×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

CTs on line side of main disconnect in MCC

CTs on line side of main disconnect in MCC

CTs on line side of main disconnect in MCC

(OP)
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.

"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: Eng-Tips.com Forum Policies  

RE: CTs on line side of main disconnect in MCC

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.

RE: CTs on line side of main disconnect in MCC

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.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: CTs on line side of main disconnect in MCC

(OP)
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: Eng-Tips.com Forum Policies  

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources