IDMT grading calculator
Plot the trip curves of inverse-time overcurrent relays.
Reads IEC 60255-151:2009 / IEEE C37.112-2018. Tables checked 26 Sept 2026.
Try a job 6
Each job fills in example values. Replace them with your values.
- Grade two relaysThe operating times of a feeder and an incomer relay at 4 kA.
- Grade a very inverse pairThe operating times of two very inverse relays at 6 kA.
- Grade three relays in seriesThe margins between three relays at 5 kA.
- Find a relay's time at ten times pick-upThe operating time of one relay at 2 kA.
- Check an instantaneous settingWhether a 4 kA instantaneous element clears an 8 kA fault.
- Grade an extremely inverse feederAn extremely inverse relay under a standard inverse incomer.
Inputs
A worked example with the default inputs.
- Fault current
- 1,200 A
- Relays
- 2 rows
Downstream trip time
0.27s
- Upstream trip time
- 0.96s
- Margin Downstream to Upstream
- 0.68s
This result is a design aid. Verify it against the current standard.
Calculation steps
- Fault current1,200A
- Relays graded2
- Downstream (IEC-SI) trip timeResult0.27s
IEC 60255-151:2009
- Upstream (IEC-SI) trip time0.96s
IEC 60255-151:2009
- Grading margin Downstream to Upstream0.68s
About this calculator
How this is calculated
The tool plots the trip curves of inverse-time overcurrent relays. It then checks the grading margin between relays at one fault current. Up to four relays are allowed, listed from downstream to upstream.
- Fault current. You enter the prospective fault current. The tool uses it for every relay.
- Multiple of pickup. For each relay, the tool divides the fault current by the pickup current. A relay does not trip at a multiple of 1 or less.
- Trip time. For an IEC curve, the tool uses the IEC 60255-151:2009 equation, with the time multiplier setting. For an IEEE curve, it uses the IEEE C37.112 equation, with the time dial. The curve constants come from the standard.
- Instantaneous element. If the fault current reaches the instantaneous setting, the relay trips at a fixed short time.
- Grading margin. The tool subtracts the trip time of each relay from that of the next one upstream.
| Step | Table or clause | What it decides |
|---|---|---|
| Curve shape | IEC 60255-151:2009 | The trip time of an IEC relay |
| Curve shape | IEEE C37.112 | The trip time of an IEEE relay |
| Margin | Your relay list | The gap between each pair of relays |
Worked example
- Two IEC standard inverse relays at 1200 A. The downstream relay has a pickup of 100 A and a multiplier of 0.1. The upstream relay has 200 A and 0.25. The downstream relay trips in 0.27 s. The upstream relay trips in 0.96 s. The margin is 0.68 s.
Limits
The tool uses the ideal curve equations. It does not model relay reset time, overshoot or current transformer saturation. It does not read curve data from a maker. An instantaneous setting of 0 means none.
The margin is for one fault current only. Check the margin at the highest and the lowest fault current at the relay. The tool does not choose settings for you.
See also: IDMT curve 100 A feeder grading, IDMT grading for a main switch and submain and IEC 60255-151 IDMT curve constants.
Questions
The result is a design aid. Verify against the current standard.
What does the IDMT calculator do?
The tool plots the trip curve of up to four inverse time overcurrent relays. It reports the trip time of each relay at the fault current you enter. It also reports the grading margin between each pair of relays in the list.
Which curves are available?
The tool offers the standard inverse, very inverse, extremely inverse and long time inverse curves of IEC 60255-151:2009. It also offers the moderately inverse, very inverse and extremely inverse curves of IEEE C37.112. Each curve has its own constants, so the same settings give different trip times.
Which inputs matter most?
The curve type, the pickup current and the time multiplier set the curve's shape and position. The fault current sets where the trip time is read. An instantaneous setting is optional. Relay order in the list sets which pairs are compared for grading.
What are the common gotchas?
A relay does not trip below its pickup current, so the tool reports no pickup there. An instantaneous setting overrides the curve above its threshold. A warning appears when a grading margin falls below 0.3 seconds, which is the default the tool uses. Relay settings sit in the page address.
Worked examples
- Dairy shed IDMT relay setting and gradingThree IDMT relays at a 3000 A fault, two grading margins and one curve inversion between the shed main and the farm incomer, read off the AmpSize relay engine.
- IEC 60255-151: IDMT curve constantsWhat IEC 60255-151 governs, which curve sets the IDMT tool carries, and a worked grading pair on standard inverse that clears in 0.21 s and 0.74 s.
- IDMT curve grading for a 100 A feederTwo standard inverse relays at 2000 A give a feeder trip at 0.23 s, an incomer trip at 0.99 s and a grading margin of 0.76 s, read off one chart.
Background
Clauses and tables this tool reads
- IEC 60255-151:2009 / IEEE C37.112-2018Inverse-time overcurrent curves
- IEC 60255-151 Table 1 / IEEE C37.112 Table 1IDMT overcurrent curve constants
Every clause and table, with how each was verified
Tables last checked on 26 Sept 2026.
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