SIST EN ISO 21857:2021

SLOVENSKI STANDARD
01-december-2021
Petrokemična industrija ter industrija za predelavo nafte in zemeljskega plina -
Preprečevanje korozije na cevovodnih sistemih zaradi blodečih tokov (ISO
21857:2021)
Petroleum, petrochemical and natural gas industries - Prevention of corrosion on pipeline
systems influenced by stray currents (ISO 21857:2021)
Erdöl-, petrochemische und Erdgasindustrie - Vermeidung von durch Streuströme
beeinflusster Korrosion an Rohrleitungssystemen (ISO 21857:2021)
Industries du pétrole, de la pétrochimie et du gaz naturel - Prévention de la corrosion sur
les systèmes de conduites soumis à l'influence de courants vagabonds (ISO
21857:2021)
Ta slovenski standard je istoveten z: EN ISO 21857:2021
ICS:
75.200 Oprema za skladiščenje Petroleum products and
nafte, naftnih proizvodov in natural gas handling
zemeljskega plina equipment
77.060 Korozija kovin Corrosion of metals
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 21857
EUROPEAN STANDARD
NORME EUROPÉENNE
October 2021
EUROPÄISCHE NORM
ICS 75.200
English Version
Petroleum, petrochemical and natural gas industries -
Prevention of corrosion on pipeline systems influenced by
stray currents (ISO 21857:2021)
Industries du pétrole, de la pétrochimie et du gaz Erdöl-, petrochemische und Erdgasindustrie -
naturel - Prévention de la corrosion sur les systèmes Vermeidung von durch Streuströme beeinflusster
de conduites soumis à l'influence de courants Korrosion an Rohrleitungssystemen (ISO 21857:2021)
vagabonds (ISO 21857:2021)
This European Standard was approved by CEN on 3 October 2021.

CEN 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 CEN-CENELEC Management Centre or to any CEN
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 CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2021 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 21857:2021 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
The text of ISO 21857:2021 has been prepared by Technical Committee ISO/TC 67 "Materials,
equipment and offshore structures for petroleum, petrochemical and natural gas industries” of the
International Organization for Standardization (ISO) and has been taken over as EN ISO 21857:2021 by
Technical Committee CEN/TC 219 “Cathodic protection” the secretariat of which is held by BSI.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by April 2022, and conflicting national standards shall be
withdrawn at the latest by April 2022.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 21857:2021 has been approved by CEN as EN ISO 21857:2021 without any modification.

INTERNATIONAL ISO
STANDARD 21857
First edition
2021-03
Petroleum, petrochemical and
natural gas industries — Prevention
of corrosion on pipeline systems
influenced by stray currents
Industries du pétrole, de la pétrochimie et du gaz naturel —
Prévention de la corrosion sur les systèmes de conduites soumis à
l'influence de courants vagabonds
Reference number
ISO 21857:2021(E)
©
ISO 2021
ISO 21857:2021(E)
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved

ISO 21857:2021(E)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Abbreviations and symbols . 3
4.1 Abbreviations . 3
4.2 Symbols . 4
5 Information exchange and co-operation . 5
6 Common sources of interference that can affect corrosion . 6
6.1 General . 6
6.2 Direct current . 7
6.2.1 General. 7
6.2.2 Traction systems . 7
6.2.3 Industrial systems . 7
6.3 Alternating current. 7
6.3.1 General. 7
6.3.2 Overhead and buried power lines. 8
6.4 High-voltage direct current transmission systems . 8
6.5 Natural interference . 8
6.5.1 General. 8
6.5.2 Geomagnetic (telluric) interference . 8
6.5.3 Tidal interference effects . 9
7 Identification and measurement of stray current interference .9
7.1 Principle . 9
7.2 Stray Current interference .10
7.2.1 Inductive and conductive coupling from remote sources .10
7.2.2 Conductive coupling from nearby sources .10
7.3 Measurement of electrical parameters .10
7.3.1 Data acquisition systems .10
7.3.2 Possible errors in AC measurements .11
7.3.3 Potential measurement .11
7.3.4 Current measurement on probes .11
7.3.5 IR-free potential measurement on coupons or probes .11
7.3.6 Duration of the measurement .11
7.3.7 Specific requirements for coupons or probes.12
7.4 Corrosion rate measurement .12
8 Acceptance criteria for DC interference .12
8.1 Overview of criteria .12
8.2 Corrosion rate .13
8.3 Criteria for steel and cast iron .14
8.3.1 Time constant interference .14
8.3.2 Time variant interference .15
8.4 Criteria for steel pipes in concrete based on potential measurements without
cathodic protection .16
8.4.1 Time constant anodic interference .16
8.4.2 Time variant interference .16
9 Reduction of DC stray current interference .16
9.1 General .16
9.2 Modifications to the source of interference .17
9.2.1 Principles .17
ISO 21857:2021(E)
9.2.2 Direct current systems at industrial sites .17
9.2.3 Direct current systems at ports.17
9.2.4 Direct current traction systems .17
9.2.5 Cathodic protection systems .18
9.2.6 Telluric interference.19
9.2.7 Direct current communication systems .19
10 Modifications to the interfered structure .19
10.1 General .19
10.2 Design prerequisites .20
10.2.1 Coatings .20
10.2.2 Isolation from other structures .20
10.2.3 Distance to be maximized .20
10.2.4 Installation of mitigation devices .20
10.2.5 Modifying the electrical continuity of the interfered structure .21
11 Inspection and maintenance .22
Annex A (informative) Use of current probes to evaluate fluctuating stray current
interference on cathodically protected structures .23
Annex B (informative) Determining the relevant position for placing reference electrodes,
coupons and probes in case of any conductive coupling caused by stray currents .26
Annex C (informative) Operating principles of electrical resistance probes .33
Annex D (informative) Geomagnetic interference .34
Annex E (informative) High voltage direct current interference .43
Annex F (informative) Alternating Current Interference .45
Annex G (informative) Tidal Effects .50
Annex H (informative) Photovoltaic interference .51
Annex I (informative) Modelling the effects of stray current interference on cathodically
protected pipelines .54
Annex J (informative) Assessment of the corrosion risk for steel in concrete or for
cathodically protected structures under time variant interference conditions .58
Annex K (informative) Principles of anodic and cathodic interference .63
Bibliography .66
iv © ISO 2021 – All rights reserved

ISO 21857:2021(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 67, Materials, equipment and offshore
structures for petroleum, petrochemical and natural gas industries, Subcommittee SC 2, Pipeline
transportation systems, in collaboration with the European Committee for Standardization (CEN)
Technical Committee CEN/TC 219, Cathodic protection, in accordance with the Agreement on technical
cooperation between ISO and CEN (Vienna Agreement).
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
ISO 21857:2021(E)
Introduction
This document provides guidance for the prevention of external corrosion when a pipeline is influenced
by electrical interference. Electrical interference can be from stray currents (defined in ISO 8044) and
from naturally occurring interference caused by geomagnetic or tidal activity.
International Standards on cathodic protection (e.g. ISO 15589-1 and ISO 15589-2) refer to a structure-
to- electrolyte potential value that is considered to indicate that cathodic protection is effective. When
the potential is influenced by stray currents, however, it is not always possible to obtain a meaningful
structure-to-electrolyte potential and other methods of assessment are needed. These other methods
can include mathematical analysis of the potentials and/or direct assessment of the corrosion rate
using electrical resistance probes.
An affected structure carrying stray currents, e.g. a pipeline or cable can itself affect other nearby
structures.
This document is not intended to inhibit the use of alternative equipment or engineering solutions for
individual applications. Where an alternative is offered, it is intended that any variations from this
document be identified and documented.
vi © ISO 2021 – All rights reserved

