ISO 24678-6:2026

International
Standard
ISO 24678-6
Second edition
Fire safety engineering —
2026-08
Requirements governing algebraic
formulae —
Part 6:
Flashover-related phenomena
Reference number
© ISO 2026
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements governing the description of physical phenomena. 2
5 Requirements governing the calculation process. 2
6 Requirements governing limitations . 2
7 Requirements governing input parameters . 2
8 Requirements governing the domain of applicability . 2
9 Example of documentation . 2
Annex A (informative) Algebraic formulae for calculating the minimum heat release rate to
cause flashover in small enclosures . 3
Bibliography .18

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
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This document was prepared by Technical Committee ISO/TC 92, Fire safety, Subcommittee SC 4, Fire safety
engineering.
This second edition cancels and replaces the first edition (ISO 24678-6:2016), which has been technically
revised.
The main changes are as follows:
— the main text has been simplified by referring to ISO 24678-1;
— distinction between formulae for heavy construction and for variable constructions have been made
clear in Annex A;
— comparisons with experimental data have been updated in Annex A.
A list of all parts in the ISO 24678 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
The ISO 24678 series is intended to be used by fire safety practitioners involved with fire safety engineering
calculation methods. It is expected that the users of this document are appropriately qualified and competent
in the field of fire safety engineering. It is particularly important that users understand the parameters
within which particular methodologies may be used.
Algebraic formulae conforming to the requirements of this document are used with other engineering
calculation methods during a fire safety design. Such a design is preceded by the establishment of a context,
including the fire safety goals and objectives to be met, as well as performance criteria when a trial fire
safety design is subject to specified design fire scenarios. Engineering calculation methods are used to
determine if these performance criteria are met by a particular design and if not, how the design needs to be
modified.
The subjects of engineering calculations include the fire-safe design of entirely new built environments, such
as buildings, ships or vehicles, as well as the assessment of the fire safety of existing built environments.
The algebraic formulae discussed in this document can be useful for estimating the consequences of design
fire scenarios. Such formulae are valuable for allowing the practitioner to quickly determine how a proposed
fire safety design needs to be modified to meet performance criteria and to compare among multiple
trial designs. Detailed numerical calculations can be carried out up until the final design documentation.
Examples of areas where algebraic formulae have been applicable include determination of convective
and radiative heat transfer from fire plumes, prediction of ceiling jet flow properties governing detector
response times, calculation of smoke transport through vent openings, and analysis of compartment fire
hazards such as smoke filling and flashover. However, the simple models often have stringent limitations
and are less likely to include the effects of multiple phenomena occurring in the design scenarios.
The general principles of fire safety engineering are described in ISO 23932-1, which provides a performance-
based methodology for engineers to assess the level of fire safety for new or existing built environments.
Fire safety is evaluated through an engineered approach based on the quantification of the behaviour of fire
and based on knowledge of the consequences of such behaviour on life safety, property and the environment.
ISO 23932-1 provides the process (i.e. necessary steps) and essential elements for conducting a robust
performance-based fire safety design.
ISO 23932-1 is supported by a set of fire safety engineering documents on the methods and data needed for
all the steps in a fire safety engineering design as summarized in Figure 1 (taken from ISO 23932-1:2018,
Clause 4). This set of documents is referred to as the Global fire safety engineering analysis and information
system. This global approach and system of standards provide an awareness of the interrelationships
between fire evaluations when using the set of fire safety engineering documents. The set of documents
includes ISO/TS 13447, ISO 16730-1, ISO 16732-1, ISO 16733-1, ISO/TS 16733-2, ISO/TR 16738, ISO 24678-1,
ISO 24679-1, ISO/TS 29761 and other supporting Technical Reports that provide examples of and information
on the application of these documents.
Each document supporting the global fire safety engineering analysis and information system includes
language in the introduction to tie that document to the steps in the fire safety engineering design process
outlined in ISO 23932-1. ISO 23932-1 requires that engineering methods be selected properly to predict the
fire consequences of specific scenarios and scenario elements (see ISO 23932-1:2018, Clause 12). Pursuant
to the requirements of ISO 23932-1, this document provides the requirements governing algebraic formulae
for fire safety engineering. This step in the fire safety engineering process is shown as a highlighted box in
Figure 1 and described in ISO 23932-1.

