The LWF Blog

ASET vs RSET Part Two| Timings of Required Safe Egress Time

November 18, 2013 5:33 pm

In our last blog article, we looked at ASET (Available Safe Egress Time) and how this can be manipulated to your advantage using Fire Safety Engineering. Now, we will look at the Required Safe Egress Time (RSET) in order to determine exit strategy safety for your build, and compare the two results.

Required Safe Egress Time

When quantifying the RSET, three components must be considered:

 detection time

 pre-movement time

 movement time

Detection Time

The detection time is the time from ignition of the fire to an alarm being raised.

The detection time and as such, the RSET, is highly dependent on the type of fire detection system provided. A manual fire alarm system incurs a potentially greater detection time than an automatic detection system with detection devices throughout the building.

Pre-Movement time

Pre-movement time is the time an occupant needs to recognise the situation and to react to the alarm. 

Recognition includes the time required to get an occupant to take fire alarm cues seriously and recognize that some response to the emergency is required.

Response consists of a range of activities such as investigating behaviour to find the source of the fire, stopping machinery, warning others, securing money or other risks, gathering children and other family members, deciding an exit route and fighting the fire.

As such, the pre-movement time is subject to influences such as; the level of assistance in an emergency evacuation (managed evacuation), the level of fire safety training undertaken by the occupants, the type of occupants (awake/sleeping, familiar/unfamiliar with the building etc.) or if the building is fitted with a PA/VA system to guide the behaviour of occupants in an emergency situation.

Movement Time

The movement time is the physical time taken for a building’s occupants to reach a place of safety once they have begun to evacuate. 

There are a number of methods available for making reasonable accurate predictions of evacuation patterns in fire emergencies. These predictable flow patterns have followed the development of egress models, which can test both egress arrangements and predict movement times.

The movement time is dependent on various factors, for example, the number and location of escape routes, widths of escape routes and fire exits, nature and number of occupants, walking speed of occupants (children, adults etc.) or relation between and dependency grade of the occupants (families, children, etc.).

Due to the complexity of the fire evacuation process and the number of factors influencing the actual movement time, hand calculations are only used to carry out assessments for simple means of escape arrangements.

To model the evacuation process in more complex buildings fire engineers normally use computer models. 

Modern fire evacuation models consider all parameters mentioned above, calculate a potential evacuation scenario and give information about the usage of escape routes, the time required to move from certain areas to a place of safety and all other criteria that are required to make a sound assumption for the movement time.

Finally, the addition of detection time, pre-movement time and movement time results in the RSET for the Building. As this is a theoretical approach using computer models the results are subject to a safety factor, to assess the RSET figure for design purposes.

Safe fire evacuation is ensured if the assessed ASET exceeds the RSET, even if the building is non-compliant with the prescriptive Building Regulation approach in terms of means of escape.

ASET vs RSET – Results

The rules on travel distance provide a powerful control on the size of compartments, the distance to exits and the number of exits provided. Stair and exit widths dictate the maximum time required for physical evacuation to protected escape routes. 

However, as Building Regulations provide guidance for the more common building situations, prescriptive guidance does not consider the specific nature of the building and its occupants. 

ASET vs RSET illustrates that an evacuation is a complex process that is dependent upon a number of factors specific to a building and its occupants. Using a performance-based approach allows a range of factors affecting fire hazard and escape time to be placed on a common framework, so that the effects of altering various design features may be calculated. 

As such, Fire Engineering can help to realise modern building layouts that are non-compliant with prescriptive guidance.

If you would like to know more about ASET vs RSET and Fire Engineering processes that may help to make your design fire safe and viable, please contact Peter Gyere at Lawrence Webster Forrest Limited on 020 8668 8663.

LWF are fire engineering and fire risk management consultants with over twenty years experience in the development of fire engineered technology and the application of fire safety standards including fire engineered techniques.

Share this post