The present invention provides a simple, reliable, comprehensive electrical phase and amplitude fault detection and response system. The system includes means for detecting any of various electrical faults, such as impermissibly low voltage levels, impermissibly high voltage levels, phase dropouts, single-phasing, phase reversals, and/or total power loss. Further, the invention provides both fault detection and response customized to the plant equipment, providing power cut-off an appropriate time after a given fault is detected, and preventing premature power restoration when the line power has not been fully returned to normal. The system provides the fault detection and response functions without the need for electrical power of its own, avoiding dependence on line voltage, batteries or back-up generators. This functioning independent of the presence of electrical power is enabled through use of a pneumatic arrangement for interrupting power to plant equipment.