist or intersect, ensuring safe coexistence of varying power supplies.

• System Interface Management
  This includes managing system separation points between a.c. and d.c. traction lines to prevent unsafe electrical interference while facilitating operational functionality.

• Infrastructure Sharing
  The standard helps define safe practices for shared use of tracks, earthing systems, and structures by different electrical systems, reducing the risk of stray currents and potential damage.

Related Standards

• IEC 62128 Series
  IEC 62128 consists of multiple parts that address railway fixed installations and electrical safety:

  • Part 1: General requirements
  • Part 2: Return circuit
  • Part 3: Mutual interaction of a.c. and d.c. traction systems (this part)

• EN 50122-3
  The European standard upon which IEC 62128-3 is based, providing localized or regional requirements that complement the international approach.

• IEC 60364 Series
  General electrical installation standards that provide guidelines on earthing and electrical safety applicable in railway settings.

• ISO/IEC Directives, Part 2
  Governs the preparation and structure of IEC standards, ensuring consistency and clarity in their international application.

---

By adhering to IEC 62128-3:2013, railway operators and infrastructure providers can minimize electrical hazards in complex traction environments, improving safety for personnel and equipment alike while supporting the efficient coexistence of diverse railway power systems. Key SEO phrases include railway electrical safety, traction system interaction, mutual interaction a.c. and d.c., railway earthing standards, and mixed traction system safety.

Standards Content (Sample)

IEC 62128-3 ®
Edition 1.0 2013-09
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Railway applications – Fixed installations – Electrical safety, earthing and the
return circuit –
Part 3: Mutual interaction of a.c. and d.c. traction systems

Applications ferroviaires – Installations fixes – Sécurité électrique, mise à la
terre et circuit de retour –
Partie 3: Interactions mutuelles entre systèmes de traction en courant alternatif
et en courant continu
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IEC 62128-3 ®
Edition 1.0 2013-09
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Railway applications – Fixed installations – Electrical safety, earthing and the

return circuit –
Part 3: Mutual interaction of a.c. and d.c. traction systems

Applications ferroviaires – Installations fixes – Sécurité électrique, mise à la

terre et circuit de retour –
Partie 3: Interactions mutuelles entre systèmes de traction en courant alternatif

et en courant continu
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX V
ICS 45.060 ISBN 978-2-8322-1034-5

– 2 – 62128-3  IEC:2013
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 7
3 Terms and definitions . 7
4 Hazards and adverse effects . 7
4.1 General . 7
4.2 Electrical safety of persons . 7
5 Types of mutual interaction to be considered . 7
5.1 General . 7
5.2 Galvanic coupling . 8
5.2.1 AC and d.c. return circuits not directly connected . 8
5.2.2 AC and d.c. return circuits directly connected or common . 8
5.3 Non-galvanic coupling . 9
5.3.1 Inductive coupling . 9
5.3.2 Capacitive coupling . 9
6 Zone of mutual interaction . 9
6.1 General . 9
6.2 AC . 9
6.3 DC . 10
7 Touch voltage limits for the combination of alternating and direct voltages . 10
7.1 General . 10
7.2 Touch voltage limits for long-term conditions . 11
7.3 AC system short-term conditions and d.c. system long-term conditions . 12
7.4 AC system long-term conditions and d.c. system short-term conditions . 13
7.5 AC system short-term conditions and d.c. system short-term conditions . 14
7.6 Workshops and similar locations . 14
8 Technical requirements and measures inside the zone of mutual interaction . 15
8.1 General . 15
8.2 Requirements if the a.c. railway and the d.c. railway have separate return
circuits . 15
8.2.1 General . 15
8.2.2 Return circuit or parts connected to the return circuit located in the
OCLZ and/or CCZ of the other system . 15
8.2.3 Common buildings and common structures . 16
8.2.4 Inductive and capacitive coupling . 17
8.3 Requirements if the a.c. railway and the d.c. railway have common return
circuits and use the same tracks . 17
8.3.1 General . 17
8.3.2 Measures against stray current . 17
8.3.3 Common structures and common buildings . 18
8.3.4 Exceptions . 18
8.3.5 Design of overhead contact line . 18
8.3.6 Inductive and capacitive coupling . 18
8.4 System separation sections and system separation stations . 18
Annex A (informative) Zone of mutual interaction . 20
Annex B (informative) Analysis of combined voltages . 26

