IEC TS 62271-315:2025

High-voltage switchgear and controlgear - Part 315: Direct current (DC) transfer switches

IEC TS 62271-315:2025 is applicable to direct current (DC) transfer switches designed for indoor or outdoor installation and for operation on HVDC transmission systems having direct voltages of 100 kV and above. DC transfer switches normally include metallic return transfer switches (MRTS), earth return transfer switches (ERTS), neutral bus switches (NBS) and neutral bus earthing switches (NBES).

General Information

• Status: Published
• Publication Date: 10-Mar-2025
• ICS: 29.130.10 - High voltage switchgear and controlgear
• Technical Committee: SC 17A - Switching devices
• Drafting Committee: WG 65 - TC 17/SC 17A/WG 65
• Current Stage: PPUB - Publication issued
• Start Date: 11-Mar-2025
• Completion Date: 03-Jan-2025

Overview

IEC TS 62271-315:2025 is a Technical Specification from the International Electrotechnical Commission (IEC) that defines requirements for direct current (DC) transfer switches used on high-voltage DC (HVDC) transmission systems. The standard applies to DC transfer switches for indoor or outdoor installation and is intended for systems with direct voltages of 100 kV and above. Typical transfer switch types addressed include metallic return transfer switches (MRTS), earth return transfer switches (ERTS), neutral bus switches (NBS) and neutral bus earthing switches (NBES).

Key topics and requirements

The document covers the full lifecycle of DC transfer switch specification, design and testing. Major technical topics include:

• Scope and definitions: clear terminology for assemblies, parts and operational characteristics of DC transfer switches.
• Ratings: rated direct voltage, rated continuous current, short-time and peak withstand currents, rated commutation voltage and dissipated energy during transfer operations.
• Design and construction: requirements for insulation, earthing, auxiliary/control circuits, enclosures, sealing (gas, liquid and vacuum tightness), nameplates, mechanical loads and degrees of protection for indoor/outdoor use.
• Special components: guidance for commutation switches, commutation capacitors, reactors, energy dissipation devices and making/charging devices where applicable.
• Type and routine tests: dielectric and partial discharge tests, continuous current, short-time and peak withstand tests, DC commutation tests, direct arc withstand tests, EMC and X‑radiation tests for vacuum interrupters, mechanical and environmental tests (temperature, humidity, icing, seismic).
• Operational & safety features: position indication, locking, stored/independent operation modes and protection verification.
• Testing criteria and reports: information required for type-test identification and reporting, pass/fail criteria for key tests.

Applications

IEC TS 62271-315 is directly applicable to:

• HVDC converter stations and transmission lines operating at ≥100 kV
• DC interconnects and point-to-point HVDC links
• HVDC and hybrid AC/DC substations where transfer of DC circuits, neutral and earthing functions are needed
• Use cases include transfer between metallic and earth-return modes, neutral bus switching and controlled earthing.

Who should use this standard

• HVDC equipment manufacturers and switchgear designers specifying DC transfer switches
• Power utilities and system operators developing HVDC project specifications
• Testing laboratories and certification bodies performing type and routine tests
• Consulting engineers, project specifiers and standards/quality engineers drafting procurement or installation documents.

Related standards

• The specification is part of the IEC 62271 (High‑voltage switchgear and controlgear) family - refer to other IEC 62271 parts for complementary requirements on insulation, switchgear testing, and auxiliary systems.

Keywords: IEC TS 62271-315:2025, DC transfer switches, HVDC, high-voltage switchgear, MRTS, ERTS, neutral bus switch, DC commutation tests.

