Detector activation time - Revision history

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	Fire detection and alarm systems can serve life safety, property protection, business continuity and environmental goals.<ref name="SMC-4-1">Schifiliti et al. (2002), p. 4-1.</ref> For life safety, the warning must come early enough to allow complete evacuation of the danger zone before conditions become untenable.<ref name="SMC-4-2">Schifiliti et al. (2002), p. 4-2.</ref> Schifiliti, Meacham and Custer therefore define a critical fire size: the heat release rate at which the fire must be detected to reach the design objective.<ref name="SMC-4-2" /> The time between this critical fire and the design limit is the total system response time.<ref name="SMC-4-2" /> It is the sum of fixed delays such as alarm verification, variable transport delays and the response lag of the actions that follow.<ref name="SMC-4-2" /> The transport delays depend on the distance from the fire to the detector, the ceiling height and the convective heat release rate.<ref name="SMC-4-2" />		Fire detection and alarm systems can serve life safety, property protection, business continuity and environmental goals.<ref name="SMC-4-1">Schifiliti et al. (2002), p. 4-1.</ref> For life safety, the warning must come early enough to allow complete evacuation of the danger zone before conditions become untenable.<ref name="SMC-4-2">Schifiliti et al. (2002), p. 4-2.</ref> Schifiliti, Meacham and Custer therefore define a critical fire size: the heat release rate at which the fire must be detected to reach the design objective.<ref name="SMC-4-2" /> The time between this critical fire and the design limit is the total system response time.<ref name="SMC-4-2" /> It is the sum of fixed delays such as alarm verification, variable transport delays and the response lag of the actions that follow.<ref name="SMC-4-2" /> The transport delays depend on the distance from the fire to the detector, the ceiling height and the convective heat release rate.<ref name="SMC-4-2" />
			[[File:fse-detector-activation-time-v1.svg\|thumb\|upright=1.5\|alt=Rising fire curve marking the critical fire, the delays after detection and the design limit, above a bar of the four parts of RSET.\|Detection on a design fire curve: the fire must be detected at the critical fire size so that all later actions finish before the design limit. Based on Schifiliti et al. (2002), pp. 4-2 to 4-3, and Ministry of Business, Innovation and Employment (2025b), p. 26.]]

	Prescriptive rules place detectors at a fixed spacing, for example one smoke detector per 84 m² or one every 9 m.<ref name="SMC-4-2" /> Such rules do not state how quickly the detectors respond or how large the fire is at that moment.<ref name="SMC-4-2" /> The listed spacing of a heat detector comes from a full-scale test with an alcohol pan fire under sprinklers rated at 71 °C on a 3 m grid.<ref name="SMC-4-3">Schifiliti et al. (2002), p. 4-3.</ref> The fire is set so that a sprinkler operates after about 2 minutes, and the listed spacing is the largest at which the detector operates first.<ref name="SMC-4-3" /> According to the same authors, this code approach does not allow a quantitative assessment of detector response.<ref name="SMC-4-4">Schifiliti et al. (2002), p. 4-4.</ref>		Prescriptive rules place detectors at a fixed spacing, for example one smoke detector per 84 m² or one every 9 m.<ref name="SMC-4-2" /> Such rules do not state how quickly the detectors respond or how large the fire is at that moment.<ref name="SMC-4-2" /> The listed spacing of a heat detector comes from a full-scale test with an alcohol pan fire under sprinklers rated at 71 °C on a 3 m grid.<ref name="SMC-4-3">Schifiliti et al. (2002), p. 4-3.</ref> The fire is set so that a sprinkler operates after about 2 minutes, and the listed spacing is the largest at which the detector operates first.<ref name="SMC-4-3" /> According to the same authors, this code approach does not allow a quantitative assessment of detector response.<ref name="SMC-4-4">Schifiliti et al. (2002), p. 4-4.</ref>
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	== Heat transfer to a heat detector or sprinkler ==		== Heat transfer to a heat detector or sprinkler ==
	Heat reaches the sensing element of a heat detector or sprinkler by conduction, convection and radiation.<ref name="SMC-4-5" /> In the early stage of a fire, radiation can be neglected.<ref name="SMC-4-5" /> The elements of most heat detectors are thermally isolated from the rest of the unit, so convection from the hot gas dominates.<ref name="SMC-4-5" /> The element is treated as a lumped mass, which means that its temperature is assumed to be the same throughout.<ref name="IS-12-6">Iqbal and Salley (2004), p. 12-6.</ref> Its heating is described by a time constant that grows with its mass and specific heat and falls with its surface area and heat transfer coefficient.<ref name="SMC-4-5" />		Heat reaches the sensing element of a heat detector or sprinkler by conduction, convection and radiation.<ref name="SMC-4-5" /> In the early stage of a fire, radiation can be neglected.<ref name="SMC-4-5" /> The elements of most heat detectors are thermally isolated from the rest of the unit, so convection from the hot gas dominates.<ref name="SMC-4-5" /> The element is treated as a lumped mass, which means that its temperature is assumed to be the same throughout.<ref name="IS-12-6">Iqbal and Salley (2004), p. 12-6.</ref> Its heating is described by a time constant that grows with its mass and specific heat and falls with its surface area and heat transfer coefficient.<ref name="SMC-4-5" />
			[[File:fse-detector-activation-time-v2.svg\|thumb\|upright=1.5\|alt=Cross-section of a heat detector under a ceiling: hot gas flows past the element; thick arrow for convection, thin arrows for radiation and conduction.\|Heat transfer to a ceiling-mounted heat detector: early in a fire, convection from the ceiling jet dominates, while radiation and conduction to the detector body are small. Based on Schifiliti et al. (2002), pp. 4-5 to 4-6.]]

