This circuit prevents damage to the load due to incorrect phasing. The three power SSR's are only permitted to turn-on for a phase sequence of phase A leading phase B. If phase A lags phase the input currents will cancel, causing the SCR and the inhibit SSR to remain off until the sequence is reversed.
The inhibit SSR is included to maintain isolation at the input.
Phase Sequence Checker for 3 Phase Supply | Electrical Projects
QR code 14 -- Phase rotation/sequence test
How to Use the Hioki Digital Phase Detector PD3259