INTERNATIONAL STANDARD ISO 21857:2021(E)
Petroleum, petrochemical and natural gas industries —
Prevention of corrosion on pipeline systems influenced by
stray currents
1 Scope
This document establishes the general principles for the evaluation and minimization of the effects of
stray current corrosion on external surfaces of buried or immersed pipeline systems caused by AC and
DC electrical interference.
Other stray current effects such as overheating, and interference with welding operations are not
covered in this document.
A brief description of AC effects, general principles and some guidelines, are provided.
NOTE 1 See ISO 18086 for the effects of alternating current on buried or immersed pipelines.
Systems that can also be affected by stray currents include buried or immersed metal structures such
as the following:
a) pipeline systems;
b) metal sheathed cables;
c) tanks and vessels;
d) earthing systems;
e) steel reinforcement in concrete;
f) sheet steel piling.
This document gives guidelines for
— the design of cathodic protection systems that might produce stray currents,
— the design of pipeline systems, or elements of pipeline systems, which are buried or immersed, and
which can be subject to stray current corrosion, and
— the selection of appropriate protection or mitigation measures.
Internal corrosion risks from stray currents are not dealt with in detail in this document but principles
and measures described here can be applicable for minimizing the interference effects.
NOTE 2 The impact of electromagnetic interference on above-ground appurtenances of pipeline systems is
covered in EN 50443, IEC 61140, IEC 60364-4-41, IEC 60479-1, IEC 60364-5-52, IEC/TS 61201 and IEC/TR 60479-5.
This document can also be used for pipeline systems outside of the petrochemical and natural gas
industries and other buried or immersed structures.
NOTE 3 EN 50162 provides guidance for railway related structures.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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.
ISO 21857:2021(E)
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
ISO 15589-1, Petroleum, petrochemical and natural gas industries — Cathodic protection of pipeline
systems — Part 1: On-land pipelines
ISO 8044, Corrosion of metals and alloys — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 15589-1, IEC 62128-2:2013,
ISO 8044 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
coating
electrically insulating covering bonded to a metal surface for protection against corrosion by preventing
contact between the electrolyte and the metal surface
3.2
remote earth
theoretical concept that refers to a ground electrode of zero impedance placed an infinite distance
away from the ground under test
Note 1 to entry: In practice, remote earth is approached when the mutual resistance between the ground under
test and the test electrode becomes negligible. Remote earth is normally considered to be at zero potential.
[1]
[SOURCE: IEEE Std 81-2012 ]
3.3
conductive coupling
transfer of energy occurring when a part of the current belonging to the interfering system returns to
the system earth via the interfered system
Note 1 to entry: Also, when the voltage to the reference earth of the ground in the vicinity of the influenced object
rises because of a fault in the interfering system, and the results of which are conductive voltages and currents.
3.4
drainage
electrical drainage
transfer of stray current from an affected structure to the current source by means of a deliberate bond
Note 1 to entry: For drainage devices see direct drainage bond (3.5), unidirectional drainage bond (3.7) and forced
drainage bond (3.6).
3.5
direct drainage bond
device that provides electrical drainage (3.4) by means of a bi-directional, metallic bond between an
affected structure and the stray current source
Note 1 to entry: The bond can include a series resistor to reduce the current.
2 © ISO 2021 – All rights reserved

ISO 21857:2021(E)
3.6
forced drainage bond
device that provides electrical drainage (3.4) by means of a bond between an affected structure and the
stray current source
Note 1 to entry: The bond includes a separate source of DC power to augment the transfer of current.
3.7
unidirectional drainage bond
device that provides electrical drainage (3.4) by means of a unidirectional bond between the affected
structure and the stray current source
Note 1 to entry: The bond includes a device such as a diode to ensure that current can only flow in one direction.
3.8
telluric interference
voltages generated by geomagnetic field variations that cause variations in the observed pipe-to-soil
potentials
3.9
electrical resistance probe
ER probe
device that measures metal loss by comparison of the calibrated resistance value of a piece of metal
with known physical characteristics
3.10
sampling rate
measuring interval set by the operator
3.11
alternating current interference
AC interference
electrical disturbance generated by AC systems that affects buried or immersed pipelines by conduction
and/or induction
Note 1 to entry: Powerlines, railway traction systems.
3.12
direct current interference
DC interference
disturbance, generated by DC systems, that affects buried or immersed metallic structures primarily
by conduction
4 Abbreviations and symbols
4.1 Abbreviations
AC Alternating current
ACVG Alternating current voltage gradient
CP Cathodic protection
DC Direct current
DCVG Direct current voltage gradient
emf Electromotive force
ISO 21857:2021(E)
GIC Geomagnetically induced currents
HVAC High voltage alternating current
HVDC High Voltage Direct Current
IR Product of the current and resistance (I and R) that indicates the voltage drop error in a
potential measurement
PV Photovoltaic
r.m.s. Root mean square
4.2 Symbols
–1
a Per annum
B Magnetic field
E Structure/soil potential for non cathodically protected structures
E Anodic potential
a
E Cathodic potential
c
ΔE Potential difference due to operation / non-operation of the interfering source
ΔE Anodic potential shift (IR drop included)
a
ΔE Average anodic potential shift
a,avg
ΔE Anodic potential shift (IR drop excluded)
a,IR free
ΔE Cathodic potential shift
c
ΔE Measured interference
m
ΔE Average cathodic potential shift
c,avg
E Structure potential of a metal in a given corrosion system (ISO 8044)
cor
E Structure potential without measurement error due to current flowing in the circuit
IR-free
E ON potential
on
E Average ON potential
on,avg
E Protection potential according to ISO 15589-1
p
E On potential required to achieve effective cathodic protection
ref
F Electric field
I Coupon current
cpn
J Current density
J Anodic current density
a
J Cathodic current density
c
4 © ISO 2021 – All rights reserved

