IDMT Overcurrent Relay Coordination - IEC Normal Inverse Characteristic ensures that the relay closest to a fault trips first, while upstream relays operate only if the downstream relay fails.
The IEC Normal Inverse curve provides faster tripping for higher fault currents while maintaining a minimum operating time, improving selectivity and system reliability.
The operating time is calculated using:
t=0.14 × TMS /(I/Is)0.02−1
Where
TMS - Time Multiplier Setting
I - Fault Current
This characteristic is widely used in
- Transformers,
- Feeders and
- Distribution systems
because it provides effective protection, proper relay coordination and minimizes unnecessary power interruptions.