Short circuit current

[Arevaengineer]

In a substation 132/33/11kV, the SC rating of 33kV CB (MOCBs) is 31.5kA. The client is being proposed replacement with vacuum CBs rated 25kA considering the fact that the neutral point on the 33kV side is isolated and grounded by 17 Ohm reactor.
Is it advisable to use the 25kA CB instead of the required 31.5kA.
Thanks in advance for your expert comments.

 

[racobb]

You need to know the available 3 phase fault duty at the bus.

Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[slavag]

Hi
Izn't correct from your client side.
What Alan saied, 3-ph bolted fault for eq. choice, not single phase fault.
Best Regards.
Slava

 

[rbulsara]

Agree with above. The answer is NO. Neutral impedance does not limit phase to phase fault currents. The CB need to meet or exceed the maximum fault current available, either L-L or L-G.