v
a
See also ISO/TR 16576 (examples).
b
See also ISO 16732-1, ISO 16733-1, ISO/TS 16733-2, ISO/TS 29761.
c
See also ISO 16732-1, ISO 16733-1, ISO/TS 16733-2, ISO/TS 29761.
d
See also ISO/TS 13447, ISO 16730-1, ISO/TR 16730-2 to ISO/TR 16730-5 (examples), ISO/TR 16738, ISO
24678-1 to ISO 24678-7 and ISO 24678-9.
e
See also ISO/TR 16738, ISO 16733-1, ISO/TS 16733-2.
NOTE Documents linked to large parts of the fire safety engineering design process: ISO 16732-1, ISO 16733-1,
ISO 24678-1, ISO 24679-1, ISO/TS 29761, ISO/TR 16732-2 and ISO/TR 16732-3 (examples), ISO/TR 24679-2 to
ISO/TR 24679-6, ISO/TR 24679-8 (examples).
SOURCE: ISO 23932-1:2018, Figure 1.
Figure 1 — Flow chart illustrating the fire safety engineering design process

vi
International Standard ISO 24678-6:2026(en)
Fire safety engineering — Requirements governing algebraic
formulae —
Part 6:
Flashover-related phenomena
1 Scope
This document specifies the requirements governing the application of a set of explicit algebraic formulae
for the calculation of specific characteristics of flashover-related phenomena.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 13943, Fire safety — Vocabulary
ISO 24678-1, Fire safety engineering — Requirements governing algebraic formulae — Part 1: General
requirements
ISO 24678-4, Fire safety engineering — Requirements governing algebraic formulae — Part 4: Smoke layers
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 13943 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
flashover
rapid transition to a state of total surface involvement in a fire of combustibles within an enclosure
3.2
fuel-controlled burning
state in which the burning rate is controlled by fuel conditions such as material properties of fuel, surface
area and orientation
3.3
opening factor
parameter determining the rate of airflow through a vent, calculated by the product of vent area and square
root of vent height
3.4
thermal inertia
ability of material to absorb heat, calculated by the product of thermal conductivity, density and specific
heat of the material
3.5
ventilation-controlled burning
state in which the burning rate is controlled by air flow incoming to an enclosure
4 Requirements governing the description of physical phenomena
4.1 The requirements governing the description of physical phenomena as specified in ISO 24678-1 shall
apply, in addition to the requirements specified in 4.2 to 4.3.
4.2 The onset of flashover is a complex thermo-physical phenomenon that can be highly transient. As a
result of burning in an enclosure, hot smoke layer develops in the upper part. This shall be as stated in
ISO 24678-4. Heat and mass transfer in an enclosure takes place. Radiative and convective heat transfer
to fuel surface may increase the heat release rate. To calculate the onset of flashover, interactions between
phenomena should be considered.
4.3 As different formulae describe different flashover characteristics or apply to different scenarios, if
there is more than one method for calculating a given quantity, guidance shall be given on the selection of
appropriate methods. A descriptive example is given in Annex A.
5 Requirements governing the calculation process
The requirements specified in ISO 24678-1 governing the calculation process shall apply.
6 Requirements governing limitations
The requirements specified in ISO 24678-1 governing limitations shall apply.
7 Requirements governing input parameters
The requirements specified in ISO 24678-1 governing input parameters shall apply.
8 Requirements governing the domain of applicability
The requirements specified in ISO 24678-1 governing the domain of applicability shall apply.
9 Example of documentation
An example of sets of algebraic formulae meeting the requirements of Clause 4, Clause 5, Clause 6, Clause 7
and Clause 8 is provided in Annex A.

