Motor Protection - Overcurrent and Earth-fault

This post describes the guidelines for setting calculations for Motor protection with respect to Motor starting and damage characteristics.

Motor Protection – Over Current and Earth Fault:

Motor Protection – If the motor is heavily loaded beyond its capacity, then it is said to be in an overloaded condition, in which it draws heavy current from the supply and there will be a simultaneous rise in temperature of winding and deterioration of the insulation resulting in damage of winding. Hence, the motor must be protected against this mechanical overload. Thermal overload relays, over current relays or miniature circuit breakers with built-in trip coils, may be used.

Due to Excessive temperature rise, the insulation may get damaged which may lead to stator earth fault or stator phase to-phase fault. For low rating motors fuse, MCB and MCCB provide sufficient protection against fault while for large motors, overcurrent, earth fault protection and differential protection are used. This blog discusses the Overcurrent and Earth Fault Protection of motors.

Over Current Protection:

51- IDMT Over Current Protection

Pickup Current (I >):

Time Multiplier Settings (T >):
TMS is selected above the starting characteristics of all the three voltages (100%, 80%, 110% voltage) of the motor to avoid mal-operation during starting and below the motor hot withstand curve.

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