IEC 60310:2016

Railway applications - Traction transformers and inductors on board rolling stock

IEC 60310:2016 applies to traction and auxiliary power transformers installed on board rolling stock and to the various types of power inductors inserted in the traction and auxiliary circuits of rolling stock, of dry or liquid-immersed design. This standard can also be applied, after agreement between purchaser and manufacturer, to the traction transformers of three-phase AC line-side powered vehicles and to the transformers inserted in the single-phase or poly-phase auxiliary circuits of vehicles, except instrument transformers and transformers of a rated output below 1 kVA single-phase or 5 kVA poly-phase. This new edition takes into account the new generic railway standards, more specifically general service conditions referring to IEC 62498-1 and shock and vibration considerations referring to IEC 61373. It also includes the following significant technical changes with regard to the previous edition:
- temperature limits;
- temperature-rise test;
- dielectric tests;
- partial discharge test;
- inductance measurement methods;
- voltage between terminals withstand test, etc.
The contents of the corrigendum of February 2018 have been included in this copy.

Applications ferroviaires - Transformateurs de traction et bobines d'inductance à bord du matériel roulant

L'IEC 60310:2016 s'applique aux transformateurs de traction et de puissance auxiliaire à bord du matériel roulant et aux divers types de bobines d'inductance d'alimentation insérées dans les circuits de traction ou auxiliaires du matériel roulant, de conception sèche ou immergée dans un liquide. La présente Norme peut également être appliquée, après accord entre l'acheteur et le fabricant, aux transformateurs de traction des véhicules à courant alternatif triphasé côté réseau et aux transformateurs insérés dans les circuits auxiliaires monophasés ou polyphasés des véhicules, exception faite des transformateurs de mesure et des transformateurs de puissance assignée inférieure à 1 kVA en monophasé ou à 5 kVA en polyphasé. Cette nouvelle édition tient compte des nouvelles normes génériques ferroviaires, en particulier en ce qui concerne les conditions générales de service en référence à l'IEC 62498-1 et les considérations en matière de chocs et de vibrations en référence à l'IEC 61373. Elle inclut également les modifications techniques majeures suivantes par rapport à l'édition précédente:
- limites de températures;
- essai d'échauffement;
- essais diélectriques;
- essai de décharges partielles;
- méthodes de mesure d'inductance;
- essai de tenue à la tension entre bornes; etc.
Le contenu du corrigendum de février 2018 a été pris en considération dans cet exemplaire.

General Information

• Status: Published
• Publication Date: 26-Jan-2016
• ICS: 45.060.01 - Railway rolling stock in general
• Technical Committee: TC 9 - Electrical equipment and systems for railways
• Drafting Committee: MT 60310 - TC 9/MT 60310
• Current Stage: PPUB - Publication issued
• Start Date: 27-Jan-2016
• Completion Date: 31-Jan-2016

Relations

• Corrected By: IEC 60310:2016/COR1:2018 - Corrigendum 1 - Railway applications - Traction transformers and inductors on board rolling stock
• Effective Date: 05-Sep-2023
• Revises: IEC 60310:2004 - Railway applications - Traction transformers and inductors on board rolling stock
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC 60310 ®
Edition 4.0 2016-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Railway applications – Traction transformers and inductors on board rolling
stock
Applications ferroviaires – Transformateurs de traction et bobines d'inductance
à bord du matériel roulant
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IEC 60310 ®
Edition 4.0 2016-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Railway applications – Traction transformers and inductors on board rolling

stock
Applications ferroviaires – Transformateurs de traction et bobines d'inductance

à bord du matériel roulant
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 45.060 ISBN 978-2-8322-3107-4

