The LWF Blog

Fire Engineering for Healthcare Premises – ASET & RSET– Part 36

May 24, 2021 11:01 am

In LWF’s blog series for healthcare professionals, our aim is to give information on best practice of fire safety in hospitals and other healthcare premises. In part 35 of Fire Engineering for Healthcare Premises, LWF considered what information can be gained from calculating smoke movement and ASET vs RSET. In part 36, we continue looking at ASET and RSET.

The Available Safe Egress Time (ASET) depends upon the fire scenario chosen and is the time available to safely exit the building (or move to a place of safety) between the start of the fire and the onset of conditions which would be hazardous to the occupants.

ASET is useful in comparison to Required Safe Egress Time (RSET) but both must be measured from the same point – i.e. whether it is measured from ignition of a fire or the first time that fire is detected. Where the time is to be measured from ignition, this means that there must be an allocated amount of time in both calculations to express the time taken from the fire igniting to the first detection.

Some fire simulation models, including CFAST and Jasmine, calculate ASET using either zone or CFD approaches.

The RSET is the time needed after a fire has started for the last person to reach a place of safety.  It must include various elements – time for the fire to be detected, the alarm to be raised, building occupants to recognise the alarm, people to respond to the alarm and begin moving and for them to evacuate safely.

As in the case of ASET, RSET depends on the fire scenario chosen. When comparing RSET with ASET, it is important to ascertain which parts of the building are under consideration. As progressive horizontal evacuation is common in larger healthcare buildings, there may be one value of RSET for evacuation from one sub-compartment to the next and another RSET value for evacuation from one compartment to an adjacent compartment.

Evacuation modelling software is available to predict the movement of people, such as CRISP, Simulex, EXODUS, EVACNET, EXIT89, EvacSim, etc. Other modelling softwares are available and consultation with a fire engineer is necessary to ascertain the most appropriate way to approach modelling and evacuation planning.

In Part 37 of LWF’s blog series, LWF will begin to look at spill plumes and smoke reservoirs. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact LWF on freephone 0800 410 1130.

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.