Applies to electronic indirect a.c. converter systems with electrical energy storage means in the d.c. link. The primary function of the uninterruptible power system (UPS) is to ensure continuity of the alternating power source. It may also serve to improve the quality of the power source by keeping it within specified characteristics. Applies to electronic UPS delivering single- or three-phase fixed frequency a.c. output voltage, with energy storage device in the d.c. link if not otherwise specified, with rated voltage not exceeding 1 000 V a.c., and movable, stationary and/or fixed equipment. Also includes the method of specifying all power switches that form integral parts of a UPS and are associated with its output.