– 2 – IEC 60310:2016 © IEC 2016
CONTENTS
FOREWORD . 6
1 Scope . 8
2 Normative references. 8
3 Terms and definitions . 9
3.1 General definitions . 10
3.2 Definitions for transformers . 11
3.3 Definitions for inductors . 11
4 Classification . 12
4.1 Classification of transformers . 12
4.2 Classification of inductors . 12
5 Service conditions . 12
6 Rated current and load profile . 13
6.1 Load profile . 13
6.2 Rated current . 13
7 Rated voltage and power of transformer windings . 13
7.1 Rated line-side voltage . 13
7.2 Rated secondary voltage . 13
7.3 Rated power of transformer . 14
8 Transformer tappings . 14
9 Cooling . 14
9.1 Identification of transformers and inductors according to cooling method . 14
9.2 Arrangement of symbols . 15
9.2.1 Enclosed transformers and inductors . 15
9.2.2 Non-enclosed transformers and inductors . 15
9.2.3 Air cooling . 15
10 Temperature limits . 15
10.1 Classification of insulating materials . 15
10.2 Temperature limits of solid insulation . 16
10.3 Temperature limits for liquid . 16
10.4 Temperature limits for other parts . 17
11 Mechanical design . 17
12 Rating plates . 17
13 Tests . 18
13.1 Categories of tests . 18
13.1.1 General . 18
13.1.2 Type tests . 18
13.1.3 Routine tests. 18
13.1.4 Investigation tests . 18
13.2 Tests on transformers . 18
13.2.1 General – List of tests . 18
13.2.2 Tolerances . 20
13.2.3 Visual checks (type, routine test) and functional tests (optional type and
routine test) . 20
13.2.4 Mass (type and optional routine test) . 20
13.2.5 Measurement of winding resistance (type and routine tests) . 21

13.2.6 Measurement of voltage ratio, polarities and vector groups (type and
routine tests) . 21
13.2.7 Measurement of no-load primary current and losses (type and routine
tests) . 22
13.2.8 Measurement of impedance voltages or short-circuit impedances (type
and routine tests) . 22
13.2.9 Measurement of fundamental load losses (type and routine tests) . 23
13.2.10 Determination of total losses (type test) . 24
13.2.11 Temperature-rise test (type test) . 24
13.2.12 Insulation resistance test (optional type and routine test) . 28
13.2.13 Dielectric tests (type and routine tests) . 28
13.2.14 Partial discharge test (type or optional routine test for dry-type,
investigation test for immersed type) . 34
13.2.15 Short-circuit withstand test (optional type test) . 36
13.2.16 Shock and vibration test (optional type test). 38
13.2.17 Voltage transmission ratio – VTR (optional type test) . 40
13.2.18 Noise measurement (type test) . 41
13.2.19 Leakage magnetic flux density measurement (optional type test) . 41
13.2.20 Electrical Frequency Response Analysis FRA (investigation test) . 42
13.2.21 Inrush current measurement (optional type test) . 42
13.3 Tests on inductors . 43
13.3.1 List of tests . 43
13.3.2 Tolerances . 43
13.3.3 Visual checks (type and routine test) . 44
13.3.4 Mass (type and optional routine test) . 44
13.3.5 Measurement of winding resistance (type and routine test) . 44
13.3.6 Determination of losses (type test) . 44
13.3.7 Measurement of inductance (type and routine tests) . 45
13.3.8 Temperature-rise test (type tests) . 48
13.3.9 Insulation resistance test (optional type and routine test) . 48
13.3.10 Dielectric tests (type and routine tests) . 48
13.3.11 Partial discharge test (type or optional routine test for the dry-type) . 49
13.3.12 Short-circuit withstand test (optional type test) . 49
13.3.13 Shock and vibration test (optional type test). 50
13.3.14 Vibration test with current flowing (investigation test) . 50
13.3.15 Noise measurement (type test) . 50
13.3.16 Leakage magnetic flux density measurement (optional type test) . 50
Annex A (informative) List of items for which an agreement between purchaser and
manufacturer is needed or for which further information or specifications shall be given
by the purchaser or by the manufacturer . 51
A.1 Items subject to agreement between purchaser and manufacturer . 51
A.1.1 Transformer and inductors . 51
A.1.2 Transformers . 52
A.1.3 Inductors . 53
A.2 Information to be given by purchaser to manufacturer . 54
A.2.1 Transformers and inductors . 54
A.2.2 Transformers . 55
A.2.3 Inductors . 55
A.3 Information to be given by manufacturer to purchaser . 56
A.3.1 Transformers and inductors . 56

