Emergency lighting is lighting for an emergency situation when the main power supply fails. The loss of mains electricity could be the result of a fire or a power cut and the normal lighting supplies fail.
This may lead to sudden darkness and a possible danger to the occupants, either through physical danger or panic.

Emergency lighting is normally required to operate fully automatically and give illumination of a sufficiently high level to enable persons of all ages to evacuate the premises safely. Most new buildings now have emergency lighting installed during construction, the design and type of equipment is normally specified by the relevant local authority,

The British Standard BS 5266: Part 1: 2005 includes in its scope residential hotels, clubs, hospitals, nursing homes, schools and colleges, licensed premises, offices, museums, shops, multi-storey dwellings etc. Although this standard recommends the types and durations of emergency lighting systems relating to each category of premise it should be remembered that the standards are minimum safe standards for the types of premises and that a higher standard may be needed for a particular installation.

Type of system
Most emergency lighting schemes fall into one of the following categories:-

•Non-maintained – The emergency light units only illuminate in the event of a mains failure.
•Maintained – The emergency light units are illuminated at all times using the same lamps for both normal and emergency operation.
•Sustained – The emergency light units are fitted with two lamps or two sets of lamps. One of which operates on mains 240V AC supply, the other which operates from the battery supply in the event of mains failure. It is basically a non-maintained system with the addition of mains lamps which should be illuminated whenever the premises are occupied.
The type of system and emergency duration is often expressed in abbreviated form as per the following examples:

•M3 – Maintained System, emergency duration 3 hours.
•NM2 – Non-maintained System, emergency duration 2 hours.
•S1 – Sustained System, emergency duration 1 hour.
Emergency Lights
An emergency lights will either be of the self-contained type, i.e. with all components integral to the lighting body (battery, charging circuit, mains ballast etc). Or “slave” to a central battery system where only the lamp (and an inverter if fluorescent) is located within the lighting body and the supply on power failure is fed from some centralised point.

The majority of systems installed in the United Kingdom are of the self-contained type, but both options have their own advantages and disadvantages.

Emergency Power Supplies
Central Battery Source
Advantages
•Maintenance and routine testing is easier with only one location to consider
•Superior battery life, between 5 and 25 years dependent upon type
•Environmentally stable in a protected environment, light able to operate at relatively high or low ambient temperatures
•Large batteries are cheaper per unit of power and lights are usually less expensive.
Disadvantages:
•High capital equipment costs
•High installation and wiring costs with essential MICC or Pirelli FP200 type cable to each slave light
•Poor system integrity – failure of battery or wiring circuit can disable a large part of the system; Requirement for ‘battery room’ to house cells and charger
•circuits etc, may also require ventilation of acid gases. Localised mains failure may not trigger operation of emergency lighting in that area.
•Voltage drop on light wired furthest from the central battery could become a problem.
Self Contained – Single Point
Advantages:
•Speedy and low cost installation
•Standard wiring material may be used (failure of mains supply due to cable burning through will automatically satisfy the requirement for a light to be lit)
•Low maintenance costs – periodic test and general cleaning only required
•Low hardware equipment costs – no requirement for extended wiring, special ventilation etc
•Greater system integrity with each light independent of the other;
•System can easily be extended with additional lights
•No special sub-circuit monitoring requirements
Disadvantages:
•Limited environmental operating range (batteries may be adversely affected by a relatively high ambient temperature)
•Battery life is limited to between 2 and 4 years dependent upon application
•Testing requires isolation and observation of luminaires on an individual basis
In general then,the decision to use either a central battery or a self-contained system is likely to be cost dominated. If an installation has longevity and low maintenance as priorities, then the higher cost of central battery may be acceptable on a very large project. Usually, lights and installation costs will be a major consideration, particularly on smaller jobs, and it is this criterion which makes the self-contained luminaire the most popular choice.

How much light?
BS5266 recommends the provision of a horizontal illuminationat floor level on the centre line of a defined escape route (permanently unobstructed) not less than 0.2 lux and 0.5 lux minimum for anti panic areas to exclude 0.5 metre border around the room. In addition, for escape routes of up to 2m wide, 50% of the route width should be lit to a minimum of 0.1 lux. Wider escape routes can be treated as a number of 2m wide bands. The actual degree of illumination should be closely related to the nature of both the premises and its occupants with special consideration being given to old person’s homes, hospitals, crowded areas such as pubs, discos and supermarkets, and to whether or not the premises are residential.

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