IEC 60695-1-40:2013

IEC 60695-1-40 ®
Edition 1.0 2013-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
BASIC SAFETY PUBLICATION
PUBLICATION FONDAMENTALE DE SÉCURITÉ
Fire hazard testing –
Part 1-40: Guidance for assessing the fire hazard of electrotechnical products –
Insulating liquids
Essais relatifs aux risques du feu –
Partie 1-40: Guide pour l’évaluation des risques du feu des produits
électrotechniques – Liquides isolants

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IEC 60695-1-40 ®
Edition 1.0 2013-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
BASIC SAFETY PUBLICATION
PUBLICATION FONDAMENTALE DE SÉCURITÉ

Fire hazard testing –
Part 1-40: Guidance for assessing the fire hazard of electrotechnical products –

Insulating liquids
Essais relatifs aux risques du feu –

Partie 1-40: Guide pour l’évaluation des risques du feu des produits

électrotechniques – Liquides isolants

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX V
ICS 13.220.40; 29.020 ISBN 978-2-8322-1170-0

– 2 – 60695-1-40 © IEC:2013
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 8
4 Classification of insulating liquids . 13
5 Types of electrotechnical equipment containing insulating liquids . 13
6 Fire parameters . 14
6.1 General . 14
6.2 Ignition . 14
6.2.1 General . 14
6.2.2 Combustion . 14
6.2.3 Potential fire growth . 14
6.2.4 Fire effluent . 14
7 Fire scenarios . 14
7.1 General . 14
7.2 Origin fire scenarios . 14
7.2.1 General . 14
7.2.2 Major causes of fire . 15
7.2.3 Minor causes of fire . 16
7.2.4 Pool fires . 16
7.2.5 Burning spray . 16
7.2.6 Ignition on hot surface . 16
7.3 Victim fire scenarios . 16
8 Protective measures against fire . 17
9 Considerations for the selection of test methods . 17
9.1 General . 17
9.2 Type tests . 18
9.3 Sampling tests . 18
9.4 Arc resistance tests . 18
9.5 Relevance of test results to fire scenario . 18
Annex A (informative) History of insulating liquids . 19
Annex B (informative) Preventive and protective measures against fire . 20
B.1 General . 20
B.2 Physical protective measures . 20
B.3 Chemical protective measures . 20
B.4 Electrical protective measures . 20
B.5 Sensing devices . 20
B.6 Maintenance and inspection . 20
Annex C (informative) Transformers. 22
C.1 General . 22
C.2 Transformer choice . 22
Annex D (informative) Power capacitors . 24

60695-1-40 © IEC:2013 – 3 –
Annex E (informative) Cables. 25
E.1 Power cables . 25
E.2 Communication cables . 26
E.3 Cables with water blocking compounds . 26
E.4 Cable terminations . 26
Annex F (informative) Bushings . 27
Annex G (informative) Switchgear . 28
Bibliography . 29

Figure E.1 – Oil viscosity . 26

Table 1 – Classification of insulating liquids . 13

– 4 – 60695-1-40 © IEC:2013
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIRE HAZARD TESTING –
Part 1-40: Guidance for assessing the fire hazard
of electrotechnical products –
Insulating liquids
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
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
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with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
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other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60695-1-40 has been prepared by IEC technical committee 89:
Fire hazard testing.
This first edition of IEC 60695-1-40 cancels and replaces the first edition of
IEC/TS 60695-1-40 published in 2002. It constitutes a technical revision and now has the
status of an International Standard.
The main changes with respect to the first edition of IEC/TS 60695-1-40 are the integration of
editorial and technical changes throughout the text.

