In recent years there has been an increasing call for the installation of emergency—sometimes termed “alternative” or “secondary”—lighting systems. These can be provided by candles or nightlights, gas, dry- or wet-battery torches, accumulators, engine-generator sets, alternative independent mains supply, or by a combination of several methods. The amount of emergency lighting may range from an intensity equal to normal down to the minimum considered necessary for safety and continuation of essential services. This paper deals mainly with the use of accumulators for standby purposes, operated either manually or automatically upon failure of the mains supply and capable of supplying loads for varying periods according to the class of installations, which are grouped under five heads.