	The heat transfer coefficient grows roughly with the square root of the gas velocity. The product of the time constant and the square root of the velocity is therefore nearly constant for a given device.<ref name="SMC-4-5" /> This product is the response time index (RTI), with units of m<sup>1/2</sup>s<sup>1/2</sup>.<ref name="SMC-4-6" /> It is measured in a plunge test, in which the device is suddenly lowered into a stream of hot gas with a constant temperature and velocity.<ref name="SMC-4-6" /> Generic values range from 34 m<sup>1/2</sup>s<sup>1/2</sup> for a quick-response sprinkler link to 235 m<sup>1/2</sup>s<sup>1/2</sup> for a standard-response bulb.<ref name="IS-10-8">Iqbal and Salley (2004), p. 10-8.</ref> For listed heat detectors, NFPA 72 gives time constants derived from the listing tests, which convert to RTI values with a reference velocity of 1.5 m/s.<ref name="IS-12-7">Iqbal and Salley (2004), p. 12-7.</ref><ref name="IS-12-8">Iqbal and Salley (2004), p. 12-8.</ref>		The heat transfer coefficient grows roughly with the square root of the gas velocity. The product of the time constant and the square root of the velocity is therefore nearly constant for a given device.<ref name="SMC-4-5" /> This product is the response time index (RTI), with units of m<sup>1/2</sup>s<sup>1/2</sup>.<ref name="SMC-4-6" /> It is measured in a plunge test, in which the device is suddenly lowered into a stream of hot gas with a constant temperature and velocity.<ref name="SMC-4-6" /> Generic values range from 34 m<sup>1/2</sup>s<sup>1/2</sup> for a quick-response sprinkler link to 235 m<sup>1/2</sup>s<sup>1/2</sup> for a standard-response bulb.<ref name="IS-10-8">Iqbal and Salley (2004), p. 10-8.</ref> For listed heat detectors, NFPA 72 gives time constants derived from the listing tests, which convert to RTI values with a reference velocity of 1.5 m/s.<ref name="IS-12-7">Iqbal and Salley (2004), p. 12-7.</ref><ref name="IS-12-8">Iqbal and Salley (2004), p. 12-8.</ref>
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	<math display="block">t_\mathrm{act} = \frac{55}{\sqrt{2.4}} \ln \left( \frac{153 - 20}{153 - 57} \right) = 35.5 \times 0.326 \approx 12\ \mathrm{s}</math>		<math display="block">t_\mathrm{act} = \frac{55}{\sqrt{2.4}} \ln \left( \frac{153 - 20}{153 - 57} \right) = 35.5 \times 0.326 \approx 12\ \mathrm{s}</math>
	A detector with the RTI of 30 m<sup>1/2</sup>s<sup>1/2</sup> that C/VM2 prescribes for heat detectors would respond in about 6.3 s in the same fire.<ref name="MBIE-27" /> From the same values, the time constant of the detector element in this flow, its RTI divided by the square root of the gas velocity, is about 35.5 s.<ref name="SMC-4-5" /> The calculated activation time is much shorter than this, because the gas at 153 °C is far hotter than the 57 °C rating, so the logarithmic term in the equation stays small.<ref name="SMC-4-8" /> Such short times also arise because the fire is assumed to burn at its full size from the start.<ref name="SMC-4-8" />		A detector with the RTI of 30 m<sup>1/2</sup>s<sup>1/2</sup> that C/VM2 prescribes for heat detectors would respond in about 6.3 s in the same fire.<ref name="MBIE-27" /> From the same values, the time constant of the detector element in this flow, its RTI divided by the square root of the gas velocity, is about 35.5 s.<ref name="SMC-4-5" /> The calculated activation time is much shorter than this, because the gas at 153 °C is far hotter than the 57 °C rating, so the logarithmic term in the equation stays small.<ref name="SMC-4-8" /> Such short times also arise because the fire is assumed to burn at its full size from the start.<ref name="SMC-4-8" />
			[[File:fse-detector-activation-time-v3.svg\|thumb\|upright=1.5\|alt=Line chart: element temperatures for RTI 55 and RTI 30 rise towards a 153 °C gas temperature and cross the 57 °C rating after 11.6 s and 6.3 s.\|Temperature of the sensing element of two heat detectors in a steady ceiling jet of 153 °C and 2.4 m/s; the 57 °C rating is reached after about 11.6 s (RTI 55) and 6.3 s (RTI 30). Based on Schifiliti et al. (2002), p. 4-8.]]

	The same authors show a quasi-steady calculation for a stack of wood pallets under the same ceiling, with the detector again 6 m from the fire.<ref name="SMC-4-9" /> The heat release rate rises from 5 kW after 10 s to 675 kW after 120 s. By then the detector has warmed only from 20 °C to about 32 °C, so a 57 °C detector has not yet responded.<ref name="SMC-4-9" /> Continuing the calculation with the tabulated heat release rates gives about 46 °C after 180 s, when the fire has reached 1,519 kW.<ref name="SMC-4-9" />		The same authors show a quasi-steady calculation for a stack of wood pallets under the same ceiling, with the detector again 6 m from the fire.<ref name="SMC-4-9" /> The heat release rate rises from 5 kW after 10 s to 675 kW after 120 s. By then the detector has warmed only from 20 °C to about 32 °C, so a 57 °C detector has not yet responded.<ref name="SMC-4-9" /> Continuing the calculation with the tabulated heat release rates gives about 46 °C after 180 s, when the fire has reached 1,519 kW.<ref name="SMC-4-9" />

	A third example compares a sprinkler and a heat detector under a 4 m ceiling.<ref name="SMC-4-21" /> The sprinkler is rated at 74 °C with an RTI of 110 m<sup>1/2</sup>s<sup>1/2</sup>, the detector at 57 °C with an RTI of 42 m<sup>1/2</sup>s<sup>1/2</sup>.<ref name="SMC-4-21" /> In a fast fire with a growth time of 50 s, the detector responds after 50 s and the sprinkler after 65 s.<ref name="SMC-4-21" /> In a slow fire with a growth time of 600 s, they respond after 300 s and 370 s.<ref name="SMC-4-21" /> The detector thus responds when the fast fire has reached about 1.06 MW, but the slow fire only about 0.27 MW (both values rounded up, to the safe side).<ref name="SMC-4-10" />		A third example compares a sprinkler and a heat detector under a 4 m ceiling.<ref name="SMC-4-21" /> The sprinkler is rated at 74 °C with an RTI of 110 m<sup>1/2</sup>s<sup>1/2</sup>, the detector at 57 °C with an RTI of 42 m<sup>1/2</sup>s<sup>1/2</sup>.<ref name="SMC-4-21" /> In a fast fire with a growth time of 50 s, the detector responds after 50 s and the sprinkler after 65 s.<ref name="SMC-4-21" /> In a slow fire with a growth time of 600 s, they respond after 300 s and 370 s.<ref name="SMC-4-21" /> The detector thus responds when the fast fire has reached about 1.06 MW, but the slow fire only about 0.27 MW (both values rounded up, to the safe side).<ref name="SMC-4-10" />
			[[File:fse-detector-activation-time-v4.svg\|thumb\|upright=1.5\|alt=Two rising fire curves, fast and slow, with markers where a heat detector and a sprinkler respond; the fast fire is much larger at response.\|Heat release rate of a fast and a slow t-squared fire with the response of a heat detector and a sprinkler under a 4 m ceiling. Based on Schifiliti et al. (2002), pp. 4-10 and 4-21.]]

