Skip to content
Upcoming courses

Available safe egress time

Tools
From Fire Safety Academy Wiki
(Redirected from ASET)
Contents
  1. 1Definition and variants
  2. 2Comparison with RSET
  3. 3Tenability criteria
  4. 4How ASET is determined
  5. 5Worked example
  6. 6Limits and uncertainty
  7. 7See also
  8. 8Notes
  9. 9References

Available safe egress time (ASET) is the time that people in a building have to escape safely during a fire. It runs from ignition until conditions in a given space become untenable, for example because smoke has made it too hard to see or the heat has become too great.[1][2] ASET is one half of the central comparison in escape design. The other half is the required safe egress time (RSET), the time people need to notice the fire, respond and reach a place of safety. A design is acceptable when ASET is longer than RSET by a sufficient margin.[1]

Timeline from ignition: a long ASET arrow above a chain of RSET steps (detection, alarm, recognition, response, movement) and a safety margin at the end.
ASET runs from ignition to the moment the first tenability limit is exceeded; RSET is the sum of the escape steps. The gap between them is the safety margin. Based on Van Rossum (2027), §7.1.

ASET describes the fire, not the people: it depends on how fast the fire grows, how quickly smoke fills the space and which limits are set for heat, smoke and toxic gases.[3] It is usually found with fire modelling, ranging from simple zone models to computational fluid dynamics.[4] In New Zealand and Australia, the verification methods fix the tenability limits used to define ASET, measured 2 m above the floor. Visibility is usually the first limit to be exceeded.[5][6] Because many inputs are uncertain, a safety factor or safety margin is applied between ASET and RSET.[1]

Definition and variants

The book Introduction to Fire Safety Engineering defines ASET as the time available for people to escape safely. The comparison with RSET can be made for a whole building or for a single room, and the available time must clearly be longer than the time needed.[1] Escape is mostly prevented because hot smoke reduces visibility, although temperature and toxicity can also play a part.[7]

Purser describes the idea behind ASET as a race between two processes that run in parallel. The first is the developing hazard from the fire: ignition, fire growth and the spread of smoke and heat. Its assessment aims at the time when an occupant would receive an incapacitating exposure. The second is the escape process, from detection and warning to movement out of the building, which gives the time required for escape.[8] If the occupants are out before the fire becomes hazardous, with a reasonable margin of safety, the design can be considered to have succeeded.[8] The time available for escape depends on the fire dynamics and on the time to incapacitation of people exposed to smoke.[3]

Written definitions differ slightly in their starting point. The International Fire Engineering Guidelines (2005) define ASET in the Australian version as the time between ignition and the onset of untenable conditions in a specific part of a building. The New Zealand version starts the clock at the start of established burning instead of ignition.[2][9] These guidelines also expand the abbreviation as available safe evacuation time; the meaning is the same.[2]

Current regulations tie the definition to a measurable criterion. In New Zealand, C/VM2 (second edition, 2025) defines ASET as the time between ignition of the design fire and the moment the first tenability criterion is exceeded in a specified room.[5] The Australian Fire Safety Verification Method Standard 2022 uses the same wording.[6] ASET is therefore not one number for a building. It belongs to a particular space, fire location and set of criteria, and C/VM2 requires several fire locations to be checked to find the limiting case.[10]

Comparison with RSET

Many inputs of both ASET and RSET are uncertain, so a safety margin is important. The book gives two common forms of the comparison.[1] The first multiplies RSET by a safety factor:

ASET≥f⋅RSET

Symbol Description Unit
ASET available safe egress time, from ignition to the first exceeded tenability limit s
RSET required safe egress time, from ignition until the occupants have left the space s
f safety factor (1.0 or more) none

The second form requires the difference ASET − RSET to be at least a fixed safety margin. Which margin is needed depends on several factors, of which the reliability of the inputs is probably the most important. The more reliable the input, the more certain the outcome and the smaller the margin needs to be.[1] A sensitivity analysis shows whether variation in the input has a large effect on the results, and so whether the margin is sufficient.[1]

In the Netherlands, the standards NEN 6060 and NEN 6079 for large fire compartments include an ASET/RSET comparison with safety factors. For new buildings the factors range from 1.2 to 2.5. They are higher when RSET exceeds 3 minutes and when a rule-of-thumb method is used with nominal (50 %) rather than conservative (80 %) parameters. For existing buildings the factors range from 1.0 to 1.5.[11] A conservative solution is possible only for ASET, because RSET can only be entered as a nominal value.[11]

Grouped bar chart of Dutch ASET/RSET safety factors from 1.0 to 2.5, highest for new buildings, nominal parameters and RSET over 3 minutes.
Safety factors on RSET in the Dutch standards NEN 6060 and NEN 6079 for large fire compartments. Based on NEN 6060 and NEN 6079, as cited in Van Rossum (2027), §7.1.

