Inverse-time overcurrent relay operating time from the standard IEC curves. Also solves the reverse problem: what TMS do I need to hit a target operating time?
Verified against published standard
Relay Setting
SI = standard inverse, VI = very inverse, EI = extremely inverse, LTI = long-time inverse.
Reverse Solve (optional)
Enter relay settings and calculate.
About This Calculation
An IDMT (Inverse Definite Minimum Time) relay trips faster as fault current rises further above its pickup setting — a large fault clears in a fraction of a second, while a marginal overload gives equipment (and downstream protection) time to react first. This inverse-time shape is what makes coordinating multiple relays in series possible: each relay downstream of another is set with just enough time margin to let the closer relay clear the fault first, so an outage stays as small as possible. Two standard curve families exist — IEC 60255 and IEEE C37.112 — both implemented on this page, since real numerical relays (ABB, Siemens, SEL, GE, and others) support both.
Frequently Asked Questions
What does TMS (Time Multiplier Setting) actually control?
TMS scales the whole curve up or down without changing its shape — it’s the main tool for coordinating relays in series: a downstream relay gets a lower TMS (faster) and an upstream relay a higher TMS (slower), so they clear a fault in the correct order.
What is PSM (Plug Setting Multiplier)?
PSM is simply the fault current expressed as a multiple of the relay’s pickup setting (fault current ÷ pickup current). It’s the x-axis of every IDMT time-current curve — the relay operates faster at higher PSM.
Which curve should I use: IEC or IEEE?
This is usually dictated by regional practice and what the rest of the protection scheme already uses — IEC curves (Standard/Very/Extremely Inverse) are common outside North America; IEEE/ANSI curves (Moderately/Very/Extremely Inverse) are common in North America. Mixing families on relays that must coordinate with each other needs careful checking.
Why does the relay never operate below its pickup setting?
Below pickup (PSM ≤ 1) there is, by definition, no meaningful overcurrent to trip on — the IDMT equations are mathematically undefined at PSM = 1 and give negative/nonsensical results below it, which is why real relays simply don’t operate there.