IEC 60695-1-20:2016

IEC 60695-1-20 ®
Edition 1.0 2016-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
BASIC SAFETY PUBLICATION
PUBLICATION FONDAMENTALE DE SÉCURITÉ
Fire hazard testing –
Part 1-20: Guidance for assessing the fire hazard of electrotechnical products –
Ignitability – General guidance

Essais relatifs aux risques du feu –
Partie 1-20: Lignes directrices pour l'évaluation des risques du feu des produits
électrotechniques – Allumabilité – Lignes directrices générales

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IEC 60695-1-20 ®
Edition 1.0 2016-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
BASIC SAFETY PUBLICATION
PUBLICATION FONDAMENTALE DE SÉCURITÉ

Fire hazard testing –
Part 1-20: Guidance for assessing the fire hazard of electrotechnical products –

Ignitability – General guidance

Essais relatifs aux risques du feu –

Partie 1-20: Lignes directrices pour l'évaluation des risques du feu des produits

électrotechniques – Allumabilité – Lignes directrices générales

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 13.220.40, 29.020 ISBN 978-2-8322-3146-3

– 2 – IEC 60695-1-20:2016 © IEC 2016
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references. 6
3 Terms and definitions . 7
4 Principles of ignitability . 12
4.1 Gases . 12
4.1.1 Overview . 12
4.1.2 Flammability limits . 12
4.1.3 Arc fires . 12
4.2 Liquids . 13
4.2.1 Overview . 13
4.2.2 Ignition parameters . 13
4.2.3 Insulating liquids . 13
4.3 Solids . 13
4.3.1 Overview . 13
4.3.2 Parameters affecting ignition . 14
4.3.3 Metals . 14
4.3.4 Carbon (graphite) and carbonaceous char . 15
4.3.5 Reactive substances . 15
4.3.6 Dust clouds . 15
5 Consideration for the selection of test methods . 15
5.1 General . 15
5.2 Fire scenario . 16
5.3 Ignition sources . 16
5.3.1 General . 16
5.3.2 Internal ignition sources . 16
5.3.3 External ignition sources . 17
5.3.4 Arc ignition of materials . 18
5.4 Types of test specimen. 19
5.5 Test procedure and apparatus . 20
6 Use and interpretation of results . 20
Annex A (informative) Examples of accidents due to arc fires in underground
hydroelectric power plants or urban substations . 21
A.1 General . 21
A.2 Examples which are generally available (non-exhaustive list) . 21
A.2.1 Underground hydroelectric power plants . 21
A.2.2 Urban substations (non-exhaustive list) . 21
Bibliography . 22

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIRE HAZARD TESTING –
Part 1-20: Guidance for assessing the
fire hazard of electrotechnical products –
Ignitability – General guidance