62128-3  IEC:2013 – 3 –
Annex C (informative) Analysis and assessment of mutual interaction . 31
Bibliography . 32

Figure 1 – Maximum permissible combined effective touch voltages (excluding
workshops and similar locations) for long-term conditions . 12
Figure 2 – Maximum permissible combined effective touch voltages under a.c. short-
term conditions and d.c. long-term conditions . 13
Figure 3 – Maximum permissible combined effective touch voltages under a.c. long-
term conditions and d.c. short-term conditions . 14
Figure 4 – Maximum permissible combined effective touch voltages in workshops and
similar locations excluding short-term conditions. 15
Figure 5 – Example of where a VLD shall be suitable for both alternating and direct
voltage . 16
Figure A.1 – Overview of voltages coupled as function of distance and soil resistivity I . 21
Figure A.2 – Overview of voltages coupled as function of distance and soil resistivity II . 22
Figure A.3 – Relation between length of parallelism and zone of mutual interaction
caused by an a.c. railway . 23
Figure B.1 – Definition of combined peak voltage. 27
Figure B.2 – Overview of permissible combined a.c. and d.c. voltages . 28
Figure B.3 – Overview of permissible voltages in case of a duration ≥ 1,0 s for both
a.c. voltage and d.c. voltage . 29
Figure B.4 – Permissible voltages in case of a duration of 0,1 s a.c. voltage and a
duration of 300 s d.c. voltage . 30

– 4 – 62128-3  IEC:2013
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
RAILWAY APPLICATIONS –
FIXED INSTALLATIONS –
ELECTRICAL SAFETY, EARTHING AND THE RETURN CIRCUIT –

Part 3: Mutual interaction of a.c. and d.c. traction systems

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 62128-3 has been prepared by IEC technical committee 9:
Electrical equipment and systems for railways.
This standard is based on EN 50122-3.
The text of this standard is based on the following documents:
FDIS Report on voting
9/1805/FDIS 9/1838/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.

62128-3  IEC:2013 – 5 –
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 62128 series, published under the general title Railway
applications – Fixed installations – Electrical safety, earthing and the return circuit, can be
found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – 62128-3  IEC:2013
RAILWAY APPLICATIONS –
FIXED INSTALLATIONS –
ELECTRICAL SAFETY, EARTHING AND THE RETURN CIRCUIT –

Part 3: Mutual interaction of a.c. and d.c. traction systems

1 Scope
This part of IEC 62128 specifies requirements for the protective provisions relating to
electrical safety in fixed installations, when it is reasonably likely that hazardous voltages or
currents will arise for people or equipment, as a result of the mutual interaction of a.c. and
d.c. electric traction systems.
It also applies to all aspects of fixed installations that are necessary to ensure electrical safety
during maintenance work within electric traction systems.
The mutual interaction can be of any of the following kinds:
– parallel running of a.c. and d.c. electric traction systems;
– crossing of a.c. and d.c. electric traction systems;
– shared use of tracks, buildings or other structures;
– system separation sections between a.c. and d.c. electric traction systems.
Scope is limited to basic frequency voltages and currents and their superposition. This
standard does not cover radiated interferences.
This standard applies to all new lines, extensions and to all major revisions to existing lines
for the following electric traction systems:
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.
The 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 or via
transformers from the contact line system and are not endangered by the traction power
supply system for railways;
f) suspended cable cars;
g) funicular railways;
h) procedures or rules for maintenance.
The rules given in this standard can also be applied to mutual interaction with non-electrified
tracks, if hazardous voltages or currents can arise from a.c. or d.c. electric traction systems.