Annex A
(informative)
Algebraic formulae for calculating the minimum heat release rate to
cause flashover in small enclosures
A.1 General
This annex describes formula sets for minimum heat release rate to cause flashover in small enclosures.
The formula sets discussed in this annex were all empirically derived using temperature and heat
flux measurements from fire tests where different fuels were burned in enclosures of similar size and
construction. The tests were generally performed to study enclosure fire behaviour, including the
phenomenon of flashover. As combustion products from fires have substantially higher temperature than
the ambient air, the enclosure gases stratify with the less dense hot combustion products forming a growing
hot layer in the ceiling volume of the enclosure. The consensus threshold heat release rate for flashover, Q ,
fo
is reached when the measured temperature of hot layer exceeds certain values such as 500 °C to 600 °C
(approximately 770 K to 870 K) and radiation heat flux to floor surface exceeds 20 kW/m which is enough
[6]
to ignite common combustible materials in a short time .
The range of application is limited by the range of experiments from which these formula sets were derived;
namely, enclosures that have moderate volume. All of these formula sets were empirically derived based on
[6][7]
small enclosures. The datasets analysed show wide variations because they are collected from similar
experimental enclosures but using different fuel packages and enclosure lining materials.
Note that the majority of algebraic relationships discussed in Clauses A.3 to A.9 have been derived from
experiments in naturally ventilated enclosures. Only one of the formula sets was developed for forced
ventilation conditions. Comparisons with experimental data are included to assess the uncertainties of
formulae.
The formulae, all normalized by the opening factorAH , relate Q to the total interior surface area of the
fo
vv
enclosure and a factor proportional to the thermal inertia of the enclosure surfaces.
A.2 Symbols used in this annex
A area of floor (m )
f
A total internal surface area of the enclosure excluding opening area (m )
T
-1/2
ratio of total internal surface area to opening factor (m )
AA/ H
T vv
A area of ventilation opening (m )
v
5/2
opening factor (m )
AH
vv
A area of walls and ceiling (m )
wc
−1 −1
c specific heat (kJ·kg ·K )
f(x) function of x
−2 −1
h effective heat transfer coefficient (kW·m ·K )
T
H height of ventilation opening (m)
v
−1 −1
k thermal conductivity (kW·m ·K )
2 −4 −2 −1
(kρc) thermal inertia of floor lining (kJ ·m ·K ·s )
f
2 −4 −2 −1
(kρc) thermal inertia of wall and ceiling linings (kJ ·m ·K ·s )
wc
−1

m mechanical ventilation rate (kg·s )
e

m
mass flow rate of air incoming to an enclosure (kg/s)
v
Q minimum heat release rate to cause flashover (kW)
fo
t characteristic time for flashover occurrence (s)
c
−3
ρ density (kg·m )
A.3 Description of physical phenomena addressed by the formula set
A.3.1 General description of the calculation method
A.3.1.1 Calculation procedure
Estimating the minimum heat release rate for flashover involves the following steps:
— determination of geometry of the enclosure (internal surface area of enclosure, width and height of an
opening, etc.);
— determination of characteristics of lining materials, etc., if applicable;
— calculation of minimum heat release rate for flashover.
A.3.1.2 Flashover characteristics to be calculated
Consider a growing fire in an enclosure as shown in Figure A.1. A combustible object has just been ignited.
Enough air is available for combustion in the early stage of fire growth. The burning is said to be fuel-
controlled because the combustion, including the resultant heat release rate, is primarily influenced by fuel
factors such as fuel type (the material) and its configuration (horizontal or vertical). Combustion products
tend to form a layer under the ceiling because they are hotter and therefore less dense than the ambient
environment. There is often a fairly sharp demarcation between a hot upper smoke layer and a relatively
cool lower layer. As burning continues, the hot layer increases in depth and temperature. This increases the
radiative and convective heat transfer to burning items below, which in turn increases their heat release rate
resulting in an increase in the upper layer temperature, further enhancing heat transfer rates to burning
items and enclosure surfaces. If sufficient fuel is present and arranged in such a way that the fire continues
to grow, the fire can enter a transitional phase known as flashover.
Flashover is the rapid transition to a state of total surface involvement in a fire of combustible materials
within an enclosure. Essentially, almost all exposed combustible surfaces ignite and start to burn during
flashover. This results in a fully developed fire. Flashover is often associated with a sharp peak of burning
rate. After the peak, fire transfers from fuel-controlled burning to ventilation-controlled burning where
the heat release rate is limited by oxygen availability, which is provided by the flow of fresh air into the
enclosure. After flashover, there is typically no longer any separation between a hot upper layer and a
cool lower layer. Rather, a single well-mixed zone of hot gases exists. The occurrence of flashover and
transition to post-flashover burning represents the most hazardous stage of an enclosure fire. Due to the
incomplete combustion in an enclosure, oxygen concentration in the generated smoke is reduced, while the
concentrations of CO and CO are considerably increased. As the smoke is full of soot particles, visibility
through smoke is reduced. Flame may emerge from the opening of the enclosure. Therefore, it is of great
practical importance to be able to estimate the onset of flashover by way of the minimum heat release rate
at which a fire will show transition to its flashover stage.

