Applies to electronic direct a.c. converter systems with electrical energy storage means in the d.c. link. Ensures continuity of an alternating power source. Also includes the method of specifying all power switches that form integral parts of a UPS and are associated with its output. Included are interrupters, bypass switches, isolating switches, load transfer switches and tie switches. does not refer to cnventional mains distirbution boards, rectifier input switches or d.c. switches or UPS based on rotating machines. Defines a complete uninterruptible power system in terms of its performance and not individual UPS functional units.