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Designing an emergency lighting scheme

Jan 26, 2005
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Given the wide variety of different products available and the need to comply with the requirements of relevant standards, designing an emergency lighting scheme can be a daunting prospect. Chris Watts, Standards Consultant for Cooper Lighting and Security, looks at the key points to bear in mind.

The prime function of an emergency lighting system is to safeguard a building’s occupants in the event of a failure of the mains supply to the normal lighting. As with most aspects of health and safety, there are national and international standards in place to ensure that such systems are able to perform the task demanded of them.

When planning an emergency lighting installation, therefore, the initial consideration should be the requirements of prevailing legislation and the relevant standards. At the same time, the installation should be economical and meet the operational and servicing needs of the building user.

The main standard covering emergency lighting is the code of practice embodied in BS 5266 Part 1 (1999); the specific light levels called up by this standard are laid out in BS 5266 Part 7/EN 1838 (1999). Also in place is BS EN 60598-2-22 (1999), which provides a product standard for luminaires used in emergency lighting and ensures that they achieve the performance required by BS 5266 Part 1.

In addition, a new European Application Standard with improved testing regimes, EN 50172 (2004), has been published and this is to be issued as a supporting part of the BS 5266 series.

The design objective for any emergency lighting system is established by BS 5266 Section 4.2, which says that, when the supply to the normal lighting fails, emergency lighting is required to:

  • indicate clearly the escape routes
  • provide illumination along such routes to allow safe movement through the exits
  • ensure that fire-alarm and fire-fighting equipment can be readily located.

Types of system

Both the size and function of the premises critically affect the type of emergency lighting system selected – not only from a design viewpoint, but also because of differing operational characteristics and economic considerations.

Self-contained luminaires generally provide the simplest and quickest solution as they contain their own batteries and operate independently. They normally use 8W fluorescent lamps as their light source, but, as some only employ two battery cells instead of three and also because the effects of the diffusers vary, there is a considerable difference in their spacing. Some will only cover 5m, while others provide a spacing of over 11m. Selection of the appropriate luminaire spacing not only defines the number needed but also, of course, the installation costs.

Typically, the light output of self-contained luminaires is between 70 and 200 lumens, which makes them suitable for escape routes through corridors and areas with low ceilings.

Many architects and building users prefer to see a single type of luminaire utilised for both normal and emergency lighting. This can be achieved either by building in a self-contained emergency circuit or by powering the luminaire from a central battery system. While this approach can be aesthetically pleasing and also save the cost of a specialist housing, there are a number of points that need consideration.

Firstly, the light source type and size and circuit must be suitable for emergency use. Secondly, is the housing suitable? If it is to be used on an escape route, the enclosure must pass an 850 degC glow wire test. Thirdly, if being used with self-contained conversion equipment, the luminaire must be physically large enough to accommodate the circuit or else a separate additional enclosure is needed. Fourthly, the heat within the housing has to be checked to ensure that the emergency circuit or batteries will not be damaged when in operation.

Finally, any modifications to the luminaire will invalidate both its warranty and its CE mark. The person carrying out such modifications assumes complete commercial and legal responsibility for the converted fittings and must therefore ensure that the modified unit is checked for electrical safety and compliance with the EMC directive.

Having satisfied these requirements, the person doing the modifications must then replace the CE mark and be ready to provide a technical file on the changes that have been made, if requested to do so. Because of the difficulties and commercial risks of such a conversion, it is normally better to approach the original luminaire manufacturer or a specialist emergency lighting company to provide the luminaires.

Alternatively, a central battery system can be used to supply either normal mains luminaires or special-voltage luminaires or indeed a combination of both. Because many of these systems power the luminaires either at full light output or at levels only slightly lower than the lamps’ nominal rating, they are particularly suitable for use when higher light outputs are required, e.g. open areas and when ceilings are above 2.5m high.