The International Fire Engineering Guidelines describe the same comparison as a timeline approach. In an absolute evaluation, ASET should exceed RSET by a margin agreed in the fire engineering brief. In a comparative evaluation, the margin of the proposed design should be the same as or greater than that of a design meeting the prescriptive rules.[12] In Australia, the 2020 data sheets for the Fire Safety Verification Method take the same comparative view. The margin ASET − RSET for a performance solution should be at least equal to that of the reference building.[13]

Where both designs have the same occupants, alarm and exits, the RSET can be assumed equal and only the ASET values need to be compared.[14] Such a comparison avoids the need to treat the variability of human behaviour explicitly.[15] New Zealand's C/VM2 (second edition, 2025) simply requires ASET to be greater than RSET for the design scenarios concerned.[16]

Tenability criteria

ASET ends when the first limit for safe escape is exceeded. The limits are called tenability criteria and usually cover heat, visibility and toxic gases.[17] According to the book, such limits are not formally laid down anywhere, but several national and international sources give guidance.[17]

In the Netherlands, the explanatory notes to the building regulations give values for non-enclosed spaces through which an escape route runs. These are a smoke-free height of 2.5 m above the highest part of the escape route and a radiant heat flux below the smoke layer of at most 2 kW/m². When people escape through mixed smoke, the temperature should not exceed 45 °C and light-emitting objects should be visible from at least 30 m.[17] The book calls these values strict. In the United Kingdom, BS 9999 lets visibility fall to 8 to 10 m, and in corridors 5 m is often accepted. A visibility of 30 m is only needed in large spaces.[17]

Heat limits follow from radiation and from hot gases. In Dutch practice 2.5 kW/m² is commonly used for escape and 1 kW/m² for long stays. A very conservative calculation shows that a smoke layer at about 200 °C radiates roughly this flux to a person below it, so the layer should be no hotter than that.[17] In mixed hot gases, about 80 °C can be endured for a short escape, but for exposures over ten minutes in humid air the limit is about 60 °C. For toxic gases, the dose taken in during escape counts. With less than 30 to 60 seconds in smoke, toxicity is usually not decisive.[17]

In New Zealand, Building Code clause C4.3 sets three limits at 2.0 m above the floor. These are a fractional effective dose (FED) of carbon monoxide of 0.3, an FED of thermal effects of 0.3, and a visibility of 10 m, or 5 m in rooms smaller than 100 m².[18][5] An FED of 1.0 is the dose expected to stop a person of average susceptibility from escaping, which affects about half of the occupants. The threshold of 0.3 corresponds to about 11 % of the population being susceptible, and no threshold is safe for every occupant.[19] Clause C4.4 drops the thermal and visibility limits in sprinklered firecells where no more than 1,000 occupants can be exposed.[18]

Room cross-section with a smoke layer, a person walking to an exit and a dashed line 2.0 m above the floor where tenability is checked.
Tenability is assessed 2.0 m above the floor: ASET ends when the smoke at that height exceeds the first limit for heat, carbon monoxide or visibility. Based on Ministry of Business, Innovation and Employment (2013), p. 28.

