Inverse definite minimum time (IDMT) relay is a form of overcurrent relay that operates in inverse time fashion with the current in the protection system. Basically, with the change in the peak value of the current, the time required for the relay to perform is decreased. The measurement of operating time starts immediately when the current exceeds the set start value and the relay triggers the START signal.

IEC and ANSI / IEEE follow standardized inverse-time characteristics for inverse-time operation. ANSI and IEC IDMT curve operating times are specified with coefficients A, B, and C. The coefficient values can be calculated by formula: