IEC 60255 IDMT Curves
Inverse Definite Minimum Time (IDMT) curves define the trip time for protection relays based on fault current magnitude. Higher fault currents result in faster trip times.
Two settings determine trip time: current setting (Is) and time multiplier setting (TMS).
| Curve Type | k | α |
|---|---|---|
| Standard Inverse | 0.14 | 0.02 |
| Very Inverse | 13.5 | 1 |
| Extremely Inverse | 80 | 2 |
| Long Time Inverse | 120 | 1 |
Formula: IEC 60255-151:2009, clause 4.4.1.3, equation (1), with c = 0 for the IEC curves. Constants: Annex A, Table A.1, curves A (SI), B (VI) and C (EI). LTI is not in Table A.1.
Worked Example
A Standard Inverse relay set at Is = 400 A and TMS = 0.1 sees a 4,000 A fault, so I/Is = 10.
t = 0.1 × 0.14 / (100.02 − 1)
= 0.1 × 0.14 / 0.04713
= 0.297 s
Enter TMS 0.1 and I/Is 10 below and the tool returns 0.297 s (SI), 0.150 s (VI), 0.081 s (EI) and 1.333 s (LTI).
Inputs
Trip Times
One relay's trip time is the start. Choosing the curve and TMS for every relay in the chain against real fault levels is a protection study. Protection studies →
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Related: Check the margin between two relays with the IDMT grading tool
Curves
Frequently Asked Questions
What is an IDMT calculator used for?
An IDMT calculator calculates protection relay trip times based on IEC 60255 inverse time curves. It determines how quickly a relay will trip based on fault current magnitude and time multiplier settings (TMS). Essential for protection coordination studies in electrical power systems.
What IEC curves are supported?
SI, VI and EI are curves A, B and C in IEC 60255-151 Annex A, Table A.1. LTI (k = 120, α = 1) is not in that table; it is a manufacturer curve, so check the relay manual before relying on it.
Is this IDMT calculator free to use?
Yes, completely free with no registration or signup required. Calculate unlimited trip times for protection coordination studies. All calculations run in your browser with instant results.
How do I calculate IDMT trip time?
Enter your TMS (Time Multiplier Setting) and I/Is (current multiplier) values, then click Calculate. The tool uses the IEC formula: t = TMS × (k / ((I/Is)^α - 1)) to calculate trip times for all four curve types simultaneously.
What is TMS in relay settings?
TMS (Time Multiplier Setting) is a multiplier that scales the trip time curve. Higher TMS values increase trip times proportionally, allowing for protection grading coordination between multiple relays.