Circumstances that cause emergency lights to switch on?

[NoSparks]

Does anybody know if during a fire the highest risk factor that would cause emergency lights to be operated would be

a) an open circuit (before the lights) eg ceiling collapse, melted coper conductors, etc or

b) a closed circuit causing the mcb to trip eg melted insulation leading to short circuit?

 

[telectrix]

either could be the cause. i think a short circuit tripping the supply would be more likely.