1336227 flashover, backdraft, and fire gas ignitions

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GRA 5.8flashover, backdraught

and fire gas ignitions

Fire and Rescue ServiceOperational Guidance



Generic Risk Assessment 5.8

Flashover, backdraught and firegas ignitions

August 2009

London: TSO



Published with the permission of the Department for Communities and Local Government on behalf of

Her Majesty’s Stationery Ofce.

© Crown Copyright 2009

ISBN 978 0 11 754011 8

Copyright in the typographical arrangement and design rests with the Crown.

This is a value added publication which falls outside the scope of the HMSO Class Licence.

Applications for reproduction should be made to Ofce of Public Sec tor Information, Information Policy Team, Kew,Richmond, Surrey, TW9 4DU.

Published by TSO (The Stationery Office) and available from:

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Contents

SECTION 1 GRA 5.8 Flashover, backdraught and fire

gas ignitions 5

Scope 5

Significant hazards and risks 5

Flashover 5

Backdraught 5

Fire gas ignitions 6

Key control measures 7

Recognition of the signs of backdraught and flashover 7

Training 8

Pre-determined attendance 8

Personal protective equipment 8

Incident command and control 9

Additional equipment 9

Technical references 10

Glossary 10

Neutral Pressure Place 10

Deflagration 10

Pyrolysis 10

SECTION 2 Summary of GRA 5.8 Flashover, backdraught and

fire gas ignitions 11

The Generic Risk Assessments in this series only apply to England.



5

SECTION 1

GRA 5.8 Flashover, backdraughtand fire gas ignitions

Scope

This generic risk assessment (GRA) examines the hazards, risks and controls that relate

to Fire and Rescue Service staff and others who may be affected when the phenomenon

of Flashover, Backdraught or Fire Gas Ignition is experienced.

As with all GRAs, this assessment provides a starting point for FRSs to conduct their ownassessments within the context of local conditions and existing organisational arrangements.

Significant hazards and risks

Flashover

In a compartment fire there may come a stage where the total thermal radiation from the

fire plume, hot gases and hot compartment boundaries (ceilings and walls) causes the

radiative ignition of all exposed combustible surfaces within the compartment. Where the

compartment is adequately ventilated, this sudden and sustained transition of a growingfire to a fully developed fire is known as a flashover .

Backdraught

A backdraught is where limited ventilation can lead to a fire in a compartment producing

fire gases containing significant proportions of partial combustion products and unburnt

pyrolysis products. If these accumulate, the admission of air when an opening is made to

the compartment can lead to a sudden deflagration. This deflagration moving through the

compartment and out of the opening is a backdraught.

There are two scenarios of which firefighters must be fully aware:

If the fire is still burning within a compartment when the door is opened,

especially if the combustion gases are not escaping, the incoming air will

mix with the gases and create an explosive mixture. If the gases within the

compartment are hot enough, they will auto-ignite and flame will spread

back into the compartment along with the fresh air. This would result in rapid

fire growth, but not necessarily in a backdraught. Alternatively, if the gases

are not sufficiently hot they will only be ignited once sufficient oxygen has

reached the gases surrounding the fire. The flame will then travel across the

compartment towards, and out of the doorway, driven by the expandinggases behind it.

S c e n a r i o 1

Generic Risk Assessment 5.8 – Flashover, backdraught and fire gas ignitions



6 Generic Risk Assessment 5.8 – Flashover, backdraught and fire gas ignitions

A more dangerous situation can occur if the fire in the compartment has

almost died out. Once the door is opened air flows in and an explosive

mixture may be created. There is the potential for ignition of these gases

not to occur immediately. Once the firefighters enter the room however, and

start to disturb the contents (e.g. turning over), an ignition source may be

exposed and result in total flame engulfment. This is defined as a ‘delayed backdraught’.

S

c e n a r i o 2

Fire gas ignitions

Fire gas ignitions occur when gases from a compartment fire are ‘leaked’ into an adjacent

compartment and mixed with the air within this additional area. This mixture may then fall

within the appropriate flammable limits that if ignited, will create an increase in pressure

either with or without explosive force. Where this process occurs it is not necessary for an

opening to be opened for such ignition to take place. If an explosive force is experienced,

this is commonly termed a ‘smoke explosion’. Where an ignition occurs with much less

pressure, the term ‘flash fire’ is more appropriate.

The backdraught, flashover and fire gas ignition phenomena described above can have

potentially fatal outcomes. It should be recognised that although their outcomes may

be similar the phenomena themselves are in fact significantly different in terms of the

following:

• their pre-conditions

• evolution

• recognisable indicators.

At one incident, in the same structure, there may well exist the preconditions for

each phenomenon. Further detailed information of the nature and preconditions for

backdraught, flashover and fire gas ignitions is provided in the Fire and Rescue Service

Manual Volume 2 Fire Service Operations, Compartment Fires and Tactical Ventilation.

Experience has shown that standard personal protective equipment (PPE), in itself, will

not afford adequate protection against the effects of either flashover or backdraught.

Therefore, it is vital that FRSs consider all the key control measures in order to reduce the

risk to personnel to a tolerable level.



7

Key control measures

FRSs should consider how they control the hazards and risks described above. Key

measures are as follows.

Recognition of the signs of backdraught and flashover

The first consideration that there is a possibility of a backdraught occurring will be

dependant upon the fire history. Therefore, as much information as possible should be

gathered in the early stages of the incident.

