Over Current Protection Relay
Working principal and Connection diagram.

  1. Over current protection relay.
  2. Working principle.
  3. Connection diagram of OCR.
  4. Relay setting parameters.
  5. Types of over current relay.

1 - Over current relay:

Over current relay(OCR): types and setting parameters.

Over current relay is an element of relay which is operated after crossing preset limit value of current and time then it provides tripping signal to the circuit breaker tripping coil. It is really current monitoring relay which is behave as per current condition.
Over current relay protects the electrical system like as transmission lines, transformers, generators from short circuit, overload, ground fault etc.
If the fault current value is extra high then it will trip instantaneous or quick. So, this type over current relay can be categorized in different types as per fault current value. Let’s discuss about categories of over current relay.

2 - Working principle of OCR relay.

If the fault current is high than trip time will be minimum. If the fault current is low than trip time is high or adjustable as per requirement. Nowadays it is available in digital type numeric relay with three phase and over current relay.
It has two no’s functions available. One is three phase over current relay and earth fault relay in HV system. It is best for using in overhead feeder and underground feeders.
It is microprocessors based with wide range field.it can be controlled with different characteristic of trip time. It has also time delayed selected program for over current and earth fault relay.

3 - Connection diagram of over current relay.

Over current relay(OCR): connection diagram and working principle.

4 - Relay setting parameters:

Sr.No. Parameters Setting range
1 Current : 1A to 5A
2 frequency : 50, +/- 2.5 %
3 Auxiliary supply : 24V to 110V AC/DC ± 15% or
95V to 240V AC/DC ± 15%
4 Over current : 0.2 - 2.0 times (in steps of 0.05 In)
5 Time : 0.1 to 1.6 in steps of 0.05
6 Pickup Current : Same as set current
7 Reset Current : 95% - 90% of pickup current
8 Reset time : 0.1- 10Sec in steps of 0.1Sec or Less than 50ms
9 Current Transformer : Less than 0.25 VA / phase

5 - Types of over current relay.

  1. Instantaneous over current relay.
  2. Definite time over current relay.
  3. Inverse time over current relay
  4. Inverse definite minimum time (IDMT) over current relay
  5. Very inverse time over current relay
  6. Extreme Inverse over current relay

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