ISO 15589-1:2003

Petroleum and natural gas industries - Cathodic protection of pipeline transportation systems - Part 1: On-land pipelines

ISO 15589-1:2003 specifies requirements and gives recommendations for the pre-installation surveys, design, materials, equipment, fabrication, installation, commissioning, operation, inspection and maintenance of cathodic protection systems for on-land pipelines, as defined in ISO 13623, for the petroleum and natural gas industries. ISO 15589-1:2003 is applicable to buried carbon steel and stainless steel pipelines on land. It can also apply to landfalls of offshore pipeline sections protected by onshore-based cathodic protection installations. ISO 15589-1:2003 is also applicable to retrofits, modifications and repairs made to existing pipeline systems. NOTE Special conditions sometimes exist where cathodic protection is ineffective or only partially effective. Such conditions can include elevated temperatures, disbonded coatings, thermal insulating coatings, shielding, bacterial attack and unusual contaminants in the electrolyte.

Industries du pétrole et du gaz naturel — Protection cathodique des systèmes de transport par conduites — Partie 1: Conduites terrestres

General Information

• Status: Withdrawn
• Publication Date: 05-Nov-2003
• Withdrawal Date: 05-Nov-2003
• ICS: 75.200 - Petroleum products and natural gas handling equipment
• Technical Committee: ISO/TC 67/SC 2 - Pipeline transportation systems
• Drafting Committee: ISO/TC 67/SC 2 - Pipeline transportation systems
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 02-Mar-2015
• Completion Date: 13-Dec-2025

Relations

• Consolidates: ISO/R 888:1968 - Title missing - Legacy paper document
• Effective Date: 28-Feb-2023
• Consolidated By: ISO 19157:2013 - Geographic information - Data quality
• Effective Date: 06-Jun-2022
• Revised: ISO 15589-1:2015 - Petroleum, petrochemical and natural gas industries - Cathodic protection of pipeline systems - Part 1: On-land pipelines
• Effective Date: 23-May-2009

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 15589-1
First edition
2003-11-15
Petroleum and natural gas industries —
Cathodic protection of pipeline
transportation systems —
Part 1:
On-land pipelines
Industries du pétrole et du gaz naturel — Protection cathodique des
systèmes de transport par conduites —
Partie 1: Conduites terrestres

Reference number
©
ISO 2003
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©  ISO 2003
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ii © ISO 2003 — All rights reserved

Contents Page
Foreword. v
Introduction . vi
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 2
4 Symbols and abbreviations . 4
5 Design requirements . 4
5.1 General. 4
5.2 Design information . 4
5.3 Criteria for CP. 5
5.4 Predesign investigations. 6
5.5 Electrical isolation . 7
5.6 Electrical earthing . 8
5.7 Electrical continuity. 8
5.8 Current requirements . 8
5.9 Type of CP system and selection of sites . 9
6 Impressed-current systems . 10
6.1 Power supply. 10
6.2 Groundbeds. 11
6.3 Current output control and distribution . 13
7 Galvanic-anode systems. 14
7.1 General. 14
7.2 Zinc anodes . 14
7.3 Magnesium anodes. 15
7.4 Anode backfill. 16
7.5 Cables and cable connections . 16
8 Monitoring facilities . 16
8.1 General. 16
8.2 Monitoring stations (test posts) . 16
8.3 Bonding to other pipelines. 17
8.4 Test facilities at cased crossings. 17
8.5 Test facilities at isolating joints. 17
8.6 Drain-point test facilities . 17
8.7 Miscellaneous monitoring facilities . 17
9 Special facilities . 17
9.1 Temporary protection. 17
9.2 Protective casings . 17
9.3 Parallel power lines or a.c. traction systems .18
9.4 Lightning protection . 18
9.5 Surge arrestors . 18
9.6 CP cables and cable connections . 18
9.7 Monitoring stations and distribution boxes. 19
10 Commissioning . 20
10.1 General. 20
10.2 Equipment tests . 20
10.3 System tests . 21
11 Inspection and monitoring .21
11.1 General .21
11.2 Frequencies of inspection.21
11.3 Monitoring plan.22
11.4 Monitoring equipment.23
11.5 Specialized surveys .23
12 Maintenance and repair .23
13 Documentation .24
13.1 Design documentation.24
13.2 Commissioning documentation.25
13.3 Inspection and monitoring documentation .25
13.4 Operating and maintenance documentation .25
13.5 Maintenance records .26
Annex A (normative) CP measurements .27
Annex B (normative) Electrical interference.31
Annex C (informative) Fault detection of impressed-current systems during operation.35
Annex D (informative) Description of specialized surveys.37
Bibliography.40

