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If a three phase motor is running and it looses a phase, it will actually generate 90% of the phase voltage back onto its feed (see rotary phase converters). So to build a loss of phase detector for a single or multiple motors is difficult. If there is a considerable non motor load on the three phases (lighting and heating. . ) Then it the motor(s) ability to generate 90% of the line volts fails, so the voltage falls. However if the faulty 3 phase is not removed rapidly the motor overload circuits will kick in and trip the motors off.