The LWF Blog

Facilities Management & Fire Safety – Fire Safety Engineering & the Prescriptive Approach – Part 7

November 27, 2018 2:59 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 6, we looked at BS 7974 and the practice directions associated with the standard. In part 7, we continue looking at BS 7974 and the framework it provides to achieve an engineered approach to fire safety in buildings.


BS 7974:2001 Application of fire safety engineering principles to the design of buildings. Code of practice provides the basis of equations which allow fire engineers and other relevantly qualified individuals to calculate the relevant parameters in fire scenarios. It also describes the principles of fire engineering in its most fundamental state.


Fire and fire safety are both, essentially, concerned with time. Fire grows the more time that passes and opposing fire with extinguishing action will extend the amount of time available for evacuation, which can happen alongside any extinguishing actions. Extinguishing can be undertaken by automatic systems or by people in this example.


BS 7974 illustrates fire engineering with the use of time lines, which compare the progress of fire development with the progress of evacuation.


The critical factor in terms of fire development and smoke spread is the available safe egress time (known as ASET). ASET is the calculated time between ignition of a fire and the time when conditions in the building become untenable for any persons left inside. However, this expanse of time cannot be considered as the escape time, due to such factors as the delay between fire ignition and fire alarm or alert and the constantly degrading conditions in which escape must be achieved. The escape time must be shorter than the ASET to ensure life safety. The time calculated for safe escape is called the required safe egress time, or RSET.


Calculation of accurate ASET can be complex and includes predicting fire growth, smoke and toxic gas production and correlating this with the reaction of the building and its fire protection systems.


LWF will discuss the elements which must be considered when calculating accurate ASET in part 8 of this blog 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.


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