Emergency lighting

Emergency lighting is lighting provided to support safe movement when normal lighting or its power supply is unavailable. In a fire, it can help occupants see the escape path, changes in level, doors and exit signs. Its contribution depends on the remaining smoke conditions and on whether the emergency supply and luminaires operate as intended.

The system addresses a different problem from fire detection or smoke control. Detection provides a warning and smoke control acts on the fire environment, while emergency lighting supports visual orientation and movement. These measures can influence one another, but the presence of one does not demonstrate that the others work.

Emergency lighting cannot guarantee a usable escape route in every smoke condition. Visibility can be reduced by extinction and irritation, and occupants can respond to unfamiliar routes in unexpected ways. The lighting arrangement must therefore be assessed together with signage, route design, smoke exposure and occupant response.

Purpose within an escape route

The basic function is to preserve useful visual information during an emergency. A person needs to identify the route and physical features while moving towards safety. Loss of normal illumination can make that task harder, especially where the route is unfamiliar or contains changes of direction.

Emergency lighting is not a substitute for a safe physical route. Means of escape also depends on travel paths, exit capacity, protected construction and usable doors. Lighting supports the use of those provisions rather than correcting an inadequate route.

Its role becomes more significant when other visual cues disappear. Normal room lighting may be unavailable while smoke reduces contrast and viewing distance. Emergency lighting can help preserve local visibility, but smoke still controls how much of a sign or surface can be seen.

Occupants do not always move immediately after an alarm. Proulx describes investigation, gathering family members and collecting belongings before departure. The lighting may therefore be needed throughout pre-movement activities as well as during directed travel along the route.

Normal and emergency power

Normal lighting and emergency lighting can have different supply arrangements. The US National Institute of Standards and Technology (NIST) describes emergency power serving lighting circuits and individual units with backup batteries. Its account of the historical New York provisions explicitly distinguishes emergency lighting with battery packs from lighting supplied by the building emergency-power system.

Emergency lighting depends on a complete backup-power chain. Based on Ferreira and Strege (2005), p. 33 and pp. 52–53.

NIST's comparison of historical New York requirements illustrates this system context. The 2003 city code required emergency power for exit signs and exit illumination in specified office buildings, while the cited 1968 provisions did not. This is a jurisdictional comparison, not a universal rule for every building.

The same NIST comparison lists emergency lighting with fire pumps, alarm equipment, smoke-control ventilation and stair pressurisation among emergency-power loads in the relevant high-rise provisions. Sharing a supply can support coordinated operation, but it also creates a dependency that belongs in reliability analysis.

The World Trade Center investigation provides an event-specific example. WTC 7 had emergency generators supplying corridor and stair lighting, with individual backup batteries for units in exit stairs, lift lobbies and lift cars. Those observations describe that building, not a standard arrangement for all premises.

This case documents installed arrangements, rather than a measured duration of useful illumination during the fire. The report's electrical-system description does not give a minimum illuminance, battery endurance or luminaire spacing for general design. Those quantities cannot be derived from the statement that backup generators and batteries were present.

The historical New York comparison also concerns particular building categories. Razza and Grill describe emergency power for exit signs and exit illumination in new buildings and existing Class E office buildings. Their separate high-rise provision includes capacity to supply several emergency loads simultaneously, which makes the connected loads part of the stated requirement.

Visibility and smoke

Visibility in smoke concerns the distance at which a relevant object or sign can be discerned. Smoke attenuates light, and the result depends on whether the target emits light or reflects it. A luminaire that operates normally does not automatically make every route feature visible through smoke.

The Australian Building Codes Board's (ABCB) 2020 data sheets give different empirical coefficient ranges for light-emitting and reflective signs. This means that the same smoke extinction coefficient does not produce the same sign visibility for both target types. The source relationship is specific to the cited visibility model.

The ABCB relationship divides an empirical visibility factor by the smoke extinction coefficient. Extinction describes the loss of light along its path, in inverse metres. The factor represents the target type within the source's model, rather than an electrical power or illuminance rating.

$$V=\frac{k_e}{C_s}$$

Symbol	Description	Unit
$V$	Modelled visibility distance	m
$k_e$	Empirical visibility factor	1
$C_s$	Smoke extinction coefficient	m⁻¹

Using the table's factors of 2 for a light-reflecting sign and 5 for a light-emitting sign reproduces its example. At an extinction coefficient of 0.67 m⁻¹, the results round to 3.0 m and 7.5 m respectively. These are optical estimates for the specified targets, not evacuation distances or limits on route length.

$$V_{\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}}=\frac{2}{0.67}\approx 3.0\mathrm{m},V_{\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{t}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}}=\frac{5}{0.67}\approx 7.5\mathrm{m}$$

For a fixed target factor, doubling extinction halves the modelled viewing distance. This inverse relationship explains why a powered sign can become difficult to see as smoke accumulates. It does not calculate whether an occupant recognises the message, accepts it, or can move safely through the remaining atmosphere.

