# 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.

##### Results

Trip times:

Standard Inverse - s

Very Inverse - s

Extremely Inverse - s

Long Time Inverse - s