– 4 – IEC 60310:2016 © IEC 2016
A.3.2 Transformers . 56
A.3.3 Inductors . 56
Annex B (informative) Thermal ageing and insulation life time . 57
B.1 Insulation life time and thermal ageing . 57
B.2 Definitions of thermal endurance . 57
B.3 Thermal endurance calculations . 58
B.4 Special considerations for thermal design and test . 59
B.4.1 General . 59
B.4.2 Cooling medium temperature at the external interface . 59
B.4.3 Rated current . 60
B.4.4 Temperature rise test of a dry-type transformer/inductor . 60
B.5 Thermal conformity of the insulation system . 60
B.6 End of life criterion . 60
Annex C (informative) Example of thermal endurance calculation to demonstrate the
suitability of an insulation system for a specified application . 61
C.1 Preliminary. 61
C.2 Example 1 – Temperature limits for a dry-type transformer/inductor. 61
C.3 Example 2 – Thermal endurance calculation . 61
C.3.1 General . 61
C.3.2 Operating conditions to be provided by the purchaser . 62
C.3.3 Thermal endurance characteristics to be provided by the manufacturer . 62
C.3.4 Temperature rise test results . 62
C.3.5 Calculations . 63
Annex D (informative) Wet dielectric tests for dry-type transformers and inductors . 65
D.1 General . 65
D.2 Wet test 1 (optional type test or optional routine test): short soaking . 65
D.3 Wet test 2 (investigation test or optional type test): misting . 66
D.4 Wet test 3 (investigation test): thermal shock – long soaking – misting . 66
D.4.1 General . 66
D.4.2 Temperature conditioning . 66
D.4.3 Thermal shock . 66
D.4.4 Dielectric test . 66
D.5 Common test procedure and criteria for wet dielectric tests . 66
Annex E (informative) Load profiles . 68
Bibliography . 69

Figure 1 – Examples of set up for induced voltage withstanding tests . 31
Figure 2 – Examples of set up for separate source voltage withstanding tests . 32
Figure 3 – Examples of impulse test connections for traction, inductor and auxiliary
transformers . 33
Figure 4 – Partial discharge test: voltage versus time . 35
Figure 5 – Configurations for VTR test . 41
Figure 6 – Example of test circuit . 42

Table 1 – Letter symbols for cooling method . 14
Table 2 – Order of symbols . 15
Table 3 – Temperature limits of solid insulation . 16

Table 4 – Temperature limits for liquid . 16
Table 5 – List of checks and tests to be made on traction transformers . 19
Table 6 – Tolerances . 20
Table 7 – Reference temperatures . 21
Table 8 – Dielectric test voltage . 30
Table 9 – Partial discharge measurements . 35
Table 10 – List of checks and tests to be made on inductors . 43
Table 11 – Tolerances . 44
Table 12 – Test method of voltage between terminals withstand test . 49
Table C.1 – Temperature limits and expected lifetime for a dry-type transformer or
inductor (examples) . 61
Table C.2 – Load cycle histogram . 62
Table C.3 – Temperature histogram . 62
Table C.4 – Temperature rise test results . 63
Table C.5 – Thermal endurance calculation . 63
Table C.6 – Equivalent current and temperatures . 64

– 6 – IEC 60310:2016 © IEC 2016
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
RAILWAY APPLICATIONS –
TRACTION TRANSFORMERS AND
INDUCTORS ON BOARD ROLLING STOCK

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 co-operation on all questions concerning standardization in the electrical and electronic fields. To
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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60310 has been prepared by IEC technical committee 9: Electrical
equipment and systems for railways.
This fourth edition cancels and replaces the third edition issued in 2004 and constitutes a
technical revision.
This edition takes into account the new generic railway standards, more specifically general
service conditions referring to IEC 62498-1 and shock and vibration considerations referring
to IEC 61373. It also includes the following significant technical changes with regard to the
previous edition:
– temperature limits;
– temperature-rise test;
– dielectric tests;
– partial discharge test;
– inductance measurement methods;
– voltage between terminals withstand test;
– thermal ageing and insulation lifetime (informative);
– examples of thermal endurance calculation (informative);
– wet dielectric tests (informative);
– load profiles (informative).
The text of this standard is based on the following documents:
FDIS Report on voting
9/2080/FDIS 9/2117/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
The contents of the corrigendum of February 2018 have been included in this copy.

