IEC 60695-8-1:2008

IEC 60695-8-1
Edition 2.0 2008-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
BASIC SAFETY PUBLICATION
PUBLICATION FONDAMENTALE DE SÉCURITÉ
Fire hazard testing –
Part 8-1: Heat release – General guidance

Essais relatifs aux risques du feu –
Partie 8-1: Dégagement de chaleur – Guide général

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IEC 60695-8-1
Edition 2.0 2008-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
BASIC SAFETY PUBLICATION
PUBLICATION FONDAMENTALE DE SÉCURITÉ
Fire hazard testing –
Part 8-1: Heat release – General guidance

Essais relatifs aux risques du feu –
Partie 8-1: Dégagement de chaleur – Guide général

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
T
CODE PRIX
ICS 13.220.40; 29.020 ISBN 2-8318-9646-0

– 2 – 60695-8-1 © IEC:2008
CONTENTS
FOREWORD.4
INTRODUCTION.6

1 Scope.7
2 Normative references.7
3 Terms and definitions .7
4 Principles of determining heat release.10
4.1 Complete combustion measured by the oxygen bomb calorimeter (ISO 1716) .10
4.2 Incomplete combustion .11
4.2.1 Measurement techniques .11
4.2.2 Heat release by oxygen consumption .11
4.2.3 Heat release by carbon dioxide generation .12
4.2.4 Heat release by increase of gas temperature.12
5 Parameters used to report heat release data.14
5.1 Heat of combustion (gross and net) .14
5.2 Heat release rate.14
5.3 Heat release.15
5.4 Heat release rate per unit area.15
5.5 Total heat release.16
5.6 Peak heat release rate.16
5.7 Time to peak heat release rate .16
5.8 Effective heat of combustion.16
5.8.1 Measurement and calculation.16
5.8.2 Examples.17
5.9 FIGRA index.18
5.10 ARHE and MARHE .19
6 Considerations for the selection of test methods .21
6.1 Ignition sources .21
6.2 Type of test specimen.21
6.3 Choice of conditions .21
6.4 Test apparatus .21
6.4.1 Small-scale fire test apparatus .21
6.4.2 Intermediate and large-scale fire test apparatus .22
6.4.3 Comparison between small-scale and intermediate/large-scale fire
test methods.22
7 Relevance of heat release data.22
7.1 Contribution to fire hazard.22
7.2 Secondary ignition and flame spread.22
7.3 Determination of self-propagating fire thresholds .22
7.4 Probability of reaching flash-over .23
7.5 Smoke and toxic gas production .23
7.6 The role of heat release testing in research and development .23

Bibliography .24

Figure 1 – Heat release rate (HRR) curve.15

60695-8-1 © IEC:2008 – 3 –
Figure 2 – Heat release (HR) curve .15
Figure 3 – Heat release rate (HRR*) per unit area curve .16
Figure 4 – Mass loss curve.17
Figure 5 – FIGRA curve derived from Figure 1.18
Figure 6 – Illustrative HRR curve .19
Figure 7 – FIGRA curve derived from Figure 6.19
Figure 8 – ARHE curve derived from Figure 1.20
Figure 9 – ARHE curve derived from Figure 6.20

Table 1 – Heat of combustion.13
-1
Table 1a – Relationship between heat of combustion expressed in units of kJ·g of fuel
-1
burned and kJ·g of oxygen consumed for a variety of fuels .13
-1
Table 1b – Relationship between heat of combustion expressed in units of kJ·g of fuel
-1
burned and kJ·g of oxygen consumed for a variety of insulating liquids .14

– 4 – 60695-8-1 © IEC:2008
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIRE HAZARD TESTING –
Part 8-1: Heat release –
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
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
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
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
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 60695-8-1 has been prepared by IEC technical committee 89: Fire
hazard testing.
This second edition cancels and replaces the first edition, published in 2001 and constitutes a
technical revision.
The main changes with respect to the first edition are listed below:
− editorial changes throughout;
− revised terms and definitions;
− new text concerning bomb calorimetry;
− revised Table 1a;
− new Clause 5 – Parameters used to report heat release data;
− introduction of intermediate scale fire test.