60695-1-40 © IEC:2013 – 5 –
The text of this standard is based on the following documents:
FDIS Report on voting
89/1191/FDIS 89/1200/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.
A list of all the parts in the 60695 series, under the general title Fire hazard testing, can be
found on the IEC web site.
This international standard is to be used in conjunction with IEC 60695-1-10.
IEC 60695-1 consists of the following parts:
– Part 1-10: Guidance for assessing the fire hazard of electrotechnical products – General
guidelines
– Part 1-11: Guidance for assessing the fire hazard of electrotechnical products – Fire
hazard assessment
– Part 1-12: Guidance for assessing the fire hazard of electrotechnical products – Fire
safety engineering
– Part 1-20: Guidance for assessing the fire hazard of electrotechnical products – Ignitability
– General guidance
– Part 1-21: Guidance for assessing the fire hazard of electrotechnical products – Ignitability
– Summary and relevance of test methods
– Part 1-30: Guidance for assessing the fire hazard of electrotechnical products –
Preselection testing process − General guidelines
– Part 1-40: Guidance for assessing the fire hazard of electrotechnical products – Insulating
liquids
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.
– 6 – 60695-1-40 © IEC:2013
INTRODUCTION
In the design of any electrotechnical product the risk of fire and the potential hazards
associated with fire need to be considered. In this respect the objective of component, circuit
and product design as well as the choice of materials is to reduce to acceptable levels the
potential risks of fire even in the event of foreseeable abnormal use, malfunction or failure.
For more than 100 years, insulating liquids based on mineral oil have been used for the
insulating and cooling of electrical transformers and some other types of electrotechnical
equipment.
During the last 70 years, synthetic insulating liquids have been developed and used in
specific electrotechnical applications for which their properties are particularly suitable.
However, for technical and economic reasons, highly refined mineral oil continues to be the
most widely used insulating liquid for use in transformers, the major end use application. Their
safe installation is covered by local, national and international regulations.
The fire safety record of electrotechnical equipment containing insulating liquids is good, for
both mineral oil and synthetic liquids. In recent years improvements in design and protective
measures against fire have reduced the fire hazard for electrotechnical equipment containing
mineral oil. However, as for all forms of electrotechnical equipment, the objective should be to
reduce the likelihood of fire even in the event of foreseeable abnormal use.
The practical aim is to prevent ignition, but if ignition occurs, to control the fire, preferably
within the enclosure of the electrotechnical equipment.

60695-1-40 © IEC:2013 – 7 –
FIRE HAZARD TESTING –
Part 1-40: Guidance for assessing the fire hazard
of electrotechnical products –
Insulating liquids
1 Scope
This international standard provides guidance on the minimization of fire hazard arising from
the use of electrical insulating liquids, with respect to:
a) electrotechnical equipment and systems,
b) people, building structures and their contents.
This basic safety publication is intended for use by technical committees in the preparation of
standards in accordance with the principles laid down in IEC Guide 104 [1] and
ISO/IEC Guide 51 [2]. It is not intended for use by manufacturers or certification bodies.
One of the responsibilities of a technical committee is, wherever applicable, to make use of
basic safety publications in the preparation of its publications.
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, International electrotechnical vocabulary
IEC 60296, Fluids for electrotechnical applications – Unused mineral insulating oils for
transformers and switchgear
IEC 60465, Specification for unused insulating mineral oils for cables with oil ducts
IEC 60695-1-10, Fire hazard testing − Part 1-10: Guidance for assessing the fire hazard of
electrotechnical products − General guidelines
IEC 60695-1-11, Fire hazard testing − Part 1-11: Guidance for assessing the fire hazard of
electrotechnical products − Fire hazard assessment
IEC 60695-4:2012, Fire hazard testing – Part 4: Terminology concerning fire tests for
electrotechnical products
IEC 60695-6-2, Fire hazard testing – Part 6-2: Smoke obscuration – Summary and relevance
of test methods
IEC 60695-7-2, Fire hazard testing – Part 7-2: Toxicity of fire effluent – Summary and
relevance of test methods
______________
Numbers in square brackets refer to the Bibliography.

