The circuit provides a simple means of determining the succession of phases of a 3-phase 120 V source used in synchro work. Terminals A, BT and C are connected to the three terminals of the source to be checked. If the neon bulb lights, interchange any two leads; the light then extinguishes and A, B, and C indicate the correct sequence. If power on any one line is lost, the neon bulb will light.
This feature may be useful for monitoring purposes.