Stand-by lighting serves to facilitate undisturbed continuation of work – in some cases only for a limited amount of time. Where tasks need to be executed even in cases of general power failure, the photometric requirements for stand-by lighting are dictated by the type of task and the visual performance derived therefrom at times of normal power supply. If work is to be shut down (stopped) within a suitable time frame, lower requirements for lighting are also acceptable. EN 1838 does not specify details for such cases, but 1/10 of the maintained illuminance stipulated for normal cases is the minimum requirement. Switchover mostly occurs without interruption within 0,5 seconds.