Fire in a compartment with limited ventilation:

• Fire has been burning for some time

• Fire gases being pushed out under pressure from gaps

• Windows blackened with no visible signs of flame

• Fire gases pulsing out from gaps.

External signs of backdraught

Fire in a compartment with limited ventilation:

• Low neutral pressure plane

• Darkening of fire gases

• Inrush of air and fire gases forced out, through the opening

• Pulsation of fire gases through an opening

• Deflagration through the opening.

Internal signs of backdraught

• Flames visible in the fire gases

• Combustible materials gassing off due to pyrolysis

• High temperatures, and increasing rate of combustion

• Neutral plane moving down

• Sudden increase in development of fire

• Pyrolysis at floor level in the compartment.

Signs of flashover

Generic Risk Assessment 5.8 – Flashover, backdraught and fire gas ignitions



Generic Risk Assessment 5.8 – Flashover, backdraught and fire gas ignitions8

Training

The level and nature of training undertaken should be based upon an informed

assessment of the operational need for each Fire and Rescue Service and within agreed

guidance for competency based training (CBT).

Personnel engaged in an operational firefighting role should receive information instruction

and training which should include:

• chemistry of combustion

• the effects of walls and the ceiling on the fire plume

• signs and symptoms of backdraught, flashover and fire gas ignitions

• door entry and gas cooling procedures

• safe system of work for dealing with incidents where flashover and backdraught

may occur

• physiological stress and its management.

The outcomes of training must be evaluated to ensure that the training is:

• appropriate

• effective

• up-to-date

• meeting the identified operational need.

Pre-determined attendance

FRSs should ensure that the operational response to a building fire will be sufficient

to allow relevant safe systems of work to undertake the activity. This will initially be

determined by information received from the caller and followed and supplemented by the

assessment of the Incident Commander.

Personal protective equipment

PPE manufactured to the current accepted standard should be worn.

When choosing suitable protective garments, the standard of clothing worn beneath the

PPE should also be considered. Retained and auxiliary personnel may not have sufficient

protection afforded by this clothing and therefore appropriate guidance/restrictions should

be provided.



9Generic Risk Assessment 5.8 – Flashover, backdraught and fire gas ignitions

Incident command and control

The National Incident Command System should be adopted on arrival at the incident.

Where a high risk of backdraught is identified, consideration should be given to the initial

adoption of defensive firefighting tactics.

Detailed guidance on operational tactics is published in the Fire and Rescue ServiceManual, Volume 2, Fire Service Operations – Incident Command, 3rd Edition 2008.

Ventilation of the premises should be conducted in a controlled and considered manner

with due account taken of wind conditions. Effective communications are essential for this

to be achieved.

Ventilation points and exposed risks should be covered by water sprays to reduce the risk

of external fire spread.

Suitable communications will be needed to ensure that the Incident Commander and

sector commanders are able to communicate at all times.

The area outside the building on fire should be controlled ( Inner Cordon Management) to

reduce the number of persons at risk to the minimum necessary should a backdraught

occur. This may include the tactical positioning of breathing apparatus entry control

boards and fire appliances.

Additional equipment

An informed assessment of the risks within a FRS area will allow FRSs to consider the

provision, and use of any additional equipment. Equipment may include:

• thermal imaging cameras

• ventilation tools

• hose reel spray branches

• PPV fans.




Technical references

1 Table of contents, Volume 3 Guide to Operational Risk Assessment

2 Fire and Rescue Service Manual – Volume 2 – Compartment Fires and Tactical

Ventilation (1997)

3 Fire and Rescue Service Manual, Volume 2, Fire Service Operations – IncidentCommand, 3rd Edition 2008 – TSO

4 The principles of Operational Training Fire Service Circular 5/96

5 Institute of Occupational Medicine; Home Office, FRDG Fire Research Reports

and Memoranda, 18/96

6 Study of the Physiological Effect of Wearing BA (Institute of Occupational

Medicine). DFM letter and DCO letter 8/1997 (Management of Physiological

Stress)

7 GRA 3.6 Fighting Fires – Using PPV, HMSO 1998

8 BS EN 469 Protective clothing for firefighters & Personal Protective Equipment

at Work: guidance on Regulations: Personal Protective Equipment at Work

Regulations 1992

9 Dynamic Management of Risk at Operational Incidents, HMSO 1998

10 Home Office Technical Bulletin 1/97 Breathing Apparatus – Command and

Control

11 Euro Firefighter – Global firefighting Strategy and Tactics, Command and Control,

Firefighter Safety – Paul Grimwood FIFireE

GlossaryNeutral Pressure Plane

Fire in a compartment causes smoke and gases to rapidly expand with the hot gases

rising due to their lower density and greater buoyancy. This results in a higher (positive)

pressure in the fire room than the areas outside of the compartment. The forces, once

fully developed, create an equilibrium whereby negative pressure relative to the outside,

exists in the lower part of the room an positive pressure exists in the higher levels.

Where these two zones meet is known as the ‘Neutral Pressure Plane’.

Deflagration

To burn or cause to burn with great heat and intense light.

Pyrolysis

Transformation of a substance produced by the action of heat.



11Generic Risk Assessment 5.8 – Flashover, backdraught and fire gas ignitions

S E

C T I O N

2

S u

m m a r y o f G R A 5 . 8

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w h e r e t h e

t h e r m a l r a d i a t i o n

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n t r o l m e a s u r e s – T h a t c o u l d b e u s e d b y t h e S e r v i c e t o r e d u c e t h e r i s k

1336227 flashover, backdraft, and fire gas ignitions

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