This calculator computes the protection trip time according to IEC 60255 and IEEE C37.112-1996 protection curves.
TOC (or IDMT - Inverse Definite Minimum Time) ensures trip times are inversely proportional to the fault current, providing faster response for higher fault currents.
IDMT (Inverse Definite Minimum Time) is a time-current characteristic where higher fault currents result in quicker tripping of the relay.
The trip time for IEC 60255 can be calculated using the formula:
\[t(I) = TMS \left(\frac{k}{\left(\left(\frac{I}{I_s}\right)^\alpha - 1\right)}\right)\]Where:
| Curve Type | k | α |
|---|---|---|
| Standard Inverse | 0.140 | 0.020 |
| Very Inverse | 13.5 | 1 |
| Extremely Inverse | 80 | 2 |
| Long Time Standard Inverse | 120 | 1 |
For IEEE C37.112-1996 standards, the trip time is given by:
\[t(I) = TD \left(\frac{A}{\left(\left(\frac{I}{I_s}\right)^p - 1\right)} + B\right)\]Where:
| Curve Type | A | B | p |
|---|---|---|---|
| Moderately Inverse | 0.0515 | 0.114 | 0.02 |
| Very Inverse | 19.61 | 0.491 | 2 |
| Extremely Inverse | 28.2 | 0.1217 | 2.0 |