	NUREG-1805 compares three methods for a smoke detector 3.05 m from a steady 1,000 kW pool fire in a compartment 3.96 m high.<ref name="IS-11-14">Iqbal and Salley (2004), p. 11-14.</ref> The method of Alpert gives 0.42 s, the method of Mowrer 0.74 s and the method of Milke 0.26 s.<ref name="IS-11-14" /> Mowrer's method, for example, adds the time for the smoke to rise to the ceiling and the time to travel along it to the detector.<ref name="IS-11-9">Iqbal and Salley (2004), p. 11-9.</ref> These results only describe smoke transport from a fire that is steady from the start.<ref name="IS-11-11">Iqbal and Salley (2004), p. 11-11.</ref> A medium t-squared fire, by comparison, needs about 300 s to grow to 1,055 kW.<ref name="Alpert-2-24" />		NUREG-1805 compares three methods for a smoke detector 3.05 m from a steady 1,000 kW pool fire in a compartment 3.96 m high.<ref name="IS-11-14">Iqbal and Salley (2004), p. 11-14.</ref> The method of Alpert gives 0.42 s, the method of Mowrer 0.74 s and the method of Milke 0.26 s.<ref name="IS-11-14" /> Mowrer's method, for example, adds the time for the smoke to rise to the ceiling and the time to travel along it to the detector.<ref name="IS-11-9">Iqbal and Salley (2004), p. 11-9.</ref> These results only describe smoke transport from a fire that is steady from the start.<ref name="IS-11-11">Iqbal and Salley (2004), p. 11-11.</ref> A medium t-squared fire, by comparison, needs about 300 s to grow to 1,055 kW.<ref name="Alpert-2-24" />

Mark: FSE wiki: corrections after independent fact-check (written by Claude Opus 5.5)

2026-11-01T23:23:01Z

FSE wiki: corrections after independent fact-check (written by Claude Opus 5.5)

Mark: New FSE wiki article: Detector activation time (written by Claude Opus 5.5)

2026-11-01T22:13:29Z

New FSE wiki article: Detector activation time (written by Claude Opus 5.5)

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'''Detector activation time''' is the time from the start of a fire until an automatic fire detector, or a sprinkler acting as a detector, responds to it.<ref name="SMC-4-12">Schifiliti et al. (2002), p. 4-12.</ref><ref name="MBIE-26">Ministry of Business, Innovation and Employment (2025b), p. 26.</ref> In performance-based design it is the first part of the [[required safe egress time]], because warning and escape can only start once the fire has been detected.<ref name="MBIE-26" /><ref name="VR-7.3">Van Rossum (2027), §7.3.</ref> A prediction needs the growth rate of the fire, the characteristics of the detector and the geometry of the building.<ref name="IFEG-2.7-2">National Research Council of Canada et al. (2005), p. 2.7-2.</ref>

For a [[heat detector]] or a sprinkler, the calculation is a heat balance. Hot gas in the [[ceiling jet]] warms the sensing element until it reaches its operating temperature.<ref name="SMC-4-5">Schifiliti et al. (2002), p. 4-5.</ref> How fast the element warms is expressed by its [[response time index]].<ref name="SMC-4-6">Schifiliti et al. (2002), p. 4-6.</ref> Smoke detectors are much harder to model, and engineers often treat them as very sensitive heat detectors that respond to a temperature rise of about 10 to 13 °C.<ref name="IS-11-7">Iqbal and Salley (2004), p. 11-7.</ref><ref name="IFEG-2.7-5">National Research Council of Canada et al. (2005), p. 2.7-5.</ref> Validation against experiments shows a wide scatter: for smoke detectors, the predicted activation times have a relative standard deviation of 0.58.<ref name="Stroup-6-57">Stroup et al. (2016), p. 6-57.</ref>

== Role in fire safety design ==
In New Zealand, C/VM2 (second edition, 2025) writes the required safe egress time as a sum of four parts.<ref name="MBIE-26" /> These are the detection time, the time to notify the occupants, the time before they start to move, and the time spent moving to a place of safety.<ref name="MBIE-26" /> The detection time in this sum must come from deterministic modelling of automatic detection or sprinkler activation.<ref name="MBIE-26" /> In the ''Introduction to Fire Safety Engineering'', the first warning for occupants likewise follows from detection and alarm, given either by installed systems or by people.<ref name="VR-7.3" />

Fire detection and alarm systems can serve life safety, property protection, business continuity and environmental goals.<ref name="SMC-4-1">Schifiliti et al. (2002), p. 4-1.</ref> For life safety, the warning must come early enough to allow complete evacuation of the danger zone before conditions become untenable.<ref name="SMC-4-2">Schifiliti et al. (2002), p. 4-2.</ref> Schifiliti, Meacham and Custer therefore define a critical fire size: the heat release rate at which the fire must be detected to reach the design objective.<ref name="SMC-4-2" /> The time between this critical fire and the design limit is the total system response time.<ref name="SMC-4-2" /> It is the sum of fixed delays such as alarm verification, variable transport delays and the response lag of the actions that follow.<ref name="SMC-4-2" /> The transport delays depend on the distance from the fire to the detector, the ceiling height and the convective heat release rate.<ref name="SMC-4-2" />

Prescriptive rules place detectors at a fixed spacing, for example one smoke detector per 84 m² or one every 9 m.<ref name="SMC-4-2" /> Such rules do not state how quickly the detectors respond or how large the fire is at that moment.<ref name="SMC-4-2" /> The listed spacing of a heat detector comes from a full-scale test with an alcohol pan fire under sprinklers rated at 71 °C on a 3 m grid.<ref name="SMC-4-3">Schifiliti et al. (2002), p. 4-3.</ref> The fire is set so that a sprinkler operates after about 2 minutes, and the listed spacing is the largest at which the detector operates first.<ref name="SMC-4-3" /> According to the same authors, this code approach does not allow a quantitative assessment of detector response.<ref name="SMC-4-4">Schifiliti et al. (2002), p. 4-4.</ref>

