IDMT grading calculator
I-plot ang trip curves ng inverse-time overcurrent relays.
Reads IEC 60255-151:2009 / IEEE C37.112-2018. Tables checked 26 Sept 2026.
Subukan ang isang job 6
Nilalagyan ng halimbawang value ang bawat job. Palitan ang mga ito ng sarili mong value.
- 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.
Mga input
Worked example gamit ang mga default na input.
- Fault current
- 1,200 A
- Mga relay
- 2 row
Downstream trip time
0.27s
- Upstream trip time
- 0.96s
- Margin mula Downstream hanggang Upstream
- 0.68s
Design aid lang ang resultang ito. I-verify ito laban sa kasalukuyang standard.
Mga hakbang ng kalkulasyon
- Fault current1,200A
- Mga relay na naka-grade2
- Trip time ng Downstream (IEC-SI)Resulta0.27s
IEC 60255-151:2009
- Trip time ng Upstream (IEC-SI)0.96s
IEC 60255-151:2009
- Grading margin Downstream hanggang Upstream0.68s
Tungkol sa calculator na ito
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.
Mga tanong
Design aid ang resulta. I-verify ito laban sa kasalukuyang standard.
Ano ang ginagawa ng IDMT calculator?
Iginuguhit ng tool ang trip curve ng hanggang apat na inverse time overcurrent relay. Iniuulat nito ang trip time ng bawat relay sa fault current na ilalagay mo. Iniuulat din nito ang grading margin sa pagitan ng bawat pares ng relay sa listahan.
Anong curve ang available?
Nag-aalok ang tool ng standard inverse, very inverse, extremely inverse at long time inverse curve ng IEC 60255-151:2009. Nag-aalok din ito ng moderately inverse, very inverse at extremely inverse curve ng IEEE C37.112. May sariling constant ang bawat curve, kaya magkaiba ang trip time kahit pareho ang setting.
Aling mga input ang pinakamahalaga?
Itinatakda ng curve type, ng pickup current at ng time multiplier ang hugis at posisyon ng curve. Itinatakda ng fault current kung saan binabasa ang trip time. Opsyonal ang instantaneous setting. Itinatakda ng relay order sa listahan kung aling pares ang ikinukumpara para sa grading.
Ano ang mga karaniwang gotcha?
Hindi nagti-trip ang relay sa ibaba ng pickup current nito, kaya walang pickup na iniuulat ang tool doon. Pinapawalang-bisa ng instantaneous setting ang curve sa ibabaw ng threshold nito. Lumalabas ang babala kapag bumaba sa 0.3 segundo ang grading margin, na default ng tool. Nasa page address ang relay settings.
Mga worked example
- 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
Mga clause at table na binabasa ng tool na ito
- IEC 60255-151:2009 / IEEE C37.112-2018Inverse-time overcurrent curves
- IEC 60255-151 Table 1 / IEEE C37.112 Table 1IDMT overcurrent curve constants
Bawat clause at table, kasama kung paano ito na-verify
Huling na-check ang mga table noong 26 Sept 2026.
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