Overcurrent Relay Settings: IDMT Curve Worked Example
An overcurrent relay has two main settings: the current at which it starts, and how fast it trips above that. The inverse-time curve links them. Here is the calculation with an example.
The curve
For the IEC standard inverse curve: t = TMS × 0.14 ÷ ((I ÷ Is)0.02 − 1).
- t: operating time in seconds
- I: fault current in primary amperes
- Is: pickup current in the same units
- TMS: time multiplier setting
Worked example
Illustrative example, not a project. A feeder carries 300 A maximum. Set pickup at 1.2 × 300 = 360 A. Use TMS = 0.1. A fault of 4,000 A occurs.
- Plug setting multiple = 4,000 ÷ 360 = 11.1
- 11.10.02 = 1.0493, so the denominator is 0.0493
- Curve time at TMS 1 = 0.14 ÷ 0.0493 = 2.84 s
- Operating time = 0.1 × 2.84 = 0.28 s
At 6,000 A the same relay operates in about 0.24 s; at 2,000 A, about 0.40 s. The relay is faster for bigger faults, which is what inverse-time means.
Grading with the upstream relay
The incomer's relay must be slower than the feeder's at every fault level on the feeder, by a grading margin that allows for breaker opening time, relay tolerances and overshoot. If the feeder relay clears the fault in 0.28 s and you adopt an assumed margin of 0.3 s, the upstream relay should not operate before about 0.58 s at that fault current. Plot both curves to check, and take the actual margin from the relay manufacturer and the project specification. See protection coordination.
Other settings and checks
- Instantaneous element: trips at once above a high fault current, often set above the largest through-fault current so downstream relays still grade.
- Transformer inrush and motor starting: pickup and delay must ride through these.
- Thermal withstand: the clearing time must be short enough to protect cables. See the adiabatic equation.
- Arc flash: longer clearing times raise incident energy. See arc flash basics.
Select the CT first: CT ratio and burden. Software for the curves: ETAP.
Written by the Vision Matrix Institute editorial team. Worked examples use stated, illustrative assumptions; check them against your project data, the applicable standards and manufacturer datasheets before use.
Frequently Asked Questions
What are the main overcurrent relay settings?
The pickup (plug setting) current, the time multiplier setting (TMS) that scales the curve, the curve type, and often an instantaneous element for high fault currents.
What is the IEC standard inverse curve?
An inverse definite minimum time curve where operating time t = TMS × 0.14 ÷ ((I ÷ Is)^0.02 − 1), with I the fault current and Is the pickup current.
How is pickup chosen?
Above the maximum load (and allowable overload) with margin so the relay does not operate on normal load, but low enough to see the minimum fault current on the circuit.
How are upstream and downstream relays graded?
The upstream relay is set to operate slower than the downstream relay by a grading margin that covers breaker opening time and relay tolerances, so only the nearest relay trips.
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