IMPORTANT – The 'colour inside' logo on the cover page of this publication 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.
– 8 – IEC 60310:2016 © IEC 2016
RAILWAY APPLICATIONS –
TRACTION TRANSFORMERS AND
INDUCTORS ON BOARD ROLLING STOCK

1 Scope
This International Standard applies to traction and auxiliary power transformers installed on
board rolling stock and to the various types of power inductors inserted in the traction and
auxiliary circuits of rolling stock, of dry or liquid-immersed design.
NOTE The requirements of IEC 60076 (all parts) are applicable to transformers and inductors where they do not
conflict with this standard, or with the specialized IEC publications dealing with traction applications.
This standard can also be applied, after agreement between purchaser and manufacturer, to
the traction transformers of three-phase a.c. line-side powered vehicles and to the
transformers inserted in the single-phase or poly-phase auxiliary circuits of vehicles, except
instrument transformers and transformers of a rated output below 1 kVA single-phase or
5 kVA poly-phase.
This standard does not cover accessories such as tap changers, resistors, heat exchangers,
fans, etc., intended for mounting on the transformers or inductors, which are tested separately
according to relevant rules.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60050-811, International Electrotechnical Vocabulary (IEV) – Chapter 811: Electric
traction
IEC 60060-1, High-voltage test techniques – Part 1: General definitions and test requirements
IEC 60060-2, High-voltage test techniques – Part 2: Measuring systems
IEC 60076-1:2011, Power transformers – Part 1: General
IEC 60076-2, Power transformers – Part 2: Temperature rise for liquid-immersed transformers
IEC 60076-3: Power transformers – Part 3: Insulation levels, dielectric tests and external
clearances in air
IEC 60076-4, Power transformers – Part 4: Guide to the lightning impulse and switching
impulse testing – Power transformers and reactors
IEC 60076-5, Power transformers – Part 5: Ability to withstand short circuit
IEC 60076-6:2007, Power transformers – Part 6: Reactors
IEC 60076-7, Power transformers – Part 7: Loading guide for oil-immersed power
transformers
IEC 60076-10, Power transformers – Part 10: Determination of sound levels
IEC 60076-11, Power transformers – Part 11: Dry-type transformers
IEC 60076-12:2008, Power transformers – Part 12: Loading guide for dry-type transformers
IEC 60076-14, Power transformers – Part 14: Liquid-immersed power transformers using
high-temperature insulation materials
IEC 60076-18, Power transformers – Part 18: Measurement of frequency response
IEC 60077-1, Railway applications – Electric equipment for rolling stock – Part 1: General
service conditions and general rules
IEC 60085, Electrical insulation – Thermal evaluation and designation
IEC 60270, High-voltage test techniques – Partial discharge measurements
IEC 60296, Fluids for electrotechnical applications – Unused mineral insulating oils for
transformers and switchgear
IEC 60836, Specifications for unused silicone insulating liquids for electrotechnical purposes
IEC 60850, Railway applications – Supply voltage of traction systems
IEC 61039, Classification of insulating liquids
IEC 61099, Insulating liquids – Specifications for unused synthetic organic esters for electrical
purposes
IEC 61373:2010, Railway applications – Rolling stock equipment – Shock and vibration tests
IEC 61378-1:2011, Convertor transformers – Part 1: Transformers for industrial applications
IEC 62497-1, Railway applications – Insulation coordination – Part 1: Basic requirements –
Clearances and creepage distances for all electrical and electronic equipment
IEC 62498-1, Railway applications – Environmental conditions for equipment – Part 1:
Equipment on board rolling stock
ISO 3746, Acoustics – Determination of sound power levels and sound energy levels of noise
sources using sound pressure – Survey method using an enveloping measurement surface
over a reflecting plane
ISO 9614-1, Acoustics – Determination of sound power levels of noise sources using sound
intensity – Part 1: Measurement at discrete points
ISO 9614-2, Acoustics – Determination of sound power levels of noise sources using sound
intensity – Part 2: Measurement by scanning
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60076-1 and
IEC 60050-811 together with the following apply.

– 10 – IEC 60310:2016 © IEC 2016
NOTE When the term “transformer” is used alone, it applies to both traction and auxiliary transformers.
The term “transformer(s)/inductor(s)” appears in clauses applicable to both transformers and inductors to avoid
duplication of text.
The term “inductor” is used in this standard with the same meaning as the term “reactor” mentioned in IEC 60050-
421, IEC 60050-811 and IEC 60076-6.
3.1 General definitions
3.1.1
load profile
current/power versus time under specified conditions including voltage
3.1.2
cooling medium
cooling medium used to extract the heat out of the transformer/inductor e.g. air, water, oil,
heat sink, etc.
3.1.3
rated insulation voltage
U
Nm
r.m.s. withstand voltage value assigned by the manufacturer to the equipment or a part of it,
characterising the specified permanent (over 5 min) withstand capability of its insulation
Note 1 to entry: U is a voltage between a live part of equipment and earth or another live part. For rolling stock,
Nm
earth refers to the car body.
Note 2 to entry: For circuits, systems and sub-systems in railway applications this definition is preferred to
"highest voltage for equipment" which is widely used in international standards.
Note 3 to entry: U is higher than or equal to the working voltage. As a consequence, for circuits directly
Nm
connected to the contact line, U is equal to or higher than U as specified in IEC 60850. For circuits
Nm max1
connected to electronic converter U is higher than or equal to the d.c. link voltage.
Nm
Note 4 to entry: U is not necessarily equal to the rated voltage which is primarily related to functional
Nm
performance.
3.1.4
nominal voltage
U
n
suitable approximate voltage used to designate or identify a given supply system
3.1.5
rated voltage
U
r
value of voltage assigned for a specific operating condition
3.1.6
rated impulse voltage
U
Ni
impulse voltage value, characterizing the specified withstand capability of its insulation
against transient over-voltages

3.1.7
test voltage
U
a
r.m.s. value derived from U used for separate source voltage, induced voltage, voltage
Nm
between terminals withstand, depending on test carried out
3.1.8
recurring peak voltage
U , U
mT mG
maximum peak value of periodic excursions of the voltage waveform between terminals (U )
mT
or between terminals and ground (U )
mG
3.2 Definitions for transformers
3.2.1
voltage transmission ratio
VTR
ratio between the secondary voltage and the primary voltage when a specified impulse or a.c.
square voltage is applied on the primary.
The VTR is expressed as a percentage of this applied voltage.
3.2.2
impedance voltage
voltage applied to reach the rated current in short-circuit.
This is expressed as a percentage of this applied voltage to the rated voltage at reference
temperature.
Note 1 to entry: When expressed as a percentage or per unit, this is equal to the short circuit impedance referred
in IEC 60076-1:2011, 3.7.
3.2.3
tolerance
permitted deviation between the declared value of a quantity and the measured value
[SOURCE: IEC 60050-411:2007, 411-36-19]

3.3 Definitions for inductors
Values of inductance for inductors are related to the different classes of utilisation and are
defined as follows, with the understanding that they include an indication of the nature and
value of the current used in their measurement.
3.3.1
a.c. inductance
inductance derived from the measurement of the alternating current carried by the inductor
when it is supplied by a sinusoidal alternating voltage of specified value and frequency
3.3.2
differential inductance
inductance defined from the derivative of the linked flux as a function of current (equal to the
slope of the magnetic characteristic)
Note 1 to entry: It is derived from the transient record of instantaneous voltage and current in the inductor or from
the measurement of the variation of magnetic flux.

