×
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

Spare primary windings on metering summation CT

Spare primary windings on metering summation CT

Spare primary windings on metering summation CT

(OP)
Could someone give me a quick sanity check?

One of our switchboards has a fairly complex old-school summation CT scheme feeding metering, and I need to remove a couple of circuits from the scheme. The scheme uses a number of summation CTs, each with multiple 5A primaries and a single 5A secondary. Several of these summation CT secondaries drive a further summation CT which then feeds into an energy meter.

We all know that a short-circuit is always applied to an unused CT secondary winding to prevent damage to the CT and avoid hazardous voltages being developed. In the case of this summation CT I will have an unused primary winding. I think we should leave it open, because shorting it will effectively turn it into an additional secondary winding and reduce the available ampere-turns available to drive current in the proper secondary winding, which will cause the meter to under-record the energy usage. The counter opinion expressed is that the spare primary should be shorted for safety like an unused CT secondary, but I think they are saying this because "you always short an unused CT winding" without considering that this is a primary winding and not a secondary.

Am I losing my marbles?

RE: Spare primary windings on metering summation CT

Marbles, what marbles? winky smile

Between now and when scottf weighs in, I think that you need to have to leave them open. I'm assuming from the description that this is a wound transformer, not some sort of window/bushing CT. You need one winding that can supply current to the meter and however many that supply the current to be summed. Each adds its own amp turns to the core, and the meter gets all of the resulting amp turns. Just what you want. The core won't know or care that an unused winding is a "primary" or a "secondary". If it's left open its not part of the amp turn balance. If it's shorted then it can provide an alternate path instead of the meter.

Connect just the "primarys" to be summed and leave everything else open and you'll have the classic open CT problem, amp turns in and none out. Short one winding that isn't supplying current and you make a path for the amp turn balance.

Consider the multi-ratio CT, you may leave most of the winding open, but there's part of it that can provide the amp turn balance and no open CT problem.

RE: Spare primary windings on metering summation CT

Yes...leave the unused primary(s) open.

RE: Spare primary windings on metering summation CT

(OP)
Great, the perfect two people to confirm what I thought. Thank you gents. smile

David - I'm not using this as evidence of my sanity!

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