Adds ground fault protection to heavy equipment powered through 3-and-4 wire circuits grounded with neutral * Meets or exceeds UL 2003 Code Requirements for increased surge immunity, corrosion resistance, resistance to false tripping and diagnostic indication for line-load wiring reversal * Exclusive, patented lockout-action: RESET button is blocked if GFCI protection has been compromised, eliminating the risk of end-users incorrectly assuming that a reset GFCI is providing ground fault protection when it actually is not * Test and Reset buttons confirm proper operation * Ideal for use with spas, heater packs, high pressure washers, industrial mixers, and other equipment