62128-3  IEC:2013 – 7 –
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 62128-1:2013, Railway applications – Fixed installations – Electrical safety, earthing and
the return circuit – Part 1: Protective provisions against electric shock
IEC 62128-2:2013, 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
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 62128-1 apply.
4 Hazards and adverse effects
4.1 General
The different requirements specified in IEC 62128-1 and IEC 62128-2, concerning
connections to the return circuit of the a.c. railway, and connections to the return circuit of the
d.c. railway, shall be harmonized in order to avoid risks of hazardous voltages and stray
currents.
Such hazards and risks shall be considered from the start of the planning of any installation
which includes both a.c. and d.c. railways. Suitable measures shall be specified for limiting
the voltages to the levels given in this standard, while limiting the damaging effects of stray
currents in accordance with IEC 62128-2.
Additional adverse effects are possible, for example:
– thermal overload of conductors, screens and sheaths;
– thermal overload of transformers due to magnetic saturation of the cores;
– restriction of operation because of possible effects on the safety and correct functioning of
signalling systems;
– restriction of operation because of malfunction of the communication system.
These effects should be considered in accordance with the appropriate standards.
4.2 Electrical safety of persons
Where a.c. and d.c. voltages are present together the limits for touch voltage given in
Clause 7 apply in addition to the limits given in IEC 62128-1:2013, Clause 9.
5 Types of mutual interaction to be considered
5.1 General
Coupling describes the physical process of transmission of energy from a source to a
susceptible device.
The following types of coupling shall be considered:

– 8 – 62128-3  IEC:2013
a) galvanic (conductive) coupling;
b) non-galvanic coupling;
1) inductive coupling;
2) capacitive coupling.
Galvanic coupling dominates at low frequencies, when circuit impedances are low. The effects
of galvanic coupling are conductive voltages and currents.
The effects of inductive coupling are induced voltages and hence currents. These voltages
and currents depend inter alia on the distances, length, inducing current conductor
arrangement and frequency.
The effects of capacitive coupling are influenced voltages into galvanically separated parts or
conductors. The influenced voltages depend inter alia on the voltage of the influencing system
and the distance. Currents resulting from capacitive coupling also depend on the frequency.
NOTE As far as the capacitive and inductive coupling is concerned, general experience is that only the influence
of the a.c. railway to the d.c. railway is significant.
5.2 Galvanic coupling
5.2.1 AC and d.c. return circuits not directly connected
A mutual interaction between the return circuits is possible by currents through earth caused
by the rail potential of both a.c. and d.c. railways, for example return currents flowing through
the return conductors, earthing installations of traction power supply substations and cable
screens.
In case a conductive parallel path to the return circuit exists in the influenced system, various
effects are possible. In case a vehicle forms part of the parallel path, return current of the
influencing railway system can flow through the propulsion system of the traction unit. The
same effects are possible when the return current of the influencing system flows, for
example, through the auto-transformer and substation transformer of an auto-transformer
system or through booster transformers or other devices.
An electric shock with combined voltages can occur when parts of the return circuits or
conductive parts which are connected to the return circuits by voltage limiting devices are
located in the overhead contact line zone of the other railway system, see 8.2.2.
5.2.2 AC and d.c. return circuits directly connected or common
In addition to the effects described in 5.2.1 current exchange will be increased where a.c. and
d.c. return circuits are directly connected or common.
EXAMPLE Direct connections can be railway level crossings, common tracks, system separation sections, etc.
Currents flowing between the a.c. railway and the d.c. railway can create mutual interaction
between the return circuits.
Both return circuits are at the same potential at the location of the connection. A short-circuit
within the a.c. system can cause a peak voltage on conductive structures connected to the
return circuit of the d.c. railway. The same effects apply for conductive structures connected
to it directly or via a voltage limiting device (VLD). The voltage across the voltage limiting
device can trip the device without a fault on the d.c. side.
The connection of the return circuit of the d.c. railway to the earthed return circuit of the a.c.
railway increases the danger of stray current corrosion.
For requirements for fixed installations see 8.3.