== Heat transfer to a heat detector or sprinkler ==
Heat reaches the sensing element of a heat detector or sprinkler by conduction, convection and radiation.<ref name="SMC-4-5" /> In the early stage of a fire, radiation can be neglected.<ref name="SMC-4-5" /> The elements of most heat detectors are thermally isolated from the rest of the unit, so convection from the hot gas dominates.<ref name="SMC-4-5" /> The element is treated as a lumped mass, which means that its temperature is assumed to be the same throughout.<ref name="IS-12-6">Iqbal and Salley (2004), p. 12-6.</ref> Its heating is described by a time constant that grows with its mass and specific heat and falls with its surface area and heat transfer coefficient.<ref name="SMC-4-5" />

The heat transfer coefficient grows roughly with the square root of the gas velocity. The product of the time constant and the square root of the velocity is therefore nearly constant for a given device.<ref name="SMC-4-5" /> This product is the response time index (RTI), with units of m1/2s1/2.<ref name="SMC-4-6" /> It is measured in a plunge test, in which the device is suddenly lowered into a stream of hot gas with a constant temperature and velocity.<ref name="SMC-4-6" /> Generic values range from 34 m1/2s1/2 for a quick-response sprinkler link to 235 m1/2s1/2 for a standard-response bulb.<ref name="IS-10-8">Iqbal and Salley (2004), p. 10-8.</ref> For listed heat detectors, NFPA 72 gives time constants derived from the listing tests, which convert to RTI values with a reference velocity of 1.5 m/s.<ref name="IS-12-7">Iqbal and Salley (2004), p. 12-7.</ref><ref name="IS-12-8">Iqbal and Salley (2004), p. 12-8.</ref>

With the RTI known, the element temperature follows from a heat balance. The element warms at a rate set by the square root of the gas velocity and by the difference between gas and element temperature, divided by the RTI.<ref name="SMC-4-6" /><ref name="IFEG-2.7-3">National Research Council of Canada et al. (2005), p. 2.7-3.</ref> A fixed-temperature device activates when the element reaches its rated temperature.<ref name="IFEG-2.7-3" /> When the gas temperature and velocity stay constant, the equation can be solved directly for the activation time:<ref name="IS-10-9">Iqbal and Salley (2004), p. 10-9.</ref><ref name="SMC-4-8">Schifiliti et al. (2002), p. 4-8.</ref>
<math display="block">\frac{\mathrm{d}T_\mathrm{d}}{\mathrm{d}t} = \frac{\sqrt{u}\,(T_\mathrm{g} - T_\mathrm{d})}{\mathrm{RTI}}</math>
<math display="block">t_\mathrm{act} = \frac{\mathrm{RTI}}{\sqrt{u}} \ln \left( \frac{T_\mathrm{g} - T_\mathrm{a}}{T_\mathrm{g} - T_\mathrm{act}} \right)</math>

{| class="wikitable"
! Symbol !! Description !! Unit
|-
| <math>T_\mathrm{d}</math> || temperature of the sensing element || °C
|-
| <math>T_\mathrm{g}</math> || gas temperature at the detector || °C
|-
| <math>T_\mathrm{a}</math> || ambient (initial) temperature || °C
|-
| <math>T_\mathrm{act}</math> || rated activation temperature || °C
|-
| <math>u</math> || gas velocity at the detector || m/s
|-
| <math>\mathrm{RTI}</math> || response time index || m1/2s1/2
|-
| <math>t</math> || time || s
|-
| <math>t_\mathrm{act}</math> || activation time || s
|}

Not every heat detector behaves like a lumped mass. Rate-of-rise detectors respond when the temperature rises faster than a set rate, typically about 7 to 8 °C per minute.<ref name="IS-12-4">Iqbal and Salley (2004), p. 12-4.</ref> Most of them contain an air chamber with a small vent, so the RTI is only an approximation of their response.<ref name="SMC-4-7">Schifiliti et al. (2002), p. 4-7.</ref> Rate-compensated detectors tend to respond when the surrounding gas reaches their rated temperature, as long as it rises by less than about 22 °C per minute.<ref name="SMC-4-7" /> Heat detectors have the lowest false alarm rate of the automatic detectors, but they are generally the slowest.<ref name="IS-12-1">Iqbal and Salley (2004), p. 12-1.</ref>

== Gas conditions at the detector ==
The heat balance needs the gas temperature and velocity at the device, which under a flat ceiling means the ceiling jet.<ref name="SMC-4-4" /> For steady fires, the [[Alpert correlations]] give both values from the heat release rate, the ceiling height and the radial distance from the fire axis.<ref name="SMC-4-7" /> They assume that the gas at the detector follows the instantaneous heat release rate, so they ignore the time the gas needs to travel from the fire.<ref name="SMC-4-8" /> The height of the device below the ceiling matters too. In tests with small pan fires about 4.3 m below a ceiling, Marrion found the highest temperatures and velocities between 25 and 76 mm below the ceiling.<ref name="SMC-4-7" />

A growing fire can be split into short intervals with a constant heat release rate, which is called quasi-steady modelling.<ref name="SMC-4-8" /> For each interval, the gas temperature and velocity are calculated, and the heat balance gives the detector temperature at the end of the interval.<ref name="SMC-4-9">Schifiliti et al. (2002), p. 4-9.</ref> The calculation continues until the detector reaches its operating temperature.<ref name="SMC-4-9" /> The computer program DETACT-QS by Evans and Stroup uses this method with Alpert's equations.<ref name="SMC-4-10">Schifiliti et al. (2002), p. 4-10.</ref>

Many fires grow roughly with the square of time, and such a [[t-squared fire]] has its own ceiling jet correlations. Heskestad and Delichatsios developed them to include the moment at which the heat front reaches the detector.<ref name="SMC-4-10" /> NFPA 72 describes these fires by a fire growth time, the time needed to reach 1,055 kW.<ref name="SMC-4-10" /> Its slow, medium, fast and ultrafast fires have growth coefficients of 0.00293, 0.01172, 0.0469 and 0.1876 kW/s².<ref name="Alpert-2-24">Alpert (2002), p. 2-24.</ref> These fires therefore reach 1,055 kW after about 600, 300, 150 and 75 s.<ref name="Alpert-2-24" /> Beyler later solved the detector heat balance for these correlations analytically, which allows the required spacing to be found by iteration.<ref name="SMC-4-11">Schifiliti et al. (2002), p. 4-11.</ref>

