Abstract: An apparatus for protecting an electrical power system supplying electrical power to an electrical load comprises a transformer for sensing an operating condition of the electrical power system and for producing an analog signal representative of the operating condition, and a microcomputer for periodically sampling the analog signal and for converting the analog signal into a series of digital signals. The microcomputer includes circuitry for deriving a digital value representative of a square root of the series of digital signals and circuitry for processing the digital value over time to determine a processed value which is a function of both the sensed operating condition and time. A circuit breaker is responsive to the microcomputer for disconnecting the power system from the load in the event that the processed value is not within preset limits.