If you have commissioned a fire risk assessment for a complex building, or if you have been involved in the design or fire engineering of a new development, you may have encountered the terms ASET and RSET.
They are concepts drawn from fire safety engineering, and while they are not part of the everyday vocabulary of most building owners and managers, understanding what they mean and why they matter can provide valuable insight into how fire safety decisions are made and justified.
What Do ASET and RSET Stand For?
ASET stands for Available Safe Egress Time.
RSET stands for Required Safe Egress Time.
Together, they form the fundamental framework for assessing whether the occupants of a building can escape safely in the event of a fire.
The relationship between the two is straightforward in principle.
For a building to be considered safe for its occupants in the event of a fire, the available safe egress time must be greater than the required safe egress time.
In other words, the time available for occupants to escape before conditions in the building become untenable must exceed the time it actually takes for all occupants to reach a place of safety.
If ASET is greater than RSET, the building can be considered safe.
If RSET exceeds ASET, occupants may be unable to escape before conditions become life-threatening, and the fire safety design or management of the building must be reviewed.
What is ASET?
ASET is the time from the ignition of a fire to the point at which conditions in the building become untenable for the occupants.
Untenable conditions are those in which it is no longer safe for a person to remain in or move through a space, typically because of the effects of heat, smoke, or toxic gases produced by the fire.
The precise definition of untenable conditions is a matter of fire engineering judgement, but commonly used criteria include a temperature at head height exceeding 60 degrees Celsius, a smoke layer descending to a height of 2 metres above the floor, a visibility in smoke falling below 10 metres in large spaces or 5 metres in smaller spaces, and the concentration of toxic gases reaching levels that would incapacitate an occupant.
ASET is determined by modelling or calculating the development of fire and smoke within the building, taking into account factors including the size and type of the fire, the geometry and ventilation of the space, the fire load present, and the performance of any active fire safety measures such as sprinklers or smoke control systems.
In a simple, well-ventilated space with an effective smoke control system, ASET may be relatively long, giving occupants a generous window of time in which to escape.
In a large, complex, or poorly ventilated space with a high fire load and no smoke control, ASET may be very short, leaving occupants with little time to react and escape before conditions become dangerous.
What is RSET?
RSET is the total time required for all occupants of a building to reach a place of safety from the moment of fire ignition.
It is not simply the time it takes to walk to a fire exit.
RSET is the sum of several distinct time components that together make up the full egress timeline.
The first component is the detection time, the time from ignition to the point at which the fire is detected, either by an automatic detection system or by a person.
The second is the alarm time, the time from detection to the point at which the alarm is communicated to occupants, either through an automatic alarm system or through manual means.
The third is the pre-movement time, sometimes referred to as the recognition and response time, which is the time from the alarm being raised to the point at which occupants begin to move towards an exit.
Pre-movement time is often the most complex and variable component of RSET.
It encompasses the time taken for occupants to recognise that the alarm signal means they should evacuate, to decide to act on that recognition, to prepare themselves for evacuation, and to begin moving.
Pre-movement time is influenced by a wide range of factors, including the clarity and nature of the alarm signal, the familiarity of occupants with the building and its evacuation procedures, whether occupants are awake or asleep, the presence of staff trained to direct evacuation, and the characteristics of the occupant population including age, mobility, and cognitive ability.
In a well-managed office building with regular fire drills and well-trained staff, pre-movement time may be relatively short.
In a hotel where guests are asleep in unfamiliar rooms, a care home where residents may have mobility or cognitive impairments, or a large public venue where the audience may be reluctant to believe that an alarm is genuine, pre-movement time can be very much longer.
The fourth component is the travel time, the time taken for occupants to move from their starting position to a place of safety, along the available escape routes.
Travel time depends on the distance to be covered, the capacity and configuration of the escape routes, the number of occupants using those routes simultaneously, and the physical characteristics of the occupant population.
RSET is the sum of all these components, and reducing any one of them reduces the overall RSET and improves the safety margin between ASET and RSET.
Why Does the Relationship Between ASET and RSET Matter?
The relationship between ASET and RSET is the fundamental measure of whether a building’s fire safety design and management arrangements are adequate to protect its occupants in the event of a fire.
A building in which ASET significantly exceeds RSET has a comfortable safety margin.
Occupants have ample time to escape before conditions become dangerous, and the building can accommodate a degree of variability in fire development and occupant behaviour without compromising safety.
A building in which ASET only marginally exceeds RSET has a very thin safety margin.
Any variation in fire development, any delay in detection or alarm, any increase in pre-movement time, or any obstruction of escape routes could result in RSET exceeding ASET, with potentially fatal consequences.
And a building in which RSET exceeds ASET is one in which the fire safety arrangements are demonstrably inadequate.
Occupants cannot escape before conditions become untenable, and urgent action is needed to either increase ASET, by improving the fire safety measures that slow the development of dangerous conditions, or to reduce RSET, by improving the speed and efficiency of the evacuation process.
How Are ASET and RSET Used in Practice?
ASET and RSET are most commonly encountered in the context of fire safety engineering, where they are used to justify fire safety designs that depart from the prescriptive guidance set out in Approved Document B or other standard design codes.
When a building cannot demonstrate compliance with standard prescriptive fire safety requirements, for example because of its size, height, complexity, or unusual occupancy, a fire engineer may use ASET and RSET analysis to demonstrate that the proposed design nonetheless provides an adequate level of safety.
