ASET and RSET are important fire engineering concepts used to assess whether people have enough time to escape before conditions become unsafe. They are commonly used in fire strategy work, evacuation analysis, smoke control design and complex building assessments.
Quick Answer
ASET means Available Safe Egress Time. RSET means Required Safe Egress Time. In simple terms, ASET is the time available before conditions become unsafe, while RSET is the time people need to become aware of a fire, respond, evacuate and reach a place of safety. For a safe design, ASET should exceed RSET with an appropriate margin of safety.
What Is ASET?
ASET stands for Available Safe Egress Time. It is the estimated time between fire ignition and the point at which conditions become untenable for occupants in the area being assessed.
Untenable conditions may relate to smoke, heat, toxic gases, visibility, radiation or other life safety criteria. The purpose of ASET analysis is to understand how long people have before the environment is no longer considered safe for escape.
What Is RSET?
RSET stands for Required Safe Egress Time. It is the time needed for occupants to detect or be warned about a fire, respond to the warning, begin evacuation, move through the escape routes and reach a place of relative safety or ultimate safety.
RSET is not just walking time. It can include detection time, alarm time, pre-movement time, queuing, travel time, assisted evacuation and management actions.
ASET vs RSET: The Core Principle
The basic principle is that ASET should be greater than RSET. In other words, people should have enough time to escape before smoke, heat or other fire conditions make the escape route unsafe.
| Term | Meaning | What It Considers |
|---|---|---|
| ASET | Available Safe Egress Time | How long conditions remain tenable for escape. |
| RSET | Required Safe Egress Time | How long people need to recognise, respond and evacuate. |
| Safety margin | Difference between ASET and RSET | The allowance between required escape time and untenable conditions. |
What Makes Up RSET?
RSET is usually made up of several stages. These may vary depending on the building, occupants and evacuation strategy.
- Detection time: how long it takes for the fire to be detected.
- Alarm time: how long it takes for a warning to be given.
- Recognition time: how long occupants take to understand that action is needed.
- Response or pre-movement time: how long people take before starting to evacuate.
- Travel time: how long it takes to move through the escape route.
- Queuing or congestion: delays at doors, corridors, stairs or exits.
- Assisted evacuation: additional time needed for people who require help.
What Affects ASET?
ASET depends on how fire and smoke develop in the building. It can be affected by the fire size, fuel load, ventilation, compartmentation, ceiling height, smoke control, detection, suppression and the geometry of the space.
For example, a large open atrium may need a different approach from a small cellular office. A building with smoke control or sprinklers may also behave differently from one without those systems.
Where ASET/RSET Analysis May Be Used
ASET/RSET analysis is most relevant where fire safety design requires more detailed consideration than a simple prescriptive approach.
- Complex buildings and unusual layouts
- Atria and large open spaces
- Shopping centres, airports, arenas and transport premises
- Residential buildings with complex evacuation strategies
- Care, healthcare or supported living environments
- Smoke control strategies
- Fire engineered designs or alternative approaches
- Projects where vulnerable occupants may need more time to evacuate
How CFD Modelling Can Support ASET/RSET Analysis
Computational Fluid Dynamics (CFD) modelling can be used in more complex fire engineering assessments to estimate smoke movement, heat conditions, visibility and tenability over time. This can help establish the Available Safe Egress Time for a specific fire scenario.
CFD may be useful for complex buildings, atria, large open spaces, smoke control strategies, unusual layouts or engineered fire safety designs. However, it is not required for every ASET/RSET review. The need for CFD depends on the building, design objectives, fire scenarios, assumptions and the level of evidence required.
Where CFD is used, the assumptions, inputs, limitations and results should be reviewed by a competent fire engineer. Poor assumptions can produce misleading results, so CFD should not be treated as a simple compliance shortcut.
Why Occupant Behaviour Matters
RSET is strongly affected by how people behave during an emergency. People may not immediately evacuate when an alarm sounds. They may investigate, gather belongings, wait for instructions, assist others or move more slowly because of mobility, sensory, cognitive or medical needs.
This is why evacuation analysis should consider the real building users, not only the geometry of the escape routes.
ASET/RSET and Vulnerable Occupants
Where occupants may need additional time or support to evacuate, RSET may increase. This can be relevant in care homes, hospitals, supported housing, high-rise residential buildings and buildings used by members of the public.
In these cases, ASET/RSET thinking may link with evacuation strategy, Personal Emergency Evacuation Plans, person-centred fire risk assessments, staffing levels and management procedures.
ASET/RSET and Fire Strategy Reports
ASET/RSET analysis may form part of a wider fire strategy where evacuation performance, smoke control or life safety assumptions need to be justified. It can support design decisions, but it should sit within a wider fire safety strategy rather than being considered in isolation.
For more context, see our guides to fire strategy reports and fire engineering consultants.
Need Fire Engineering or Strategy Support?
Fire Risk Assessment Network can help arrange fire engineering consultancy and fire strategy support for buildings where evacuation strategy, smoke control or life safety assumptions need competent review.
Important Limitations
ASET/RSET calculations depend heavily on assumptions about the fire scenario, detection, alarm, occupant behaviour, management, mobility, smoke spread, travel speeds and tenability criteria. Small changes in assumptions can significantly affect the result.
For that reason, ASET/RSET analysis should be undertaken by competent professionals where it is needed. It should not be used to justify weak fire safety arrangements, poor management or unsuitable escape provisions.
Relevant Guidance and Standards
ASET/RSET analysis may be considered alongside recognised fire engineering guidance such as BS 7974 and the PD 7974 series, including guidance on human factors, occupant evacuation behaviour and life safety strategies.
The exact guidance used depends on the project, building type, design objectives and fire engineering approach.
Conclusion
ASET and RSET are useful concepts for understanding whether occupants have enough time to escape before fire conditions become unsafe. ASET considers the time available before conditions become untenable, while RSET considers the time people need to detect, respond and evacuate.
For simple buildings, detailed ASET/RSET analysis may not be needed. For complex buildings, engineered designs, smoke control strategies or vulnerable occupants, it can be an important part of competent fire engineering review.
Frequently Asked Questions
What does ASET mean?
ASET means Available Safe Egress Time. It is the estimated time before conditions become untenable for occupants during a fire scenario.
What does RSET mean?
RSET means Required Safe Egress Time. It is the time occupants need to become aware of a fire, respond, evacuate and reach a place of safety.
Should ASET be greater than RSET?
Yes. The available safe egress time should exceed the required safe egress time, with an appropriate margin of safety.
Is CFD always needed for ASET/RSET analysis?
No. CFD modelling is not always needed. It may be useful for complex buildings, smoke control strategies, atria, large open spaces or engineered designs, but simpler assessments may use other appropriate methods.
Who should carry out ASET/RSET analysis?
ASET/RSET analysis should be carried out by competent fire engineering professionals where it is needed, especially for complex buildings or engineered fire safety strategies.
Is ASET/RSET part of a normal fire risk assessment?
Not usually. A typical fire risk assessment may identify evacuation concerns, but detailed ASET/RSET analysis is normally a fire engineering task.
Can ASET/RSET be used for existing buildings?
Yes, but it must be used carefully. Existing building analysis depends on reliable information about the building, fire safety systems, management arrangements, occupants and fire scenarios.
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