The Guidelines in Brief

The rules and regulations around emergency lighting are complex and whilst I will not attempt to cover all aspects here I will just bring out a few salient points.

In the eventuality of a power failure the emergency lighting must come on for a minimum of 3 hours. This can be achieved through 2 types of lights:-

1. Maintained Lights , where the existing light has a backup battery which will be automatically turned on in the eventuality of a power failure.

2. Non-Maintained Lights, which are never on unless there is a power failure and then will light for 3 hours.

The requirement then in broad terms is to provide a minimum of 1 Lux illumination along all the escape routes. This can be reduced to 0.5 Lux for other areas but in most cases experts prefer to keep the level to 1 Lux in all areas as you are never sure which route might become an escape route if objects are left in a planned escape route.

The Non-Maintained Emergency Light

A cost effective and aesthetically pleasing way to fit a room with emergency lights is to use small non-intrusive emergency lights like these 3w emergency light

The lights is powered by its own battery back up unit and driver so that it is automatically lit in the eventuality of a power failure.

These are cost effective and efficient but how many do you need in a room and what should the spacing be?

Spacing of Emergency Lights on a Square Grid

We did some actual tests with a room with a ceiling height of 3.4m and measured the LUX level with an accurate Lux meter of a single light and then here I apply some mathematics to calculate the spacing needed to provide a minimum level of 1 Lux.

The lights used was have a lumens output of 110lm.
The distance from the position on the floor directly underneath the downlight, to where light level was 1 Lux was 2.9m and to 0.5 Lux was 4.1m.
This provides the size of circle that is always lit to these levels in a dark room. However with a square grid spacing then some trigonometry is needed to work out the spacing.
The maximum spacing between 2 lights would then be 4.1 x cos45 x 2 = 5.8m.
At the edge of the room this is significantly reduced as there are no other lights to contribute to the light level and accordingly the maximum distance would be 2.9 x cos45 = 2.05m .

Spacing of Emergency Lights on a Hexagonal Grid

If the second row of lights is spaced to be midway between the first row then a more efficient spacing can be achieved and consequently less emergency lights are needed in the room. The calculation is as follows:-

The maximum spacing to achieve 1 Lux would be 4.1 x cos30 x 2 = 7.1m. The distance from the wall would remain the same but you would need to add an extra light for every other row so this means for a small room this might prove inefficient.

Clearly tests of Lux levels need to be performed but these measurements should provide some starting guidelines.