IEC 61869-20:2025

IEC 61869-20 ®
Edition 1.0 2025-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Instrument transformers –
Part 20: Safety requirements of instrument transformers for high voltage
applications
Transformateurs de mesure –
Partie 20: Exigences de sécurité des transformateurs de mesure pour
applications à haute tension
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IEC 61869-20 ®
Edition 1.0 2025-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Instrument transformers –
Part 20: Safety requirements of instrument transformers for high voltage

applications
Transformateurs de mesure –
Partie 20: Exigences de sécurité des transformateurs de mesure pour

applications à haute tension
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 17.220.20  ISBN 978-2-8327-0273-4

– 2 – IEC 61869-20:2025 © IEC 2025
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Protection against electric shock . 6
4.1 General . 6
4.2 Earthing of enclosure, terminals and connection to carrier-frequency
accessory . 6
4.2.1 General. 6
4.2.2 Earthing of high voltage circuit transformers (HV CTs) . 6
4.2.3 Earthing of high-voltage VTs (HV VTs) . 8
4.3 Access to secondary terminals . 12
4.3.1 General. 12
4.3.2 Indoor instrument transformers . 12
4.3.3 Outdoor instrument transformers . 12
4.3.4 Protection of equipment against mechanical impact under normal
service conditions . 13
4.4 Risk of open secondary terminals in CTs and shorted secondary terminals in VTs 13
4.4.1 General. 13
4.4.2 Protections against the risk of open secondary terminals in CTs . 13
4.4.3 Protections against the risk of shorted secondary terminals in VTs . 13
5 Protection against thermal and chemical hazards . 13
5.1 General . 13
5.2 Risk of fire . 13
5.3 Risk of internal arc . 14
5.4 Risk of leakage of oil or release of gases . 14
6 Protection against mechanical hazards . 14
6.1 General . 14
6.2 Hazards during mounting and dismounting . 14
Bibliography . 15

Figure 1 – Earthing of a CT with one secondary circuit (example) . 7
Figure 2 – Earthing of a CT with one secondary circuit having a tap (example) . 7
Figure 3 – Earthing of a CT with two secondary circuits (example) . 7
Figure 4 – Earthing terminal connected to the steel support structure . 8
Figure 5 – Earthing terminal connected to earth via an earthing conductor . 8
Figure 6 – Open delta earthing. 9
Figure 7 – Earthing of an earthed VT with one secondary circuit (example) . 9
Figure 8 – Earthing of an unearthed VT with one secondary circuit (example) . 10
Figure 9 – Earthing of an earthed VT with two secondary circuits (example) . 10
Figure 10 – Earthing of an earthed VT with a multi-tap secondary circuit (example) . 10
Figure 11 – Earthing of an earthed VT with carrier-frequency accessory (example) . 11
Figure 12 – Earthing terminal connected to the steel support structure . 11
Figure 13 – Earthing terminal connected to earth via an earthing conductor. 12

Table 1 – Fire hazard of electrotechnical products . 14

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INSTRUMENT TRANSFORMERS –
Part 20: Safety requirements of instrument transformers for high voltage
applications
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
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shall not be held responsible for identifying any or all such patent rights.
IEC 61869-20 has been prepared by IEC technical committee 38: Instrument Transformers. It
is an International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
38/808/FDIS 38/814/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.

– 4 – IEC 61869-20:2025 © IEC 2025
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 61869 series, published under the general title Instrument
transformers, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

INSTRUMENT TRANSFORMERS –
Part 20: Safety requirements of instrument transformers for high voltage
applications
1 Scope
This part of IEC 61869 specifies the requirements for the safe design, operation and tests for
the safety of instrument transformers whose highest voltage for equipment is higher than
1 kV AC or 1,5 kV DC.
Low-power instrument transformers are not covered by this document.
NOTE The possibility of including additional safety requirements, specific to low-power instrument transformers, is
currently under consideration.
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.
IEC 60529, Degrees of protection provided by enclosures (IP Code)
IEC TS 60815-1, Selection and dimensioning of high-voltage insulators intended for use in
polluted conditions – Part 1: Definitions, information and general principles
IEC 61869-1, Instrument transformers – Part 1: General requirements
IEC 61869-99, Instrument transformers – Part 99: Glossary
IEC 62262, Degrees of protection provided by enclosures for electrical equipment against
external mechanical impacts (IK code)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61869-99 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1
high voltage instrument transformer
HV IT
instrument transformer whose highest voltage for equipment is a high voltage

