Over current Relays – concept, theory and applications.

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:

  1. Instantaneous Over current Relays
  2. Definite Time Over current Relays
  3. Inverse Time Over current Relays
  4. Directional Over current Relays
01

Instantaneous Over current Relays

Instantaneous over current relay characteristics
Instantaneous over current relay characteristics

Its operation is time independent. When fault occurs it immediately trips / break the circuit. It is mainly applied to feeder circuit.

02

Definite Time Over current Relays

definite time over current relay
Definite time over current relay – after sensing the over current, at first the timer activated, then elapsing a minimum setting time the trip circuit activated.

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.

Characteristics of definite time over current relay
Characteristics of definite time over current relay
03

Inverse Time Over current Relays

Characteristics of inverse time over current relays
Characteristics of 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
types.

04

Directional Over current Relays

Three conditions must be satisfied for its operation: current magnitude, time delay and directionality.
different types of overcurrent relay


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