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 |

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