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
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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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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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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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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
members of its technical committees and IEC National Committees for any personal injury, property damage or
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 60695-1-20 has been prepared by IEC technical committee 89:
Fire hazard testing.
This first edition of IEC 60695-1-20 cancels and replaces the first edition of IEC TS 60695-1-
20 published in 2008. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) ISO 5660-1 has been added to the normative references;
b) definitions of “pyrolysis” and “short-circuit” have been added to Clause 3;
c) some text from the introduction has been moved to Clause 5 and is now part of the
normative text;
– 4 – IEC 60695-1-20:2016 © IEC 2016
d) Clause 5 now contains several mandatory statements.
The text of this standard is based on the following documents:
FDIS Report on voting
89/1296/FDIS 89/1302/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.
It has the status of a basic safety publication in accordance with IEC Guide 104 and
ISO/IEC Guide 51.
In this standard, the terms defined in Clause 3 are printed in bold type.
A list of all parts in the IEC 60695 series, published under the general title Fire hazard testing,
can be found on the IEC website.
The IEC 60695-1 series, under the general title Fire hazard testing, 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 procedures – 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 website 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.
INTRODUCTION
Fires are responsible for creating hazards to life and property as a result of the generation of
heat (thermal hazard), and also as a result of the production of toxic effluent, corrosive
effluent and smoke (non-thermal hazard). Fires start with ignition and then can grow, leading
in some cases to flash-over and a fully developed fire. Ignition resistance is therefore one of
the most important parameters of a material to be considered in the assessment of fire
hazard. If there is no ignition, there is no fire.
For most materials (other than metals and some other elements), ignition occurs in the gas
phase. Ignition occurs when combustible vapour, mixed with air, reaches a high enough
temperature for exothermic oxidation reactions to rapidly propagate. The ease of ignition is a
function of the chemical nature of the vapour, the fuel/air ratio and the temperature.
In the case of liquids, the combustible vapour is produced by vaporization of the liquid, and
the vaporization process is dependent on the temperature and chemical composition of the
liquid.
In the case of solids, the combustible vapour is produced by pyrolysis when the temperature
of the solid is sufficiently high. The vaporization process is dependent on the temperature and
chemical composition of the solid, and also on the thickness, density, specific heat, and
thermal conductivity of the solid.
The ease of ignition of a test specimen depends on many variables. Factors to be considered
for the assessment of ignitability are:
a) the geometry of the test specimen, including thickness and the presence of edges,
corners or joints;
b) the surface orientation;
c) the rate and direction of air flow;
d) the nature and position of the ignition source;
e) the magnitude and position of any external heat flux; and
f) whether the combustible material is a solid or a liquid.
The primary aims are to prevent ignition caused by an electrically energized component part,
and in the event of ignition, to confine any resulting fire within the bounds of the enclosure of
the electrotechnical product.
Secondary aims include the minimization of any flame spread beyond the product’s enclosure
and the minimization of harmful effects of fire effluents including heat, smoke, and toxic or
corrosive combustion products.
Fires involving electrotechnical products can also be initiated from external non-electrical
sources. Considerations of this nature are dealt with in an overall fire hazard assessment.
This international standard gives an overview of ignitability and its relevance to the fire hazard
of electrotechnical products.
– 6 – IEC 60695-1-20:2016 © IEC 2016
FIRE HAZARD TESTING –
Part 1-20: Guidance for assessing the
fire hazard of electrotechnical products –
Ignitability – General guidance

1 Scope
This part of IEC 60695 provides guidance on the ignitability of electrotechnical products and
the materials from which they are formed. It gives guidance on:
a) the principles of ignitability;
b) the selection of appropriate test methods, and
c) the use and interpretation of results.
This part of IEC 60695 is intended for use by technical committees in preparation of standards
in accordance with the principles laid down in IEC Guide 104 and ISO/IEC Guide 51.
One of the responsibilities of a technical committee is, wherever applicable, to make use of
basic safety publications in the preparation of its publications. The requirements, test methods
or test conditions of this basic safety publication will not apply unless specifically referred to
or included in the relevant 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 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 60695-1-12, Fire hazard testing – Part 1-12: Guidance for assessing the fire hazard of
electrotechnical products – Fire safety engineering
IEC TR 60695-1-21, Fire hazard testing – Part 1-21: Guidance for assessing the fire hazard of
electrotechnical products – Ignitability – Summary and relevance of test methods
IEC 60695-2-11, Fire hazard testing – Part 2-11: Glowing/hot-wire based test methods –
Glow-wire flammability test method for end-products (GWEPT)
IEC 60695-2-12, Fire hazard testing – Part 2-12: Glowing/hot-wire based test methods –
Glow-wire flammability index (GWFI) test method for materials
IEC 60695-2-13, Fire hazard testing – Part 2-13: Glowing/hot-wire based test methods –
Glow-wire ignition temperature (GWIT) test method for materials

IEC 60695-4:2012, Fire hazard testing – Part 4: Terminology concerning fire tests for
electrotechnical products
IEC 60695-11-5, Fire hazard testing – Part 11-5: Test flames – Needle-flame test method –
Apparatus, confirmatory test arrangement and guidance
IEC 60695-11-10, Fire hazard testing – Part 11-10: Test flames – 50 W horizontal and vertical
flame test methods
IEC TS 60695-11-11, Fire hazard testing – Part 11-11: Test flames – Determination of the
characteristic heat flux for ignition from a non-contacting flame source
IEC 60695-11-20, Fire hazard testing – Part 11-20: Test flames – 500 W flame test methods
IEC Guide 104, The preparation of safety publications and the use of basic safety publications
and group safety publications
ISO/IEC Guide 51, Safety aspects – Guidelines for their inclusion in standards
ISO 13943, Fire safety – Vocabulary
ISO 871:2006, Plastics – Determination of ignition temperature using a hot-air furnace
ISO 2592, Determination of flash and fire points – Cleveland open cup method
ISO 2719, Determination of flash point – Pensky-Martens closed cup method
ISO 5657, Reaction to fire tests – Ignitability of building products using a radiant heat source
ISO 5660-1, Reaction to fire tests – Heat release smoke production and mass loss rate –
Part 1: Heat release rate (cone calorimeter method)
ISO 10840, Plastics – Guidance for the use of standard fire tests
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 13943:2008 and
IEC 60695-4:2012 (some of which are reproduced below), as well as the following, apply.
3.1
auto-ignition
spontaneous ignition
self-ignition CA, US
unpiloted ignition CA, US
DEPRECATED: spontaneous combustion
ignition (3.20) resulting from a rise in temperature without a separate ignition source (3.22)
Note 1 to entry: The ignition can be caused either by self-heating or by heating from an external source.
Note 2 to entry: In North America, “spontaneous ignition” is the preferred term used to designate ignition caused
by self-heating.
[SOURCE: ISO 13943:2008, 4.18]

