The LWF Blog

Facilities Management & Fire Safety – Calculating RSET – Part 10

December 20, 2018 1:16 pm

In LWF’s 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 ASET and RSET (Available Safe Egress Time and Required Safe Egress Time). In part 9 of this series, it was established that in many cases, a fire engineered approach to ASET would not begin with a first principles approach, but rather would assume compliance with a prescriptive code where possible and work on a fire engineered approach for any excess distance. In part 10, we continue from that point.


The excess distance, over and above that allowed by the prescriptive guidance, would be translated into units of time. For example, 20 metres excess travel distance might become 20 to 30 seconds of extra time which will be required by occupants of the building needing to evacuate that extra distance.


The first steps would be to consider whether it is possible to increase the ASET by 20 to 30 seconds, or reduce the components comprising RSET time by 20 to 30 seconds.


ASET could be increased by the installation of a smoke-control system. Such systems work to keep the smoke layer above the heads of the evacuating occupants and this would allow at the very least an additional 20 to 30 seconds, probably considerably more.


For the smoke control system to engage promptly, there must be an automatic fire detection system to indicate the presence of smoke or flame.


Predictions would need to be made regarding fire size in order for the smoke-control system to be effective and while this can be calculated based upon likely load, a more ‘future proof’ method might be to provide an automatic sprinkler system which would certainly limit the size the fire could grow to.


The combination of the three systems – smoke control, fire detection and fire suppression – would almost certainly make conditions in the building safer for building occupants in the extra time needed to safely evacuate and while this is not necessarily what is required in every situation, a fire engineered solution can open up new potential design features for the architect.


Of course, the above scenario is not a fully fire engineered design. A design taken from first principles is likely to be more complex but potentially allow even greater design freedoms.


When consideration is being given to reducing RSET, the potential for a voice fire alarm system cannot be underestimated. It has already been established that a human voice giving an instruction is much more effective at reducing pre-movement time than a standard alarm sounder.


It should also be noted that management can have an impact upon RSET too. Effective and vigorous fire safety training for the building occupants really can make a substantial difference.


In next week’s Facilities Management blog, LWF will turn their attention to the role of the insurer in property protection. 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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