iv © ISO 2003 — All rights reserved

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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 15589-1 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.
ISO 15589 consists of the following parts, under the general title Petroleum and natural gas industries —
Cathodic protection of pipeline transportation systems:
 Part 1: On-land pipelines
 Part 2: Offshore pipelines
Introduction
Pipeline cathodic protection is achieved by the supply of sufficient direct current to the external pipe surface,
so that the steel-to-electrolyte potential is lowered to values at which external corrosion is reduced to an
insignificant rate.
Cathodic protection is normally used in combination with a suitable protective coating system to protect the
external surfaces of steel pipelines from corrosion.
External corrosion control in general is covered by ISO 13623.
Users of this part of ISO 15589 should be aware that further or differing requirements may be needed for
individual applications. This part of ISO 15589 is not intended to inhibit alternative equipment or engineering
solutions to be used for the individual application. This may be particularly applicable where there is innovative
or developing technology. Where an alternative is offered, any variations from this part of ISO 15589 should
be identified.
vi © ISO 2003 — All rights reserved

INTERNATIONAL STANDARD ISO 15589-1:2003(E)

Petroleum and natural gas industries — Cathodic protection
of pipeline transportation systems —
Part 1:
On-land pipelines
1 Scope
This part of ISO 15589 specifies requirements and gives recommendations for the pre-installation surveys,
design, materials, equipment, fabrication, installation, commissioning, operation, inspection and maintenance
of cathodic protection systems for on-land pipelines, as defined in ISO 13623, for the petroleum and natural
gas industries.
This part of ISO 15589 is applicable to buried carbon steel and stainless steel pipelines on land. It can also
apply to landfalls of offshore pipeline sections protected by onshore-based cathodic protection installations.
This part of ISO 15589 is also applicable to retrofits, modifications and repairs made to existing pipeline
systems.
NOTE Special conditions sometimes exist where cathodic protection is ineffective or only partially effective. Such
conditions can include elevated temperatures, disbonded coatings, thermal-insulating coatings, shielding, bacterial attack
and unusual contaminants in the electrolyte.
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.
ISO 8044, Corrosion of metals and alloys — Basic terms and definitions
ISO 13623, Petroleum and natural gas industries — Pipeline transportation systems
ISO 13847, Petroleum and natural gas industries — Pipeline transportation systems — Welding of pipelines

1)
ASTM G 97 , Standard test method for laboratory evaluation of magnesium sacrificial anode test specimens
for underground applications
1) American Society for Testing and Materials, 100 Barr Harbour Drive, West Conshohocken, PA 19428-2959, USA
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8044 and the following apply.
3.1
anode backfill
material with a low resistivity, which may be moisture-retaining, immediately surrounding a buried anode, for
the purpose of decreasing the effective resistance between the anode and the electrolyte and to prevent
anode polarization
3.2
bond
metal conductor, usually copper, connecting two points on the same or on different structures, usually with the
intention of providing electrical continuity between the points
3.3
cathodic protection system
system consisting of a d.c. current source and an anode in order to provide protective current to a metallic
structure
3.4
coupon
representative metal sample of known surface area used to quantify the extent of corrosion or the
effectiveness of applied cathodic protection
3.5
d.c. decoupling device
protective device that conducts electricity when predetermined threshold voltage levels are exceeded
EXAMPLE Polarization cells, spark gaps and diode assemblies.
3.6
drain point
location of the negative cable connection to the protected structure through which the protective current
returns to its source
3.7
galvanic anode
electrode that provides current for cathodic protection by means of galvanic action
3.8
groundbed
system of buried or immersed galvanic or impressed-current anodes
3.9
impressed-current anode
electrode that provides current for cathodic protection by means of impressed current
3.10
impressed-current station
station containing the equipment which provides cathodic protection by means of impressed current
3.11
impressed-current system
system which provides cathodic protection by means of impressed current
2 © ISO 2003 — All rights reserved