== Smoke detectors ==
The response of a [[smoke detector]] is much harder to predict than that of a heat detector.<ref name="SMC-4-22">Schifiliti et al. (2002), p. 4-22.</ref> It depends on the smoke particles, their transport to the detector, the aerodynamics of the detector housing and the type of sensor.<ref name="SMC-4-21">Schifiliti et al. (2002), p. 4-21.</ref> Ionisation and scattering detectors do not work by light attenuation, while research data mostly describe smoke by its optical density, which is a measure of attenuation.<ref name="SMC-4-22" /> Detectors also use response algorithms that manufacturers generally do not publish.<ref name="SMC-4-22" /> For smoke detectors there is therefore no measure that plays the role of the RTI.<ref name="IS-11-6">Iqbal and Salley (2004), p. 11-6.</ref>

The most common approximation treats a smoke detector as a sensitive heat detector with little or no thermal lag.<ref name="IS-11-6" /> NUREG-1805 uses a temperature rise of 10 °C, based on work by Heskestad and Delichatsios for typical fuels.<ref name="IS-11-7" /> The International Fire Engineering Guidelines (2005) use 13 °C above ambient with an RTI below 10 m1/2s1/2.<ref name="IFEG-2.7-5" /> The 2016 validation study for nuclear power plant fire models used an RTI of 5 m1/2s1/2 and a rise of 10 °C, as do the zone models CFAST and MAGIC.<ref name="Stroup-6-53">Stroup et al. (2016), p. 6-53.</ref> This approach assumes that temperature rise and smoke concentration go together.<ref name="IFEG-2.7-5" />

A second approach compares the optical density of the smoke at the detector with the density needed for an alarm.<ref name="IFEG-2.7-5" /> C/VM2 uses an optical density at alarm of 0.097 m−1 for spot and air sampling detectors, a value for normal sensitivity.<ref name="MBIE-27">Ministry of Business, Innovation and Employment (2025b), p. 27.</ref><ref name="MBIE-28">Ministry of Business, Innovation and Employment (2025b), p. 28.</ref> The value at alarm depends strongly on the smoke. A scattering detector that alarms at 0.029 m−1 in smoke from a smouldering cotton wick may need 0.15 m−1 in the darker smoke of a kerosene fire.<ref name="SMC-4-23">Schifiliti et al. (2002), p. 4-23.</ref>

Smoke also needs time to enter the sensing chamber. Heskestad described this entry lag as a characteristic length of the detector divided by the gas velocity, so the lag grows when the ceiling jet is slow.<ref name="SMC-4-24">Schifiliti et al. (2002), p. 4-24.</ref><ref name="McG-108">McGrattan et al. (2026), p. 108.</ref> Residential detectors respond well to flaming fires that create gas velocities above 0.25 m/s, but may fail in smouldering fires with velocities below 0.15 m/s.<ref name="IS-11-2">Iqbal and Salley (2004), p. 11-2.</ref> Aspirating systems can be up to ten times as sensitive as spot detectors, but dilution and travel through the pipework usually delay them considerably.<ref name="IFEG-2.7-6">National Research Council of Canada et al. (2005), p. 2.7-6.</ref> For gas sensing detectors, the guidelines know of no accepted method to determine activation times.<ref name="IFEG-2.7-6" />

== Worked examples ==
Schifiliti, Meacham and Custer give the example of a kerosene pool fire of 4 m² under a flat ceiling 6 m high, in a room at 20 °C.<ref name="SMC-4-8" /> With a burning rate of 67 g/(m²·s) and a heat of combustion of 40.3 kJ/g, the heat release rate is about 10,800 kW. A heat detector with an RTI of 55 m1/2s1/2 and a rating of 57 °C hangs 6 m from the fire axis. The Alpert correlations give a gas temperature of 153 °C and a velocity of 2.4 m/s at the detector.<ref name="SMC-4-8" /> The steady-state equation then gives the activation time:
<math display="block">t_\mathrm{act} = \frac{55}{\sqrt{2.4}} \ln \left( \frac{153 - 20}{153 - 57} \right) = 35.5 \times 0.326 \approx 12\ \mathrm{s}</math>
A detector with the RTI of 30 m1/2s1/2 that C/VM2 prescribes for heat detectors would respond in about 6.3 s in the same fire.<ref name="MBIE-27" /> Both times are shorter than the time constant of the first element in this flow, about 35.5 s, because the gas is far hotter than the rating.<ref name="SMC-4-8" /> Such short times also arise because the fire is assumed to burn at its full size from the start.<ref name="SMC-4-8" />

The same authors show a quasi-steady calculation for a stack of wood pallets under the same ceiling, with the detector again 6 m from the fire.<ref name="SMC-4-9" /> The heat release rate rises from 5 kW after 10 s to 675 kW after 120 s. By then the detector has warmed only from 20 °C to about 32 °C, so a 57 °C detector has not yet responded.<ref name="SMC-4-9" /> Continuing the calculation with the tabulated heat release rates gives about 46 °C after 180 s, when the fire has reached 1,519 kW.<ref name="SMC-4-9" />

A third example compares a sprinkler and a heat detector under a 4 m ceiling.<ref name="SMC-4-21" /> The sprinkler is rated at 74 °C with an RTI of 110 m1/2s1/2, the detector at 57 °C with an RTI of 42 m1/2s1/2.<ref name="SMC-4-21" /> In a fast fire with a growth time of 50 s, the detector responds after 50 s and the sprinkler after 65 s.<ref name="SMC-4-21" /> In a slow fire with a growth time of 600 s, they respond after 300 s and 370 s.<ref name="SMC-4-21" /> The detector thus responds when the fast fire has reached about 1.06 MW, but the slow fire only about 0.27 MW.<ref name="SMC-4-10" />

NUREG-1805 compares three methods for a smoke detector 3.05 m from a steady 1,000 kW pool fire in a compartment 3.96 m high.<ref name="IS-11-14">Iqbal and Salley (2004), p. 11-14.</ref> The method of Alpert gives 0.42 s, the method of Mowrer 0.74 s and the method of Milke 0.26 s.<ref name="IS-11-14" /> Mowrer's method, for example, adds the time for the smoke to rise to the ceiling and the time to travel along it to the detector.<ref name="IS-11-9">Iqbal and Salley (2004), p. 11-9.</ref> These results only describe smoke transport from a fire that is steady from the start.<ref name="IS-11-11">Iqbal and Salley (2004), p. 11-11.</ref> A medium t-squared fire, by comparison, needs about 300 s to grow to 1,055 kW.<ref name="Alpert-2-24" />