62128-3  IEC:2013 – 9 –
5.3 Non-galvanic coupling
5.3.1 Inductive coupling
An a.c. voltage can be induced on a d.c. contact line system and on the d.c. system’s return
circuit. This effect needs to be considered in case the d.c. railway is within the zone of mutual
interaction.
Consequently an a.c. voltage can occur within the d.c. substation at the busbars versus earth
(i.e. at the rectifier or in the feeder cubicles).
Interaction can occur in terms of impermissible touch voltages. See Clause 7.
Perpendicular crossings do not result in inductive effects in the d.c. system.
5.3.2 Capacitive coupling
Within small distances an a.c. voltage can be influenced on a d.c. contact line system when it
is isolated with a disconnector or circuit-breaker open. The possibility shall be considered that
the flash-over voltage of the insulators or of the surge arrestors can be reached.
Distance depends inter alia on geometry and voltage.
An a.c. voltage can occur within the d.c. substation at the d.c. busbars versus earth, i.e. in the
feeder cubicles.
Interaction can occur in terms of impermissible touch voltages. See Clause 7.
6 Zone of mutual interaction
6.1 General
The a.c. railway affects the d.c. railway and vice-versa by galvanic, inductive and/or
capacitive coupling (see Clause 5). The zone of mutual interaction indicates a distance and a
length of parallelism between an a.c. railway and a d.c. railway (see Annex A). The limits of
zone of mutual interaction are based on the limits of the touch voltage given in Clause 7.
If a zone of mutual interaction exists the requirements given in this standard shall be fulfilled.
When the distance between both a.c. and d.c. railways is less than 50 m a zone of mutual
interaction is assumed. Distances in excess of 50 m are dealt with in 6.2 and 6.3.
When the distance between a.c. and d.c. railways becomes less than 50 m effects as
described in 5.2.1 or even 5.2.2 can be expected.
Distances between a.c. railway and the d.c. railway cannot be given in a generic way and
should be addressed separately depending on the local conditions.
NOTE For information on analysis and assessment of zone of mutual interaction see Annex C.
6.2 AC
In case of an a.c. railway influencing a d.c. railway the zone of mutual interaction is based on
voltages coupled into the affected system.
For planning purposes the zone of mutual interaction has to be investigated either by
calculation or by the following procedure.