– 8 – IEC 60695-1-20:2016 © IEC 2016
3.2
auto-ignition temperature
spontaneous ignition temperature
minimum temperature at which auto-ignition (3.1) is obtained in a fire test
Note 1 to entry: The typical units are degrees Celsius (°C).
[SOURCE: ISO 13943:2008, 4.19]
3.3
combustion
exothermic reaction of a substance with an oxidizing agent
Note 1 to entry: Combustion generally emits fire effluent accompanied by flames (3.11) and/or glowing.
[SOURCE: ISO 13943:2008, 4.46]
3.4
fire 〈general〉
process of combustion (3.3) characterized by the emission of heat and fire effluent and
usually accompanied by smoke, flame (3.11), 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
(3.5) and fire (3.6), 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]
3.5
fire 〈controlled〉
self-supporting combustion (3.3) that has been deliberately arranged to provide useful
effects and is limited in its extent in time and space
[SOURCE: ISO 13943:2008, 4.97]
3.6
fire 〈uncontrolled〉
self-supporting combustion (3.3) that has not been deliberately arranged to provide useful
effects and is not limited in its extent in time and space
[SOURCE: ISO 13943:2008, 4.98]
3.7
fire hazard
physical object or condition with a potential for an undesirable consequence from fire (3.4)
[SOURCE: ISO 13943:2008, 4.112]
3.8
fire point
minimum temperature at which a material ignites and continues to burn for a specified time
after a standardized small flame (3.11) has been applied to its surface under specified
conditions
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).

Note 3 to entry: See flash point (3.16)
[SOURCE: ISO 13943:2008, 4.119]
3.9
fire retardant, noun
substance added, or a treatment applied, to a material in order to delay ignition (3.20) or to
reduce the rate of combustion (3.3)
Note 1 to entry: The use of (a) fire retardant(s) does not necessarily suppress fire (3.4) or terminate combustion
(3.3).
Note 2 to entry: See flame retardant (3.12)
[SOURCE: ISO 13943:2008, 4.123]
3.10
fire scenario
qualitative description of the course of a fire (3.6) 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 (3.20) and fire growth processes, the fully developed fire (3.17)
stage, the fire decay stage, and the environment and systems that impact on the course of the fire.
[SOURCE: ISO 13943:2008, 4.129]
3.11
flame, noun
zone in which there is rapid, self-sustaining, sub-sonic propagation of combustion (3.3) in a
gaseous medium, usually with emission of light
[SOURCE: ISO 13943:2008, 4.133 – modified – The words "zone in which there is" have been
added at the beginning of the definition.]
3.12
flame retardant, noun
substance added, or a treatment applied, to a material in order to suppress or delay the
appearance of a flame (3.11) and/or reduce the flame-spread rate
Note 1 to entry: The use of (a) flame retardant(s) does not necessarily suppress fire (3.6) or terminate
combustion (3.3).
Note 2 to entry: See fire retardant (3.9).
[SOURCE: ISO 13943:2008, 4.139]
3.13
flaming combustion
combustion (3.3) in the gaseous phase, usually with emission of light
[SOURCE: ISO 13943:2008, 4.148]
3.14
flash-ignition temperature
FIT
minimum temperature at which, under specified test conditions, sufficient flammable gases
are emitted to ignite momentarily on application of a pilot flame (3.11)
Note 1 to entry: This note applies to the French language only.

