Design and performance requirements for d.c. and single-phase a.c. double-pole change-over contactors for low-voltages up to 250 volts and a range of standard current ratings, which are used to provide the automatic transfer of current supplied to safety lights from the normal source to an alternative source when the normal source fails; reconnection is automatic when the normal supply conditions are restored.Cross References:BS 2752BS 2757BS 5750