== Requirements and design methods ==
In New Zealand, C/VM2 (second edition, 2025) requires the detection time of an automatic system to be modelled with a ceiling jet correlation or with a CFD model.<ref name="MBIE-27" /> Detectors must be modelled with fixed values, regardless of the make and installation of the devices that will actually be fitted.<ref name="MBIE-27" /> A heat detector, for example, has an RTI of 30 m1/2s1/2, an activation temperature of 57 °C and a radial distance of 4.2 m.<ref name="MBIE-27" /> Smoke detectors are placed at a radial distance of 7.0 m, and all devices at least 25 mm below the ceiling.<ref name="MBIE-27" />

C/VM2 allows a manual alarm only in spaces with a ceiling height of at least 5 m, with fewer than 50 occupants who are awake and familiar with the building.<ref name="MBIE-28" /> Occupants are then assumed to become aware of the fire once the ceiling jet has crossed the whole space.<ref name="MBIE-28" /> For storage up to 5 m high, this time in seconds is 10 plus 2.4 times the length of the space, when the length is at most 1.4 times the width.<ref name="MBIE-28" /> A space 20 m long and 15 m wide would therefore give a detection time of 58 s.<ref name="MBIE-28" />

Schifiliti, Meacham and Custer distinguish two kinds of problem.<ref name="SMC-4-11" /> In a design problem, the required response time or fire size is given, and the engineer searches for the detector spacing. That spacing equals 1.41 times the largest radial distance that still gives activation in time.<ref name="SMC-4-11" /> In an analysis problem, the spacing is known and the activation time or the fire size at activation is calculated.<ref name="SMC-4-11" /><ref name="SMC-4-12" /> In one of their examples, a 50% larger sprinkler spacing, from 3.0 to 4.6 m, raised the fire size at activation by about 25%.<ref name="SMC-4-21" />

== Limits and uncertainty ==
Many sources of error affect these calculations, including uncertainty in the operating temperature, the ambient temperature and the ceiling jet correlations.<ref name="SMC-4-9" /> Because the size of these errors is largely unknown, the SFPE Handbook chapter states that a confidence interval for the result cannot be estimated.<ref name="SMC-4-9" /> The RTI concept is itself a simplification. The heat transfer coefficient varies with the Reynolds number to a power between 0.330 and 0.805, and up to 10% of the heat gained may be lost by conduction to the detector body.<ref name="SMC-4-6" /> The heat detector model in FDS does not account for thermal radiation.<ref name="McG-107">McGrattan et al. (2026), p. 107.</ref>

For t-squared fires, errors in the predicted dimensionless gas temperature can reach 50%, which corresponds to 5 to 10 °C.<ref name="SMC-4-12" /> The largest errors occur for fast fires and low ceilings.<ref name="SMC-4-12" /> The ceiling jet methods also assume a flat ceiling without walls. In one handbook example, the leading edge of the ceiling jet travelled about 48 m in 120 s. The authors use such a distance to check whether the limits of the model have been exceeded.<ref name="SMC-4-9" /> NUREG-1805 adds that detector response times increase as forced ventilation increases.<ref name="IS-11-11" />

The 2016 supplement to NUREG-1824 compared predictions with experiments.<ref name="Stroup-6-57" /> For sprinkler activation time, the empirical correlation had a bias factor of 1.11 and a relative standard deviation of 0.41.<ref name="Stroup-6-57" /> For smoke detector activation with the temperature rise method, these values were 1.07 and 0.58, while the experimental uncertainty itself was 0.34.<ref name="Stroup-6-57" /> The evaluation only included sprinklers under smooth, flat and horizontal ceilings, and smoke detectors in the room of origin.<ref name="Stroup-6-58">Stroup et al. (2016), p. 6-58.</ref>

For smoke detectors, the SFPE Handbook chapter goes further. Because the accuracy of the estimation methods is unknown, it advises against comparing their results with egress time calculations or with the time to untenable conditions.<ref name="SMC-4-23" /> It recommends using them instead to compare how the response of one detector changes with its spacing or location.<ref name="SMC-4-23" /> C/VM2, by contrast, uses the detection time directly in the required safe egress time, with fixed detector values for every design.<ref name="MBIE-26" /><ref name="MBIE-27" />

== See also ==
* [[Aspirating smoke detection]]
* [[Beam smoke detector]]
* [[Flame detector]]
* [[Multi-sensor detector]]
* [[Fire alarm system]]

== Notes ==
<references />

== References ==
* Alpert, R. L. (2002). Ceiling jet flows. 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. 2-18–2-31). National Fire Protection Association; Society of Fire Protection Engineers.
* 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.
* McGrattan, K., Hostikka, S., Floyd, J., McDermott, R., Vanella, M., Mueller, E., & Paul, C. (2026). ''Fire Dynamics Simulator technical reference guide: Vol. 1. Mathematical model'' (6th ed., NIST Special Publication 1018-1). National Institute of Standards and Technology. https://doi.org/10.6028/NIST.SP.1018
* 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.
* Schifiliti, R. P., Meacham, B. J., & Custer, R. L. P. (2002). Design of detection systems. 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. 4-1–4-43). 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.

[[Category:Detection & Alarm]]

← Older revision		Revision as of 00:23, 2 November 2026
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	'''Detector activation time''' is the time from the start of a fire until an automatic fire detector, or a sprinkler acting as a detector, responds to it.<ref name="SMC-4-12">Schifiliti et al. (2002), p. 4-12.</ref><ref name="MBIE-26">Ministry of Business, Innovation and Employment (2025b), p. 26.</ref> In performance-based design it is the first part of the [[required safe egress time]], because warning and escape can only start once the fire has been detected.<ref name="MBIE-26" /><ref name="VR-7.3">Van Rossum (2027), §7.3.</ref> A prediction needs the growth rate of the fire, the characteristics of the detector and the geometry of the building.<ref name="IFEG-2.7-2">National Research Council of Canada et al. (2005), p. 2.7-2.</ref>		'''Detector activation time''' is the time from the start of a fire until an automatic fire detector, or a sprinkler acting as a detector, responds to it.<ref name="SMC-4-12">Schifiliti et al. (2002), p. 4-12.</ref><ref name="MBIE-26">Ministry of Business, Innovation and Employment (2025b), p. 26.</ref> In performance-based design it is the first part of the [[required safe egress time]], because warning and escape can only start once the fire has been detected.<ref name="MBIE-26" /><ref name="VR-7.3">Van Rossum (2027), §7.3.</ref> A prediction needs the growth rate of the fire, the characteristics of the detector and the geometry of the building.<ref name="IFEG-2.7-2">National Research Council of Canada et al. (2005), p. 2.7-2.</ref>