3.12
instant-on potential
structure-to-electrolyte potential measured immediately after turning on all sources of applied cathodic
protection current
3.13
intensive measurement technique
technique which simultaneously measures pipe-to-electrolyte potentials and associated perpendicular
potential gradients
NOTE The intensive measurement technique identifies coating defects and enables calculation of IR-free potentials
at the defects.
3.14
IR drop
voltage, due to any current, developed between two points in the metallic path or in the lateral gradient in an
electrolyte such as the soil, measured between a reference electrode and the metal of the pipe, in accordance
with Ohm’s Law
3.15
IR-free potential
polarized potential
structure-to-electrolyte potential measured without the voltage error caused by the IR drop from the protection
current or any other current
3.16
isolating joint
electrically-insulating component inserted between two lengths of pipe to prevent electrical continuity between
them
EXAMPLE Monobloc isolating joint, isolating flange, isolating coupling.
3.17
monitoring station
test post
station where measuring and test facilities for the buried pipeline are located
3.18
on-potential
structure-to-soil potential measured while the cathodic protection system is continuously operating
3.19
off potential
instant-off potential
structure-to-electrolyte potential measured immediately after interruption off all sources of applied cathodic
protection current
NOTE This potential is normally measured immediately after the cathodic protection system is switched off and the
applied electrical current stops flowing to the bare steel surface, but before polarization has decreased.
3.20
protection potential
structure-to-electrolyte potential for which the metal corrosion rate is insignificant
3.21
reference electrode
electrode whose open circuit potential is constant under similar conditions of measurement, used to measure
the structure-to-electrolyte potential
3.22
remote earth
that part of the electrolyte in which no measurable voltages, caused by current flow, occur between any two
points
NOTE This condition generally prevails outside the zone of influence of an earth electrode, an earthing system, an
impressed-current groundbed or a protected structure.
3.23
stray current
current in the path other than the protective current under consideration
4 Symbols and abbreviations
a.c. alternating current
CP cathodic protection
CSE copper−copper sulfate (saturated) reference electrode
d.c. direct current
SCC stress corrosion cracking
SCE calomel reference electrode
5 Design requirements
5.1 General
For new construction projects, the design of the CP system shall be part of the total pipeline design and
corrosion management. The details of the pipeline isolation (e.g. location of isolating joints) and the protective
coating system shall be included.
Design, fabrication, installation, operation and maintenance of CP systems shall be carried out by experienced
and qualified personnel.
5.2 Design information
The following technical information shall be collected and considered when designing a CP system:
 detailed information on the pipeline to be protected, e.g. length, diameter, wall thickness, type and grade
of material, protective coating, operating temperature profile, design pressure;
 products to be transported;
 the required design life of the CP system;
 relevant drawings of the pipeline route, showing existing CP systems, existing foreign structures/pipelines
etc.;
 environmental operating conditions for the CP equipment;
 topographical details and soil conditions, including soil resistivity;
4 © ISO 2003 — All rights reserved

 climatic conditions, e.g. frozen soil;
 the possibility of telluric current activity;
 location, route and rating of high-voltage overhead or buried power lines;
 valves and regulating station locations;
 water, railway and road crossings;
 casing pipes that will remain after construction;
 types of pipeline bedding material;
 types and locations of isolating joints;
 characteristics of neighbouring a.c. and d.c. traction systems (e.g. electrical substations and their
operating voltages and polarities) and other interference-current sources;
 types and locations of earthing systems;
 availability of power supply.
The following information should be considered in the design of the pipeline CP system:
 soil pH, and the presence of bacteria which can cause corrosion;
 types and locations of neighbouring telemetry systems which can be used for remote monitoring.
5.3 Criteria for CP
5.3.1 General
The metal-to-electrolyte potential at which the corrosion rate is less than 0,01 mm per year is the protection
potential, E . This corrosion rate is sufficiently low so that corrosion will be within acceptable limits for the
p
design life. The criterion for CP is therefore
E u E
p
The protection potential of a metal depends on the corrosive environment (electrolyte) and on the type of
metal used.
The protection potential criterion applies at the metal/electrolyte interface, i.e. a potential which is free from the
IR drop in the corrosive environment (IR-free potential/polarized potential).
Some metals can be subject to hydrogen embrittlement at very negative potentials, and coating damage can
also increase at very negative potentials. For such metals, the potential shall not be more negative than a
limiting critical potential E . In such cases, the criterion for CP is
l
E u E u E
l p
5.3.2 Protection criteria
5.3.2.1 The CP system shall be capable of polarizing all parts of the buried pipeline to potentials more
negative than −850 mV referred to CSE, and to maintain such potentials throughout the design life of the
pipeline. These potentials are those which exist at the metal-to-environment interface, i.e. the polarized
potentials.
To prevent damage to the coating, the limiting critical potential should not be more negative than −1 200 mV
referred to CSE, to avoid the detrimental effects of hydrogen production and/or a high pH at the metal surface.
For high strength steels (specified minimum yield strength greater than 550 MPa) and corrosion-resistant
alloys such as martensitic and duplex stainless steels, the limiting critical potential shall be determined with
respect to the detrimental effects in the material due to hydrogen formation at the metal surface. Stainless
steels and other corrosion-resistant alloys generally need protection potentials more positive than −850 mV
referred to CSE; however, for most practical applications this value can be used.
For pipelines operating in anaerobic soils and where there are known, or suspected, significant quantities of
sulfate-reducing bacteria (SRB) and/or other bacteria having detrimental effects on pipeline steels, potentials
more negative than −950 mV referred to CSE should be used to control external corrosion.
For pipelines operating in soils with very high resistivity, a protection potential more positive than −850 mV
referred to CSE may be considered, e.g. as follows:
 −750 mV for 100 < ρ < 1 000;
 −650 mV for ρ W 1 000
where ρ is the soil resistivity, expressed in ohm metres.
As an alternative to the protection potentials given above, a minimum of 100 mV of cathodic polarization
between the pipeline surface and a reference electrode contacting the electrolyte may be used. The formation
or decay of polarization shall be measured in accordance with A.2.3.
5.3.2.2 The application of the 100 mV polarization criterion shall be avoided at higher operating
temperatures, in SRB-containing soils, or with interference currents, equalizing currents and telluric currents.
The conditions should be characterized prior to using this criterion. Furthermore, the criteria shall not be used
in case of pipelines connected to or consisting of mixed metal components
5.3.2.3 Under certain conditions, pipelines suffer from high-pH SCC in the potential range −650 mV to
−750 mV, and this shall be considered when using protective potentials more positive than −850 mV.
5.3.2.4 Care should be exercised in the use of all protection criteria where the pipeline is electrically
continuous with components manufactured from metals more noble than carbon steel, such as copper
earthing systems.
5.3.2.5 For pipelines operating at temperatures above 40 °C, the above values may not provide adequate
protection potential. In these cases, alternative criteria shall be verified and applied.
5.3.3 Measurements of protection potentials
Measurement techniques shall be in accordance with Annex A.
Other practical reference electrodes to CSE may be used for the various criteria provided that their properties
are reliable and documented.
If a.c. interference is present on a pipeline, a.c. corrosion can occur even though the protection potential is
achieved (see Annex B).
5.4 Predesign investigations
A site survey shall be carried out before preparing the pipeline CP design. Information obtained during
previous site surveys relevant to the proposed pipeline route may be used provided that the date and source
of such surveys are documented. If the area to be surveyed is affected by seasonal changes, these shall be
taken into account and the most severe conditions with respect to the soil conditions shall be used for the
design.
6 © ISO 2003 — All rights reserved

