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 a overloaded condition, in which it draws heavy current from the supply and there will be 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 breaker 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 provides sufficient protection against fault while for large motors, overcurrent, earth fault protection and differential protection are used. This blog discusses about the Overcurrent and Earth Fault protection of Motor.

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.

TMS value can be as close as possible to the starting current and starting time of the motor with consideration of a 20% positive tolerance of the time and current for the motor.

The time discrimination between the motor characteristics and relay characteristics has to be decided by what’s the error in the motor starting characteristics. So, you can take 20% tolerance (IEC 60034) as an error in the motor manufacturer data. For Example: If the starting time is 10 seconds it will take 12 seconds to start and starting current is 6 times it will take 7.2 times the starting current.

Curve Type:

IEC Extremely Inverse (Curve which is as close to starting characteristics prevents maloperation and safeguards equipment)

50 – Instantaneous Over Current Protection

Pickup Current (I>>):

Time Dial:
If you specify a time, it is no longer Instantaneous protection. No intentional time delay. There will be a delay like 20ms within the relay itself to identify the fault.

Earth Fault Protection:

Predominately Single line to ground fault occurs more than 75% in a system

51N- IDMT Earth fault Protection

Pickup Current (I >):

Time Multiplier Settings (T >):
TMS is to be selected like during the Unbalance happened during the motor starting should not results in maloperation. Typically, can take 0.1s

Curve Type:
IEC Normal Inverse

50N – Instantaneous Earth Fault Protection

Pickup Current (I>>):

Time Dial:
No intentional time delay. Minimum time available in the relay. Few Cases there will be some maloperation
in that case we can increase to 100ms.

Manual Calculation:

Rated Voltage : 3.3 kV
CT Ratio : 350/1A
Protection Relays : ABB REF615
Motor Full Load Current : 262.7A

Non-directional over current protection 51 (I>) REF615 relay:

Required Current Setting = FLA x 110% / CT Primary Amps
Pickup = 0.83 (290.5A)
Curve = IEC Extremely Inverse
TMS = 1.5 (From ETAP Graph)

Instantaneous Phase protection 50 (I>>) REF615 relay:

Required Current Setting = LRA x 130% x 1.6 / CT Primary Amps
Pickup = 10.15 (3552A)
DT = 0.02

Non-Directional Earth fault protection (51N) REF615 relay:

Required Current Setting = 10% of Load Current / CT primary amps
Pickup = 0.08 (28A)
Curve = IEC Normal Inverse
TMS = 0.1

Instantaneous Earth fault protection (50N) REF615 relay:

Required Current Setting = 100% of Load Current / CT primary amps
Pickup = 1 (350A)
DT = 0.02

Graph: