What is the IDMT relay?

IDMT relays means Indefinite Mean Time Relay.. It operateduring overload conditions.The Formula is Current inversely proportional to the time.It means If fault Current is High then the Time to Trip theCircuit will be less or vice versa..

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IDMT stands for Inverse Definite Minimum Time - and is the normal type of time/current graded overcurrent protection used in the supply industry and elsewhere. Basically, the more current you put through the relay, the faster it goes. But eventually friction and magnetic saturation limit the maximium speed it can go, hence giving the definite minimum time (2.2s from memory at unity time multiplier). You can tap the coils in the relay (usually% to% of rating) to alter the current sensitivity, and you can alter the distance the disc travels (or its electronic equivalent) to vary the time of operation at any given current setting.

Clearly setting this is a little tricky as you have two variables - current through the relay cf its setting, and time multiplier. Historically settings were done graphically on log graph paper (or was it log-log....) because of this, using clear templates of the operating curve to get the grading. Sounds hard but was quite easy once you'd grasped it. It's standard power system text book stuff.

a) Long Time Inverse Definite Minimum Time is typically used for standby earth fault protection (ie it operates more slowly than the other curve types of IDMT protection and provides backup protection in case of failure by main protection)

b) Useful references to this can be found in manufacturers data sheets (may possibly be available on the web) Areva T&D MCGG range of relays has some useful information (Areva is the new name for what was called Alstom T&D).

c) I've found The Protection of Industrial Power Sysatems by T Davies a very easily digested (ie light on the maths) reference book on protection issues for ~£. For more detail Areva publish Network & Protection Relal Application Guide (NPRAG) which costs about twice as much.

d) The Reclose Initiation I think is the arming function of an auto reclose relay (eg RI being instigated by a tripping of the (closed) circuit breaker on fault). I have limited experience of these but I'd expect autoreclose to operate for transient faults only which are typically short-circuit faults (eg branch falling across an overhead line where the fault is cleared by the time the reclose occurs). I definately would not expect it to be initiated by a Long time, standby relay which is indicaticative of a persistent fault (it's possible that the LTI relay may have been intended to block the RI function).