– 10 – IEC 60695-1-20:2016 © IEC 2016
[SOURCE: ISO 871:2006, 3.1]
3.15
flashover, 〈stage of fire〉
transition to a state of total surface involvement in a fire (3.4) of combustible materials within
an enclosure
[SOURCE: ISO 13943:2008, 4.156]
3.16
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 (3.11) under specified test conditions
Note 1 to entry: The typical units are degrees Celsius (˚C).
[SOURCE: ISO 13943:2008, 4.154]
3.17
fully developed fire
state of total involvement of combustible materials in a fire (3.6)
[SOURCE: ISO 13943:2008, 4.164]
3.18
glowing combustion
combustion (3.3) of a material in the solid phase without flame (3.11) but with emission of
light from the combustion zone
[SOURCE: ISO 13943:2008, 4.169]
3.19
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]
3.20
ignition, 〈general〉
DEPRECATED: sustained ignition
initiation of combustion (3.3)
[SOURCE: ISO 13943:2008, 4.187]
3.21
ignition, 〈flaming combustion〉
DEPRECATED: sustained ignition
initiation of sustained flame (3.11)
[SOURCE: ISO 13943:2008, 4.188]
3.22
ignition source
source of energy that initiates combustion (3.3)

[SOURCE: ISO 13943:2008, 4.189]
3.23
lower flammability limit
LFL
minimum concentration of fuel vapour in air below which propagation of a flame (3.11) does
not occur in the presence of an ignition source (3.22)
Note 1 to entry: The concentration is usually expressed as a volume fraction at a defined temperature and
pressure, and expressed as a percentage.
[SOURCE: ISO 13943:2008, 4.216]
3.24
minimum ignition temperature
ignition point
minimum temperature at which sustained combustion (3.3) can be initiated under specified
test conditions
Note 1 to entry: The minimum ignition temperature implies the application of a thermal stress for an infinite length
of time.
Note 2 to entry: The typical units are degrees Celsius (°C).
[SOURCE: ISO 13943:2008, 4.231]
3.25
pyrolysis
chemical decomposition of a substance by the action of heat
Note 1 to entry: Pyrolysis is often used to refer to a stage of fire (3.4) before flaming combustion (3.13) has
begun.
Note 2 to entry: In fire science, no assumption is made about the presence or absence of oxygen.
[SOURCE: ISO 13943:2008, 4.266]
3.26
short-circuit
unintended connection of two nodes of an electrical circuit
Note 1 to entry: Current flow might occur, which could cause circuit damage, overheating, fire or explosion.
3.27
spontaneous-ignition temperature
SIT
minimum temperature at which, under specified test conditions, ignition (3.20), is obtained by
heating, in the absence of any additional ignition source (3.22)
[SOURCE: ISO 871:2006, 3.2]
3.28
thermal inertia
product of thermal conductivity, density and specific heat capacity
8 2 -1 -4 -2
EXAMPLES The thermal inertia of steel is 2,3 × 10 J ⋅s ⋅m ⋅K . The thermal inertia of polystyrene foam is
3 2 -1 -4 -2
1,4 × 10 J ⋅s ⋅m ⋅K .
Note 1 to entry: When a material is exposed to a heat flux, the rate of increase of surface temperature depends
strongly on the value of the thermal inertia of the material. The surface temperature of a material with a low thermal
inertia rises relatively quickly when it is heated, and vice versa.

– 12 – IEC 60695-1-20:2016 © IEC 2016
Note 2 to entry: The typical units are joules squared per second per metre to the fourth power per Kelvin squared
2 -1 -4 -2
(J ⋅s ⋅m ⋅K ).
[SOURCE: ISO 13943:2008, 4.326]
3.29
upper flammability limit
UFL
maximum concentration of fuel vapour in air above which propagation of a flame (3.11) will
not occur in the presence of an ignition source (3.22)
Note 1 to entry: The concentration is usually expressed as a volume fraction at a defined temperature and
pressure, and expressed as a percentage.
[SOURCE: ISO 13943:2008, 4.349]
4 Principles of ignitability
4.1 Gases
4.1.1 Overview
Ignition of a gas depends on how the gas is mixed with air. If the gas is mixed with air before
ignition, the subsequent reaction is known as premixed combustion. In a burner, the
combustion is controlled, but if a large volume of a gas/air mixture is ignited, a gas explosion
results.
In most fires, ignition results in the development of diffusion flames where combustible gas
comes in contact with air without being previously mixed.
Gas mixtures can be ignited in two basic ways:
a) auto-ignition – where the temperature of all the gas mixture is raised, and
b) piloted ignition – where a local source of heat is introduced, e.g. a flame or an electrical
spark.
Some fires are the result of the ignition of a material which is already in the gaseous state,
but combustible gases can also be produced by the vaporization of liquids (see 4.2) or by the
pyrolysis of solids (see 4.3).
4.1.2 Flammability limits
Flame propagation cannot occur in a fuel/air gas mixture if the fuel concentration is too low or
too high. The limiting concentration values are known as the lower flammability limit (LFL) and
the upper flammability limit (UFL). These limits arise because flames need a minimum
temperature to exist. Too much air or fuel prevents the temperature being maintained at a
sufficiently high level. Flammability limits are normally expressed as the percentage of fuel, by
volume, in the fuel/air mixture.
4.1.3 Arc fires
Faults in some electrical equipment such as junction boxes and power transformers can result
in disruptive electrical discharges (electric arcs) which can pyrolyse insulation materials to
produce high temperature combustible gases. Such gases expand rapidly and in contact with
air can result in an explosion (see 5.3.4.4).

