In IDMT relays the time of operation is inversely proportional to the fault current level and the actual characteristics is a function of both “time” and “current” settings.
Each phase and ground current is separately compared with the pickup values of the inverse time over current protection elements. If a current exceeds 1.1
times the setting value, the corresponding element picks up and is signaled individually.
If the inrush restraint feature is applied, either the normal pickup signals or the corresponding inrush signals are output as long as inrush current is detected. Pickup of
a relay element is based on the RMS value of the fundamental harmonic. When the element picks up, the time delay of the trip signal is calculated using an integrated
measurement process.
The calculated time delay is dependent on the actual fault current flowing and the selected tripping characteristics. Once the time delay elapses, a trip signal is issued assuming that no inrush current is detected or inrush restraint is disabled.
If the inrush restraint feature is enabled and an inrush condition exists, no tripping takes place, but a message is recorded and displayed indicating when the over current element time delay elapses.