– 8 – 60695-1-40 © IEC:2013
IEC 60695-8-2, Fire hazard testing – Part 8-2: Heat release – Summary and relevance of test
methods
IEC 60944, Guide for the maintenance of silicone transformer liquids
IEC 61039, Classification of insulating liquids
IEC 61203, Synthetic organic esters for electrical purposes – Guide for maintenance of
transformer esters in equipment
IEC/TS 60695-5-2, Fire hazard testing – Part 5-2: Corrosion damage effects of fire effluent –
Summary and relevance of test methods
IEC/TS 60695-8-3, Fire hazard testing – Part 8-3: Heat release – Heat release of insulating
liquids used in electrotechnical products
ISO 1716, Reaction to fire tests for products – Determination of the gross heat of combustion
(calorific value)
ISO 2592, Determination of flash and fire points − Cleveland open cup method
ISO 13943:2008, Fire safety − Vocabulary
3 Terms and definitions
For the purposes of this document, terms and definitions given in ISO 13943:2008 and
IEC 60695-4:2012, some of which are reproduced below for the user’s convenience, as well
as the following additional definitions, apply.
3.1
arc
electrical breakdown of a gas which produces a sustained plasma discharge, resulting from
an electric current flowing through a normally nonconductive medium such as air
3.2
bund
outer wall or tank designed to retain the contents of an inner container in the event of leakage
or spillage
Note 1 to entry: A bund should be designed to capture well in excess of the volume of liquids held within the bund
area.
3.3
bushing
insulating liner in an opening through which a conductor passes
3.4
combustion
exothermic reaction of a substance with an oxidizing agent
Note 1 to entry: Combustion generally emits fire effluent accompanied by flames and/or glowing.
[SOURCE: ISO 13943:2008, 4.46]
3.5
corrosion damage
physical and/or chemical damage or impaired function caused by chemical action

60695-1-40 © IEC:2013 – 9 –
[SOURCE: ISO 13943:2008, 4.56]
3.6
enclosure
〈electrotechnical〉 external casing protecting the electrical and mechanical parts of apparatus
Note 1 to entry: The term excludes cables.
[SOURCE: IEC 60695-4:2012, 3.2.6]
3.7
fire
〈general〉 process of combustion characterized by the emission of heat and fire effluent and
usually accompanied by smoke, flame, glowing or a combination thereof
Note 1 to entry: In the English language the term “fire” is used to designate three concepts, two of which, fire and
fire, relate to specific types of self-supporting combustion with different meanings and two of them are designated
using two different terms in both French and German.
[SOURCE: ISO 13943:2008, 4.96]
3.8
fire effluent
totality of gases and aerosols, including suspended particles, created by combustion or
pyrolysis in a fire
[SOURCE: ISO 13943:2008, 4.105]
3.9
fire growth
stage of fire development during which the heat release rate and the temperature of the fire
are increasing
[SOURCE: ISO 13943:2008, 4.111]
3.10
fire hazard
physical object or condition with a potential for an undesirable consequence from fire
[SOURCE: ISO 13943:2008, definition 4.112]
3.11
fire Ioad
quantity of heat which can be released by the complete combustion of all the combustible
materials in a volume, including the facings of all bounding surfaces
Note 1 to entry: Fire load may be based on effective heat of combustion, gross heat of combustion, or net heat of
combustion as required by the specifier.
Note 2 to entry: The word “load” can be used to denote force or power or energy. In this context, it is being used
to denote energy.
Note 3 to entry: The typical units are kilojoules (kJ) or megajoules (MJ).
[SOURCE: ISO 13943:2008, 4.114]
3.12
fire point
minimum temperature at which a material ignites and continues to burn for a specified time
after a standardized small flame has been applied to its surface under specified conditions

