The Available Safe Egress Time (ASET) is the amount of time that elapses between fire ignition and the development of untenable conditions.
ASET is determined from smoke simulation. First, a design fire is established by considering the types of combustibles present and their associated product yields (soot and carbon monoxide). Next, this design fire is provided as input to a CFD tool such as a FDS to determine the time after ignition at which the space through which occupants must pass becomes untenable due to the presence of smoke or heat.
Sample tenability criteria for determining ASET might be:
1. Visibility must remain above 10 m.
2. Temperature must remain below 60° C.
3. Carbon monoxide concentration must remain below 800 ppm for short exposure and must be less than 125 ppm for any exposure Period of up to 30 minutes
The Required Safe Egress Time (RSET) can be defined as the time period from the ignition of fire until the time the last occupant reaches a place of safety. RSET is a function of four time based values, which consists of detection time, Alarm time, Pre movement time & Travel time.
Alarm time is the time at which occupants first become aware of fire through a building's automatic or manual fire alarm system. The pre-movement time, is the time that elapses between activation of the occupant notification system and the time at which occupants make the decision to begin evacuating. Pre-movement activities might include investigation to determine if the fire is real, gathering belongings etc.
Finally, the movement time is the time required for occupants to reach a protected exit of the building.
The RSET has been determined through evacuation simulation carried out using FDS-Evac software. In the context of an ASET/RSET analysis, An Evacuation analysis is acceptable if the ASET is greater than the RSET, after applying an appropriate safety factor.