Indepandant emergency supply
Many buildings must have some form of emergency lighting to come on if the electric supply to the ordinary lights fails. BS 5266: Emergency lighting, BS EN 1838, BS 5266–7: Lighting applications. Emergency lighting are the codes of practice to which to refer. One of the possible causes of failure is a breakdown in the supply authority’s service to the building and, therefore, the emergency supply must be independent of the service into the building.
Electric lighting for emergency use can be provided if the building has a standby generator. A generator can be installed to take over the entire supply to a building, so that the only special provision for emergency lights need made is to cover the time between the mains supply fails and the standby generator is up to speed, but for economy the standby generator is often rated at less than the ordinary mains service to the building.
The distribution then has to be arranged so that only a part of the service within the building is fed by the generator, and only a few of the lights should be included in this part. There is no need for full lighting under emergency conditions, and lighting in the main corridors and staircases is usually enough. High risk task areas need special consideration. Emergency supplies are of particular importance in hospitals and no new hospital should be built without a standby generator, but buildings like schools, offices, theatres and blocks of flats seldom justify the expense.
For these buildings emergency lighting is almost invariably provided, by self-contained battery luminaires. Emergency lights are fitted throughout the building. They come on only when the mains fail (a non-maintained system) and cannot be used while the mains are healthy.
They are not intended to give full illumination, but only to provide sufficient light for people to make their way out of the building safely. A minimum of 1 lux should be achieved, and 5 lux near fire-fighting equipment.
Emergency lighting in high-risk task areas such as near hot vats should have an illumination of 10 lux. Manufacturers’ data will give luminaire spacings at given heights to achieve these lighting levels. One light on each landing and perhaps one in the centre of any particular corridor should be perfectly adequate. A number of low-power luminaires is generally better than one large luminaire. These lights work on low voltage d.c. and are fed from a battery. A trickle charger permanently connected to the mains ensures that the battery is always fully charged.
The lights are cabled from the battery through a relay, the contacts of which are closed when the coil is de-energized. The coil is fed from the mains and as long as the mains are on, the contacts are held open. Thus as long as the main supply is healthy, the battery lighting circuit is kept open, but immediately the mains fail the relay contacts close and the emergency lights come on.
The circuit diagram is given in Figure 1. There are battery chargers and relays purpose-made for this kind of application. The charger must be left permanently switched on, and contains the relays necessary to stop the charging current when the battery is at full charge. It can be supplied by a final circuit from any convenient distribution board, but there should be no other outlets on the same final circuit. Alternatively, it can be fed directly from a switch fuse at the main intake.
Because the emergency lights work at a low voltage, the voltage drop in the cable to them can become considerable and may present something of a problem. Whereas a 60W bulb on a 230V supply takes 0.26A, a 24W bulb on a 24V supply takes 1.0A and the voltage drop in a cable of a given size is about four times as great. At the same time, a drop in a cable of 2.4V in 230 may reduce the light output by perhaps 2 per cent but the same drop of 2.4V in a 24V system is proportionately ten times as great and could reduce the light output by a fifth or a quarter. Low-voltage cables must, therefore, be adequately sized.
It is in any case inadvisable to design an emergency system for less than 48V d.c., which is a convenient standard battery-output voltage. Even with a 48V system and ample cable sizes, there must obviously be a limit to the number of lights which can be served from one battery and to the distance the furthest light can be from the battery.
A large building may, therefore, need several separate battery systems. Legislation has made it more essential to provide lights to mark fire-escape routes from buildings. The lights used for this are of very low wattage and, consequently, the voltage-drop problems are somewhat eased.