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Hi All,
       We have some 415v/3 phase vent fans and have recently had some trouble with them. A couple have s/c to earth which has caused the main 11kv cb to trip via its s/c current protection (wirewound resistor and voltage sensing relay). The vent fans are protected by a 4-6A cb and 32A fuses from the 415v supply transformer. WHat I want to do is possibly use rccb's instead of the cb/fuses to protect the vent fans, but also stop the faults tripping the 11kv cb. Any ideas.


Earth faults on the 415V system should not operate the 11kV earth fault protection, assuming that the step-down transformer has a delta-wye configuration.  
Installing sensitive earth fault protection on the motor feeders sounds like a good idea, but I would also be interested in finding out why the 11kV protection operated.


I am of the view that RCCBs are not really suitable for your application with industrial fans. Could lead to more problems than its worth. Properly rated fuses and overload protection are adequate.
As peterb states 11kv cb should not operate because of an earth fault in a final circuit.


what's an rccb?


The need for and desirability of sensitive earth fault protection on the motor feeders is a function of the system earthing and the fault level.  
If the fuse/cb combination is sensitive enough to trip for minimum arcing earth faults, then no other protection should be needed.  On the other hand, if the minimum earth fault level is below the capacity of the fuse/cb to trip, then separate earth fault protection is warranted.  This would preferably be of the zero sequence current detection type, such as provided by an RCCB (Residual Current Circuit Breaker, if memory serves me).


Assuming a Delta/star connection on your transformer, your 11kv protection should not have tripped. An earth fault on your secondary side will manifest itself in the primary as an overcurrent of approx 58% of maximum through fault current.  Your protection on the 415v system needs to be adaquate to deal with any 415v faults and your primary protection should be chosen to discriminate with this as well as protect primary circuits. I would suggest looking at your relay settings.  I have made a few assumptions here and do not have the full details but hope this sheds some light on the subject.


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