The LWF Blog

Facilities Management & Fire Safety – Calculating ASET as per BS 7974 – Part 8

December 6, 2018 12:40 pm

In LWFs blog series for those who work in Facilities Management or who have an interest in or responsibility for fire safety, we have been looking at the concept of fire safety engineering. In part 7, we looked at BS 7974 and the framework it provides to achieve an engineered approach to fire safety in buildings and escape time. In part 8, LWF will consider escape time by breaking it down into the different components which it comprises.


The escape time is considered by sub-dividing events into components, as follows:


Detection Time

The time span between fire ignition and discovery of a fire.



Time to sound alarm

This can be expected to take up to 10 seconds if the detection and alarm system are automatic, due to current standards. Substantially longer delays can be expected if the fire is detected by a person or people, due in part to a reticence by individuals to sound the fire alarm even if they can see fire and it has been known that people will delay sounding a fire alarm to undertake other tasks too.



Recognition time

After the fire alarm has sounded, occupants of the building sometimes continue with the task they were previously undertaking, such as eating a meal, working on something, shopping etc. Improvements upon time taken to reach recognition can be achieved with good fire safety management and training within the building.



Response time

After occupants have recognised there is a fire situation and they need to respond to the alarm signal or simply their knowledge that there is a fire, there is a further delay known as the ‘response time’ before people begin to move towards an exit. The duration of the response time can vary significantly from a few seconds to grab their phone or bag to a longer period during which they might turn off machinery, look for family members or lock a cash till, for instance.


It should be noted that the vast majority of actions undertaken during the response time are not recommended or accepted but should still be taken into account by the building owner/occupier when considering timings.


The escape time has further elements, known as ‘pre-movement time’ and ‘travel time’. Both elements will be discussed by LWF in part 9 of this series. In the meantime, if you have any queries about your own facilities or wish to discuss this blog series, please contact Peter Gyere in the first instance on 0208 668 8663.


Lawrence Webster Forrest is a fire engineering consultancy based in Surrey with over 25 years’ experience, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings. 


While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.


Share this post