Standards Content (Sample)

IEC TS 62271-315 ®
Edition 1.0 2025-03
TECHNICAL
SPECIFICATION
High-voltage switchgear and controlgear –
Part 315: Direct current (DC) transfer switches

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IEC TS 62271-315 ®
Edition 1.0 2025-03
TECHNICAL
SPECIFICATION
High-voltage switchgear and controlgear –

Part 315: Direct current (DC) transfer switches

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.130.10  ISBN 978-2-8327-0244-4

– 2 – IEC TS 62271-315:2025 © IEC 2025
CONTENTS
FOREWORD . 7
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 10
3.1 General terms and definitions . 10
3.2 Assemblies of switchgear and controlgear . 10
3.3 Parts of assemblies . 10
3.4 Switching devices . 10
3.5 Parts of switchgear and controlgear . 12
3.6 Operational characteristics of DC transfer switches . 13
3.7 Characteristic quantities . 13
3.8 Index of definitions . 14
4 Normal and special service conditions . 15
5 Ratings . 16
5.1 General . 16
5.2 Rated direct voltage (U ) . 16
rd
5.3 Rated insulation level (U , U , U ) . 16
dd p s
5.4 Rated continuous current (I ) . 17
rd
5.5 Rated values of short-time withstand current . 17
5.5.1 Typical waveform of short-circuit current . 17
5.5.2 Rated short-time withstand direct current (I ) . 17
kd
5.5.3 Rated peak withstand current (I ) . 17
pd
5.5.4 Rated duration of short circuit (t ) . 17
kd
5.6 Rated supply voltage of auxiliary and control circuits (U ) . 17
a
5.6.1 General . 17
5.6.2 Rated supply voltage (U ) . 17
a
5.7 Rated supply frequency of auxiliary and control circuits . 17
5.8 Rated pressure of compressed gas supply for controlled pressure systems . 18
5.101 Rated direct voltage of transfer switch (U ) . 18
rts
5.102 Rated transfer current (I ) . 18
t
5.103 Rated commutation voltage (U ) . 18
c
5.104 Rated dissipated energy during transfer operation (E ) . 18
rd
5.105 Rated operating sequence . 19
5.106 Rated open-close time . 19
6 Design and construction . 19
6.1 Requirements for liquids in switchgear and controlgear . 19
6.2 Requirements for gases in switchgear and controlgear . 19
6.3 Earthing of switchgear and controlgear . 19
6.4 Auxiliary and control equipment and circuits . 19
6.5 Dependent power operation . 20
6.6 Stored energy operation . 20
6.7 Independent unlatched operation (independent manual or power operation) . 20
6.8 Manually operated actuators . 20
6.9 Operation of releases . 20

6.10 Pressure/level indication . 20
6.11 Nameplates. 20
6.12 Locking devices . 22
6.13 Position indication . 22
6.14 Degrees of protection by enclosures . 22
6.15 Creepage distances for outdoor insulators . 22
6.16 Gas and vacuum tightness . 22
6.17 Tightness for liquid system . 22
6.18 Fire hazard (flammability) . 22
6.19 Electromagnetic compatibility (EMC) . 22
6.20 X-ray emission . 22
6.21 Corrosion . 22
6.22 Filling levels for insulation, switching and operation . 22
6.101 Seismic requirement for operation . 22
6.102 Commutation switch . 22
6.102.1 General requirement for operation . 22
6.102.2 Pressure limits of fluids for operation. . 23
6.102.3 Vent outlets . 23
6.102.4 Time quantities . 23
6.102.5 Static mechanical loads . 23
6.102.6 Classification . 24
6.102.7 Arcing withstand capability . 24
6.103 Insulated platform . 25
6.104 Commutation capacitor . 25
6.105 Energy dissipation device . 25
6.106 Reactor . 25
6.107 Making switch (if applicable) . 25
6.108 Charging device (if applicable) . 25
7 Type tests . 25
7.1 General . 25
7.1.1 Basics . 25
7.1.2 Information for identification of test objects . 27
7.1.3 Information to be included in type-test reports . 27
7.2 Dielectric tests . 27
7.2.1 General . 27
7.2.2 Ambient air conditions during tests . 27
7.2.3 Wet test procedure . 27
7.2.4 Arrangement of the equipment . 27
7.2.5 Criteria to pass the test . 27
7.2.6 Application of the test voltage and test conditions . 27
7.2.7 Tests of switchgear and controlgear . 28
7.2.8 Artificial pollution tests for outdoor insulators . 29
7.2.9 Partial discharge tests . 29
7.2.10 Dielectric tests on auxiliary and control circuits . 29
7.2.11 Voltage test as condition check . 29
7.3 Resistance measurement . 29
7.3.1 Measurement of the resistance of auxiliary contacts class 1 and class 2. 29
7.3.2 Measurement of the resistance of auxiliary contacts class 3 . 29
7.3.3 Electrical continuity of earthed metallic parts test . 29