4.2 Liquids
4.2.1 Overview
With the exception of some unstable or reactive substances, bulk liquids do not generally
ignite. Normally it is combustible vapour which ignites. The combustible vapour is produced
by vaporization of the liquid, and the vaporization process is dependent on the temperature
and chemical composition of the liquid.
4.2.2 Ignition parameters
Temperature is normally used to define the ignitability of a liquid. Three different temperatures
are used. These are the auto ignition temperature (see 3.2), the fire point (see 3.8) and the
flash point (see 3.16). Auto-ignition refers to ignition in the absence of a localized heat
source. Flash point concerns momentary ignition. Fire point concerns sustained combustion
after ignition.
Several different test methods are used to measure these characteristic temperatures. The
measured temperature depends on the particular details of the test apparatus used. It is
therefore important to define the test method when quoting these parameters.
4.2.3 Insulating liquids
4.2.3.1 Flash point measurement
ISO 2719 (Pensky-Martens closed cup method) is cited in IEC standards for the measurement
of the flash point of insulating liquids. It measures the flash point in a confined space and is
intended to detect minor amounts of volatile material. An alternative method is ISO 2592
(Cleveland open cup method) which is used to measure the flash point over an open liquid
surface. The flash point measured by ISO 2592 is significantly lower than that measured by
ISO 2719.
4.2.3.2 Cone calorimeter measurements
IEC 60695-8-3 was developed to measure the quantity of heat released from burning
insulating liquids. The test specimen is exposed to a uniform heat flux in the presence of a
spark ignition source. Ignition related properties can be defined as the time to ignition at a
specified heat flux, or the minimum incident heat flux that will support ignition.
NOTE IEC 60695-8-3 is to be withdrawn. The ISO intends to develop a related test method with a wider scope.
4.3 Solids
4.3.1 Overview
With some exceptions (see below) solids do not generally ignite. Normally, the material that
ignites is in the gas phase and can be a mixture of gases, aerosols and suspended particles.
The combustible vapour is produced by pyrolysis of the solid, and the vaporization process is
dependent on the temperature and chemical composition of the solid.
The exceptions to this general statement are:
– metals (see 4.3.3);
– some non-metallic elements, for example carbon (see 4.3.4), sulphur and phosphorous;
– certain reactive substances (see 4.3.5); and
– dust clouds (see 4.3.6).
– 14 – IEC 60695-1-20:2016 © IEC 2016
4.3.2 Parameters affecting ignition
In the case of a solid, the generation of flammable volatiles from the material is function of the
temperature of that material. This is affected by the nature of the heat input which may be, for
example, a radiant heat flux, a convective heat flux, a conductive heat flux, an imposed flame,
a hot wire or a combination of these sources.
The ease of ignition will also depend on the chemical nature of the flammable volatiles, which
in turn will depend on the chemical nature of the solid.
The rate of heating of the material is a function of a number of properties of the solid:
a) thickness;
b) thermal conductivity, (k);
c) density, (ρ);
d) specific heat, (c);
e) absorptivity (in the case of radiative heating).
In a thick test specimen, material below the surface is able to conduct heat away thus
reducing the rate of surface heating and increasing the resistance to ignition. In a thin
specimen, conduction of heat from the surface is negligible, and so resistance to ignition is
lower.
Thermoplastic materials have a tendency to melt away from the heat source (e.g. flame or
hot-wire) often resulting in non-ignition. Because of this behaviour special consideration
should be given to the testing of the ignitability of thermoplastics. The problems that can arise
when thermoplastics are tested in standard fire tests are discussed in ISO 10840.
The product, k ⋅ ρ ⋅ c, is known as 'thermal inertia'. If the thermal inertia is high, for example as
in the case of a solid metal, the rate of surface heating will be relatively low and it will
therefore take a relatively long time for the ignition temperature to be reached. If the thermal
inertia is low, e.g. as in the case of some foamed plastics or low density combustible