In Australia, the Fire Safety Verification Method uses an FED of thermal effects of 0.3 and the same visibility limits of 10 m and 5 m. The 5 m value also applies where the distance to an exit is 5 m or less.[6] The handbook adds that visibility should be calculated for back-lit exit signs and that smouldering fires need a carbon monoxide criterion.[20] Purser's suggested limits point the same way: visibility of 5 m for small enclosures with short travel distances, and 10 m for large enclosures with long travel distances.[21]

How ASET is determined

ASET is mostly calculated with a model that simulates the effects of a fire on a space, in particular the smoke layer and the smoke-free layer below it. The model shows how long it takes before conditions become too poor for escape.[4] The limits must be chosen before the simulation is run.[22] In order of complexity, the book lists a steady-state zone model (NEN 6093), dynamic zone models such as OZone and CFAST, and computational fluid dynamics with FDS.[4] The choice depends on whether the model can represent the key features of the scenario. Examples are a fire in an adjacent room, a post-flashover fire or a complex geometry.[4]

In a single room, the simplest picture is smoke filling: the smoke layer under the ceiling grows downwards until it reaches head height. A simple one-room smoke-filling computer program was itself called ASET. It calculates the thickness and temperature of the smoke layer and the concentrations of combustion products for a user-defined fire.[23] For a room without ventilation and a constant heat release rate, the related ASET correlation predicts the height of the hot layer interface.[24]

Verification methods add rules so that ASET is not overestimated. C/VM2 requires the fire to be placed away from walls and corners, with its base no higher than 0.5 m above the floor. A higher fire would be detected earlier and its smoke layer would descend more slowly, which would give a longer ASET.[5] Several fire locations must be checked to find the lowest ASET for an escape route.[10] In both C/VM2 and the Australian method, visibility is generally the first criterion to be exceeded unless an exception applies.[5][6]

Purser illustrates the order in which hazards arise with an analysis of a furniture fire in a room. Smoke obscuration was the first hazard and exceeded its limit after the second minute. Irritant gases followed in the third minute, heat crossed its limit during the fourth minute, and the radiant flux reached 2.5 kW/m² during the fifth.[25] Escape would become difficult in the third minute. Large-scale tests suggest that even a rapidly growing flaming fire leaves at least three minutes before conditions become untenable, and often much longer.[26]

Worked example

The commentary to the first edition of C/VM2 (New Zealand, 2013) gives an example of a sprinklered hospital ward with 12 single patient rooms. A fire starts in a patient room of 8.0 m by 4.0 m with an open door; all other doors are closed. The corridor and nurses' area are modelled as one room of 5.0 m by 32 m, both rooms being 2.4 m high.[27] The zone model results give smoke detection at 46 s and sprinkler activation at 112 s, which caps the fire at 565 kW.[27]

In the patient room, visibility drops below 10 m after 23 s and the thermal FED exceeds 0.3 after 118 s. In the corridor this happens after 86 s and 628 s. Because the building is sprinklered, only the carbon monoxide FED counts. It exceeds 0.3 after 586 s in the patient room and after 863 s in the corridor, and these are the ASET values.[27][28]

Two timelines for a sprinklered hospital ward: visibility, thermal dose and carbon monoxide limits reached at different times, compared with RSET markers.
Times at which the tenability limits are exceeded in the fire room and the corridor of a sprinklered hospital ward, with the RSET of each space. Based on Ministry of Business, Innovation and Employment (2013), pp. 98 and 100.

All patients are assumed unable to walk, so staff must move them. The RSET follows from detection (46 s), alarm verification (30 s), the time for nurses to arrive (60 s) and 120 s per patient moved by two staff. This gives 256 s for the fire room and 856 s for the whole ward with four staff.[28]

For the fire room the margin is 586 − 256 = 330 s, a factor of 2.29 on RSET. For the ward the margin is only 863 − 856 = 7 s, a factor of 1.01. The ward just meets the bare requirement that ASET exceeds RSET.[28] It would not meet a safety factor of 1.2, the lowest factor in the Dutch table for new buildings.[11] The example also shows how strongly ASET depends on the criterion chosen: without the sprinkler exception, visibility would end the ASET in the corridor after 86 s.[27]

Limits and uncertainty

Tenability limits contain deliberate conservatism. One reason is that the endpoints cannot be measured precisely, because humans cannot be exposed to dangerous smoke in experiments. Another is the wide range of sensitivity in a mixed population. Purser names the elderly and people with asthma, about 15 % of children and 5 % of adults, as groups that are more sensitive to fire gases.[3] The C/VM2 commentary therefore notes that the FED limit may be set lower for vulnerable populations.[29]

