Phase loss is simple: the loss of a phase (or pole, leg) in a 3 phase system often cause by a blown fuse, overload relay malfunction, old worn contacts or simply mechanical failure. This resulting loss causes excessive AMP draw on the remaining phases which can quickly cause damage to your motor windings. For example, if you had 50 AMP contactor, each leg (or phase) is passing through a 50 AMP current. If a phase is lost, this 50 AMPS is then distributed over the remaining two phases causing overheating and oftentimes melting of the two remaining contacts so the contactor cannot be shut off.
So what can be done to prevent phase loss or at least mitigate the damage when it occurs? The simplest and often overlooked prevention method is to ensure the correct tightening torque when installing the contactor or starter (we have written an article on this topic which can be found here). For an electrical solution to the problem, you can add an overload relay with phase loss protection. When phase loss occurs, the overload will trip, cutting voltage to the contactor coil and shutting the motor off to prevent any permanent damage from an over current situation.