The survey report shall contain the design information specified in 5.2.
Representative soil resistivity values should be obtained at pipeline depth along the route of the pipeline, and
shall be obtained at various depths at prospective locations for anode groundbeds. The number of
measurements should be based on local soil conditions. If there are changes in soil characteristics, more
measurements shall be taken.
If corrosive conditions are anticipated due to bacterial activity, appropriate action shall be taken which might
include chemical and bacterial analyses of the soil. This requirement shall be extended to the imported soil
used for pipeline trench construction.
Possible sources of detrimental d.c. and a.c. interference currents shall be investigated, and the design shall
include measures to mitigate the effect of such currents. Annex B shall apply with respect to the detection and
control of interference currents.
Locations where high-voltage a.c. transmission lines or a.c.-powered train systems cross, or run parallel to,
the pipeline shall be identified.
5.5 Electrical isolation
Isolating joints should be installed above ground whenever possible at both extremities of a pipeline, and
should also be considered at the following locations:
 at connections to branch lines;
 between pipeline sections with different external coating systems;
 between pipeline sections running in different types of electrolyte (e.g. at river crossings);
 in areas of high telluric activity;
 on pipeline sections which are differently affected by a.c. or d.c. interference currents;
 between cathodically-protected pipelines and non-protected facilities.
Monobloc isolating joints should be used wherever possible. Each isolating joint/isolating flange should be
provided with test facilities.
Safety-earthing and instrument-earthing shall be mutually compatible with the CP system. In areas where
there could be an unacceptable risk of high voltages on the pipeline exceeding the joint's electrical capacity,
e.g. caused by nearby power systems or lightning, the isolating joints or flanges shall be protected using
electrical earthing or surge arrestors.
The design, materials, dimensions and construction of the isolating joints shall meet the design requirements
of ISO 13623.
If the pipeline is transporting any fluids that are electrically conductive, the isolating joints shall be internally
coated on the cathodic (most expected negative potential) side, for a length sufficient to avoid
interference-current corrosion. All sealing, coating and insulation materials shall be resistant to the fluid
transported.
The electrical resistance across isolating joints should be more than 10 MΩ measured at 1 000 V (d.c.) in dry
air, before installation.
If the use of monobloc isolating joints is not practical, electrical isolation should be provided using isolating
flange kits. Isolating flanges should be protected against ingress of dirt and moisture by the use of flange
protectors or protective tape.
The pipeline under CP should be electrically isolated from common or plant earthing systems to avoid
inadequate pipeline CP, unless measures are taken to ensure that adequate current is provided to the pipeline
system to account for the galvanic effects of the other systems.
5.6 Electrical earthing
Electrical earthing of devices installed on the protected pipeline might be required for safety reasons or
pipeline earthing might be required to mitigate the effect of induced electrical voltages.
If electrical safety-earthing is required, this shall be made compatible with the CP system by installing
polarization cells or diode circuits, suitably specified and rated for the purpose, in the earthing circuit or by
installing separate earthing zinc or galvanized steel electrodes, buried in low-resistivity backfill and not in
direct electrical continuity with other earthing systems.
If earthing is to be installed to mitigate the effect of a.c.-induced voltages on the pipeline, this should be done
at the locations where the anticipated or measured voltages to ground are highest, and where the pipeline is
exposed and can be touched by personnel.
The requirements for detection and control of electrical interference are contained in Annex B.
5.7 Electrical continuity
Under certain circumstances it might be necessary to bond across isolating devices for measurement or other
purposes. If electrical continuity is to be established permanently, the bonding should be done in a monitoring
station.
If CP is to be applied on non-welded pipelines, the continuity of the pipeline shall be ensured. This shall be
done by installing permanent bonds across the high-resistance mechanical connectors, using suitable
attachment methods. The continuity of non-welded pipelines shall be checked by carrying out resistance and
potential measurements, see Annex A.
5.8 Current requirements
For new pipelines, the total current demand, I , shall be determined by evaluating the design parameters
tot
and/or from previous experience with similar systems, using Equation (1):
I J ⋅F ⋅2πrL (1)
tot = c
where:
J is the design electric current density for bare steel, expressed in milliamperes per square metre;
F is the coating breakdown factor, dimensionless;
c
r is the outer radius of the pipeline, expressed in metres;
L is the length of pipeline, expressed in metres.
Table 1 give values for the combined effects of design current density and coating breakdown that can be
used if relevant previous experience is not available. The coating breakdown factor, F , includes the effects of
c
factory coating and a compatible field-joint coating.
To determine the current demand for existing pipelines, where the actual condition of the applied coating is
unknown, a current drainage test should be carried out.
8 © ISO 2003 — All rights reserved