ASET is also limited by the fire model. The result holds only for the design fire, fire location and space that were analysed, and different fire locations can give different values.[10] A model without the relevant features, such as a fire in an adjacent room or flashover, can give a misleading ASET.[4] In the C/VM2 example, the choice of criterion alone changed the corridor ASET from 86 s for visibility to 863 s for carbon monoxide.[28]

The criteria assume that people are in the smoke at a given height. In Purser's analysis the occupant's head was assumed to be in the smoke at all times. In a real room the hot layer might settle at 1 to 1.2 m above the floor, and a person lower down would be exposed later.[25] The book stresses that the limits must be judged in context. Navigating a corridor is easier than crossing a large hall, so a lower visibility can be accepted there.[17]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Van Rossum (2027), §7.1.
  2. ↑ 2.0 2.1 2.2 National Research Council of Canada et al. (2005), p. 0.5-2 (Australia).
  3. ↑ 3.0 3.1 3.2 Purser (2002), p. 2-90.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Van Rossum (2027), §8 (Method for determining ASET).
  5. ↑ 5.0 5.1 5.2 5.3 5.4 Ministry of Business, Innovation and Employment (2025b), p. 17.
  6. ↑ 6.0 6.1 6.2 6.3 Australian Building Codes Board (2022), p. 10.
  7. ↑ Van Rossum (2027), §7.
  8. ↑ 8.0 8.1 Purser (2002), p. 2-85.
  9. ↑ National Research Council of Canada et al. (2005), p. 0.5-2 (New Zealand).
  10. ↑ 10.0 10.1 10.2 Ministry of Business, Innovation and Employment (2025b), p. 18.
  11. ↑ 11.0 11.1 11.2 NEN 6060 and NEN 6079, as cited in Van Rossum (2027), §7.1.
  12. ↑ National Research Council of Canada et al. (2005), p. 1.2-22.
  13. ↑ Australian Building Codes Board (2020a), Data Sheet D1, p. 241.
  14. ↑ Australian Building Codes Board (2020b), pp. 63–64.
  15. ↑ Australian Building Codes Board (2020b), p. 69.
  16. ↑ Ministry of Business, Innovation and Employment (2025b), p. 14.
  17. ↑ 17.0 17.1 17.2 17.3 17.4 17.5 17.6 Van Rossum (2027), §7.7.
  18. ↑ 18.0 18.1 Ministry of Business, Innovation and Employment (2013), p. 28.
  19. ↑ Ministry of Business, Innovation and Employment (2013), p. 29.
  20. ↑ Australian Building Codes Board (2020b), p. 110.
  21. ↑ Purser (2002), p. 2-118.
  22. ↑ Van Rossum (2027), §7.6.
  23. ↑ Iqbal and Salley (2004), p. B-85.
  24. ↑ Stroup et al. (2016), p. 4-2.
  25. ↑ 25.0 25.1 Purser (2002), p. 2-133.
  26. ↑ Purser (2002), p. 2-136.
  27. ↑ 27.0 27.1 27.2 27.3 Ministry of Business, Innovation and Employment (2013), p. 98.
  28. ↑ 28.0 28.1 28.2 28.3 Ministry of Business, Innovation and Employment (2013), p. 100.
  29. ↑ Ministry of Business, Innovation and Employment (2013), p. 31.

References

  • Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
  • Australian Building Codes Board. (2020b). Handbook: Fire Safety Verification Method (Print version 1.1).
  • Australian Building Codes Board. (2022). Fire Safety Verification Method Standard 2022.
  • Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
  • Ministry of Business, Innovation and Employment. (2013). Commentary for Building Code clauses C1–C6 and Verification Method C/VM2.
  • Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
  • National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
  • Purser, D. A. (2002). Toxicity assessment of combustion products. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-83–2-171). National Fire Protection Association; Society of Fire Protection Engineers.
  • Stroup, D., Lindeman, A., McGrattan, K., Peacock, R., Overholt, K., Joglar, F., LeStrange, S., & Montanez, S. (2016). Verification and validation of selected fire models for nuclear power plant applications: Supplement 1 (NUREG-1824, Supplement 1; EPRI 3002002182). U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research; Electric Power Research Institute.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
Retrieved from "https://fire-safety.academy/w/index.php?title=Available_safe_egress_time&oldid=2605"