Tools > IEC IDMT Curves

Inverse Definite Minimum Time (IDMT) curves are used in both mechanical and electronic protection devices. These curves define the amount of time which should pass at a given current, before initiating an action, usually the tripping of a circuit breaker.

The IEC who develop international standards for all electrical, electronic and related technologies, have defined 4 curves to be used. The IEC standards are commonly used throughout Europe and also other parts of the world.

Two settings are used to determine the time for a relay to initiate a trip: current setting (Is) and time multiplier setting (TMS). For any of the curves and a current multiplier (I/Is), a time is calculated, this time is the point at which the relay sends a signal.

The lower the current multiplier, the longer the trip time, allowing for slight overloading of equipment like motor starting, transformer energisation, or a number of other functions. Inversely, a higher current multiplier will cause a shorter trip time, which helps in the case of faults or sever overloading of equipment.

The formula and curve constants used to determine the trip times are shown below:

This convenient tool calculates the expected trip time for a given TMS and I/Is multiplier, and should give some guidance when testing a relay, as the usual method is to pick a multiple of Is and ensure that the actual trip time is close to the calculated value.