ASET vs RSET in Fire Safety

ASET vs RSET in Fire Safety Engineering

In performance-based fire engineering, ASET (Available Safe Egress Time) and RSET (Required Safe Egress Time) are two critical metrics that determine whether occupants can evacuate safely during a fire emergency. Understanding and applying the relationship between ASET and RSET is essential for fire safety engineers developing performance solutions.

What is ASET?

Available Safe Egress Time (ASET) is the amount of time available for occupants to evacuate a space before conditions become life-threatening due to smoke, heat, or toxic gases.

ASET is typically calculated using CFD simulation software, such as the Fire Dynamics Simulator (FDS) or Pyrosim, which models the development and spread of fire, smoke layers, and temperature changes over time.

Key factors influencing ASET include:

Smoke layer height – The vertical distance from the floor to the base of the smoke layer. If this drops too low, visibility and tenability are compromised.
Visibility distance – Low visibility due to smoke impairs the ability of occupants to find exits quickly.
Toxic gas concentration – Especially carbon monoxide (CO), which can incapacitate at high levels.
Temperature thresholds – High temperatures can result in burns, disorientation, or even loss of consciousness.

What is RSET?

Required Safe Egress Time (RSET) is the total time occupants need to recognise the fire, respond to it, and safely evacuate the building. RSET is broken into distinct stages and depends heavily on occupant behaviour and building layout.

The three core components of RSET are:

Detection and alarm time – The delay between the fire starting and the alarm being triggered and heard.
Response time – The time occupants take to understand the situation, decide to act, and begin evacuating.
Movement time – The actual time taken to travel from the current location to a place of safety, such as a final exit or refuge area.

To model RSET accurately, engineers use egress simulation tools like Pathfinder, which factor in real-world elements such as:

Occupant density and demographics – Elderly occupants, children, and persons with disabilities may take longer to evacuate.
Exit widths and travel distances – Bottlenecks and long routes can delay movement.
Stairwell and corridor capacities – Flow rate limitations may lead to queuing.
Evacuation strategy – Whether evacuation is simultaneous (all at once) or phased (by zones or floors) has a significant impact on RSET.

The ASET > RSET Principle

For a design to be considered safe and compliant, ASET must be greater than RSET with a sufficient safety margin

This means occupants must have more time available than they need to evacuate, and ideally with a safety margin built in to account for uncertainties or unexpected delays.

If RSET approaches or exceeds ASET, it indicates that occupants could be exposed to hazardous conditions before reaching safety. In such cases, design improvements must be made, such as:

Upgrading fire detection and alarm systems for faster activation
Adding more exits or increasing exit widths
Implementing smoke control systems (e.g. pressurised stairwells or smoke curtains)
Enhancing passive fire protection, like fire-rated walls, doors, and fire-stopping details

Margin of Safety or Factor of Safety

The margin of safety in fire safety engineering refers to the buffer between the ASET and the RSET. It accounts for uncertainties in both fire development and human behaviour, ensuring that occupants have sufficient time to evacuate even if conditions deviate from expected scenarios.

A larger margin provides greater assurance that the design will remain effective under real-world conditions such as delayed detection, slower occupant response, or unexpected obstructions. While ASET simply being greater than RSET satisfies the basic requirement, incorporating a reasonable safety margin, typically between a factor of 1.5 to 2 reflects a more robust and resilient fire safety strategy.

Conclusion

ASET and RSET are more than just technical terms—they are life-saving metrics at the heart of modern fire safety design. By understanding the delicate balance between them, fire safety engineers can go beyond code compliance and deliver resilient, occupant-centred solutions.

If you are developing a project that requires fire safety analysis or evacuation modelling, get in touch with Lava Consultants to explore how a performance-based approach can improve safety, flexibility, and confidence in your design.