– 6 – IEC 61869-20:2025 © IEC 2025
3.2
high voltage
HV
voltage exceeding the conventionally adopted limit for low voltage
[SOURCE: IEC 60050-195:2021, 195-05-26]
3.3
low voltage
LV
voltage not exceeding a conventionally adopted limit
Note 1 to entry: For AC, the conventionally adopted limit is 1 000 V.
Note 2 to entry: For DC, the conventionally adopted limit is 1 500 V.
[SOURCE: IEC 60050-195:2021, 195-05-25]
4 Protection against electric shock
4.1 General
The protections considered in this Clause 4 are those concerning inductive instrument
transformers (capacitor voltage transformers included) and also cover the protection of the
secondary circuit. In terms of access to the primary terminals, safety regulations applicable for
the HV circuit can apply.
4.2 Earthing of enclosure, terminals and connection to carrier-frequency accessory
4.2.1 General
Each secondary circuit of the HV ITs shall be earthed at one point only. The neutral primary
terminal (usually marked as "N") of the earthed HV VTs shall be earthed at one point only,
except when it is connected to a carrier-frequency accessory.
NOTE 1 The carrier-frequency accessory is also known as coupling device for power line carrier systems
(IEC 60481).
NOTE 2 The absence of earthing on a secondary circuit will generate a capacitance between the secondary winding
and the earth. This capacitance can potentially increase the voltage in the secondary winding which can damage the
connected low-voltage equipment.
The enclosure of the HV ITs made of conductive material shall be earthed independently from
the secondary circuits.
NOTE 3 The supporting structure, if made of steel or other conducting material, usually has an independent earthing
and is electrically connected to the enclosure.
4.2.2 Earthing of HV current transformers (HV CTs)
One point for each secondary circuit of the HV current transformers shall be earthed.
NOTE 1 Usually, the CT has several secondary circuits, and each circuit is connected to a different secondary
winding dedicated to a specific function. Some secondary circuits have additional taps which increase the number of
secondary terminals for the same circuit. However, only one point for each circuit must be earthed.
If an earthing conductor plate is installed in the secondary terminal box, it should be used in
order to create a common point between each circuit for the earthing. Otherwise, an earthing
terminal shall be made available in a defined position and clearly marked, as described in the
installation manual.
Figure 1, Figure 2 and Figure 3 provide examples of earthing of the secondary circuits.

Figure 1 – Earthing of a CT with one secondary circuit (example)

Figure 2 – Earthing of a CT with one secondary circuit having a tap (example)

Figure 3 – Earthing of a CT with two secondary circuits (example)
Figure 4 and Figure 5 provide examples of earthing terminals connected to earth from the
supporting structure of a CT in an air insulated switchgear (AIS).

– 8 – IEC 61869-20:2025 © IEC 2025

Figure 4 – Earthing terminal connected to the steel support structure

Figure 5 – Earthing terminal connected to earth via an earthing conductor
The cross-section of the cable used as earthing conductor shall be defined according to the
short-circuit level of the substation.
NOTE 2 The document IEC 60949 gives guidance on the dimensioning of the cross-section of the cable.
4.2.3 Earthing of HV voltage transformers (HV VTs)
One terminal for each secondary circuit of the HV voltage transformers shall be earthed.
NOTE 1 Usually, the VT has several secondary circuits, and each circuit is connected to a different secondary
winding dedicated to a specific function. Alternatively, the same secondary winding can have multiple applications
and be connected to several secondary circuits. However, only one terminal for each independent circuit must be
earthed.
When VT secondary windings are connected in open-delta, the earthing shall be done only in
one of the open terminals of the open-delta connection, to avoid any risk to connect VT
secondary terminals in short-circuit condition (Figure 6).
Example of correct connection
Examples of incorrect connections

Figure 6 – Open-delta earthing
If an earthing conductor plate is installed in the secondary terminal box, it should be used in
order to create a common point between each circuit for the earthing. Otherwise, an earthing
terminal shall be made available in a defined position and be clearly marked, as described in
the installation manual.
The neutral primary terminal of an earthed HV VT shall be earthed together with the secondary
circuits, except when it is connected to a carrier-frequency accessory.
NOTE 2 The carrier-frequency accessory is used only with capacitive voltage transformers (CVTs), because they
allow the injection of carrier frequency signal through the neutral primary terminal; this terminal is generally located
outside the secondary terminal box. In this case, the connection to earth of the primary circuit is accomplished by
means of the line carrier circuit.
Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11 provide examples of earthing of the
neutral primary terminal and the secondary circuits.

Figure 7 – Earthing of an earthed VT with one secondary circuit (example)

– 10 – IEC 61869-20:2025 © IEC 2025

Figure 8 – Earthing of an unearthed VT with one secondary circuit (example)

Figure 9 – Earthing of an earthed VT with two secondary circuits (example)

Figure 10 – Earthing of an earthed VT with a multi-tap secondary circuit (example)

Figure 11 – Earthing of a CVT with carrier-frequency accessory (example)
Figure 12 and Figure 13 provide examples of earthing terminals connected to earth from the
supporting structure of a CT in an open-type substation.