– 10 – 60695-1-40 © IEC:2013
Note 1 to entry: In some countries, the term “fire point” has an additional meaning: a location where fire-fighting
equipment is sited, which may also comprise a fire-alarm call point and fire instruction notices.
Note 2 to entry: The typical units are degrees Celsius (°C).
[SOURCE: ISO 13943:2008, 4.119]
3.13
fire risk
probability of a fire combined with a quantified measure of its consequence
Note 1 to entry: It is often calculated as the product of probability and consequence.
[SOURCE: ISO 13943:2008, 4.124]
3.14
fire scenario
qualitative description of the course of a fire with respect to time, identifying key events that
characterise the studied fire and differentiate it from other possible fires
Note 1 to entry: It typically defines the ignition and fire growth processes, the fully developed fire stage, the fire
decay stage, and the environment and systems that impact on the course of the fire.
[SOURCE: ISO 13943:2008, 4.129]
3.15
flame, noun
zone in which there is rapid, self-sustaining, sub-sonic propagation of combustion in a
gaseous medium, usually with emission of light
[SOURCE: ISO 13943:2008, 4.133 − modified by addition of "zone in which there is"]
3.16
flammability
ability of a material or product to burn with a flame under specified conditions
[SOURCE: ISO 13943:2008, 4.151]
3.17
flash point
minimum temperature to which it is necessary to heat a material or a product for the vapours
emitted to ignite momentarily in the presence of flame under specified conditions
Note 1 to entry: The typical units are degrees Celsius (°C).
[SOURCE: ISO 13943:2008, 4.154]
3.18
gross heat of combustion
heat of combustion of a substance when the combustion is complete and any produced water
is entirely condensed under specified conditions
-1
Note 1 to entry: The typical units are kilojoules per gram (kJ⋅g ).
[SOURCE: ISO 13943:2008, 4.170]
3.19
heat of combustion
thermal energy produced by combustion of unit mass of a given substance

60695-1-40 © IEC:2013 – 11 –
-1
Note 1 to entry: The typical units are kilojoules per gram (kJ⋅g ).
[SOURCE: ISO 13943:2008, 4.174]
3.20
heat of gasification
thermal energy required to change a unit mass of material from the condensed phase to the
vapour phase at a given temperature
-1
Note 1 to entry: The typical units are kilojoules per gram (kJ⋅g ).
[SOURCE: ISO 13943:2008, 4.175]
3.21
heat release
thermal energy produced by combustion
Note 1 to entry: The typical units are joules (J).
[SOURCE: ISO 13943:2008, 4.176]
3.22
heat release rate
burning rate (deprecated)
rate of burning (deprecated)
rate of thermal energy production generated by combustion
Note 1 to entry: The typical units are watts (W).
[SOURCE: ISO 13943:2008, 4.177]
3.23
high voltage
HV
voltage greater than 1 kV (a.c.) or greater than 1,5 kV (d.c.)
3.24
ignitability
ease of ignition
measure of the ease with which a test specimen can be ignited, under specified conditions
[SOURCE: ISO 13943:2008, 4.182]
3.25
ignition
sustained ignition (deprecated)
〈general〉 initiation of combustion
[SOURCE: ISO 13943:2008, 4.187]
3.26
mineral oil
liquid conforming to IEC 60296 or IEC 60465
3.27
net heat of combustion
heat of combustion when any water produced is considered to be in the gaseous state

– 12 – 60695-1-40 © IEC:2013
Note 1 to entry: The net heat of combustion is always smaller than the gross heat of combustion because the heat
released by the condensation of water vapour is not included.
-1
Note 2 to entry: The typical units are kilojoules per gram (kJ⋅g ).
[SOURCE: ISO 13943:2008, 4.237]
3.28
opacity of smoke
ratio of incident light intensity to transmitted light intensity through smoke, under specified
conditions
Note 1 to entry: Opacity of smoke is the reciprocal of transmittance.
Note 2 to entry: The opacity of smoke is dimensionless.
[SOURCE: ISO 13943:2008, 4.243]
3.29
origin fire scenario
fire scenario involving electrotechnical equipment where the electrotechnical equipment is the
source of ignition
3.30
PCB
polychlorinated biphenyl
Note 1 to entry: PCB mixtures were developed as insulating liquids in the 1930s. They are known by various trade
names, e.g. Aroclor, Askarel, Clophen, Inerteen and Pyranol .
3.31
pool fire
fire characterized by diffusion flames formed above a horizontal body of liquid fuel where
buoyancy is the controlling mechanism for transport of fire effluent from the fire and transport
of air to the fire
3.32
routine test
test on a number of items taken at random from a batch
3.33
sampling test
conformity test made on each individual item during or after manufacture
[SOURCE: IEC 60050-151:2001, 151-16-17, modified – original term was “routine test”]
3.34
tapchanger
device fitted to power transformers for regulation of the output voltage to required levels
3.35
toxic hazard
potential for harm resulting from exposure to toxic combustion products
[SOURCE: ISO 13943:2008, 4.337]
______________
Aroclor, Askarel, Clophen, Inerteen and Pyranol are examples of suitable products available
commercially. This information is given for the convenience of users of this document and does not constitute
an endorsement by IEC of these products.