– 4 – IEC TS 62271-315:2025 © IEC 2025
7.3.4 Resistance measurement of contacts and connections in the main
circuit as a condition check . 29
7.4 Continuous current tests . 30
7.4.1 Condition of the test object . 30
7.5 Short-time withstand current and peak withstand current tests . 30
7.5.1 General . 30
7.5.2 Arrangement of the equipment and of the test circuit . 30
7.5.3 Test current and duration . 30
7.5.4 Conditions of the test object after test . 30
7.6 Verification of the protection . 30
7.7 Tightness tests . 30
7.8 Electromagnetic compatibility tests (EMC) . 31
7.9 Additional tests on auxiliary and control circuits . 31
7.10 X-radiation test for vacuum interrupters . 31
7.101 Mechanical and environmental tests . 31
7.101.1 Miscellaneous provisions for mechanical and environmental tests . 31
7.101.2 Mechanical operation tests at ambient air temperature . 33
7.101.3 Low and high temperature tests . 35
7.101.4 Humidity test . 40
7.101.5 Test to prove the operation under severe ice conditions . 42
7.102 Seismic tests . 42
7.103 Direct current commutation tests . 42
7.103.1 General . 42
7.103.2 Arrangement of the transfer switch during test . 43
7.103.3 Test circuit . 44
7.103.4 Test procedure . 45
7.103.5 Behaviour of DC transfer switch during test . 46
7.103.6 Criteria to pass the test . 46
7.104 Direct arc withstand tests . 46
7.104.1 General . 46
7.104.2 Arrangement of the transfer switch during test . 47
7.104.3 Test circuit . 47
7.104.4 Test procedure . 48
7.104.5 Behaviour of DC transfer switch during test . 48
7.104.6 Criteria to pass the test . 48
8 Routine tests . 49
8.1 General . 49
8.2 Dielectric test on the main circuit . 49
8.3 Tests on auxiliary and control circuits . 49
8.4 Measurement of the resistance of the main circuit . 49
8.5 Tightness test . 49
8.6 Design and visual checks . 49
8.101 Mechanical operating tests . 49
9 Guide to the selection of DC transfer switches （informative） . 51
9.1 General . 51
9.2 Selection of rated values . 51
9.2.101 General . 51
9.2.102 Selection of rated direct voltage of transfer switch and rated insulation
level . 51

9.2.103 Selection of rated operating sequence . 51
9.2.104 Selection of rated continuous current . 52
9.2.105 Selection of rated values of short time withstand current . 52
9.2.106 Selection of rated transfer current . 52
9.2.107 Selection of rated commutation voltage . 52
9.3 Cable-interface considerations . 52
9.4 Continuous or temporary overload due to changed service conditions . 52
9.5 Environmental aspects . 52
9.101 Selection of the application . 52
9.101.1 MRTS . 52
9.101.2 ERTS . 53
9.101.3 NBS . 53
9.101.4 NBES . 54
10 Information to be given with enquiries, tenders and orders (informative) . 55
10.1 General . 55
10.2 Information with enquiries and orders . 55
10.3 Information with tenders . 56
11 Transport, storage, installation, operating instructions and maintenance. 58
12 Safety . 58
13 Influence of the product on the environment . 58
Annex A (normative) Test requirements for component of DC transfer switches . 59
A.1 Commutation switch . 59
A.2 Making switch . 59
A.3 Commutation capacitor . 59
A.4 Energy dissipation device . 60
A.5 Reactor . 60
A.6 Charging device . 60
A.6.1 Type tests for charging device . 60
A.6.2 Routine test for charging device . 61
Annex B (informative) Additional information about test circuit and measured signals
for DC transfer switches having an oscillation branch . 62
B.1 Test circuits for direct current transfer test . 62
B.2 Test circuits for direct arc withstand test . 64
Annex C (normative) Tolerances on test quantities during tests . 66
Annex D (normative) Records and reports of type tests. 69
D.1 Information and results to be recorded . 69
D.2 Information to be included in type test reports . 69
D.2.1 General . 69
D.2.2 Apparatus tested . 69
D.2.3 Rated characteristics of DC transfer switch, including its operating
devices and auxiliary equipment . 69
D.2.4 Test conditions (for each series of tests) . 70
D.2.5 Direct current commutation tests . 70
D.2.6 Direct arc withstand tests . 70
D.2.7 Short-time withstand current test . 70
D.2.8 No-load operation . 71
D.2.9 Oscillographic and other records . 71
Annex E (informative) Extension of validity of type tests . 72

– 6 – IEC TS 62271-315:2025 © IEC 2025
E.1 General . 72
E.2 Dielectric tests . 72
E.3 Continuous current test . 72
E.4 Electromagnetic immunity test on auxiliary and control circuits . 72
E.5 Environmental tests on auxiliary and control circuits . 73
E.6 DC transfer current test. 73
Bibliography . 74

Figure 1 – Test sequence for low temperature test . 37
Figure 2 – Test sequence for high temperature test . 39
Figure 3 – Humidity test . 41
Figure 4 – General layout of test circuit for direct current commutation test . 44
Figure 5 – Current commutation test with low-frequency alternating current . 45
Figure 6 – General layout of test circuit for direct arc withstand test. 47
Figure 7 – Equivalent transfer circuit of MRTS and ERTS . 53
Figure 8 – Equivalent transfer circuit of NBS . 54
Figure 9 – Equivalent transfer circuit of NBES . 55
Figure B.1 – Test circuit with measuring points . 62
Figure B.2 – Measured signals during test (overview) . 63
Figure B.3 – Measured signals during test (expanded in time) . 64
Figure B.4 – Test circuit of direct arc withstand tests . 65

Table 1 – Rated insulation levels for transfer switches . 17
Table 2 – Nameplate information . 21
Table 3 – Classification of commutation switches and making switches . 24
Table 4 – Type tests . 26
Table 5 – Number of operating sequences for commutation switches . 34
Table 6 – Number of operating sequences for making switches . 35
Table B.1 – Legend of measuring points . 65
Table C.1 – Tolerances on test quantities for type test . 66

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
HIGH-VOLTAGE SWITCHGEAR AND CONTROLGEAR –

Part 315: Direct current (DC) transfer switches

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
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 TS 62271-315 has been prepared by subcommittee 17A: Switching devices, of IEC
technical committee 17: High-voltage switchgear and controlgear. It is a Technical Specification.
The text of this Technical Specification is based on the following documents:
Draft Report on voting
17A/1412/DTS 17A/1417/RVDTS
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 Technical Specification is English.

– 8 – IEC TS 62271-315:2025 © 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.
This document is to be read in conjunction with IEC TS 62271-5:2024, to which it refers and
which is applicable unless otherwise specified. In order to simplify the indication of
corresponding requirements, the same numbering of clauses and subclauses is used as in
IEC TS 62271-5. Amendments to these clauses and subclauses are given under the same
references whilst additional subclauses are numbered from 101.
A list of all parts of IEC 62271 series, under the general title High-voltage switchgear and
controlgear 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.
HIGH-VOLTAGE SWITCHGEAR AND CONTROLGEAR –