ISO 21857:2021(E)
J Coupon current density
cpn
J Reference value for current density (analogous to I )
ref ref
ρ Soil resistivity (Ω·m)
Q Anodic charge during the period of anodic interference
a
Q Cathodic charge during the period of cathodic interference
c
R Coupon element resistance
c
R Isolation resistance, usually of a cable insulation
iso
s Seconds
-1
S⋅km Siemens per unit length
t Time
T Interval when the structure is anodic with respect to the selected value of E or J
a ref ref
T Maximum duration of the anodic period
a,max
T Interval when the structure is cathodic with respect to the selected value of E or J
c ref ref
v Corrosion rate
cor
V Voltage with respect to a copper/copper sulfate reference electrode
CSE
5 Information exchange and co-operation
Common sources of interference that can cause stray current corrosion are given in Clause 6. During
the design stage of buried or immersed metallic pipeline systems, the possibility of both causing and
suffering from stray current interference shall be taken into consideration and documented. The
pipeline system should achieve the acceptance criteria identified in Clause 8. Construction work, major
changes on existing structures, regenerative braking, etc. can require a detailed consideration of the
interference situation.
Electrical interference problems on buried or immersed metallic pipeline systems shall be considered,
and documented, with the following points in mind:
— The operator of the pipeline system can protect a structure against corrosion with the method
that the operator considers to be the most suitable. However, levels of electrical interference on
neighbouring structures shall be maintained within the defined limits given in Clause 8
— Stray currents, especially from DC traction systems, are directly related to the design of the traction
return circuits. This means that it is possible to limit the stray current by traction circuit design, but
not to eliminate it.
— Where other structures that might be affected are present, the requirement to maintain interference
within the defined limits applies to all affected structures.
— Utility-scale photovoltaic (PV) installations can develop a steady state DC interference to adjacent
buried pipelines. It is expected that the operator of the PV installation will maintain constant
monitoring of the R value to verify the isolation resistance between the PV panels and the earth.
ISO
The pipeline operator should be informed of any changes in the R values outside the threshold value.
ISO
— The operating characteristics of HVDC systems can change under fault and maintenance conditions.
These changes can affect the corrosion risk to buried pipelines and such changes should be
communicated in a timely manner to the pipeline operator.
ISO 21857:2021(E)
These goals are best achieved by agreement, co-operation and information exchange between the
parties involved. Information exchange and co-operation are important and shall be carried out both at
the design stage and during operation of the systems. In this way possible effects, suitable precautions
and remedies can be assessed.
The following information is required to make a sound engineering judgement:
— details of buried metallic structures;
— cathodic protection installations or significant modifications to existing installations;
— DC traction system installations or significant modifications to existing installations;
— HVDC transmission line installation or modification to existing installations or modes of operation;
— details of any sources of DC installations that can cause interferences to buried pipelines;
— utility scale photovoltaic systems.
Agreement and co-operation is more effectively achieved and maintained by periodic meetings
between interested parties, committees or other associations who can establish information exchange
procedures and protocols.
6 Common sources of interference that can affect corrosion
6.1 General
DC systems that can cause currents to flow in the earth or any other electrolyte, whether intentional or
unintentional, include the following:
a) traction systems;
b) overhead lines for vehicles;
c) trolley bus systems;
d) power systems;
e) equipment at industrial sites, e.g. welding;
f) communication systems;
g) instrumentation systems;
h) cathodic protection systems;
i) high voltage transmission systems. See Annex E;
j) track circuit signalling systems. (For stray currents from traction systems, IEC 62128-2 gives
requirements for minimizing their production and for the effects within the railway system);
k) photovoltaic power systems. See Annex H;
l) offshore wind farm power systems;
m) geomagnetic interference (telluric currents). See Annex D;
n) tidal fluctuations. See Annex G.
AC systems (see Annex F) that can induce voltages into buried structures include
— three phase power transmission overhead cables,
6 © ISO 2021 – All rights reserved

