IDMT, inverse definite minimum time, describes overcurrent characteristics where operating time falls as current rises and levels off at a definite minimum for very high multiples of setting. Standard curves such as IEC normal inverse, very inverse and extremely inverse are scaled by a time multiplier setting (TMS) and a current pickup. Grading IDMT relays along a radial feeder with a margin of roughly 0.3 to 0.4 s gives discrimination without any communication link.
Set the incomer to IDMT standard inverse, 600 A pickup, TMS 0.15, that leaves us 0.35 seconds over the feeder relays.
In ANSI device numbers the time-overcurrent element is a 51 and the instantaneous is a 50, so a setting sheet full of 50/51 means IDMT plus a high-set.
Also heard as
- inverse time overcurrent
- 51
Related terms
Protection relay
Device that watches currents and voltages and orders a circuit breaker to trip when it detects a fault.
Selectivity
Arranging protection so only the device nearest a fault operates and the rest of the system stays in service.
Pickup
Threshold at which a protection element starts to operate, and the moment the measured value crosses it.