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Short circuit bus bracing on 4kV switchgears connected by N.O. tie

Short circuit bus bracing on 4kV switchgears connected by N.O. tie

Short circuit bus bracing on 4kV switchgears connected by N.O. tie

(OP)
Each of the two separate switchgears are fed by a 10MVA 15kV-4160V transformer. There is a normally open tie connecting them.
The switchgears are loaded such that one switchgear can handle all the load (less than 1/2 loaded each).

When they want to take a transformer out of service for any reason they temporarily close the tie and operate in parallel, then open the main that is connected to the transformer they want to take out of service.

I know this is a common configuration. A short circuit analysis for these switchgears, reveals that when they are operating in parallel the short circuit current available is of course much greater, and is higher than the rating of the gear. The excuse I hear is that its ok because they don't operate like this all the time just during this brief switching period.  

We are thinking of replacing this gear. Does this parallel configuration have to be taken into account when specifying the new gear? Is there any Code reference or IEEE standard that I could point to say this higher bus bracing must be done for compliance? Or is it just good design practice, or is the same excuse as described above just common practice?

RE: Short circuit bus bracing on 4kV switchgears connected by N.O. tie

Strict interpretation of the NEC is that this is never allowed, even during brief switching operations.  

See NEC 110.9 and 110.10.  

In practice, exceptions are sometimes made.  Often interlocking is provided to automatically trip one of the three breakers after a very brief time delay.

Depending on what you want the result to be, you could ask the local AHJ for an interpretation.  


 

David Castor
www.cvoes.com

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