This approach is sometimes referred to as performance-based or fire engineering design, and it is governed by the methodology set out in BS 7974:2019, Application of Fire Safety Engineering Principles to the Design of Buildings.
ASET and RSET analysis may also be used in the context of fire risk assessment for existing buildings, particularly where the standard approaches may not adequately capture the specific fire safety challenges of a complex, high-risk, or unusual premises.
In a large shopping centre, a complex mixed-use development, a historic building with an unusual layout, or a premises with a particularly vulnerable occupant population, a more rigorous analysis of the relationship between ASET and RSET may be needed to properly assess the adequacy of the existing fire safety arrangements and to justify any departures from standard guidance.
What Factors Influence ASET and RSET?
Understanding what influences ASET and RSET helps to identify the levers available for improving the safety margin between the two.
ASET can be increased by installing or improving active fire safety systems, particularly automatic fire suppression systems such as sprinklers, which slow the development of fire and smoke and can dramatically increase the time available for evacuation.
It can be increased by improving smoke control systems, which extract smoke from the building and maintain tenable conditions in escape routes for longer.
It can be increased by improving passive fire protection measures, including compartmentation, fire doors, and cavity barriers, which contain the fire and limit the spread of smoke.
And it can be influenced by the fire load present in the building, with reductions in the quantity and combustibility of materials within the space reducing the rate at which a fire develops and extends the time before conditions become untenable.
RSET can be reduced by improving fire detection, ensuring that fire is detected at the earliest possible stage and that the alarm is communicated to occupants without delay.
It can be reduced by improving the evacuation procedures and the training of staff, so that occupants respond quickly and effectively when the alarm is raised.
It can be reduced by improving the design and management of escape routes, ensuring that they are adequate for the number of occupants, clearly signed, well-lit, and free from obstruction.
And it can be influenced by the characteristics of the occupant population, with targeted measures to support the evacuation of occupants with mobility or cognitive impairments reducing the overall RSET for the building.
Personal Emergency Evacuation Plans
The management of RSET for occupants with disabilities or mobility impairments deserves particular attention.
Under the Equality Act 2010, employers and building managers have a duty to make reasonable adjustments to ensure that disabled people are not placed at a substantial disadvantage in comparison with non-disabled people.
In the context of fire safety and evacuation, this means ensuring that the evacuation needs of all occupants are considered and planned for, including those who may not be able to use standard escape routes or who may require assistance to evacuate.
A Personal Emergency Evacuation Plan, commonly referred to as a PEEP, is an individual plan that sets out the specific evacuation arrangements for a person who may need assistance or who cannot follow the standard evacuation procedure.
PEEPs should be developed in consultation with the individuals concerned, reviewed regularly, and communicated to all staff with responsibilities for evacuation.
Where a significant proportion of the occupant population has mobility or cognitive impairments, for example in a care home, a hospital, or a sheltered housing scheme, the collective impact on RSET must be carefully assessed and the evacuation strategy designed accordingly.
ASET, RSET and the Fire Risk Assessment
For most straightforward commercial and residential premises, the fire risk assessor will not conduct a formal quantitative analysis of ASET and RSET.
Instead, the assessor will apply their professional judgement to determine whether the fire safety measures in place provide an adequate safety margin, drawing on the relevant guidance documents and their knowledge and experience of similar premises.
However, where the fire risk assessment identifies concerns about the adequacy of the means of escape, the performance of fire detection or suppression systems, or the ability of the occupant population to evacuate safely, a more detailed analysis of ASET and RSET may be warranted.
In these circumstances, the fire risk assessor may recommend engagement with a fire engineer to carry out a more rigorous assessment, or to develop and justify alternative fire safety measures that address the identified concerns.
For complex, high-risk, or non-standard premises, the commissioning of a formal fire engineering analysis of ASET and RSET as part of the fire risk assessment process is a mark of a thorough and responsible approach to fire safety management.
It provides the Responsible Person with a robust, evidence-based justification for the fire safety measures in place, and a clear framework for identifying and addressing any shortfalls in the safety margin between available and required egress time.
We Can Help
At ESI: Fire Safety, we carry out fire risk assessments for a wide range of premises types, from straightforward commercial and residential buildings through to complex, high-risk, and non-standard properties where a more detailed analysis of fire safety performance may be required.
Where the specific characteristics of a premises require a more rigorous approach to the assessment of evacuation safety, we work with specialist fire engineers to ensure that the analysis is thorough, robust, and properly documented.
If you have concerns about the adequacy of the evacuation arrangements for your premises, or if you would like to discuss whether a more detailed fire engineering analysis would be appropriate for your building, get in touch with our team today.
Further Reading and Key Legislation:
BS 7974:2019 Application of Fire Safety Engineering Principles to the Design of Buildings: https://www.bsigroup.com
Regulatory Reform (Fire Safety) Order 2005 (SI 2005/1541): https://www.legislation.gov.uk/uksi/2005/1541/contents/made
Approved Document B, Fire Safety: https://www.gov.uk/government/publications/fire-safety-approved-document-b
Equality Act 2010: https://www.legislation.gov.uk/ukpga/2010/15/contents
NFCC Guidance on Evacuation and PEEP: https://nfcc.org.uk
Institution of Fire Engineers: https://www.ife.org.uk
Society of Fire Safety Engineers: https://www.sfse.org.uk
This post is intended as a general guide. The application of ASET and RSET analysis to specific buildings is a specialist area requiring professional fire engineering expertise. Always seek advice from a qualified fire safety engineer or fire risk assessor for guidance specific to your premises and circumstances.
