People escape to fire exit door
How long should it take to evacuate your premises in the event of a fire?
Fire safety practitioners are often asked, particularly after a fire drill, how long it should take to evacuate the premises.
What, on the face of it, appears to be a simple question has quite a complex answer, particularly in view of the UK’s risk-based fire safety regime.
Those responsible for fire safety need to know what would be deemed to be a safe evacuation time and the factors that influence this.
Legislation, such as the Regulatory Reform (Fire Safety) Order 2005 (RRFSO), does not prescribe any evacuation times. Rather, the RRFSO states that “in the event of danger, it must be possible for persons to evacuate the premises as quickly and as safely as possible”.
The reason for this is that, unlike fire certification arrangements under the old regime, where each certificate was building specific, the RRFSO applies to a wide range of buildings, therefore making it impossible to prescribe evacuation times.
Guidance to the RRFSO does contain further information on evacuation times. For example, guidance for office environments notes that “escape routes in a building should be designed so that people can escape quickly enough to ensure they are not placed in any danger from fire”.
The key phrase here is “should be designed”. This clearly indicates that designers of buildings need to consider the design of the means of escape as part of the overall design of the property so that occupants can move to a place of reasonable and/or total safety before the conditions in the property become untenable.
Certainly, designers should be following the requirements of the respective Building Regulations and associated guidance such as that contained in Approved Document B: Fire safety, BS9999:2017 Fire Safety in the Design, Management and Use of Buildings and BS ISO 20414:2020 Fire Safety Engineering. Verification and Validation Protocol for Building Fire Evacuation Models.
It is key that the fire safety practitioner responsible for fire evacuation in a property as occupied has access to and can share necessary information in relation to the design of the means of escape are provided.
Although this information will not provide an exact evacuation time, it will enable:
This information is particularly important given the current emphasis on the “golden thread” of information approach for buildings that is recommended following a review into the Grenfell fire.
Human behaviour
It is widely recognised that up to two-thirds of actual evacuation time consists of occupants delaying when the evacuation signal is given.
These factors are built into what are known as the:
For safe evacuation to occur, the ASET must be significantly longer than the RSET. There are a number of factors that will affect the RSET and ASET. These include the time:
In theory the fire safety practitioner should be able to rely on the RSET and ASET and be confident that the evacuation time will be within this scope.
However, there are many factors that may affect the RSET and ASET in “real world” conditions, not least:
If occupants are unable to evacuate in a timely manner this could result in a breach of relevant fire safety legislation and increase the risk of injury or death.
PAS 79-1:2020 Fire Risk Assessment. Premises Other Than Housing. Code of Practice, notes that “an assessment should be made of the likely consequences of fire”.
The assessment should “understand that all persons within the premises should be able to reach a place of ultimate safety before life threatening conditions arise; either unaided or with the assistance of staff — without FRS assistance (RSET versus ASET)”.
In assessing this aspect, the fire risk assessor may consider a number of inputs into the process including:
These could then be used to determine (usually in a qualitative manner) whether the ASET/RSET requirements are being met or whether real-life evacuation takes longer, creating a situation where occupants may be at risk of harm.
