# Tools > IEC IDMT Curve Grading

##### IEC 60255 IDMT Curve Grading

When specifying settings for 2 relays, it is usually required that one relay trips prior to another, and with a minimum grading time which would be specified by the engineer or calculated.

This tool allows for 2 IEC IDMT curves, with or without a definite time (DT) element, to be set. A fault level (FL) can then be input to determin the grading between the 2.

The resulting plot can be saved and used in engineer reports by clicking Download image.

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
##### Fault level

Relay 1 trip time: - s

Relay 2 trip time: - s

Difference: - s

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