I am a fairly long-in-the-tooth electrical inspecting engineer. My remit often extends to to deliberations on fire alarm and emergency lighting. I regularly find serious defects in emergency lighting systems, many relating to recommended illuminance levels. Where possible, I endeavour to measure light levels as relying on spacing ratios and photometric data on existing systems is often a difficult chore, especially where documentation is not available. It is perhaps for this reason that my Reports are often at odds with Reports issued by contractors, many of whom seem content that an emergency luminaire is physically present but ignore the first principles enshrined in the inverse square law and cosine rule which result in rapid diminution of illuminance levels as one moves further away from the light source. It is quite remarkable to witness the starkness of this in the field and a valuable lesson for those who often just cast their eye over an emergency lighting system. Relying on illuminated exit signs to provide required illuminance levels is dubious unless there is clear, ratified design data.
I do not design systems but I know some who do. They normally ignore the light output of exit boxes in their calculations which is why you may often find a non-maintained unit fairly close to an illuminated exit sign in many installations.
By the way, if the exit sign is over a final exit, stick your head outside (open the door first) to see if there is any evidence of emergency luminaires positioned to facilitate dispersal. The absence of these is a pet hate of mine and a very common defect!
Regards,
Lyle Dunn