60695-8-1 © IEC:2008 – 5 –
The text of this standard is based on the following documents:
FDIS Report on voting
89/856/FDIS 89/863/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 standard is to be used in conjunction with IEC 60695-8-2.
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.
A list of all the parts in the IEC 60695 series, under the general title Fire hazard testing, can be
found on the IEC website.
Part 8 consists of the following parts:
Part 8-1: Heat release – General guidance
Part 8-2: Heat release – Summary of test methods
Part 8-3: Heat release – Heat release of insulating liquids used in electrotechnical products
The committee has decided that the contents of this publication will remain unchanged until the
maintenance result 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 – 60695-8-1 © IEC:2008
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 equipment 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. The
)
future IEC 60695-1-10 [1] , together with its companion the future IEC 60695-1-11 [2] provide
guidance on how this is to be accomplished.
The primary aims are as follows:
1) to prevent ignition caused by an electrically energized component part, and
2) 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 the overall risk assessment.
Fires are responsible for creating hazards to life and property as a result of the generation of
heat (thermal hazard), toxic and/or corrosive compounds and obscuration of vision due to
smoke. Fire risk increases as the heat released increases, possibly leading to a flash-over fire.
One of the most important measurements in fire testing is the measurement of heat release,
and it is used as an important factor in the determination of fire hazard; it is also used as one
of the parameters in fire safety engineering calculations.
The measurement and use of heat release data, together with other fire test data, can be used
to reduce the likelihood of (or the effects of) fire, even in the event of foreseeable abnormal
use, malfunction or failure of electrotechnical products.
When a material is heated by some external source, fire effluent can be generated and can
form a mixture with air, which can ignite and initiate a fire. The heat released in the process is
carried away by the fire effluent-air mixture, radiatively lost or transferred back to the solid
material, to generate further pyrolysis products, thus continuing the process.
Heat may also be transferred to other nearby products, which may burn, and then release
additional heat and fire effluent.
The rate at which thermal energy is released in a fire is defined as the heat release rate. Heat
release rate is important because of its influence on flame spread and on the initiation of
secondary fires. Other characteristics are also important, such as ignitability, flame spread and
the side-effects of the fire (see the IEC 60695 series of standards).
___________
1)
Figures in square brackets refer to the bibliography.

60695-8-1 © IEC:2008 – 7 –
FIRE HAZARD TESTING –
Part 8-1: Heat release –
General guidance
1 Scope
This part of IEC 60695 provides guidance on the measurement and interpretation of heat
release from electrotechnical products and materials from which they are constructed.
Heat release data can be used as part of fire hazard assessment and in fire safety engineering,

as described in the future IEC 60695-1-10 [1] and the future IEC 60695-1-11[2].
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 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 referenced documents are indispensable for the application of this document. For
dated references, only the edition cited applies. For undated references, the latest edition of
the referenced document (including any amendments) applies.
IEC 60695 (all parts), Fire hazard testing
IEC/TS 60695-8-2, Fire hazard testing – Part 8-2: Heat release – Summary and relevance of
test methods.
IEC Guide 104:1997, The preparation of safety publications and the use of basic safety
publications and group safety publications.
ISO 1716, Reaction to fire tests for building products – Determination of the heat of
combustion.
ISO/IEC Guide 51:1999, Safety aspects – Guidelines for their inclusion in standards.
ISO/IEC 13943:2000, Fire safety – Vocabulary
EN 13823, Reaction to fire tests for building products – Building products, excluding floorings,
exposed to thermal attack by a single burning item.
3 Terms and definitions
For the purposes of this document, the following definitions apply.

