Circuit detects incorrect phase sequence of motor driving pump, compressor, conveyor, or other quipment that can be damaged by reverse rotation.Circuit also protects motor from phase loss that could cause rapid temperature rise and heat damage. LED is on when phasing is correct. For phase loss or incorrect sequence, output goes low and LED is dark. Diodes and zeners change sine waves for all phases to rectangular logiclevel pulses that feed gates. When phases are correct, output of G4 is train of rectangular pulses about 2.5 ns wide. Output is zero for incorrect sequences. Since leading edge of output pulse coincides with positive zero crossing of phase B, output pulses can be used to trigger SCR connected across phase B and driving relay-coil load. SCR then energizes relay only when sequence is correct.-H. Normet, Detector Protects 3.Phase-Powered Equipment, EDN Magazine, Aug. 5, 1978, p 78 and 80.