IDMT Tripping Time Calculator
Enter fault current, pickup current, and time dial to calculate IDMT relay tripping time using IEC 60255 curves.
Relay Parameters
The actual fault current seen by the relay.
Relay plug/pickup setting current (must be < If).
Scales the curve; typical range 0.05 – 1.0.
Result
Tripping Time
—
seconds
Current Multiple (M)
—
If / Ip
| Parameter | Value |
|---|
IEC 60255-151 Formula
t = TMS × K / (Mα − 1)
IEC 60255-151 Curve Constants
| Curve | Abbrev. | K | α |
|---|---|---|---|
| Standard Inverse | SI | 0.14 | 0.02 |
| Very Inverse | VI | 13.5 | 1.0 |
| Extremely Inverse | EI | 80 | 2.0 |
| Long-Time Inverse | LTI | 120 | 1.0 |
Formula: t = TMS × K / (Mα − 1) where M = If / Ip
Summary
Enter fault current, pickup current, and time dial to calculate IDMT relay tripping time using IEC 60255 curves.
How it works
- Select the IDMT curve type (Standard Inverse, Very Inverse, Extremely Inverse, or Long-Time Inverse).
- Enter the fault current (If) in amperes.
- Enter the relay pickup current (Ip) — the minimum current at which the relay starts timing.
- Enter the Time Dial Setting (TDS/TMS) to scale the tripping time.
- Click Calculate to get the relay trip time in seconds based on the IEC 60255-151 formula.
- Review the formula breakdown and curve constants to verify the calculation.
Use cases
- Size and coordinate overcurrent relay settings in power distribution systems.
- Verify IDMT relay selectivity during protection studies.
- Calculate grading margins between upstream and downstream relays.
- Educational reference for IEC 60255 protection relay students and engineers.
- Check relay operating time against manufacturer datasheets.
- Support power system protection engineering reports and documentation.
Frequently Asked Questions
Last updated: 2026-06-11 ·
Reviewed by Nham Vu