60695-1-40 © IEC:2013 – 13 –
3.36
type test
conformity test made on one or more items representative of the production
[SOURCE: IEC 60050-581:2008, 581-21-08]
3.37
victim fire scenario
fire scenario involving electrotechnical equipment where the electrotechnical equipment is the
victim of a fire of external origin
4 Classification of insulating liquids
Insulating liquids have been classified in IEC 61039 according to fire point and net heat of
combustion, as shown in Table 1.
Table 1 – Classification of insulating liquids
Fire point Net heat of combustion
Class O ≤300 °C Class 1 ≥42 MJ/kg
<42 MJ/kg
Class K >300 °C Class 2
≥32 MJ/kg
No measurable
Class L Class 3 <32 MJ/kg
fire point
EXAMPLE Mineral transformer oil (IEC 60296) has a classification of O1.

NOTE 1 Fire point is measured using the Cleveland open cup method, ISO 2592, and is used as the primary
method of classification.
NOTE 2 The determination of the flash point is sometimes used as a secondary method of classification.
IEC TC10 usually adopts ISO 2719:2002 [3] in order to measure the flash point using the Pensky-Martens
methodology (closed cup). If the value of the flash point determined by this method is < 250 °C, then the product is
classified with the letter “O”; if the flash point is ≥ 250 °C, then the product is classified with the letter “K”, and, if
there is no detectable flash point, the product is classified with the letter “L”.
5 Types of electrotechnical equipment containing insulating liquids
Insulating liquids are used in some designs of:
– transformers and reactors,
– capacitors,
– cables,
– bushings,
– switchgear, and
– miscellaneous power electronics (and in some other electrotechnical applications in which
the liquid serves partly as an insulant, but primarily as a coolant)
In many cases, alternative designs use solid or gaseous insulation materials as an alternative
to liquids. This international standard does not discuss the relative advantages and
disadvantages of these alternatives.
NOTE As insulating liquids are always part of an insulating system, the fire hazard assessment of the complete
system could also be of interest.

– 14 – 60695-1-40 © IEC:2013
6 Fire parameters
6.1 General
The main parameters which relate to the ignition and combustion of insulating liquids are
described in 6.2.
6.2 Ignition
6.2.1 General
Ignitability can be measured by fire point as described in ISO 2592.
6.2.2 Combustion
Combustion characteristics are be considered in terms of the contribution to the fire load, the
potential fire growth, and the fire hazards caused by fire effluent.
NOTE A fire may not cause the insulating liquid to burn but may cause leakage of the insulating liquid. In this
case, the hazards caused by leakage should also be considered.
6.2.3 Potential fire growth
Important parameters relating to the potential fire growth are net heat of combustion, heat
release rate and heat of gasification.
6.2.4 Fire effluent
The important hazardous effects of fire effluent are opacity of smoke, corrosion damage and
toxic hazard.
7 Fire scenarios
7.1 General
Fire scenarios for electrotechnical equipment containing insulating liquids are described
below. These fire scenarios are particularly relevant for transformers, the major end use
application for insulating liquids, and in some cases for other types of electrotechnical
equipment.
The fire hazard shall be assessed with reference to IEC 60695-1-10 and IEC 60695-1-11.
For electrotechnical equipment containing insulating liquids, the two types of scenario that are
considered are:
a) when the electrotechnical equipment is the source of ignition, known as an “origin fire
scenario”, and
b) when the electrotechnical equipment is the victim of a fire of external origin, known as a
“victim fire scenario”.
In the origin fire scenario, fire is initiated by failure within the electrotechnical equipment. In
the victim fire scenario, the insulating liquid contributes to the fire load for a fire of external
origin.
7.2 Origin fire scenarios
7.2.1 General
Consideration shall be given to

