IDMT Relay Trip Time Calculator – IEC & IEEE Curves
Use this free IDMT relay trip time calculator to estimate inverse time over current relay operating time using IEC 60255 and IEEE C37.112-1996 trip curve equations.
IDMT Relay Trip Time Calculator
Select a relay curve, enter pickup current, fault current, and TMS or TD to calculate estimated trip time.
IDMT Relay Trip Time Calculator
The Inverse Time Over Current (TOC/IDMT) relay trip time calculator estimates protection trip time according to IEC 60255 and IEEE C37.112-1996 trip curves.
Parameters
- Trip curve: Select from IEC and IEEE inverse curves.
- Relay pickup current (A): Current setting in relay (Is).
- Fault current (A): Expected short circuit current (I).
- TMS (IEC) / TD (IEEE): Time setting multiplier used in formulas.
IEC 60255 Trip Formula
For IEC curves:
t = TMS × k / ((I/Is)α - 1)
| Curve type | k | α |
|---|---|---|
| IEC standard inverse | 0.140 | 0.020 |
| IEC very inverse | 13.5 | 1 |
| IEC extremely inverse | 80 | 2 |
| IEC long time inverse | 120 | 1 |
IEEE C37.112-1996 Trip Formula
For IEEE curves:
t = TD × (A / ((I/Is)p - 1) + B)
| Curve type | A | B | p |
|---|---|---|---|
| IEEE moderately inverse | 0.0515 | 0.114 | 0.02 |
| IEEE very inverse | 19.61 | 0.491 | 2 |
| IEEE extremely inverse | 28.2 | 0.1217 | 2 |
How to Use
- Select the relay trip curve you want to use.
- Enter relay pickup current and fault current values.
- Enter the time multiplier setting for IEC or time dial for IEEE curves.
- Click Calculate to view the trip time result.
How This Calculator Works
This IDMT relay trip time calculator first finds the current ratio I/Is, then applies either the IEC 60255 inverse-time formula or the IEEE C37.112-1996 inverse-time formula depending on the selected curve.
Frequently Asked Questions
What is an IDMT relay?
An IDMT relay is an inverse definite minimum time overcurrent relay. It trips faster as fault current increases.
What happens if fault current is less than pickup current?
The relay will not trip, because the current ratio is not greater than 1.
Does this calculator support both IEC and IEEE curves?
Yes. It supports IEC standard, very inverse, extremely inverse, long-time inverse, and IEEE moderately, very, and extremely inverse curves.