– 8 – 60695-8-1 © IEC:2008
3.1
combustion
exothermic reaction of a substance with an oxidizer
NOTE Combustion generally emits effluent accompanied by flames and/or visible light.
[ISO/IEC 13943: 2000, definition 23]
3.2
combustion products
solid, liquid and gaseous material resulting from combustion
NOTE Combustion products may include fire effluent, ash, char, clinker and/or soot.
3.3
complete combustion
combustion in which all the combustion products are fully oxidized
NOTE 1 This means that, when the oxidizing agent is oxygen, all carbon is converted to carbon dioxide and all
hydrogen is converted to water.
NOTE 2 If elements other than carbon, hydrogen and oxygen are involved in the combustion process then it may
not be possible to uniquely define complete combustion.
3.4
controlled fire
fire which has been deliberately arranged to provide useful effects and which is controlled in its
extent in time and space
[ISO/IEC 13943:2000, definition 40, modified]

3.5
effective heat of combustion
heat released from a burning test specimen in a given time interval divided by the mass lost
from the test specimen in the same time period
NOTE 1 The value is the same as the net heat of combustion if the entire test specimen is converted to volatile
combustion products and if all the combustion products are fully oxidized.
-1
NOTE 2 The typical units are kJ·g .
3.6
fire
process of combustion characterized by the emission of heat and fire effluent accompanied by
smoke, and/or flame, and/or glowing
3.7
fire effluent
totality of gases and/or aerosols (including suspended particles) created by combustion or
pyrolysis
[ISO/IEC 13943:2000, definition 45]
3.8
fire hazard
physical object or condition with a potential for an undesirable consequence from fire

60695-8-1 © IEC:2008 – 9 –
3.9
fire safety engineering
application of engineering methods based on scientific principles to the development or
assessment of designs in the built environment through the analysis of specific fire scenarios
or through the quantification of risk for a group of fire scenarios
3.10
fire test
procedure designed to measure or assess either fire behaviour or the response of a test
specimen to one or more aspects of fire
3.11
flash-over
transition to a state of total surface involvement in a fire of combustible materials within an
enclosure
[ISO/IEC 13943: 2000, definition 77]
3.12
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
[ISO/IEC 13943: 2000, definition 86.2]
3.13
heat of combustion
thermal energy produced by combustion of unit mass of a given substance
-1
NOTE The typical units are kJ·g .
See also 3.3, 3.5, 3.12 and 3.18.
3.14
heat release
thermal energy produced by combustion
NOTE The typical units are joules.
3.15
heat release rate
rate of thermal energy production generated by combustion
NOTE The typical units are watts.
3.16
intermediate-scale fire test
fire test performed on a test specimen of medium dimensions
NOTE This definition usually applies to a fire test performed on a test specimen of which the maximum dimension
is between 1 m and 3 m.
3.17
large-scale fire test
fire test, which cannot be carried out in a typical laboratory chamber, performed on a test
specimen of large dimensions
NOTE This definition usually applies to a fire test performed on a test specimen of which the maximum dimension
is greater than 3 m.
– 10 – 60695-8-1 © IEC:2008
3.18
net heat of combustion
heat of combustion when any water produced is considered to be in the gaseous state
NOTE The net heat of combustion is always smaller than the gross heat of combustion because the heat released
by the condensation of the water vapour is not included.
3.19
oxidation
chemical reaction in which the proportion of oxygen or other electronegative element in a
substance is increased
NOTE In chemistry, the term has the broader meaning of a process which involves the loss of an electron or
electrons from an atom, molecule or ion.
3.20
oxidizing agent
substance capable of causing oxidation
NOTE Combustion is an oxidation.
3.21
oxygen consumption principle
proportional relationship between the mass of oxygen consumed during combustion and the
heat released
-1
NOTE A value of 13,1 kJ·g is commonly used.
3.22
pyrolysis
chemical decomposition of a substance by the action of heat
NOTE 1 The term is often used to refer to a stage of fire before flaming combustion has occurred.
NOTE 2 In fire science no assumption is made about the presence or absence of oxygen.
3.23
small-scale fire test
fire test performed on a test specimen of small dimensions
NOTE This definition usually applies to a fire test performed on a test specimen of which the maximum dimension
is less than 1 m.
3.24
test specimen
item subjected to a procedure of assessment or measurement
NOTE In a fire test the item may be a material, product, component, element of construction, or any combination
of these. It may also be a sensor which is used to simulate the behaviour of a product.
3.25
uncontrolled fire
fire which spreads uncontrolled in time and space
4 Principles of determining heat release
4.1 Complete combustion measured by the oxygen bomb calorimeter (ISO 1716)
The most important device for measuring heats of combustion is the adiabatic constant volume
bomb calorimeter. The “bomb” is a central vessel which is sufficiently strong to withstand high
pressures so that its internal volume remains constant. The bomb is immersed in a stirred

60695-8-1 © IEC:2008 – 11 –
water bath, and the combination of bomb and water bath is the calorimeter. The calorimeter is
also immersed in an outer water bath. During a combustion reaction, the temperature of the
water in the calorimeter and in the outer water bath is continuously monitored and adjusted by
electrical heating to the same value. This is to ensure that there is no net loss of heat from the
calorimeter to its surroundings, i.e. to ensure that the calorimeter is adiabatic.
To carry out a measurement, a known mass of sample is placed inside the bomb in contact
with an electrical ignition wire. The vessel is filled with oxygen under pressure, sealed and
allowed to attain thermal equilibrium. The sample is then ignited using a measured input of
energy. Combustion is complete because it takes place in an excess of high pressure oxygen.
The heat released is calculated from the known heat capacity of the calorimeter and the rise of
temperature which occurs as a result of the combustion reaction.
The experiment gives the heat released at constant volume, i.e. the change in internal energy,
ΔU. The gross heat of combustion is the enthalpy change, ΔH, where:
ΔH = ΔU + Δ()PV
where Δ(PV) is calculated using the ideal gas law;
Δ(PV ) = Δ()nRT
Bomb calorimeter measurement of the heat of combustion of building products is described in
ISO 1716.
4.2 Incomplete combustion
4.2.1 Measurement techniques
Combustion in fires, which usually occur in air and at atmospheric pressure, is almost always
incomplete and therefore the heat released will be less than the combined heats of combustion
of the materials involved.
The heat released can be determined indirectly using one of the following techniques:
a) oxygen consumption;
b) carbon dioxide generation;
c) gas temperature increase.
4.2.2 Heat release by oxygen consumption
For a large number of organic fuels, a more or less constant amount of heat is released per
-1
unit of oxygen consumed [4], [5]. The average value for this constant is 13,1 kJ·g of oxygen
and this value is widely used for practical applications both in small-scale and large-scale
testing. This relationship implies that it is sufficient to measure the oxygen consumed in a
combustion system, and the mass flow rate in the exhaust duct in order to determine heat
release.
Table 1a lists some net heat of combustion values [5]. With the exception of three materials:
ethene, ethyne and poly(oxymethylene), all the calculated heats of combustion per gram of
oxygen consumed lie between 12,5 kJ and 13,6 kJ. The values in Table 1a are calculated

assuming complete combustion. However, Huggett [5] does discuss the effects of possible
incomplete combustion and calculates values of ΔHc for several such cases. For example, in
the case of cellulose burning to give a 9:1 ratio of CO2 to CO:

-1
(C H O ) + 5,7 O → 5,4 CO + 0,6 CO + 5 H O  ΔH = –13,37 kJ·g of O
c
6 10 5 2 2 2 2
– 12 – 60695-8-1 © IEC:2008
or burning to give an appreciable amount of carbonaceous char:
-1
(C H O ) + 3 O → 3 CO + 3 C + 5 H O ΔH = –13,91 kJ·g of O
c
6 10 5 2 2 2 2
compared with complete combustion:
-1
(C H O ) + 6 O → 6 CO + 5 H O ΔH = –13,59 kJ·g of O
6 10 5 2 2 2 c 2
Huggett discusses several other examples and concludes that the assumption of a constant
heat release per unit of oxygen consumed will be sufficiently accurate for most applications.
Of course, if the correct value of ΔH per gram of O is known for a particular material then this
c
should be used instead of the approximate value [6].
4.2.3 Heat release by carbon dioxide generation
This technique is based on the concept that the energy released in a combustion reaction is
approximately proportional to the amount of carbon dioxide generated, provided that
combustion is complete or nearly complete (i.e. with very small CO/CO ratios). The average
-1
value for the proportionality constant is 13,3 kJ·g of carbon dioxide generated. If a more
accurate value is known for the material or product, it should be used in calculating heat
release.
In general, heat release values determined by carbon dioxide generation agree well with heat
release rate values determined by oxygen consumption.
4.2.4 Heat release by increase of gas temperature
The gas temperature technique is based on the assumptions that there are no heat losses and
that all the heat generated by the fire is used to increase the temperature of the hot flowing
mixture of air and fire effluent, and that their temperatures can be determined downstream
from the flaming zone. If the heat losses, mainly from thermal radiation, are negligible, then the
gas temperature rise technique (also called the thermopile technique) would represent the
same heat release value as the oxygen consumption or the carbon dioxide generation method.
The heat release is determined by measuring the increase in the temperature of the gases, at
the thermopile, with respect to a reference temperature, generally the ambient temperature.
This is converted to heat release by means of measurements of the total flow of the air and fire
effluent mixture using the specific heat of the mixture at the appropriate air temperature, or
simply by calibration with a constant flow of a material of well-known heat release, such as
methane.
In general, heat release values determined by temperature measurement are lower than heat
release values determined by oxygen consumption or carbon dioxide generation calorimetry
techniques, because heat losses are generally not negligible. In a small-scale test, these heat
losses can, with care, be minimized by attempting to make the system as adiabatic as possible.

60695-8-1 © IEC:2008 – 13 –
Table 1 – Heat of combustion for fuels and insulating liquids
-1
Table 1a – Relationship between heat of combustion expressed in units of kJ·g
-1
of fuel burned and kJ·g of oxygen consumed for a variety of fuels
a
ΔH
c
Fuel Formula
-1 -1
kJ·g of fuel kJ·g of O
Methane (g) CH 50,0 12,5
Ethane (g) C H 47,5 12,7
2 6
Butane (g) C H 45,7 12,8
4 10
Octane (l) C H 44,4 12,7
8 18
Ethene (g) C H 47,1 13,8
2 4
Ethyne (g) C H 48,2 15,7
2 2
Benzene (l) C H 40,1 13,1
6 6
Polyethylene –(–C H –)– 43,3 12,6
2 4 n
Polypropylene –(–C H –)– 43,3 12,7
3 6 n
Polyisobutylene –(–C H –)– 43,7 12,8
4 8 n
Polybutadiene –(–C H –)– 42,7 13,1
4 6 n
Polystyrene –(–C H –)– 39,8 13,0
8 8 n
PVC –(–CH CHCl–) – 16 12
2 n
PMMA –(–C H O –) – 24,9 12,9
5 8 2 n
PAN –(–C H N–) – 30,8 13,6
3 3 n
Polyoxymethylene –(–CH O–) – 15,4 14,5
n
PET –(–C H O –) – 22,0 13,2
10 8 4 n
Polycarbonate –(–C H O –) – 29,7 13,1
16 14 3 n
Cellulose triacetate –(–C H O –) – 17,6 13,2
12 16 8 n
Nylon 66 –(–C H NO–) – 29,5 12,6
6 11 n
Cellulose –(–C H O –) – 16 13
6 10 5 n
Cotton – 15,5 13,6
Paper (newsprint) – 18,4 13,4
Wood (maple) – 17,7 12,5
Lignite – 24,8 13,1
Coal (bituminous) – 35,2 13,5
NOTE 1 (g) = gas, (l) = liquid.
NOTE 2 Most of the values in column 3 are calculated from thermodynamic data. The values in column 4
are calculated from those in column 3 assuming complete combustion.
NOTE 3 For values calculated from thermodynamic data, carbon is assumed to be converted to carbon
dioxide, hydrogen to water, nitrogen to nitrogen dioxide and chlorine to hydrogen chloride.
a
Reactants and products at 25 °C, all products gaseous.