Key
X time
Y heat release rate
1 pre-flashover stage, fuel-controlled burning
2 flashover
3 post-flashover stage, ventilation-controlled burning
Figure A.1 — enclosure fire development including flashover
A.3.1.3 Minimum heat release rate for flashover to which formulae apply
While the phenomenon of flashover is universally accepted, the critical conditions defining the threshold
for flashover are not. Hot layer temperature between 500 °C and 600 °C, radiative heat flux to the floor of
20 kW/m and appearance of flames exiting vents are all used to signal the event of enclosure flashover.
[5]
Flashover shall be considered to have occurred when any two of these conditions have been met. As most
enclosure fire tests involve extensive temperature measurements, models are designed to predict vertical
temperature gradients and the attainment of upper layer temperatures in the above range is most often
used to indicate when flashover occurs. The algebraic formulae for flashover always use this criterion.
Roughly speaking, the onset of flashover in an enclosure occurs when the radiant heat flux to the floor
averages 20 kW/m . This heat flux level corresponds approximately to an upper layer temperature of around
500 °C to 600 °C, noting that the radiant flux emitted by a 500 °C black body is about 20 kW/m .
A.3.2 Scenario elements to which the formula set is applicable
The set of formulae is relevant to the prediction of the minimum heat release rate necessary to cause
flashover in an enclosure. Except where noted, these formulae apply to naturally ventilated fires, i.e. those
where the effect of forced ventilation due to heating, ventilating and air conditioning system is negligible. In
addition, the effect of external wind is neglected.
A.3.3 Self-consistency of the formula sets
The formula set provided in this annex has been derived and reviewed (see Clause A.5) to ensure that
calculations resulting from different formulae in the set are consistent (i.e. do not produce conflicts).
A.3.4 International standards and other documents where the formula set is used
ISO 16733-2 refers to this document for the description of flashover.

A.4 Formula-set documentation of calculation procedure
A.4.1 General description of minimum heat release rate for flashover
The minimum heat release rate necessary to cause flashover in an enclosure is usually predicted by using an
energy balance to calculate the upper layer temperature as a function of heat release rate (and sometimes
time) and then relating the onset of flashover to the attainment of some critical condition in the enclosure.
For calculation purposes, flashover is usually assumed to occur when the upper layer temperature rise is
between 500 °C and 600 °C or the radiative heat flux to the floor exceeds approximately 20 kW/m .
Consider an enclosure as shown in Figure A.2. Empirically and theoretically, the most important factors
affecting the minimum heat release rate for flashover, Q , are the area, A , and height, H , of any enclosure
fo v v
vents, the total area of internal surfaces, A , and the thermal inertia of the enclosure lining materials, kρc.
T
For this reason, the flashover correlations that have appeared in the literature can be expressed consistently
in the form of Formula (A.1):
 
Q A
fo T
 
 f ,kc (A.1)
 
AH AH
 
vv vv
where the total area of internal surfaces, A , may be determined by taking the net surface area of enclosure
T
boundary walls, floors and ceilings. Some formulae include opening area in the total area of internal
surfaces, while some do not. For conservative calculations, opening area should be exclud
...