Bushing CT Symbol?

[reENGR]

I use this CT symbol all the time but have not found a reference to what it means. The symbol is used for medium/high voltage power circuit breakers. What I am trying to figure out is why one of the turns is larger than the others?

I looked at the physical construction of one of the CTs and it is one physical torroidal CT. I assume that the manufacturer took a "more common" ratio, say 800:5, and added more turns to make it 1200:5.

What I am trying to figure out is why one of the turns is larger than the others?

 

[potteryshard]

If you look at the ways that the different CT ratios are tapped out of the secondary winding as a whole, you will see that one portion is always larger than the others.

For example, in the 1200:5 multi-ratio CTs that we use you would see that from X1 to X2 is 2/12 of the winding, from X2 to X3 is 1/12 of the secondary, from X3 to X4 is 5/12 of the winding, and from X4 to X5 is 4/12 of the total secondary winding.

I suspect that the symbol is intended to represent via the oversized loop and the bar that it is comprised of different sized tapable sections and is consequently a MR CT. By this putative logic, a single ratio CT would have equally sized loops and no bar.

 

[dpc]

I'm not sure it has to have any physical significance. It is just a symbolic way to distinguish bushing CTs from other CTs. The symbol represents bushing CTs, regardless of whether they are single ratio or multi-ratio. As to one of the turns being larger than the others, I do not normally see it drawn this way.

There are (in the ANSI world, anyway), standard ratios for multi-ratio CTs as well as single ratio CTs.

David Castor

http://www.cvoes.com

 

[stevenal]

I think the large arc shows it is a bushing device. Note the similarity to capacitor bushings and capacitor-bushing potential devices in IEEE 315.