Abstract: A circuit for protecting electrical apparatus from undervoltage of a source voltage by providing disconnection of the apparatus from the same. The circuit includes a relay controlled by a transistorized portion of the circuit responsive to conduction of a gas-discharge tube having specific breakdown and extinguishing voltage characteristics. The tube senses a voltage of that portion of the circuit having selectively adjustable A.C. and D.C. components proportionate to the source voltage, wherein a plurality of source voltage differentials can be selectively chosen at which connection and disconnection of the apparatus will be effected. The relay is also coupled to a capacitor to provide a time delay to prevent relay operation upon momentary voltage fluctuations.