Figure 12 – Earthing terminal connected to the steel support structure

– 12 – IEC 61869-20:2025 © IEC 2025

Figure 13 – Earthing terminal connected to earth via an earthing conductor
The cross-section of the cable used as earthing conductor shall be defined according to the
short-circuit level of the substation.
NOTE 3 IEC 60949 gives guidance on the dimensioning of the cross-section of the cable.
4.3 Access to secondary terminals
4.3.1 General
The access to secondary terminals is limited by means of an enclosure with the degree of
protection of persons indicated by means of a designation specified in accordance with
IEC 60529.
The recommended level of protection of the enclosure against effects of mechanical impacts
shall be defined in accordance with IEC 62262.
4.3.2 Indoor instrument transformers
The minimum degree of protection on secondary terminals for indoor instrument transformers
shall be IP20.
This requirement does not apply to installations where personnel cannot access the instrument
transformer without previously de-energising the transformer and making it safe through some
safety measures. In this case, the requirement for such external safety measures shall be
clearly stated in the user's manual.
NOTE Usual external safety measures are interlocking devices or documented operating instructions.
4.3.3 Outdoor instrument transformers
The minimum degree of protection on secondary terminals for outdoor instrument transformers
shall be IP44. However, in installations with heavy or very heavy site pollution severity as
defined in IEC TS 60815-1, the recommended minimum degree of protection is IP55.

4.3.4 Protection of equipment against mechanical impact under normal service
conditions
For indoor installations, the minimum level of protection against the effects of mechanical
impacts shall be IK07.
For outdoor installations, the minimum level of protection against the effects of mechanical
impacts shall be IK09, except if by means of additional mechanical protection the degree IK07
is considered acceptable by the customer.
4.4 Risk of open secondary terminals in CTs and shorted secondary terminals in VTs
4.4.1 General
The secondary terminals of CTs and VTs shall be accessible, when installed for service, only
by means of a terminal block, where the relevant terminals shall be properly marked, so as to
avoid the risk of confusing them with other terminals in the same block or cabinet, and to ease
the identification of the polarity of the terminals.
The secondary terminals of the CTs shall not in any case be opened or left open during service,
nor shall the secondary terminals of the VTs be shorted or left in short-circuit during service.
NOTE Secondary terminals of the CT that are open in service can present hazardous voltages which compromise
the integrity of the instrument transformer and are a serious risk for the safety of personnel. Secondary terminals of
the VT that are shorted in service can cause the circulation of high intensity currents and compromise the integrity
of the instrument transformer.
4.4.2 Protections against the risk of open secondary terminals in CTs
The terminals of the secondary circuit of the CTs should be connected to a test disconnect
terminal block, compliant with IEC 60947-7-1, so that the secondary circuit is automatically
shorted when the secondary load is disconnected from the terminal block. Otherwise, the user
shall adopt appropriate procedures and rules to avoid opening the secondary circuits in
operation.
4.4.3 Protections against the risk of shorted secondary terminals in VTs
The secondary winding of the VTs should be protected by a circuit-breaker, compliant with
IEC 60947-2, or by fuses, so that the secondary circuit is automatically disconnected from the
VT when a short-circuit occurs.
5 Protection against thermal and chemical hazards
5.1 General
This Clause 5 contains the requirements about the protection against thermal and chemical
hazards during the operation in service of HV inductive instrument transformers.
5.2 Risk of fire
Instrument transformers shall be constructed in such a way that they limit the harm that can
arise from high temperature, flame or arc.
The instrument transformer shall be designed to prevent it from catching fire as far as
reasonable, both under conditions of normal use, and even in the event of foreseeable abnormal
use, malfunction or failure.
NOTE The first objective is to prevent ignition due to an electrically energised part. The second objective is to limit
the impact of the fire.
– 14 – IEC 61869-20:2025 © IEC 2025
When possible, materials should be chosen, or the parts be designed in such a way that they
retard the propagation of fire in the equipment and reduce harmful effects on the local
environment. In cases where product performance requires the use of flammable materials,
product design shall take flame retardation into account, where possible.
The information supplied by the manufacturer shall enable the purchaser to carry out a risk
evaluation during normal and abnormal operation. Guidance is given in Table 1.
Table 1 – Fire hazard of electrotechnical products
Guidance for assessing the fire hazard Minimization of toxic hazards due to fire
IEC 60695-1-10; IEC 60695-1-11 IEC 60695-7-1

5.3 Risk of internal arc
The requirements for protection against the risk of an internal arc during a fault are specified in
IEC 61869-1.
5.4 Risk of leakage of oil or release of gases
The instrument transformers shall be designed so as to avoid any leakage of insulating liquid
or to limit the insulating gas leakage to an acceptable level, in accordance with IEC 61869-1.
The design of gas-filled instrument transformers shall be tested with an enclosure tightness test
at ambient temperature, as defined in IEC 61869-1.
6 Protection against mechanical hazards
6.1 General
The protections considered in this Clause 6 are in relation to the installation and operation of
the instrument transformers on site. The risks and the corresponding protections in place during
the operation at the test laboratory can be different and should be addressed by the party
responsible for testing.
6.2 Hazards during mounting and dismounting
The operations of mounting and dismounting of the instrument transformers must be conducted
in compliance with the rules and requirements indicated in the installation manual.
NOTE The installation manual is usually a part of the user's manual provided by the manufacturer.

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