Abstract: An uninterruptible power supply capability is provided for line regulators of a variety synthesizing a desired waveshape for presentation to a critical load. A static switch is active intermediate the input of the line regulator and the line power supply to provide a power path during normal operation. Upon the occurrence of a line defect, a stand-by power path is provided which includes a source of d.c. stand-by energy such as rechargeable batteries which supply input to an unregulated inverter which, in turn, provides an unregulated input through an inductance to the input of the line regulator. Transfer is achieved by preliminary voltage level adjustment at the inverter and the inverter itself is employed to assure commutation off of the switching components of the static switch. Detection of line fault is made by detecting any instantaneous phase shift in the line input.