60695-1-40 © IEC:2013 – 15 –
a) whether the insulating liquid can be heated to its fire point under equipment overload
conditions. This could result in fire initiation if exposed to an external source of ignition;
b) whether fire can be initiated by an uncontrolled high-energy internal arc.
Either of these situations may create internal pressure sufficient to rupture the insulating
liquid container in the electrotechnical equipment. The liquid is then ejected, normally as a
spray, which may be ignited. The spray burns intensely for a short period but then forms a
pool, which may or may not be burning at the base of the electrotechnical equipment.
Experience with Class O1 insulating liquids has shown that burning of a resultant pool fire
causes most damage but no pool fires have been reported for Class K liquids.
Tests on Class K insulating liquids (known as less-flammable insulating liquids) have shown
that even if spray ignites, the resulting pool of liquid rapidly ceases to burn. This is largely due
to its high fire point. However, mineral oils (Class O1) are much more likely to continue to
burn as a pool fire. Therefore, much of the information relating to fire damage applies to
Class O1 liquids.
PCB mixtures (see 3.30 and Annex A) exhibit similar behaviour to Class K insulating liquids.
The spray and dissolved gases can ignite, even though PCB mixtures are rated as Class L.
The resulting pool will not continue to burn.
For many types of electrotechnical equipment, Class O1 insulating liquids are almost always
used for technical and/or economic reasons. Protection against fire can then be provided by
appropriate design and safe location of the electrotechnical equipment, including physical and
electrical control devices (see Annex B).
Class K insulating liquids require less stringent protective measures than Class O insulating
liquids (see Annexes A and C).
The major use of insulating liquids is in transformers. The following lists of major and minor
fire scenarios apply to transformers and in some cases to other types of electrotechnical
equipment containing insulating liquids.
Provisions shall be made for protection of people against fire effluent or other effluent from
equipment containing PCB mixtures or mineral oil contaminated by PCBs. Such equipment
shall be identified and dealt with in accordance with local regulations which may result in
decommissioning. This is important because PCBs present a toxic hazard if decomposed
thermally with or without combustion of the carrier liquid [4].
Although failures leading to a fire in electrotechnical equipment containing insulating liquids
are rare, it is evident that any equipment transmitting a high level of electrical energy and
containing significant quantities of flammable solid and/or liquid insulating materials presents
a potential fire hazard. With good protective measures, damage caused is usually small and
confined to within the container, with possible ejection of a small quantity of insulating liquid.
7.2.2 Major causes of fire
The major causes of fire in origin fire scenarios are as follows:
a) Container damage leading to a leakage of insulating liquid, possibly in the form of a liquid
spray.
b) An increase in internal container pressure due to thermal expansion under overload or to
the production of gases from the decomposition of the insulating liquid. This can result in
the release of liquid and vapours from a pressure relief valve.
c) Undetected leakage leading to a lack of circulation, resulting in overheating and a change
in liquid characteristics, eventually leading to breakdown due to arcing from exposed
conductors.
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d) A high energy arc, or arcs, between incoming HV terminations caused by high voltage
transients, lightning or a switching surge.
e) Low magnitude faults in the centre of HV windings, causing breakdown and decomposition
of the insulating liquid into flammable gaseous components.
f) Failure of protection to clear a fault, resulting in severe overheating and winding failure.
g) Tapchanger faults – failure may spread to the transformer.
h) Bushing faults in an overheated connection resulting in a cracked insulator. This can
result in the slow release of insulating liquid on to the overheated connection, which may
cause a fire if not detected.
i) Cable box faults – cable boxes may be either compound-filled or oil-filled. Failure of the
insulation may cause a phase-to-phase arc and the resulting high pressure could cause
the cable box to burst.
j) Oil-filled cable faults.
7.2.3 Minor causes of fire
The minor causes of fire in origin fire scenarios are as follows:
a) An overheated connection resulting in a cracked insulator.
b) A slow release of insulating liquid on to an overheated connection. Depending on the
combustion characteristics of the liquid, this may cause a fire if not detected.
7.2.4 Pool fires
Experience with mineral oil-filled transformers has shown that, if the transformer tank is
ruptured by a catastrophic failure caused by a high energy internal arc, the insulating liquid
can be ejected as a spray. This spray burns intensely for a short time and can itself cause
damage, but, in most recorded accidents, a considerable contribution to total fire damage was
caused by the high heat release rate from the resulting burning pool of oil. For this reason,
the possibility of a pool fire must be a matter for particular consideration.
7.2.5 Burning spray
Spray may burn intensely for only a short period of time. Pressure is limited by comparison
with e.g. hydraulic applications, because the container in most electrotechnical equipment has
only a limited pressure withstand capability.
7.2.6 Ignition on hot surface
A fault in a high current connection, external to the electrotechnical equipment, can result in a
high local temperature, possibly exceeding 500 °C. If insulating liquid leaks from the
electrotechnical equipment and runs over such an overheated surface, it may ignite. This will
be dependent on the temperature of the surface, the ignition temperature of the liquid, and the
rate of flow.
7.3 Victim fire scenarios
The electrotechnical equipment under consideration can be involved when a fire begins
externally. This could include collapse of a building causing damage to the container and
release of the insulating liquid into a pool which can ignite.
Another type of victim fire scenario is an interactive fire, which begins in adjacent associated
electrotechnical equipment, such as connecting cables, capacitors or switchgear. For example,
fire damage to connecting cables can result in a short-circuit.
Consideration shall be given to the probability that the insulating liquid can be exposed to an
external fire, whether the liquid is fully contained within the electrotechnical equipment or is
released after physical damage to the equipment. Important parameters are the ignitability of

