Over current relays are to protect an electrical system or equipment from over current faults like thermal current or short circuit current. For the different characteristics of circuit and equipment over current relays are of different types. Such as the motor takes 3 to 5 times of its rated current at the time of starting.
Types of Over Current relays:
- Instantaneous Over current Relays
- Definite Time Over current Relays
- Inverse Time Over current Relays
- Directional Over current Relays
Instantaneous Over current Relays
Definite Time Over current Relays
Here two conditions is needed to be satisfied for operation (tripping),
- Fault current must exceed the setting value.
- Fault must be continuous at least a time equal to time setting of the relay.
Modern relays may contain more than one stage of protection each stage includes each own current and time setting.
Inverse Time Over current Relays
In this type of relays, operating time is inversely changed with current. So, high current will operate
over current relay faster than lower ones. There are standard inverse, very inverse and extremely inverse
Directional Over current Relays