	For a [[heat detector]] or a sprinkler, the calculation is a heat balance. Hot gas in the [[ceiling jet]] warms the sensing element until it reaches its operating temperature.<ref name="SMC-4-5">Schifiliti et al. (2002), p. 4-5.</ref> How fast the element warms is expressed by its [[response time index]].<ref name="SMC-4-6">Schifiliti et al. (2002), p. 4-6.</ref> Smoke detectors are much harder to model, and engineers often treat them as very sensitive heat detectors that respond to a temperature rise of about 10 to 13 °C.<ref name="IS-11-7">Iqbal and Salley (2004), p. 11-7.</ref><ref name="IFEG-2.7-5">National Research Council of Canada et al. (2005), p. 2.7-5.</ref> Validation against experiments shows a wide scatter~~: for~~ smoke detectors, the predicted activation times have a relative standard deviation of 0.58.<ref name="Stroup-6-57">Stroup et al. (2016), p. 6-57.</ref>		For a [[heat detector]] or a sprinkler, the calculation is a heat balance. Hot gas in the [[ceiling jet]] warms the sensing element until it reaches its operating temperature.<ref name="SMC-4-5">Schifiliti et al. (2002), p. 4-5.</ref> How fast the element warms is expressed by its [[response time index]].<ref name="SMC-4-6">Schifiliti et al. (2002), p. 4-6.</ref> Smoke detectors are much harder to model, and engineers often treat them as very sensitive heat detectors that respond to a temperature rise of about 10 to 13 °C.<ref name="IS-11-7">Iqbal and Salley (2004), p. 11-7.</ref><ref name="IFEG-2.7-5">National Research Council of Canada et al. (2005), p. 2.7-5.</ref> Validation against experiments shows a wide scatter. For smoke detectors modelled with the empirical temperature rise method, the predicted activation times have a relative standard deviation of 0.58, against 0.34 to 0.39 for the fire models CFAST, MAGIC and FDS.<ref name="Stroup-6-57">Stroup et al. (2016), p. 6-57.</ref>

	== Role in fire safety design ==		== Role in fire safety design ==
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	Schifiliti, Meacham and Custer give the example of a kerosene pool fire of 4 m² under a flat ceiling 6 m high, in a room at 20 °C.<ref name="SMC-4-8" /> With a burning rate of 67 g/(m²·s) and a heat of combustion of 40.3 kJ/g, the heat release rate is about 10,800 kW. A heat detector with an RTI of 55 m<sup>1/2</sup>s<sup>1/2</sup> and a rating of 57 °C hangs 6 m from the fire axis. The Alpert correlations give a gas temperature of 153 °C and a velocity of 2.4 m/s at the detector.<ref name="SMC-4-8" /> The steady-state equation then gives the activation time:		Schifiliti, Meacham and Custer give the example of a kerosene pool fire of 4 m² under a flat ceiling 6 m high, in a room at 20 °C.<ref name="SMC-4-8" /> With a burning rate of 67 g/(m²·s) and a heat of combustion of 40.3 kJ/g, the heat release rate is about 10,800 kW. A heat detector with an RTI of 55 m<sup>1/2</sup>s<sup>1/2</sup> and a rating of 57 °C hangs 6 m from the fire axis. The Alpert correlations give a gas temperature of 153 °C and a velocity of 2.4 m/s at the detector.<ref name="SMC-4-8" /> The steady-state equation then gives the activation time:
	<math display="block">t_\mathrm{act} = \frac{55}{\sqrt{2.4}} \ln \left( \frac{153 - 20}{153 - 57} \right) = 35.5 \times 0.326 \approx 12\ \mathrm{s}</math>		<math display="block">t_\mathrm{act} = \frac{55}{\sqrt{2.4}} \ln \left( \frac{153 - 20}{153 - 57} \right) = 35.5 \times 0.326 \approx 12\ \mathrm{s}</math>
	A detector with the RTI of 30 m<sup>1/2</sup>s<sup>1/2</sup> that C/VM2 prescribes for heat detectors would respond in about 6.3 s in the same fire.<ref name="MBIE-27" /> ~~Both times are shorter than~~ the time constant of the ~~first~~ element in this flow, about 35.5 s, because the gas is far hotter than the rating.<ref name="SMC-4-8" /> Such short times also arise because the fire is assumed to burn at its full size from the start.<ref name="SMC-4-8" />		A detector with the RTI of 30 m<sup>1/2</sup>s<sup>1/2</sup> that C/VM2 prescribes for heat detectors would respond in about 6.3 s in the same fire.<ref name="MBIE-27" /> From the same values, the time constant of the detector element in this flow, its RTI divided by the square root of the gas velocity, is about 35.5 s.<ref name="SMC-4-5" /> The calculated activation time is much shorter than this, because the gas at 153 °C is far hotter than the 57 °C rating, so the logarithmic term in the equation stays small.<ref name="SMC-4-8" /> Such short times also arise because the fire is assumed to burn at its full size from the start.<ref name="SMC-4-8" />

	The same authors show a quasi-steady calculation for a stack of wood pallets under the same ceiling, with the detector again 6 m from the fire.<ref name="SMC-4-9" /> The heat release rate rises from 5 kW after 10 s to 675 kW after 120 s. By then the detector has warmed only from 20 °C to about 32 °C, so a 57 °C detector has not yet responded.<ref name="SMC-4-9" /> Continuing the calculation with the tabulated heat release rates gives about 46 °C after 180 s, when the fire has reached 1,519 kW.<ref name="SMC-4-9" />		The same authors show a quasi-steady calculation for a stack of wood pallets under the same ceiling, with the detector again 6 m from the fire.<ref name="SMC-4-9" /> The heat release rate rises from 5 kW after 10 s to 675 kW after 120 s. By then the detector has warmed only from 20 °C to about 32 °C, so a 57 °C detector has not yet responded.<ref name="SMC-4-9" /> Continuing the calculation with the tabulated heat release rates gives about 46 °C after 180 s, when the fire has reached 1,519 kW.<ref name="SMC-4-9" />