Sign type changes modelled visibility in smoke. Based on Australian Building Codes Board (2020), p. 281.

Irritant smoke changes the problem further. ABCB notes that eye irritation and psychological effects can make walking speed more closely related to smoke concentration than to the visibility of a particular surface. Restoring illumination does not remove those effects.

Smoke can also make a route unfamiliar even to regular occupants. A person may see only a short section at a time and need repeated directional cues. Emergency lighting therefore supports local route recognition, while signage and spatial layout help connect those observations into a usable path.

Relationship with escape signage

Escape signage identifies exits and helps direct occupants along the route. Signs can be illuminated or designed to emit light themselves, but they serve an information function distinct from general path illumination. Both forms of visual information need to remain available for the intended evacuation period.

A lit sign is useful only if occupants can see and interpret it. Route complexity, viewing direction and smoke can affect whether it provides timely guidance. Sign placement should be assessed along the path from which occupants need information rather than as an isolated fixture.

Proulx reports that people often use familiar entry routes during evacuation. A clearly marked emergency exit that has never been used may receive less attention than the familiar way out. Signage and lighting must therefore work with route familiarity and staff direction rather than assuming automatic compliance.

Escape lighting works with signage and route guidance. Based on Proulx (2002), pp. 3-342–3-343 and pp. 3-350–3-351.

Emergency power for both signs and illumination can preserve a consistent visual message after normal power loss. A dark route with a visible exit marker presents different information from an illuminated path without a recognisable direction. The design should consider the combined visual field.

Occupant movement and route use

Evacuation includes detection, response, delay and movement. Emergency lighting mainly supports the visual conditions for movement, although it can also help occupants investigate and decide what to do. It does not remove the behavioural time before directed evacuation begins.

Proulx describes how lighting can help redirect attention in venues focused on a performance or screen. In the cited discussion, stopping the focal activity and restoring full lighting can help occupants attend to emergency information. This is a specific behavioural measure linked to management and communication.

Movement speed is affected by crowding and route geometry as well as illumination. Proulx distinguishes travel to a place of safety from flow through a restricted element. A well-lit queue can still be limited by the capacity of a doorway or stair.

Smoke can reduce speed and cause occupants to seek another route or assistance. ABCB's modification factors apply to its Fire Safety Verification Method and should not be used as universal speeds. The underlying point is that visual conditions influence behaviour as well as optical visibility.

Staff directions can help occupants interpret an unfamiliar route. Their visibility and authority can complement emergency lighting and signage, particularly in buildings used by visitors. The engineering assessment should include that operating arrangement when it forms part of the evacuation strategy.

Design and regulatory context

The applicable lighting performance and emergency-power duration must come from the jurisdiction and building type. NIST's historical comparison shows that provisions can change between code editions and may apply differently to new and existing buildings. A value from one edition should not be presented as universal.

Smoke-control and suppression scenarios can alter the environment around the lighting. A smoke layer can obscure signs even when luminaires remain on, and spray can redistribute smoke. These conditions belong to the evacuation analysis rather than being handled as unrelated building services.

Limits and common misunderstandings

Emergency lighting is not a guarantee of clear visibility. Dense or irritant smoke can reduce viewing distance and make movement difficult despite functioning luminaires. Its performance must be assessed with the expected smoke environment and the actual target being viewed.

It is also not the same as an exit sign. General lighting supports seeing the route, while signage communicates direction and exit location. Both need suitable placement, operation and visual interpretation to support evacuation.

More light does not automatically produce immediate evacuation. Occupants can spend time interpreting ambiguous alarms, seeking information or gathering others. Lighting supports action after and during those decisions, but it does not replace intelligible warning and management.

A historical code requirement is not a worldwide technical threshold. NIST documents the provisions applicable to particular New York buildings at particular dates. Other jurisdictions and editions must be checked separately when establishing a project's required performance.

See also

• Protective action decision model
• Affiliative behaviour
• Evacuation lift

References

• Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Australian Building Codes Board. (2020). Fire safety verification method data sheets (Version 1.1).
• Evans, D. D., Peacock, R. D., Kuligowski, E. D., Dols, W. S., & Grosshandler, W. L. (2005). Active fire protection systems (NIST NCSTAR 1-4). National Institute of Standards and Technology.
• Ferreira, M. J., & Strege, S. M. (2005). Smoke management systems (NIST NCSTAR 1-4D). National Institute of Standards and Technology.
• Proulx, G. (2002). Movement of people: The evacuation timing. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-342–3-366). National Fire Protection Association.
• Razza, J. C., & Grill, R. A. (2005). Comparison of the 1968 and current (2003) New York City building code provisions (NIST NCSTAR 1-1F). National Institute of Standards and Technology.