Overcurrent Relay (ANSI 50/51): Working, IDMT Curves & Settings
1 - Over current Protection relay(OCR Relay):
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 such 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.
ANSI 50 vs ANSI 51 Overcurrent Protection Comparison
| ANSI Code | Relay Type | Operating Time | Typical Application |
|---|---|---|---|
| ANSI 50 | Instantaneous Overcurrent | Zero intentional delay (< 20–50 ms) | High-magnitude dead short-circuit faults |
| ANSI 51 | IDMT Overcurrent | Inversely proportional to fault current (via TMS) | Overload & graded downstream feeder coordination |
If the fault current is high then trip time will be minimum. If the
fault current is low then 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.
4 - Over current Relay(OCR) 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.
- Instantaneous over current relay.
- Definite time over current relay.
- Inverse time over current relay
- Inverse definite minimum time (IDMT) over current relay
- Very inverse time over current relay
- Extreme Inverse over current relay