ISO 21857:2021(E)
— buried three phase power cables, and
— AC operated railways.
6.2 Direct current
6.2.1 General
Sources of DC that can affect the structure-to-electrolyte potentials on pipelines can either originate
from industrial or natural sources.
6.2.2 Traction systems
There are various configurations of DC traction systems that are in common use. They generally
differ in respect of the way that the current is returned to the substation(s). Whichever system
configuration is used there will be some current that returns via the earth. IEC 62128-2 gives guidance
on permissible limits.
6.2.3 Industrial systems
6.2.3.1 General
Industrial systems that use, or generate, DC should be provided with earthing systems that neither rely
on long earth return paths nor deliberately utilize third-party structures for earthing purposes.
6.2.3.2 Welding
Welding return circuits should be configured to ensure that the return paths are as short as possible
and do not exacerbate the risk of currents returning via third-party structures.
6.2.3.3 Photovoltaic interference on buried pipelines
Leakage currents in photovoltaic systems originate from a fault or from the systematic and inevitable
flow of DC where there is cable insulation damage to PV modules and other array components.
Under certain conditions, the DC leakage currents, if left unattended, or not detected at all, can cause
accelerated stray current corrosion on metallic underground infrastructure, such as pipelines, buried
near large, utility-scale PV systems.
6.3 Alternating current
6.3.1 General
AC powered systems can cause interference on pipelines due to inductive, conductive and capacitive
coupling mechanisms, which are described in References [3] and [6].
It is possible that the voltage resulting from interference on the pipe can exceed acceptable levels of
touch-potential and/or current densities that will lead to corrosion damage of exposed steel surfaces.
The potentials and current densities that are used to determine the risk of corrosion from AC
interference are detailed in References [3] and [6].
Annex F provides additional information and one method to calculate the induced voltage in a section
of pipe.
ISO 21857:2021(E)
6.3.2 Overhead and buried power lines
6.3.2.1 General
Overhead power lines can generate unacceptable voltages onto buried pipelines, primarily by induction.
The induction is a result of magnetic coupling. The magnitude of the induced voltage depends on the
distance, length of parallelism, inducing current magnitude, frequency and phase relationship.
6.3.2.2 Buried power cables
Buried power lines can generate unacceptable voltages onto buried pipelines, primarily by induction,
in the same way as overhead power lines. It is preferable if buried cables are laid with the phase
cables close to each other and formed in a trefoil configuration. Trefoil formation refers to a method
of arranging the individual phase cables to reduce the net inductance because the phases are in anti-
phase and cancel each other.
6.3.2.3 Railway systems
AC railway systems can be a source of interference. Where the pipeline is parallel to the railway,
the coupling is primarily inductive. The rails of AC powered railways are earthed, and this can also
result in conductive coupling to adjacent buried structures. AC railways can operate at 60 Hz, 50 Hz
and 16,67 Hz. When evaluating the risks resulting from the effects of electromagnetic interference
on buried pipelines running near AC electrified traction systems, the harmonic distortion in railway
systems should be considered. The presence of harmonics can exacerbate voltages induced on buried
[5]
pipelines .
6.4 High-voltage direct current transmission systems
There are two main configurations for high voltage direct current transmission systems, monopolar
and bipolar. Bipolar HVDC systems should be given preference to avoid stray current interference. The
earthing of HVDC systems shall be designed in such a way as to avoid current flowing through the earth
during normal operation and to minimize earth current during faulty or unbalanced load conditions.
The entire system design shall consider the possible high-level of stray currents to which buried or
immersed metal structures can be exposed, even at a substantial distance from the electrode station.
Buried HVAC and HVDC cables are joined together in joint bays installed along the cable route. The
separation distance between joint bays is dependent on the cable operating voltage, conductor size and
construction. Not all joint bays will have an earth local to the joint bay, but the cable screens will be
bonded in each joint bay. The location of all earths should be advised by the cable system operator.
Where an operator decides to install an earth at a joint bay the earth should be installed at a distance
from buried pipelines that will ensure that the touch voltage created on a pipeline during fault
conditions is within safe limits. AC and DC leakage currents through earth systems can also result in
interference on buried utilities and should be minimized.
Additional information is given in Annex E.
6.5 Natural interference
6.5.1 General
Natural low frequency interference is caused by geomagnetic field variations and by tidal water
movements.
6.5.2 Geomagnetic (telluric) interference
Geomagnetic field variations are variations in the earth’s magnetic field. The geomagnetic field
variations induce electric currents in the Earth and in long conductors such as pipelines and power
8 © ISO 2021 – All rights reserved

ISO 21857:2021(E)
transmission lines. These induced currents are generally referred to as telluric currents when
related to pipelines and as GIC by the electric power industry. Both terms are used in literature and
to be consistent with present pipeline practice this document will use the term telluric currents. (See
Annex D for additional information).
6.5.3 Tidal interference effects
The movement of conductive seawater through the Earth’s magnetic field acts like a dynamo and
generates an electric field in the seawater. This drives an electric current (a flow of charge) in the
seawater, perpendicular to the direction of water movement. Where this electric current meets the
land, there is a build-up of electrical charge that creates a potential gradient both along the seafloor and
inland perpendicular to the coast. (See Annex G for additional information).
7 Identification and measurement of stray current interference
7.1 Principle
The identification of the stray current interference is achieved by analysis of the measurements. The
evaluation of the interfere
...