40:1 – Emergency Lighting Uniformity – Maximum Diversity Ratio Explained

Emergency lighting illumination of escape routes and open areas is most effective when not only are the prescribed minimum levels of illuminance achieved, but also the illumination is uniform along the route or over the area.

There will of course be variations in the levels of illuminance achieved along any route or in any open area due to the position of luminaires

Representation of intensity of lighting typically provided by mains lighting luminaires Representation of intensity of lighting typically provided by mains lighting luminaires

Emergency escape is faster in uniform emergency Lighting
Emergency lighting illumination that varies greatly along the route or over the open area has the effect of slowing some occupants in their escape since they become concerned that they will not see obstacles in the darker areas. Their confidence to escape at a constant speed is affected.

Similarly, where a group of people are escaping there is inevitably some shielding of the illumination of the escape route floor and significant variations in levels of illuminance will again have the effect of slowing escape.

The underlying principal is that emergency lighting is better provided by a number of low output luminaires rather than a few high light output luminaires.

What the standards require
Emergency lighting standards prescribe not only minimum illuminance levels but also the uniformity in terms of a “maximum diversity ratio”. This is the difference between the maximum level of illuminance along an escape route or in an open area and the minimum value along the same route or throughout the same area.

BS5266 Part 1 specifies the maximum diversity ratio set out in BSEN 1838/BS5266 Part 7, which is a value of 40:1.

If escape route emergency lighting has been designed to achieve a minimum value of 1 Lux on the centre line then the maximum permissible value is 40 Lux.

Similarly in an open area, if the minimum value achieved is 0.5 Lux, the maximum value cannot be greater than 20 Lux. The maximum values can be anything less than these values.

Representation of intensity of lighting provided by emergency lighting luminaires Representation of intensity of lighting provided by emergency lighting luminaires

Logically it would be expected that the maximum values will occur directly below the installed emergency lighting luminaires and the minimum values at the mid points between luminaires. The separation of luminaires having normally been determined to be the maximum distance apart that the still achieves 1 Lux or 0.5 Lux as appropriate, at floor level.

Purpose designed emergency lighting luminaires are generally designed to distribute as much of their light output as possible sideways rather than down to the floor.

The Maximum Diversity Ratio must always be considered and observed. Therefore if the minimum design value or the minimum value achieved is higher than that prescribed by the standards, the maximum permissible value is also increased pro-rata.

e.g. If a design is provided to achieve a minimum level of illuminance on escape routes of say 5 Lux, the maximum permissible value would be increased to 40 x 5 = 200 Lux. Where high light output luminaires are used for emergency lighting and are producing in excess of 40 Lux on the floor under in emergency operation, then the minimum value must be no less than 1/40th of the maximum value.

e.g. If luminaires are powered by a static inverter and produce 400 Lux on the floor beneath them, the minimum design value that will be permissible will be 400/40 = 10 Lux. So the whole route or area will need to be designed to achieve at least this value, which may mean a much less cost effective installation than could be achieved with lower output luminaires.

The diversity of installations using purpose designed self contained emergency lighting luminaires is rarely a problem but must always be considered. Look at the maximum illuminance value directly under a luminaire (in emergency operation) shown in luminaire spacing tables or similar to see whether there will be a problem when it is used to provide 0.5 Lux or 1 Lux schemes.

Completion certification includes confirmation of uniformity
The completion certificate sign-off of an installation confirms that it meets at least the minimum levels of illuminance required by the current standards, but also that it meets the maximum diversity ratio requirements.