– 14 – 60695-8-1 © IEC:2008
-1
Table 1b – Relationship between heat of combustion expressed in units of kJ·g
-1
of fuel burned and kJ·g of oxygen consumed for a variety of insulating liquids
a
ΔH
c
Insulating liquids Formula
-1 -1
kJ·g of fuel kJ·g of O
Silicone oil (1) – 25 14,5
b
Pentaerythritol ester (2) – 36,8
b
Mixture of mono- and – 39,5
dibenzyl toluene (3)
b
Paraffinic mineral (4) – 46,1
(1) Silicone transformer liquid, type T1, IEC 60836 [7]
[8]
(2) Transformer esters, Type T1, IEC 61099
[9]
(3) Capacitor insulating liquid, IEC 60867
[10]
(4) Transformer and switchgear mineral oil, IEC 60296
NOTE Technical Committee 10 has found a range of values from different sources for
-1 -1
the heat of combustion of silicone oil of 25 kJ⋅g to 27 kJ⋅g .
a
Reactants and products at 25 °C, all products gaseous.
b
No data are currently available.

5 Parameters used to report heat release data
5.1 Heat of combustion (gross and net)
The standard heat of combustion of a substance is defined in thermochemical terms as the
enthalpy change that occurs when one mole of a substance undergoes complete combustion
under standard conditions. In the fire science community, heat of combustion is also referred to
as “gross heat of combustion”, and the units used are energy per unit mass rather than energy
per mole.
NOTE Older terms, now deprecated, are “calorific potential” and “gross calorific value”.
The water formed as a product of combustion is considered to be in the liquid state. For a
compound containing carbon and hydrogen, for example, complete combustion means the
conversion of all the carbon to carbon dioxide gas, and conversion of all the hydrogen to liquid
water.
Gross heat of combustion is measured by oxygen bomb calorimetry in which all the sample is
completely converted to fully oxidized products – see 4.1. In real fires this is rarely the case.
Some potentially combustible material is often left as char and products of combustion are
often only partly oxidized, for example, soot particles in smoke and carbon monoxide.
Net heat of combustion is similar to gross heat of combustion except that any water formed is
assumed to be in the vapour state. The difference is the latent heat of vaporization of water at
-1
298 K which is 2,40 kJ·g . Net heat of combustion is therefore always smaller than gross heat
of combustion. In flames and fire, water remains as vapour and therefore it is more appropriate
to use net heat of combustion values.
5.2 Heat release rate
Heat release rate is defined (see 3.15) as the thermal energy released per unit time in a fire or
fire test. It is a particularly useful parameter because it can be used to quantify the intensity of
a fire.
60695-8-1 © IEC:2008 – 15 –
Heat release rate is commonly reported in the form of a graph against time. A heat release rate
curve is shown in Figure 1.
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0 100 200 300 400 500 600 700
Time/s
IEC  328/08
Figure 1 – Heat release rate (HRR) curve
5.3 Heat release
Heat release is defined (see 3.14) as the thermal energy that is produced in a fire or fire test. It
is a particularly useful parameter because it can be used to quantify the size of a fire. Heat
release is usually calculated by integration, with respect to time, of heat release rate data.
Figure 2 shows the curve calculated from Figure 1. However, usually only the total heat release
(see 5.5) is reported.
1 000
Total heat release = 900 kJ
0 100 200 300 400 500 600 700
Time/s
IEC  329/08
Figure 2 – Heat release (HR) curve
5.4 Heat release rate per unit area
Sometimes, in the case of flat test specimens, heat release rate is reported in terms of the rate
-2
of heat release per unit area of the exposed surface. Typical units are kW·m . Data from the
cone calorimeter [11] are usually reported in this way. A heat release rate per unit area curve
is shown in Figure 3. (It is based on the curve of Figure 1 assuming an exposed surface area
of 100 cm .)
HR/kJ
HRR/kW
– 16 – 60695-8-1 © IEC:2008
0 100 200 300 400 500 600 700
Time/s
IEC  330/08
Figure 3 – Heat release rate per unit area (HRR*) curve
5.5 Total heat release
Total heat release is the heat release value at the end of the time period of interest. It can be
obtained by integrating the rate of heat release, usually from the time of ignition to the end of
the fire test. It can be used to quantify the size of a fire.
The total heat release in the curve of Figure 2 is 900 kJ.
5.6 Peak heat release rate
Peak heat release rate is the maximum value of the heat release rate that is observed during a
fire test. Peak heat release rate may be used for comparing the effectiveness of some flame
retardant treatments. However, it should be treated with some caution in cases where there are
multiple maxima in the heat release rate curve.
The peak heat release rate in the curve of Figure 1 is 3 kW.
5.7 Time to peak heat release rate
As well as the amount of heat produced, the time it takes for the heat to be produced is
important.
A simple guide to this is the time to peak heat release rate. However, it should be treated with
some caution in cases where there are multiple maxima in the heat release rate curve.
The time to peak heat release rate in the curve of Figure 1 is 300 s.
5.8 Effective heat of combustion
5.8.1 Measurement and calculation
Effective heat of combustion is defined (see 3.5) as the heat released from a burning test
specimen in a given time interval divided by the mass lost from the test specimen in the same
time period. Effective heat of combustion is a measure of the heat released per unit mass of
the burning volatile fuel which is produced from the test specimen. In most cases, it is not the
same as the net heat of combustion of the test specimen. The only case where it is the same is
when all the test specimen is consumed (i.e. all converted to volatile fuel) and when all the
combustion products are fully oxidized.

