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CT dimensioning

CT dimensioning

CT dimensioning

(OP)
How to specify correctly a protection CT for a light load (150kW/4,16kV) connected at a heavy busbar(Isc = 40kA).
Thanks

RE: CT dimensioning

First step would be to have some idea what the applicable CT standards might be.  Different places will have different ideas about how to specify CTs, even without worrying about the correctness of the specification.

RE: CT dimensioning

(OP)
Mr.David
The summary of my problem is:
Typical CT values:
Short time mech rating: 90 In
One second thermal rating: 80 In
One cycle: 150In
Bus short circuit : 40kA
Circuit breaker protection.
Full Load (motor) In: 30A
CT applied: 400/5A
Microprocessed relay current min value  for 46/49RMS
protection can be solved "fooling" the relay CT ratio set: 80/1A instead of 400/5A.
How about the CT accuracy?
In cases like this(high short circuit capacity / low load current) which would be the best solution for the CT dimensioning?
Thanks
 

RE: CT dimensioning

electrolitic,
This is a typical example where a low ratio CT has to be used at a place with very high SC rating.In your exampale the CT will get saturated within no time at 40 kA and whatever the protection connected to this CT might not work correctly even you specify a CT with a voltage class of C800.Therefore,in my opinion use a CT with a ratio 40/5 C100.I donot think you can get more than C100 due to low ratio.

Therefore,use a fused contactor where SC and or GF  protection is provided by fuses and overload protection is provided by the relay connected to the above CT.

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