– 10 – 62128-3  IEC:2013
Where the following preconditions apply the limit of the distance between a.c. and d.c. railway
is 1 000 m:
– double track line, where only the four running rails of the a.c. railway are used for the
return circuit;
– the inducing current is 500 A per overhead contact line (1 000 A in total);
– the length of parallelism between a.c. and d.c. railway is 4 km;
– the soil resistivity is 100 Ωm;
– the rated frequency is 50 Hz;
– the affected system is insulated versus earth along its entire length and connected to
earth at one end only;
– screening effects of other parallel metallic objects have not been taken into account.
Where other preconditions apply the dimension of the zone of mutual interaction shall be
calculated.
A method for the calculation is given in Annex A.
NOTE The example above is based on a 35 V limit for a.c. with a time duration longer than 300 s.
In case a d.c. railway is within the zone of mutual interaction of an a.c. railway, the level of
voltages or currents coupled into the d.c. system is not necessarily too high; in this case
further analysis of the situation shall be carried out.
6.3 DC
For the effects of d.c. railway systems on a.c. railway systems the dimension of the zone of
mutual interaction can be neglected due to the steep voltage gradient in the soil, caused by
the insulated rails.
However if the possibility of a voltage transfer exists, either permanently or temporary, due to
a galvanic connection towards conductive or partly conductive parts, the zone of mutual
interaction is given by the dimensions of those parts. In this case the level of voltages or
currents coupled into the a.c. system is not necessarily too high; further analysis of the
situation shall be carried out.
7 Touch voltage limits for the combination of alternating and direct voltages
7.1 General
The limits given in 7.2 to 7.6 are based on touch voltage only and shall not be exceeded.
Other effects with respect to electrical installations are not taken into account.
Limits for electrical installations cannot be given in a generic way and should be addressed
separately if necessary, depending on the sensitivity of the affected installations.
Where either an alternating or a direct voltage is present the touch voltage limits given in
IEC 62128-1 apply.
The direct and the alternating components of a combined voltage u(t) for time duration in
excess of 1 s are calculated as follows:
a+T
U = ⋅ u(t)⋅ dt
(1)
dc
∫
T
a
62128-3  IEC:2013 – 11 –
a+T
U = ⋅ (u(t)−U ) ⋅ dt (2)
ac dc
∫
T
a
where
T = 1 s;
t is the time;
u(t) is the combined voltage;
U is the direct component of combined voltage;
dc
U is the alternating component of combined voltage.
ac
NOTE 1 Formula (1) gives the moving average value of the direct component, Formula (2) gives the moving r.m.s.
value of the alternating component.
Only for short-duration phenomena t ≤ 1 s the following definitions for alternating voltage and
direct voltage are used:
– U is defined as that part of the combined voltage that is caused by the d.c. system;
dc
– U is defined as that part of the combined voltage that is caused by the a.c. system.
ac
NOTE 2 Further information on combined voltages is given in Annex B.
NOTE 3 Long-term conditions are associated with operation conditions and short-term conditions are associated
with fault conditions or for example switching operations.
7.2 Touch voltage limits for long-term conditions
The following approach shall be used to check whether the combined voltage is permissible:
a) the alternating part of the combined voltage shall not exceed the maximum permissible
alternating body voltage as given in IEC 62128-1, Table 3 for the applicable duration;
b) the direct part of the combined voltage shall not exceed the maximum permissible direct
body voltage as given in IEC 62128-1, Table 5 for the applicable duration;
c) the combined voltage is permissible if it is within the envelope as given for the applicable
duration in Figure 1;
d) for time durations in excess of 1 s the combined peak value (see explanation in Annex B)
shall be less than 2 × 2 times the maximum permissible alternating body voltage as
given in IEC 62128-1, Table 3 for the applicable duration irrespective of frequency
content.
EXAMPLE Assuming the maximum permissible direct touch voltage of 120 V being present in the d.c. system the
alternating voltage limit is 35 V, see Figure 1. Assuming the maximum permissible alternating touch voltage of 60 V
being present in the a.c. system the direct voltage limit is 85 V, see Figure 1.