– 12 – IEC 60310:2016 © IEC 2016
3.3.3
incremental inductance
inductance seen by the a.c. current of a particular value and frequency superimposed on a
direct current through the inductor
Note 1 to entry: It should be mentioned that the ripple factor of a pulsating current, expressed as a percentage, is
conventionally defined by the formula:
I − I
max min
×100
I + I
max min
where I and I respectively represent the maximum and minimum values of the current wave.
max min
Note 2 to entry: It is derived from a record of the terminal voltage.
4 Classification
4.1 Classification of transformers
The transformers mentioned above may be:
– traction transformers to supply the propulsion circuits, and optionally also other
equipment;
– auxiliary transformers to supply electrical equipment except propulsion circuits.
Windings mentioned in the standard may be:
– line side windings which are directly connected to the power supply line;
– traction windings which supply the propulsion circuits;
– auxiliary windings which are used for other purposes.
4.2 Classification of inductors
According to their use, inductors can be classified as follows:
– inductors for alternating current:
inductors that carry alternating current, such as transition inductors used for transition
between tappings of tap changers, inductors for a.c. commutator motor braking circuits,
interference suppression inductors, tuned filter inductors, etc.;
– inductors for direct current:
inductors that carry direct current with small or negligible a.c. components, such as d.c.
line filter inductors, inductive shunts for traction motors, inductors for d.c. motor braking
circuits, etc.;
– inductors for pulsating current:
inductors that carry direct or alternating current with a significant periodic ripple, such as
smoothing inductors for traction motors, sinusoidal filter inductors in auxiliary converters,
etc.
5 Service conditions
The normal service conditions for transformers and inductors shall be in accordance to
IEC 62498-1. Special service conditions shall be agreed between the purchaser and
manufacturer.
6 Rated current and load profile
6.1 Load profile
A transformer/inductor is designed to operate on the train in steady-state and transient
(surge) conditions, both in normal and overload conditions.
The purchaser should specify the load profile according to Annex E (informative). The current
frequency spectrum shall be specified by the responsible party.
6.2 Rated current
The rated current of a winding is the current this winding can sustain permanently at the
reference temperature for continuous load.
The rated current shall be calculated according to either of following methods:
a) r.m.s. current derived from the load profiles;
b) considering thermal ageing of insulating materials according to Annex B (informative).
Special attention should be paid to varying cooling modes and to the averaging time window.
The reference temperature for continuous load is the cooling medium temperature at the
external interface of the transformer/inductor. It shall:
c) either be directly specified by the purchaser;
Specified values should be based on the air temperature external to vehicle as defined in
IEC 62498-1.
d) or calculated by the manufacturer based on the temperature histogram provided by the
purchaser and the method of Annex B (informative): see cooling medium reference
temperature for lifetime calculation in B.4.2.
For a traction winding, the rated current shall correspond to the principal tapping. This
definition of rated current applies when other windings, which are normally on load, deliver
their rated loads.
7 Rated voltage and power of transformer windings
7.1 Rated line-side voltage
The rated line-side voltage is the r.m.s. voltage applicable in normal operating conditions to
the line-side winding group. If this winding has tappings, the rated voltage shall be referred to
the principal tapping.
Unless otherwise agreed between purchaser and manufacturer, the rated line-side voltage is
specified as being equal to the nominal voltage of the traction system.
NOTE IEC 60850 gives the list of the nominal voltages of traction systems.
7.2 Rated secondary voltage
The rated voltage of a secondary winding of a transformer is the r.m.s. no-load voltage at the
terminals of the winding when the principal tapping of the line-side winding of the transformer
is fed at its rated voltage and frequency.

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7.3 Rated power of transformer
The rated power of a transformer winding is defined as the product of the rated voltage of this
winding and its rated current.
NOTE Transformers usually have several secondary windings (e.g. traction, auxiliaries, train heating). The rated
power of the line-side winding of a transformer can be less than the sum of the rated powers of its various
secondary windings.
8 Transformer tappings
In order to permit variation of the voltage ratio of the transformer, one or more of its windings
may be equipped with intermediate tappings, which shall be indicated on the diagram and in
the specification, with a statement of their maximum operating characteristics.
The principal tapping is that which enables the rated traction motor voltage to be obtained at
the terminals of the motors when they are taking their rated current, the transformer line-side
winding being supplied at rated voltage and frequency.
When both line-side windings and tracti
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