How many Emergency lights are needed?

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

2w_emergency_3hr_01

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.2w_emergency_3hr

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.

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