	A third example compares a sprinkler and a heat detector under a 4 m ceiling.<ref name="SMC-4-21" /> The sprinkler is rated at 74 °C with an RTI of 110 m<sup>1/2</sup>s<sup>1/2</sup>, the detector at 57 °C with an RTI of 42 m<sup>1/2</sup>s<sup>1/2</sup>.<ref name="SMC-4-21" /> In a fast fire with a growth time of 50 s, the detector responds after 50 s and the sprinkler after 65 s.<ref name="SMC-4-21" /> In a slow fire with a growth time of 600 s, they respond after 300 s and 370 s.<ref name="SMC-4-21" /> The detector thus responds when the fast fire has reached about 1.06 MW, but the slow fire only about 0.27 MW.<ref name="SMC-4-10" />		A third example compares a sprinkler and a heat detector under a 4 m ceiling.<ref name="SMC-4-21" /> The sprinkler is rated at 74 °C with an RTI of 110 m<sup>1/2</sup>s<sup>1/2</sup>, the detector at 57 °C with an RTI of 42 m<sup>1/2</sup>s<sup>1/2</sup>.<ref name="SMC-4-21" /> In a fast fire with a growth time of 50 s, the detector responds after 50 s and the sprinkler after 65 s.<ref name="SMC-4-21" /> In a slow fire with a growth time of 600 s, they respond after 300 s and 370 s.<ref name="SMC-4-21" /> The detector thus responds when the fast fire has reached about 1.06 MW, but the slow fire only about 0.27 MW (both values rounded up, to the safe side).<ref name="SMC-4-10" />

	NUREG-1805 compares three methods for a smoke detector 3.05 m from a steady 1,000 kW pool fire in a compartment 3.96 m high.<ref name="IS-11-14">Iqbal and Salley (2004), p. 11-14.</ref> The method of Alpert gives 0.42 s, the method of Mowrer 0.74 s and the method of Milke 0.26 s.<ref name="IS-11-14" /> Mowrer's method, for example, adds the time for the smoke to rise to the ceiling and the time to travel along it to the detector.<ref name="IS-11-9">Iqbal and Salley (2004), p. 11-9.</ref> These results only describe smoke transport from a fire that is steady from the start.<ref name="IS-11-11">Iqbal and Salley (2004), p. 11-11.</ref> A medium t-squared fire, by comparison, needs about 300 s to grow to 1,055 kW.<ref name="Alpert-2-24" />		NUREG-1805 compares three methods for a smoke detector 3.05 m from a steady 1,000 kW pool fire in a compartment 3.96 m high.<ref name="IS-11-14">Iqbal and Salley (2004), p. 11-14.</ref> The method of Alpert gives 0.42 s, the method of Mowrer 0.74 s and the method of Milke 0.26 s.<ref name="IS-11-14" /> Mowrer's method, for example, adds the time for the smoke to rise to the ceiling and the time to travel along it to the detector.<ref name="IS-11-9">Iqbal and Salley (2004), p. 11-9.</ref> These results only describe smoke transport from a fire that is steady from the start.<ref name="IS-11-11">Iqbal and Salley (2004), p. 11-11.</ref> A medium t-squared fire, by comparison, needs about 300 s to grow to 1,055 kW.<ref name="Alpert-2-24" />
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	In New Zealand, C/VM2 (second edition, 2025) requires the detection time of an automatic system to be modelled with a ceiling jet correlation or with a CFD model.<ref name="MBIE-27" /> Detectors must be modelled with fixed values, regardless of the make and installation of the devices that will actually be fitted.<ref name="MBIE-27" /> A heat detector, for example, has an RTI of 30 m<sup>1/2</sup>s<sup>1/2</sup>, an activation temperature of 57 °C and a radial distance of 4.2 m.<ref name="MBIE-27" /> Smoke detectors are placed at a radial distance of 7.0 m, and all devices at least 25 mm below the ceiling.<ref name="MBIE-27" />		In New Zealand, C/VM2 (second edition, 2025) requires the detection time of an automatic system to be modelled with a ceiling jet correlation or with a CFD model.<ref name="MBIE-27" /> Detectors must be modelled with fixed values, regardless of the make and installation of the devices that will actually be fitted.<ref name="MBIE-27" /> A heat detector, for example, has an RTI of 30 m<sup>1/2</sup>s<sup>1/2</sup>, an activation temperature of 57 °C and a radial distance of 4.2 m.<ref name="MBIE-27" /> Smoke detectors are placed at a radial distance of 7.0 m, and all devices at least 25 mm below the ceiling.<ref name="MBIE-27" />

	C/VM2 allows a manual alarm only in spaces with a ceiling height of at least 5 m, with fewer than 50 occupants who are awake and familiar with the building.<ref name="MBIE-28" /> Occupants are then assumed to become aware of the fire once the ceiling jet has crossed the whole space.<ref name="MBIE-28" /> For storage up to 5 m high, ~~this~~ time in seconds is 10 plus 2.4 times the length of the space, ~~when~~ the length is at most 1.4 times the width.<ref name="MBIE-28" /> A space 20 m long and 15 m wide would therefore give a detection time of 58 s.<ref name="MBIE-28" />		C/VM2 allows a manual alarm only in spaces with a ceiling height of at least 5 m, with fewer than 50 occupants who are awake and familiar with the building.<ref name="MBIE-28" /> Occupants are then assumed to become aware of the fire once the ceiling jet has crossed the whole space.<ref name="MBIE-28" /> C/VM2 gives an equation for this time when zone modelling is used.<ref name="MBIE-28" /> For storage up to 5 m high, the time in seconds is 10 plus 2.4 times the length of the space, if the length is at most 1.4 times the width.<ref name="MBIE-28" /> A space 20 m long and 15 m wide would therefore give a detection time of 58 s.<ref name="MBIE-28" />

	Schifiliti, Meacham and Custer distinguish two kinds of problem.<ref name="SMC-4-11" /> In a design problem, the required response time or fire size is given, and the engineer searches for the detector spacing. That spacing equals 1.41 times the largest radial distance that still gives activation in time.<ref name="SMC-4-11" /> In an analysis problem, the spacing is known and the activation time or the fire size at activation is calculated.<ref name="SMC-4-11" /><ref name="SMC-4-12" /> In one of their examples, a 50% larger sprinkler spacing, from 3.0 to 4.6 m, raised the fire size at activation by about 25%.<ref name="SMC-4-21" />		Schifiliti, Meacham and Custer distinguish two kinds of problem.<ref name="SMC-4-11" /> In a design problem, the required response time or fire size is given, and the engineer searches for the detector spacing. That spacing equals 1.41 times the largest radial distance that still gives activation in time.<ref name="SMC-4-11" /> In an analysis problem, the spacing is known and the activation time or the fire size at activation is calculated.<ref name="SMC-4-11" /><ref name="SMC-4-12" /> In one of their examples, a 50% larger sprinkler spacing, from 3.0 to 4.6 m, raised the fire size at activation by about 25%.<ref name="SMC-4-21" />