Legally speaking, the minimum duration of the emergency lighting depends on the application it is intended for. A duration of at least 1 hour is required to allow a building to be evacuated, but this rises to 3 hours wherever people are not evacuated immediately on supply failure or if they are allowed to re-enter the building before the system has recharged, e.g. in hotels, dormitories, hospitals and places of entertainment. However, in practice, nearly all emergency lighting products provide a duration of 3 hours.

Integrated fire safety

Emergency luminaires should not only illuminate the exit route but also highlight particular hazards and fire safety equipment. Specifically, BS 5266 Section 4.1 says that emergency luminaires are essential for the following locations (where the term ‘near’ means within 2m measured horizontally):

  • at each exit door
  • near stairs or any other change of level
  • mandatory exit and safety signs
  • at escape route change of direction or intersection of corridors
  • outside and near each final exit
  • near each first aid post
  • near fire fighting equipment and call points.

Clearly, it is logical for all fire safety systems to be integrated and, with careful planning, a single luminaire will often meet a number of the requirements above. For example, it might be preferable to relocate a fire extinguisher near to a luminaire, rather than installing an additional luminaire. After these points have been covered, the emergency lighting for rest of the escape route must be designed such that a minimum illuminance of 1 lux is achieved on the centre line, bearing in mind that smoke control doors will be shut.

Theoretically, it is possible to ensure that these illuminance requirements are met by performing photometric calculations for each emergency luminaire. This would done by obtaining the luminaire distribution in candela per 1000 lumens as required in the luminaire product standard BS EN 60598–2-22, then calculating the actual lamp output by multiplying the initial luminaire lamp output by a factor for the reduction of light to the end of discharge and also factors for the ageing of the lamp and the accumulation of dust in use. This resultant worst-case luminaire output would then be corrected by a cosine factor for the angle the light ray hits the floor and also by the inverse square law for the distance to the point on the floor from the luminaire. Not surprisingly, this approach is rarely used.

To simplify matters, ICEL (the manufacturers’ trade organisation) has developed a registration scheme which checks that the luminaire has been tested against the product standard and that the photometric results are converted to easy-to-use spacing tables under ICEL’s own ISO 9002 scheme. The spacing tables for each registered luminaire show the distance to the first fitting, then the distance between subsequent luminaires for different mounting heights. As these tables are the authenticated data required by BS 5266 to verify designs, their use confirms to fire authorities and building control inspectors that the correct lighting levels are being achieved.
If ICEL spacing tables are not available or the application is outside the scope of the tables, many manufacturers can offer a suitable computer program to aid the design process, providing a particularly useful tool for installations involving high ceilings, wall mounting or a combination of normal luminaires and emergency lighting circuits.

Regular testing

All emergency lighting systems should be maintained in line with the manufacturers’ instructions and regularly tested in accordance with the requirements of BS 5266-1. At present, this standard calls for a monthly functional test not exceeding 25% of the rated duration, a six-monthly test of at least one hour for a three-hour rated system, and, after the first three years, an annual test for the full rated duration. However, BS 5266-1 will be revised in the near future to bring it into line with EN 50172, which has no requirement for a six-monthly test but does require the full duration test to be carried out annually, even in the first three years.

From a practical point of view, the major consideration is that testing must be carried out at a time of minimum risk - remembering that the risk continues until the batteries have been recharged. Test results should be recorded in a log book, together with details of any remedial work that may be needed. Finally, the system should be checked to ensure that the mains supply has been restored and that the batteries are being recharged.

The BS EN 60598-2-22 emergency luminaire product standard says that test facilities need to be appropriate to the installation and must fully simulate a normal supply failure; while it may be acceptable to switch off the mains power in an empty village hall, it is obviously not acceptable in a fully occupied old people’s home.

Although it is possible to carry out the tests manually, this can be an onerous, time-consuming and expensive process, so many building users are now adopting emergency-lighting self-test systems to achieve the highest levels of safety.

Designed to automatically execute the test regimes required by BS5266-1 and EN 50172, these systems ensure that warning is given when batteries need replacing and check at the correct intervals that the luminaires are working satisfactorily, thereby increasing safety levels as well as enabling significant time and cost savings to be made.