Central line

Changes in the way many commercial buildings are used and maintained, together with a more stringently imposed risk assessment and care of duty climate, have caused the emergency lighting industry to re-evaluate the design and performance of its products.

How have these circumstances influenced changes in equipment standards? The introduction of BS EN 50171 2001 for centrally powered systems now enables installers to be confident that equipment that has been designed and built to comply will be fit for purpose and totally reliable in all areas of: safety; quality of construction; charger performance and compatibility with battery type; changeover operation; inverter performance and compatibility with load; metering and monitoring.

This confidence has given manufacturers the impetus to develop a new generation of central powered systems providing a normal 230/240 volts ac in a supply failure. These systems enable most normal luminaires to operate at full light output and minimise the supply current that needs to be wired to the luminaires.

A major advance of these systems has been the ability to use distribution relays, which can either permit local switching of the emergency luminaire together with the normal lighting while the local supply is healthy, or hold off relays to enable emergency circuits to be isolated until the supply fails.

These local controls have been shown to be more suited to larger buildings, and when fed from a maintained central system enable the luminaires to be supplied indefinitely through local failures until the supply is restored.

What has not gone away, alas, is the potential danger of non-compliant products.

One of the risks that the new market opportunities had presented was the use of inappropriate equipment for this safety application. Typical of this are ups units designed for computer back up. The standard defines that in addition to the ups standard, any units for emergency lighting application must also comply to EN 50171.

This standard requires that the unit must be able to blow all protective devices, remove the short circuit and re-supply all other parts of the circuit. The unit must also be able to supply the full starting load of the luminaries while being powered from the battery.

There are several other reasons why standard produced ups units may not be satisfactory:

• the chargers are normally only designed for batteries with a 5-10 min output and may not be adequate for the larger battery needed for three hours standby;
  
• the batteries used need to be of a ten-year design life, reducing capacity at the end of their life according to battery manufacturers' ripple limits. Many ups systems only use five-year design life cells, which can fail suddenly, and some designs continuously power the inverter creating a potentially harmful load ripple;
  
• the standard requires monitoring indicators specific to this application;
  
• supplies of service replacement boards need to be assured for system operation.

With attention turning increasingly towards central systems, it is worth restating some of their virtues. They use a single long-life, low cost battery so when it finally does need replacing it can be done with the minimum of cost and disruption to site working. Self-contained units will probably be on their second battery change of high cost recombination nickel cadmium cells, which have to be fitted in each luminaire.

Central control provides a much easier option for system maintained or non-maintained outputs that have to be inhibited at a single source, when the building is empty. It saves the user having to switch, for instance, individual self contained exit signs on and off.