Fault Level comparison

[FernandoCFE]

In a Short Circuit Study, the fault level in the different busbars and his contributions are known.

If we think in the contribution of any circuit, only one part of the short circuit current, will flow through the switchgear in this busbar.

It is correct to take the fault level from the busbar to verify the capacity of the switchgaer?, Or only the contribution from these circuit?, Or it is necessary to calculate the fault level “after” the switchgear?

Any help and references are appreciated!

 

[Bung]

It all depends where the fault is. A circuit breaker on a circuit which does not contribute to the fault must be able to interrupt the full fault current - because the fault may be on the line side terminals of the CB, not the busbar side.


Bung
Life is non-linear...

 

[Mstrvb19]

As far as references, the IEEE Brown Book has some good coverage of performing short circuit studies, as well as the EPRI (Electric Power Research Institute) series has a good bood on coorination that may be of some service. I agree with Bung-the proper thing to do would be to compare the interrupting capacity of each breaker (or at least the breaker with the smallest capacity of your switchgear breakers have more than one rating) with the maximum postulated fault on the bus, becuase there is negligible difference in the impedance to the fault of it is on the line side of any breaker connected to the bus. Good luck!

 

[CJFlatters]

If you have rotating loads, you should consider the fault contributions from these (approx same as LRA) when checking / determining the rating of the switchgear / busbars as this is the fault current seen by the busbars at the instant of the fault (at least during the sub-transient period).

IEC 60909 is a common standard, there are others with which I am not sufficiently familiar to comment.

_______________________________________
Regards -

Colin J Flatters
Consulting Engineer & Project Manager