# The LWF Blog

## Facilities Management & Fire Safety – Calculating RSET – Part 9

December 13, 2018 11:33 am

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 fire safety engineering. In part 8, we looked at how ASET (Available Safe Egress Time) is calculated and how this should always be more than the RSET (Required Safe Egress Time). While ASET involves a series of calculations relating to fire growth and the building itself, RSET, also known as the escape time, is calculated based on factors such as detection time, recognition time and response time. Part 9 continues from that point.

When recognition time and response time have both been calculated, the combination is known as ‘pre-movement’ time and can be defined as the interval between the warning of fire being given and the building occupants first move towards an exit.

While fire safety engineering methods incorporate pre-movement time into their calculations on an individual scenario basis, prescriptive codes make no allowance or variation for this parameter, with the exception of PD 7974-6:2004 – The application of fire safety engineering principles to fire safety design of buildings. Human factors. Life safety strategies. Occupant evacuation, behaviour and condition (Sub-system 6) which gives typical pre-movement times for the first and last few occupants.

Logically, after pre-movement time comes movement time, which is known as ‘travel time’. This is the time needed for all the occupants of a specified part of the building to reach a place of safety. It is usual for this to be the furthest point in the building away from a safe exit.

While some fire engineering solutions are approached from a ‘first principles’ point of view, i.e. they do not rely on prescriptive guidance at all and use calculations to establish the safest solution, some fire engineering solutions are a combination of fire engineered solutions to areas where prescriptive codes cannot be met.

Travel distance is one such area in which a hybrid solution might be seen, using equivalence. For example, where prescriptive codes say that a travel distance of 45 metres is acceptable and any more than that would be detrimental to the safe escape of any occupants, an excess of 20 metres above the 45 (so a total of 60 metres) might be mitigated by other factors.

The travel distance would be converted to units of time rather than distance, so an excess of 20 metres might increase travel time by 30 seconds. This increase of time taken to evacuate would need to be offset by either increasing the ASET or decreasing the other parts of the RSET by the necessary amount of time.