IDMT ग्रेडिंग कैलकुलेटर

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.

1. Fault current. You enter the prospective fault current. The tool uses it for every relay.
2. 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.
3. 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.
4. Instantaneous element. If the fault current reaches the instantaneous setting, the relay trips at a fixed short time.
5. Grading margin. The tool subtracts the trip time of each relay from that of the next one upstream.

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.