One of the phase loss alarm circuits

Phase loss in the three-phase power supply will endanger the safety of electrical equipment (especially
asynchronous motors). In order to detect phase loss faults in time, you can make a self-made phase failure alarm.
    The circuit is shown in Figure 13-110. When the three-phase power supply is normal, the voltage at point A is about
ov, and no alarm occurs; when a phase is missing (one or two phases are missing), the voltage at point A rises,
the signal light H lights up, and the bell HA sounds an alarm.