materials, the rate of surface heating will be relatively high and it will therefore take a
relatively short time for the ignition temperature to be reached.
After ignition of the test specimen, flame propagation will occur if the flame transfers sufficient
heat flux, mostly as thermal radiation, ahead of the pyrolysis front so as to continue pyrolysis
and ignition at a sufficient rate.
The magnitude of the heat flux transferred ahead of the pyrolysis front depends on the heat
release rate of the test specimen and on whether there is a continuing imposed heat flux,
whereas the resistance to ignition is a function of the minimum ignition temperature of the test
specimen and the rate of heating of the surface.
4.3.3 Metals
When a metal burns in air the product of combustion is the metal oxide. Many metals have a
film of metal oxide on the surface which is formed from low temperature oxidation. The oxide
film cannot burn because it is already the product of the metal's oxidation and so before the
bulk metal can burn, the surface layer of oxide must be removed in some way.
Metals can be classified into three groups with respect to their ignition characteristics.
a) Metals that ignite at or below their melting point (for example iron and magnesium). These
metals all have melting points above 650 °C. These metals generally do not form a
protective oxide layer.
b) Metals that ignite after they melt (for example aluminium, lead, tin and zinc). These metals
all have melting points below 660 °C. These metals generally form a protective oxide
layer.
c) Metals of low reactivity which do not ignite (for example mercury, silver, gold and
platinum).
The ease of ignition is also governed by the surface area/volume ratio of the metal. Thin films
of metal and finely divided powders are much easier to ignite than bulk pieces of metal. This
is because the heat released by the oxidation process is proportional to the burning surface
area, whereas the initial removal of heat from the surface by conduction is proportional to the
volume of the metal.
4.3.4 Carbon (graphite) and carbonaceous char
4.3.4.1 Graphite
Pure carbon in the form of graphite can ignite in air above a temperature of about 800 °C. In
the range 800 °C to 1 200 °C, non-flaming surface combustion (glowing combustion) is found
to occur. Above about 1 200 °C flaming combustion occurs with a CO flame being observed.
4.3.4.2 Carbonaceous char
Carbonaceous chars are impure forms of carbon. Volatile content and porosity are two
important variables which contribute to the wide range of observed ignition temperatures. As
with graphite both flaming combustion and non-flaming combustion may be observed. Many
carbon-containing materials tend to form a carbonaceous char on their surface when they
burn, and at the early stages of fire this char layer can, to some extent, protect the underlying
material. A correlation has been observed between ignition resistance, as measured by
limiting oxygen index, and char yield for a range of organic polymers [1].
4.3.5 Reactive substances
In most fires the oxidising agent is the oxygen in air. However, in some materials the oxidising
agent, usually oxygen, is part of the molecular structure of the material or is mixed with the
solid fuel in the form of a solid oxidising agent. These materials are usually deliberately made
to be combustible or explosive. Some examples are:
– “blue touch paper” (cellulose and potassium nitrate);
– gunpowder (carbon, sulphur and potassium nitrate);
– cigarettes (tobacco and potassium nitrate);
– TNT (trinitrotoluene).
4.3.6 Dust clouds
Dust clouds are mixtures of air (or some gas or gases) and fine solid particles which are
microscopically dispersed in it, and their ignition behaviour is more like that of a premixed gas
than that of a solid.
5 Consideration for the selection of test methods
5.1 General
Important factors to be considered when selecting the test method to be used include; the fire
scenario or scenarios of concern, the possible ignition sources, the type of test specimen, and
the type of test procedure and apparatus.
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Numbers in square brackets refer to the bibliography.

– 16 – IEC 60695-1-20:2016 © IEC 2016
IEC TR 60695-1-21 gives a summary and relevance of test methods associated with
ignitability.
5.2 Fire scenario
In the design o
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