Part 315: Direct current (DC) transfer switches

1 Scope
This part of IEC 62271 is applicable to direct current (DC) transfer switches designed for indoor
or outdoor installation and for operation on HVDC transmission systems having direct voltages
of 100 kV and above.
DC transfer switches normally include metallic return transfer switches (MRTS), earth return
transfer switches (ERTS), neutral bus switches (NBS) and neutral bus earthing switches (NBES).
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 60060-1:2010, High-voltage test techniques – Part 1: General definitions and test
requirements
IEC 60068-2-1:2007, Environmental testing – Part 2-1: Tests – Test A: Cold
IEC 60068-2-2:2007, Environmental testing – Part 2-2: Tests – Test B: Dry heat
IEC 60068-2-30:2005, Environmental testing – Part 2-30: Tests – Test Db: Damp heat, cyclic
(12 h + 12 h cycle)
IEC 60071-11:2022, Insulation co-ordination – Part 11: Definitions, principles and rules for
HVDC system
IEC 60076-6, Power transformers– Part 6: Reactors
IEC 60099-9, Surge arresters – Part 9: Metal-oxide surge arresters without gaps for HVDC
converter stations
IEC 60255-21-1:1988, Electrical relays – Part 21: Vibration, shock, bump and seismic tests on
measuring relays and protection equipment – Section One: Vibration tests (sinusoidal)
IEC 60270:2000, High-voltage test techniques – Partial discharge measurements
IEC 60633, High-voltage direct current (HVDC) transmission – Vocabulary
IEC 60871-1, Shunt capacitors for a.c. power systems having a rated voltage above 1 000 V –
Part 1: General
IEC 61000-4-18:2019, Electromagnetic compatibility (EMC) – Part 4-18: Testing and
measurement techniques – Damped oscillatory wave immunity test

– 10 – IEC TS 62271-315:2025 © IEC 2025
IEC TS 62271-5:2024, High-voltage switchgear and controlgear – Part 5: Common
specifications for direct current switchgear and controlgear
IEC 62271-100:2021, High-voltage switchgear and controlgear – Part 100: Alternating-current
circuit-breakers
IEC 62271-102:2018, High-voltage switchgear and controlgear – Part 102: Alternating-current
disconnectors and earthing switches
IEC 62271-207, High-voltage switchgear and controlgear – Part 207: Seismic qualification for
gas-insulated switchgear assemblies for rated voltages above 52 kV
IEC TS 63014-1, High-voltage direct current (HVDC) power transmission – System
requirements for DC-side equipment Part 1: Using line-commutated converters
3 Terms and definitions
For the purposes of this document, the terms and definitions given in in IEC 60633,
IEC TS 63014-1 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 General terms and definitions
Subclause 3.1 of IEC TS 62271-5:2024 is applicable.
3.2 Assemblies of switchgear and controlgear
Subclause 3.2 of IEC TS 62271-5:2024 is applicable.
3.3 Parts of assemblies
Subclause 3.3 of IEC TS 62271-5:2024 is applicable.
3.4 Switching devices
Subclause 3.4 of IEC TS 62271-5:2024 is applicable with the following additions:
3.4.101
active DC transfer switch
DC transfer switch with charging device in oscillating branch, installed in parallel to the
commutation switch
3.4.102
blank DC transfer switch
DC transfer switch with a sole commutation switch only, without any additional external
branches or components
Note 1 to entry: Some commutation switches use internal components to increase voltage drop across the switching
units during transfer operation.

