Abstract: Performance failure in an uninterruptible power supply (UPS) is determined independently by itself. A selective tripping can be done within a shorter time than one cycle of an AC output. A UPS converts a DC voltage to an AC voltage and supplies the AC voltage to a load device. A UPS has a control unit conducting a failure determination by using an instantaneous value of an internal voltage and current. It is preferred that a UPS includes an inverter unit and a trip switch. The inverter unit includes a semiconductor bridge circuit generating a sinusoidal AC voltage by modulating a DC voltage with voltage instruction values, and a filter circuit inserted between the semiconductor bridge circuit and the load device. The trip switch connects and trips a connection between the inverter unit and the load device according to the failure determination of the control unit.

Abstract: A solid state power control scheme is provided with a ground fault interrupt function. Essentially, if a neutral wire is associated with the power supply, the current is sensed in that neutral wire. The sensed neutral wire current is subtracted from a summed current on the power supply lines. If that result is greater than a predetermined limit, then the ground fault function is actuated and the circuit is opened. For power supplies, both those with and without neutral wires, the current is sensed by a shunt resistor on the power supply line.