In the world of fire safety, every second counts. But how do engineers and safety experts know just how many seconds we have? They don’t guess. They use a critical calculation that forms the very foundation of modern building design and evacuation planning. This calculation is the relationship between ASET and RSET.
Understanding this concept is like understanding the maths behind a magic trick. It’s the hidden logic that dictates everything from the width of a corridor to the type of alarm system installed, all to ensure people can get out safely.
The Quick Answer
ASET and RSET are two competing clocks in a fire emergency. ASET (Available Safe Egress Time) is the total time available from the start of a fire until conditions in a building become deadly (e.g., too much smoke, too hot). RSET (Required Safe Egress Time) is the total time it takes for people to actually get out. For a building to be safe, ASET must always be greater than RSET. It’s a simple, non-negotiable race: the “time you have” must be longer than the “time you need.”
What is ASET (Available Safe Egress Time)?
ASET is the “time you have.” It’s a measure of the building, not the people.
The Building’s Clock
Think of ASET as the countdown timer on a bomb. It starts the moment a fire ignites. Fire engineers calculate this time by modelling how a fire will grow and how smoke, heat, and toxic gases will spread.
This “tenability limit” is the point where human life can no longer be supported. This is determined by factors like:
Visibility: When smoke becomes so thick you can’t see the exit signs.
Heat: When the air temperature becomes high enough to cause burns or incapacitation.
Toxicity: When the level of carbon monoxide and other toxic gases becomes lethal.
The time it takes to reach this point is the ASET. A large atrium with a smoke-venting system will have a much longer ASET than a small, windowless office.
What is RSET (Required Safe Egress Time)?
RSET is the “time you need.” It’s a measure of the people, not the building.
The People’s Clock
RSET is the total time it takes from the start of the fire until the last person has reached a place of safety. This is not just “how fast can you run.” Fire safety professionals, like those at the Fire Protection Association (FPA), know that RSET is a multi-stage process. It’s broken down into three parts:
1. Alarm Time (Detection Time): The time between the fire starting and an alarm (automatic or manual) being raised. A building with advanced smoke detectors will have a shorter alarm time than one that relies on someone spotting the flames and finding a manual call point.
2. Pre Movement Time (Recognition & Response Time): This is often the longest and most unpredictable part. It’s the “human delay” between the alarm sounding and people actually starting to move. This includes time spent:
Recognising the alarm is real (not a drill).
Figuring out what to do.
Shutting down computers or gathering belongings (which you should never do!).
Helping others.
3. Movement Time (Travel Time): This is the part we all think of. It’s the time it physically takes to walk or move from where you are to the final place of safety (like the assembly point). This is dictated by the building’s design, as laid out in UK government guidance like Approved Document B (Fire Safety).
RSET = Alarm Time + Pre-Movement Time + Movement Time
The Golden Rule: ASET > RSET
The entire goal of fire engineering is to ensure a simple equation is always true: ASET > RSET.
If the time available to escape is 10 minutes (ASET) and it takes everyone 6 minutes to get out (RSET), the building is safe. You have a 4-minute “safety factor.”
If the time available is 5 minutes (ASET) and it takes 6 minutes to get out (RSET), you have a tragedy.
How We Tip the Scales in Our Favour
A good fire risk assessment is all about improving this equation. If the RSET is too close to the ASET, we have two options:
1. Increase the ASET (Give People More Time):
Install smoke extraction systems to keep escape routes clear.
Use fire-resistant doors and walls to slow the spread of smoke and fire.
Improve compartmentation to contain the fire in one area.
2. Decrease the RSET (Get People Out Faster):
Install a better fire alarm system (reduces Alarm Time).
Conduct regular fire drills and training (massively reduces Pre Movement Time).
Ensure corridors are wide and free of clutter (reduces Movement Time).
Install clearer signage.
Conclusion
ASET vs RSET is not just technical jargon; it’s the life and death calculation at the heart of fire safety. It is the scientific method that proves a building’s evacuation strategy will actually work. By understanding that we are in a race against the fire (ASET) and a race against our own human response (RSET), we can design and manage buildings that give everyone the one thing they need in a fire: enough time.