– 12 – 62128-3  IEC:2013
V
0,7 s
0,8 s
0,9 s
1,0 s
300 s
∞
U
ac
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 V 190
U
dc
IEC  2012/13
All values are r.m.s.
Figure 1 – Maximum permissible combined effective touch voltages
(excluding workshops and similar locations) for long-term conditions
7.3 AC system short-term conditions and d.c. system long-term conditions
The following approach shall be used to check whether the combined voltage is permissible:
a) the short-duration alternating part of the combined voltage shall not exceed the maximum
permissible alternating touch voltage as given in IEC 62128-1, Table 4 for the applicable
duration;
b) the direct part of the combined voltage shall not exceed the maximum permissible direct
touch voltage as given in IEC 62128-1, Table 6 for the applicable duration;
c) the combined voltage is permissible if it is within the envelope as given for the applicable
durations in Figure 2.
62128-3  IEC:2013 – 13 –
V
0,02 s
0,05 s
0,1 s
0,2 s
U
ac 0,3 s
0,4 s
0,5 s
0,6 s
0 50 100 150 V 200
U
dc
IEC  2013/13
All values are r.m.s.
Figure 2 – Maximum permissible combined effective touch voltages
under a.c. short-term conditions and d.c. long-term conditions
EXAMPLE An example of the use of Figure 2 is given in Annex B.
7.4 AC system long-term conditions and d.c. system short-term conditions
The following approach shall be used to check whether the combined voltage is permissible:
a) the alternating part of the combined voltage shall not exceed the maximum permissible
alternating touch voltage as given in IEC 62128-1, Table 4 for the applicable duration;
b) the short-duration direct part of the combined voltage shall not exceed the maximum
permissible direct touch voltage as given in IEC 62128-1, Table 6 for the applicable
duration;
c) the combined voltage is permissible if it is within the envelope as given for the applicable
durations in Figure 3.
∞
300 s
1,0 s
0,9 s
0,8 s
0,7 s
– 14 – 62128-3  IEC:2013
V
0,7 s
0,8 s
0,9 s
1,0 s
300 s
60 ∞
UU
acac
0 100 200 300 400 500 600 700 800 V 900
UU
dcdc
IEC  2014/13
All values are r.m.s.
Figure 3 – Maximum permissible combined effective touch voltages
under a.c. long-term conditions and d.c. short-term conditions
7.5 AC system short-term conditions and d.c. system short-term conditions
Simultaneous short-term phenomena in the a.c. system and in the d.c. system do not need to
be considered.
NOTE It is unlikely that short-term phenomena occur at the same time in both the a.c. system and the d.c.
system, and that the return circuit is touched at the same moment.
7.6 Workshops and similar locations
For long-term conditions the following approach shall be used to check whether the combined
voltage is permissible:
a) the alternating part of the combined voltage shall comply with IEC 62128-1, 9.2.2.3;
b) the direct part of the combined voltage shall comply with IEC 62128-1, 9.3.2.3;
c) the combined voltage is permissible if it is within the envelope as defined in Figure 4.
For short-term conditions 7.3, 7.4 and 7.5 apply.
EXAMPLE Assuming the maximum permissible direct touch voltage of 60 V being present in the d.c. system the
alternating voltage limit is 8 V. Assuming the maximum permissible alternating touch voltage of 25 V being present
in the a.c. system the direct voltage limit is 35 V, see Figure 4.
0,6 s
0,5 s
0,4 s
0,3 s
0,2 s
0,1 s
0,05 s
0,02 s
62128-3  IEC:2013 – 15 –
V
U
ac
0 5 10 15 20 25 30 35 40 45 50 55 60 V 65
U
dc
IEC  2015/13
All values are r.m.s.
Figure 4 – Maximum permissible combined effective touch voltages
in workshops and similar locations excluding short-term conditions
8 Technical requirements and measures inside the zone of mutual interaction
8.1 General
All installations shall comply with the requirements of IEC 62128-1 and IEC 62128-2.
Additional provisions are described below.
8.2 Requirements if the a.c. railway and the d.c. railway have separate return circuits
8.2.1 General
This subclause applies if there is no return circuit connection between the a.c. railway and the
d.c. railway.
8.2.2 Return circuit or parts connected to the return circuit located in the OCLZ
and/or CCZ of the other system
An assessment shall be made of whether the application of IEC 62128-1, Clause 6 will require
conductive connections to be made between the return circuit of the a.c. railway and the
return circuit of the d.c. railway. If such connections are required, then the voltage-limiting
devices which are required by IEC 62128-1, 6.2.2 shall be specified to detect and operate
with alternating voltages and currents in addition to direct voltages and currents, so that
compliance is achieved with Clause 7 of this standard. See IEC 62128-1, Annex F.
If the return circuit, parts connected to the return circuit or the vehicles of the a.c. railway are
within the overhead contact line zone or the current collector zone of the d.c. railway, or vice
versa, then voltage-limiting devices (minimum function VLD-F) shall be connected between
the return circuit of the d.c. railway and the return circuit of the a.c. railway.

