IDMT Protection
Plot the trip curves of inverse-time overcurrent relays.
Questions
Each answer describes what this tool calculates. 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 shape and the position of the curve. 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.