Table 1 — Design current densities for coated pipe, (J ⋅F ) in Equation (1), for steel in soils
c
with various pipeline coatings to be used in the design
of CP systems for operating temperatures uuuu 30 °C
Design current density
Pipeline coating mA/m
10-year design life 20-year design life 30-year design life
Asphalt/coal-tar enamel 0, 4 0,6 0,8
Cold-applied tape
Fusion-bonded epoxy 0, 4 0,6 0,9
Liquid epoxy
3-layer epoxy-polyethylene 0,08 0,1 0,4
3-layer epoxy-polypropylene
For a design life of more than 30 years, correspondingly greater factors should be used.

It is assumed that pipeline construction and operation is carried out in such a manner that coating damage is minimized.
For pipelines operating at elevated temperatures, the current density values shall be increased by 25 % for each 10 °C
rise in operating temperature above 30 °C.
Alternative design current values may be used if reliable and properly documented.
Current density requirements also depend upon the oxygen content and resistivity of the soil.

The CP system can be designed so that the increasing current demand due to progressive coating
deterioration is catered for by a phased installation of additional CP facilities. Pipeline attenuation calculations
can be carried out to define the spacing between drain points and anodes.
Consideration should be given to the risk of the coating becoming disbonded during its service life and the
resulting possibility for corrosion due to shielding of CP current. Typical examples where this could occur
include excessively negative potentials (5.3.2) or cases of extreme soil stress.
5.9 Type of CP system and selection of sites
5.9.1 General
The CP should preferably be accomplished by installing impressed-current systems. Alternatively, the CP may
be accomplished by the use of galvanic anodes, the limitations of which are given in Clause 7.
The following factors shall be taken into account for sites of impressed-current CP systems:
 availability of a power supply;
 soil resistivity in the area of prospective groundbeds;
 possible current distribution problems and shielding effects by rock outcrops, high-resistivity soils, nearby
structures, non-uniform geological features, etc;
 impact of the system on existing or future pipelines (including those owned by others) and other
developments;
 good access for the installation of rectifiers and good access to monitoring stations;
 sufficient distance between the proposed groundbed sites and the pipeline to obtain adequate current
distribution along the pipeline;
 existence of areas classified as hazardous areas along the pipeline route;
 existence of interference-current sources.
5.9.2 CP for thermally-insulated pipelines
Thermal insulation systems are generally defined as coating systems which include a layer to provide thermal
insulation. This can be a dedicated layer in addition to a corrosion-protection layer or it can be a layer such as
polyurethane or rubber which provides both corrosion protection and thermal insulation.
The need for, and the type of, CP for thermally-insulated pipelines shall be subject to an additional evaluation
taking into account the following.
a) Thermally-insulating materials such as polyurethane foam have an extremely high electrical resistance
and it is likely that, even if they become waterlogged, attempts to cathodically protect the underlying steel
pipe will be unsuccessful due to the shielding effect of the thermal insulation. Alternative corrosion control
methods should be considered in such cases.
b) The installation of a CP system, solely to protect an insulated pipeline, is normally difficult to justify, unless
there is concern that the thermal insulation can suffer significant mechanical damage by third party action,
which will lead to direct exposure of the pipe to the soil.
c) If a CP system already exists on an adjoining, or nearby structure and has sufficient spare capacity, then
it can be considered to bond in the insulated pipeline to this system.
d) Thermal insulation prevents the natural earthing of high voltages induced by adjacent power lines, etc. If
steps are not taken to earth the pipeline in the vicinity of the induced voltage, then these voltage changes
can occur for considerable distances and can cause corrosion and/or be a safety hazard to personnel
who make direct contact with the pipe.
CP potentials measured on thermally-insulated pipelines are usually not indicative of the potentials which exist
at the metal-to-electrolyte interface beneath the coating. As such, these potentials should not be used to
determine the effectiveness of the CP on the pipeline. In this case, some other method should be used to
verify the integrity of the pipeline.
6 Impressed-current systems
6.1 Power supply
The d.c. voltage source should be a transformer/rectifier unit, fed by an a.c. power supply, but alternative
voltage sources may be considered. Before specifying the d.c. voltage source, the following shall be taken into
account:
 availability and type of connection to a.c. supply;
 type of rectifier;
 measuring devices, e.g. voltmeters, ammeters;
 number of output terminals;
 type of cooling (air or oil);
 type of output control;
 need for the installation of a current interrupter;
10 © ISO 2003 — All rights reserved