– 16 – 62128-3  IEC:2013
EXAMPLE See Figure 5.
1 3
2 4
IEC  2016/13
Key
1 current collector zone of a.c. line
2 overhead contact line zone of a.c. line
3 current collector zone of d.c. line
4 overhead contact line zone of d.c. line
5 fence or other conductive part (bonded to the return circuit of the a.c. line)
6 voltage limiting device
Figure 5 – Example of where a VLD shall be
suitable for both alternating and direct voltage
The design of the systems shall be such that the voltage-limiting devices will not conduct
during operating conditions in order to meet the requirements of IEC 62128-2.
The risks associated with the conductive state of voltage-limiting devices shall be assessed.
8.2.3 Common buildings and common structures
8.2.3.1 Selection of the strategy for earthing
An assessment shall be made, at an early planning stage, whether it is desirable and possible
to separate the part of the structure earth associated with the a.c. railway from the part of the
structure earth associated with the d.c. railway, and to separate either part of the structure
earth from earthing systems outside the common building or structure. In all cases the paths
taken by earth fault current from the a.c. contact line and the d.c. contact line shall be
identified, and conductors of sufficient cross-sectional area shall be provided. See
IEC 62128-1, Clause 7 for non-traction power supplies.
8.2.3.2 Separation of structure earths
If the structure earths are separated, then insulating gaps or joints are required.
To prevent bypassing of the insulating gaps, PE conductors of electricity supply cables, the
screens of communications cables, metal pipes and similar items which pass from one part of
the building to another, or enter the building from outside require an insulating joint. The
insulating gaps and relating equipment shall be installed at the borders of the separated
structure earths. The relevant systems shall be designed so that they will function safely when
the insulating gaps are in place.
Where it is necessary to include insulating gaps in underground parts of the building, the
insulating sections shall be of sufficient length that significant current will not flow past them
by conduction through the soil.

62128-3  IEC:2013 – 17 –
It has been found that a distance of 1 m is sufficient if the resistivity of the soil is greater than
500 Ωm; otherwise a distance of 2 m is required.
Provision should be made to detect unintended connections between the two structure earths.
8.2.3.3 Common structure earth
If the structure earths are connected, then attention shall be paid to the risk of stray currents
in the a.c. railway and in the earthing systems outside the railway structures. See
IEC 62128-2, Clause 7.
Provision should be made to detect the possible danger caused by stray currents in the a.c.
railway, the structure earth and the outside earths. See IEC 62128-2, Clause 10.
8.2.4 Inductive and capacitive coupling
The voltages induced or influenced by the a.c. railway into the contact lines and cables of the
d.c. railway, and the voltages induced into the rails and cables beside the d.c. railway, shall
be evaluated and corrective measures shall be applied as needed.
NOTE The rails of the d.c. railway can pick up significant induced voltages if they are well insulated from the
earth according to IEC 62128-1. Communication circuits beside the d.c. railway can need the same kind of
measures as are needed by the communication circuits beside the a.c. railway.
Precautions shall be taken against excessive a.c. voltages on the d.c. contact lines when they
are disconnected from the substations and are not earthed.
When assessing compliance, the indivisible sub-section of the d.c. contact line which is most
closely coupled with the a.c. railway shall be considered. The voltages coupled into the d.c.
system shall be taken into consideration in the design of the d.c. system.
8.3 Requirements if the a.c. railway and the d.c. railway have common return circuits
and use the same tracks
8.3.1 General
This subclause applies to a.c. and d.c. electric traction systems on the same tracks as well as
to level crossings between an a.c. railway and a d.c. railway.
8.3.2 Measures against stray current
Measures shall be applied to prevent the flow of significantly damaging levels of stray current
between the tracks electrified only with a.c. and the tracks electrified only with d.c. The
running rails of the tracks equipped with both systems shall be insulated from the earth in
accordance with IEC 62128-2, 6.4. The connections which are required by IEC 62128-1, 6.2,
shall be made using voltage-limiting devices (VLD) which can conduct both a.c. and d.c. The
voltage-limiting devices shall be applied in accordance with IEC 62128-1, Annex F.
EXAMPLE Examples in accordance with IEC 62128-1, 6.2 are the connections via VLD from the running rails to
the contact line supports and the structure earth, which are required by the presence of high-voltage contact lines.
The design of the railway traction power supply systems shall be such that the voltage-limiting
devices will not conduct except during extraordinary operating conditions in order to meet the
requirements of IEC 62128-2.
The risks associated with the conductive state of voltage-limiting devices shall be assessed.
The above measures cause a lack of earthing of the tracks. Therefore special measures can
be necessary in order to achieve sufficiently low
...