–2
HRR*/ kW ⋅ m
60695-8-1 © IEC:2008 – 17 –
In order to calculate the effective heat of combustion from heat release rate data, it is
necessary to measure the rate of mass loss of the test specimen. This is done by mounting the
test specimen holder on a load cell so that mass measurements can be recorded as a function
of time.
If the mass loss curve associated with the data shown in Figure 1 has the form shown in
–1
Figure 4, the effective heat of combustion will have a constant value of 25 kJ·g .
If the effective heat of combustion is approximately constant throughout the burning of a test
specimen, it implies that the mechanism of combustion is unchanged. However, it is often the
case that combustion mechanisms change with different stages of a fire and so the effective
heat of combustion will also change. Changes in the effective heat of combustion can be a
useful indication of the effectiveness of flame retardants.
NOTE At the start and towards the end of a fire test when mass loss rates have very small values, division by zero
(or near zero) errors can lead to nonsensical values of the effective heat of combustion.

0 100 200 300 400 500 600 700
Time/s
IEC  331/08
Figure 4 – Mass loss curve
5.8.2 Examples
The following examples illustrate the difference between net heat of combustion and effective
heat of combustion.
Example 1: Toluene
-1
The net heat of combustion of toluene is 40,99 kJ·g and is a measure of the thermal energy
released by the chemical reaction:
C H (liquid) + 9 O (gas) → 7 CO (gas) + 4 H O (gas), T = 298 K
7 8 2 2 2
If toluene is burned in a cone calorimeter it burns inefficiently with the production of soot,
carbon monoxide and other partially oxidized products. A typical value for the effective heat of
-1
combustion of toluene (without external heat flux) is about 36 kJ·g reflecting the incomplete
combustion. In this case, all of the test specimen volatilizes and, as a result, the effective heat
of combustion of the volatile fuel is also the same as the effective heat of combustion of the
test specimen. This would not be so if some of the test specimen remained as a residue (see
Example 2).
Example 2: Wood
Mass/g
– 18 – 60695-8-1 © IEC:2008
Consider a 100 g sample of wood that burns to leave a carbonaceous char of mass 20 g and
-1
that releases 960 kJ of heat. The effective heat of combustion will be 12 kJ·g (i.e.
960 kJ/80 g) and is a measure of the heat released per gram when the 80 g of volatile
degradation products is burned. This is not the same as the heat released per gram of test
-1
specimen which will be 9,6 kJ·g (i.e. 960 kJ/100 g). It should be noted that the net heat of
-1 -1
combustion of wood is a significantly higher figure, typically between 16 k
...