It is an inverse definite minumum time relay. In IDMT relay
its operating is inversely proportional to fault current
and also a characteristic of minumum time after which this
relay definately operates.
It is inverse in the sense , the tripping time will decrease as the magnitude of fault current increase.
"for a fault current of 25KA-the time delay may be 2s.
& for a fault current of 30KA the tme delay may be 1.5s.thus as the fault current increases the time delay decreases.
is an inverse definite minimum time relay
there are 2 precautions to be considered when using this these precautions are the time setting and the current tap setting
these are to be considered since the relay is dependent on the inverse of time
The IDMT Inverse definite minimum time relay is one such
type of relay where the delay time of the relay is
inversely proportional to the magnitude of the fault
current. For ex. If the fault current is of the magnitude
of 8KA the delay taken by the relay to tripp the ckt may be
3 sec likewise if a fault current of 16KA persists in the
system the IDMT delay time will automatically reduce to 1.5
sec and so on. It is used in applications of overcurrent
relays and earth fault relays.
it is inverse definite time relay.and in the name it self
inverse means when fault current is more then its operating
time is less.because if it takes more time the equipment
may damage.so to avoid this the relay is made with inverse