CTs on line side of main disconnect in MCC

[jraef]

I'm looking for opinions or even standards I am unaware of on this issue.

I am being asked to move the CTs for a power monitor to the LINE side of a 600A main fused disconnect switch of an MCC. This is a non standard configuration for us and requires significant re-engineering, something I'd like to avoid (not because I'm lazy but because the customer wants it FAST). Are there any reasons I'm not thinking of for NOT doing this? I was thinking safety, but if the disconnect was open (so the door could open), there could not be any current flowing so no output potential on the CTs. So this issue doesn't hold much water. I also know that utility CTs are generally on the line side of a service disconnect, so it's not unheard of. Anything else? Or am I all wet and we should develop a standard configuration this way because lots of people are going to ask for it? To that issue, we have been making MCCs for 50+ years, this has apparently never come up before.

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[rcw retired EE]

I don't know of any code that you could cite to keep the CT's on the load side other than good practice with today's environment of arc flash concerns. A load side CT can be tested or changed without de-energizing the line side. However, the arc flash level is the same in either case since the line side of the fused switch is in the same compartment.

Current will be the same on the line or load side, except for a fault in the switch so there's no technical reason either side would not work.

I would push the safety during testing issue.

Good luck.

 

[ScottyUK]

I could understand the request if he is implementing some form of unit protection scheme where he wants the breaker in-zone. Allied to a similar unit scheme on the transformer LV winding he could get the arc flash hazard right down by very fast detection and fault clearance. Seems a lot of cost on a 600A service though.

But for a power monitor I'm not sure what he gains by changing location.

Taking a contrary position to rcwilson's comment about de-energised working on the CT, if the bus is dual-fed then it may be possible to take an incomer out of service for maintenance more easily than taking the bus down.


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[jraef]

Thanks guys. I told him he could have it either way, but it would cost him more to do it on the line side and it will delay the project for re-engineering. That should quiet him down...

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376