Spare primary windings on metering summation CT

[ScottyUK]

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?

 

[davidbeach]

Marbles, what marbles? ;-)

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.

 

[scottf]

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

 

[ScottyUK]

Great, the perfect two people to confirm what I thought. Thank you gents.

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