One method is to incorporate a combined overload and single-phase relay in the control gear. A typical relay of this type includes three overload relays with trip contacts so arranged that it will trip if the displacement of one overload element differs from that of the others.
This type of relay will operate if single-phasing occurs at or near full load with the same time delay as on overload, but at light loads, the time delay for single-phase protection is longer. Another device is a phase-failure relay in the control gear. Its principle is based on the fact that the currents in the supply lines or the voltages between them at the motor terminals are unbalanced when the motor is single-phasing.
The phase-failure relay may be of the current or voltage-operated type which trips out the line switch when one of the supply lines becomes open circuited.