The abnormal operation that occurs when one line of a three-phase supply opens due to a blown fuse, broken wire, or faulty contactor. A running motor will continue to spin on the remaining two phases, but it will draw excessive current to maintain the mechanical load. This causes rapid overheating and will burn out the windings if protective relays do not trip the circuit.
Heard on the job
The contactor dropped a leg and the pump motor cooked itself from single-phasing before the thermal overload tripped.
Insider tip
A three-phase motor cannot start during a single-phasing event; it will just sit there humming and drawing locked-rotor current until it trips or burns.