60695-1-40 © IEC:2013 – 17 –
the insulating liquid and, if ignition occurs, the contribution to the fire hazard of heat release
and fire effluent. In a victim fire scenario, Class K (less-flammable) insulating liquids can be
heated to a higher temperature than Class O insulating liquids before they will ignite in
contact with an external flame and continue to burn.
8 Protective measures against fire
Protective measures against fire are as follows:
a) the retention of the insulating liquid within the electrotechnical equipment, allowing for
thermal expansion in service;
b) provision to retain any liquid released, by means of a sump or bund;
c) ensuring that there is a sufficient distance to the nearest building (for outdoor installations);
d) the use of fire barriers or fire compartments;
e) provision of a fire extinguisher or extinguishers actuated by excess temperature rise;
f) provision of a circuit breaker or breakers actuated by pressure relief valves;
g) provision of over-current protection; and
h) provision of fast-acting short-circuit protection.
Annex B describes these in more detail. Some are specified by regulatory or advisory bodies
with responsibility for particular geographical regions, e.g., USA, Europe and Japan.
For electrotechnical equipment installed in areas of particular fire hazard (e.g., in buildings),
less stringent measures are required in the case of less flammable liquids.
Electrotechnical equipment containing quantities of insulating liquids below a specified
minimum (usually about 4 litres) is exempted from many of the restrictions in such regulations,
even when the liquid is Class O. In a victim fire scenario, the small quantity of insulating liquid
will provide only a small addition to the fire load.
However, the
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