Tools > IEC IDMT Curves

IEC 60255 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:

\begin{equation*} t(I)= TMS \displaystyle \left( \frac {k}{\displaystyle \left(\frac{I}{Is}\right)^\alpha-1}\right) \end{equation*}
Curve type k α
Standard inverse 0.140 0.020
Very inverse 13.5 1
Extremely inverse 80 2
Long time inverse 120 1

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.


Trip times:

Standard Inverse - s

Very Inverse - s

Extremely Inverse - s

Long Time Inverse - s

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