 electrical and safety requirements for the equipment;
 need for a.c. and/or d.c. surge protection;
 need for environmental protection and housing;
 a.c. content of the d.c. output (acceptable ripple factor);
 identification and rating plate details.
NOTE High voltage gradients in the soil in the vicinity of groundbeds can be a hazard to animals and persons.
Generally, voltages higher than 50 V (rectifier output) should be avoided. If this is impossible, then the likely
consequences with regard to safety shall be assessed.
Transformer/rectifiers shall be specifically designed for CP service and shall be suitable for continuous
operation under the prevailing service conditions.
6.2 Groundbeds
6.2.1 General
The groundbeds of an impressed-current CP system shall be of the deep-well or shallow type and shall be
designed and located so as to satisfy the following.
a) The mass and material quality shall be suitable for the specified design life of the CP system.
b) The resistance to remote earth of each groundbed shall allow the maximum predicted current demand to
be met at no more than 70 % of the voltage capacity of the d.c. source during the design life of the CP
system. The calculation shall be carried out for the unused anode bed.
c) Harmful interference on neighbouring buried structures shall be avoided.
In selecting the location and type of groundbeds to be installed, the following local conditions shall be taken
into account:
 soil conditions and the variation in resistivity with depth;
 groundwater levels;
 any evidence of extreme changes in soil conditions from season to season;
 nature of the terrain;
 shielding (especially for parallel pipelines);
 likelihood of damage due to third party intervention.
The basic design shall include a calculation of the groundbed resistance based upon the most accurate soil
resistivity data available.
The current output from anodes should be independently adjustable.
6.2.2 Deep-well groundbeds
Deep-well groundbeds should be considered where
 soil conditions at depth are far more suitable than at surface,
 there is a risk of shielding by adjacent pipelines or other buried structures,
 available space for a shallow groundbed is limited,
 there is a risk of interference currents being generated on adjacent installations.
The detailed design shall include a procedure for drilling the deep-well, establishing the resistivity of the soil at
various depths, completing the borehole and method of installing the anodes and conductive backfill.
The borehole design and construction shall be such that the undesirable transfer of water between different
geological formations and the pollution of underlying strata is prevented.
Where necessary, metallic casings should be used for stabilizing the borehole in the active section of the
groundbed. The metallic casing shall be electrically isolated from any structures on the surface.
NOTE Metallic casings only provide temporary borehole stabilization, as the metal will be consumed by the d.c.
current flow.
If permanent stabilization is required, non-metallic, perforated casings should be used.
In the calculation of the groundbed resistance, the soil resistivity data corresponding to the depth at the mid
point of the active length shall be used and the possibility of multi-layered soils with significantly different soil
resistivities considered.
Deep-well groundbeds should be provided with adequate vent pipes to prevent gas blocking between anodes
and the conductive backfill. Vent pipe material shall be manufactured from a non-metallic chlorine-resistant
material.
6.2.3 Shallow groundbeds
Shallow groundbeds should be considered where
 soil resistivities near the surface are far more suitable than at the depths of a deep-well groundbed,
 there is no risk of shielding by adjacent pipelines or other buried structures,
 space is available for a shallow groundbed,
 there is no risk of interference currents being generated on adjacent installations.
Shallow groundbed anodes shall be installed horizontally or vertically. In either case, the top of the conductive
backfill shall be at least 1 m below ground level.
In the calculation of the groundbed resistance, the soil resistivity data corresponding to the centre-line
(horizontal groundbed) or mid-point (vertical groundbed) of the anodes shall be used and the possibility of
multi-layered soils with significantly diffe
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