3.4.103
passive DC transfer switch
DC transfer switch without charging device in oscillating branch, installed in parallel to the
commutation switch
3.4.104
earth return transfer switch
ERTS
DC transfer switch used to transfer direct current from a metallic return path to an earth return
path
Note 1 to entry: “DC transfer switch” is used instead of “DC commutation switch” to refer to whole switch, including
oscillating branch and energy dissipation branch.
Note 2 to entry: Although the term “earth return transfer breaker” (ERTB) has been widely used in the industry for
many years, it is misleading since such switches have no ability to interrupt fault current.
[SOURCE: IEC 60633:2019, 9.23, modified – The terms “earth return transfer breaker” and
“ERTB” have been removed, “DC commutation switch” has been replaced by “DC transfer
switch”, DC current is replaced by direct current, Note 1 to entry and reference to Figure 7 have
been deleted.]
3.4.105
metallic return transfer switch
MRTS
DC transfer switch used to transfer direct current from an earth return path to a metallic return
path
Note 1 to entry: “DC transfer switch” is used instead of “DC commutation switch” to refer to whole switch, including
oscillating branch and energy dissipation branch.
Note 2 to entry: Although the term “metal return transfer breaker” (MRTB) has been widely used in the industry for
many years, it is misleading since such switches have no ability to interrupt fault current.
[SOURCE: IEC 60633:2019, 9.22, modified – The terms “metallic return transfer breaker” and
“MRTB” have been removed, “DC commutation switch” has been replaced by “DC transfer
switch”, DC current is replaced by direct current, Note 1 to entry is moved to Note 2, and
reference to Figure 7 has been deleted.]
3.4.106
neutral bus switch
NBS
DC transfer switch connected in series with the neutral bus on a bipolar HVDC scheme,
designed to commutate current out of the pole conductor or neutral bus and into the electrode
line or dedicated metallic return conductor or earth e.g. in response to a fault in a converter or
neutral bus
Note 1 to entry: “DC transfer switch” is used instead of “DC commutation switch” to refer to whole switch, including
oscillating branch and energy dissipation branch.
[SOURCE: IEC 60633:2019, 9.26, modified, “DC commutation switch” has been replaced by
“DC transfer switch” and Note 1 has been replaced by a new Note 1.]

– 12 – IEC TS 62271-315:2025 © IEC 2025
3.4.107
neutral bus earthing switch
NBES
DC transfer switch connected from the neutral bus to the station earth mat on a bipolar HVDC
scheme, designed to provide a temporary earth connection, e.g. in the event of an open circuit
fault on the electrode line until the imbalance of current between the two poles can be reduced
to a safe minimum level or the electrode line connection can be restored
Note 1 to entry: Although the term “Neutral Bus Grounding Switch” (NBGS) has been widely used in the industry
for many years.
Note 2 to entry: In some applications, NBES and high-speed earthing switch (HSES) are used in series.
[SOURCE: IEC 60633:2019, 9.27, modified – The terms “neutral bus grounding switch” and
“NBGS” have been removed, “DC commutation switch” has been replaced by “DC transfer
switch”, Note 1 and Note 2 have been replaced by new notes.]
3.5 Parts of switchgear and controlgear
Subclause 3.5 of IEC TS 62271-5:2024 is applicable with the following additions:
3.5.101
commutation switch
mechanical switching device used in the main current path of DC transfer switches
Note 1 to entry: A single pole of an AC circuit-breaker or its modification was often used as commutation switch in
DC transfer switch.
Note 2 to entry: Some commutation switches use internal components to increase voltage drop across the switching
units during transfer operation.
3.5.102
oscillating branch
circuit in parallel with the commutation switch in DC transfer switches, consisting of
– capacitors and reactors, in case of passive DC transfer switches;
– capacitors including a charging device and a making switch, in case of active DC transfer
switches.
Note 1 to entry: The oscillating branch forces a current oscillation between itself and the commutation switch branch
in order to produce current zeros in the last one.
Note 2 to entry: Depending on the stray inductance of the arrangement reactors are not necessarily needed to be
installed.
Note 3 to entry: Passive DC transfer switches having a making device in series with the oscillating branch are also
known.
3.5.103
current zero device
oscillating circuit in case of passive DC transfer switch or current impulse generator in case of
active DC transfer switch
3.5.104
energy dissipation branch
impedance circuit in parallel with the commutation switch of DC transfer switches which
dissipates the energy stored in the energy storage components (e.g. reactors, stray inductance,
stray capacitance, etc.) in DC system after successful commutation of current from commutation
switch branch to oscillating branch
Note 1 to entry: In real transfer switch, metal oxide surge arrester commonly is used as energy dissipation device.

3.5.105
charging device
device used in active DC transfer switches to charge capacitors in current zero device
3.5.106
making switch
mechanical switch in series with oscillating branch or current injection branch, designed for fast
closing
Note 1 to entry: A making switch is used to close the oscillation branch to excite oscillation during current transfer
operation.
3.5.107
...