Apparatus and method for protecting a load from fluctuations in supply voltage

A power switch connecting a load to an ac source is turned off when the RMS value of the supply voltage is outside selected limits and is turned back on when the RMS voltage returns to within the limits. Adaptive hysteresis is applied to the switch controller to eliminate chattering of the power switch when the voltage fluctuates around the selected limits. To this end, the selected limits are repetitively, incrementally narrowed each time the power switch is turned off and back on a selected number of times, such as twice, within a selected time interval. The RMS voltage is rapidly determined from samples gathered over ¼ cycle so that the voltages in a three phase system can be successively repetitively calculated. Phase loss is rapidly determined by checking for the leading edge of a square wave generated from the supply voltage wave form.