medium voltage cable question?

[ibew441dc]

I am a wireman who does strictly 600vac and less.

A question was asked to me about the concentric strands on the outside of medium voltage utility cables (actually directly inside the outer insulation. ) I wish I had a picture , but I don't. The first thing that comes to mind that has a similar look would be SE Cable. Now to the question.

What is it for? Since the utilities don't bond anything, its not that . I don't think it is used for a neutral, for a wye ?

My thought was that it may be for static or possibly to drain a capacitive charge?

Any input would be great?


ibew441dc

Just when you think you know it all.........

 

[infinity]

Is this what the cable looks like?

 

[celtic]

I am a wireman who does strictly 600vac and less.

A question was asked to me about the concentric strands on the outside of medium voltage utility cables ...

What is it for? Since the utilities don't bond anything, its not that . I don't think it is used for a neutral, for a wye ?

My thought was that it may be for static or possibly to drain a capacitive charge?

It's a drain for any stray voltage.

 

[Nick]

The cable you are asking about is known as jacketed concentric neutral (JCN). The concentric wires are copper (even though in the utilities case the cables conductor is aluminum) and are bonded and grounded throughout the utility system. This is known as a multi grounded neutral system. The concentric wires can be rated anywhere from 1/3 the capacity of the cables conductor up to 100%. SCE used to use unjacketed concentric neutral cable but recently changed to jacketed.
The cable in infinty's post is not used by utilities. That is a tape shielded cable that you will see in customer owned systems on the load side of the meter. This copper tape shield is just that, a shield. It is not designed to cary neutral current like the JCN.

http://www.okonite.com/Product_Catalog/section2/sheet31.html
Note: The cable in this link has EPR insulation. The utilities around us (SCE, Anaheim, DWP etc.) use XLP. XLP looks white and translucent.

 

[ibew441dc]

Nick,

Thanks for the info. That is exactly the cable I was talking about.

So the concentric strands are used for bonding everything together to reduce a difference in potential between equipment and other associated parts? Or is it to carry neutral current?


Thanks ,

Ibew441dc

 

[peter d]

Or is it to carry neutral current?

It's the neutral.

In my area (and probably everywhere else,) this cable is used to feed single phase pad mount transformers in areas that have underground service.

If you look up on a pole where this cable connects to overhead lines, you can see the strands on the u/g cable connected to the o/h neutral wire.

 

[Nick]

It actually has a number of uses. It is the shield for this cable as this is considered shielded cable. As Peter D says it is a neutral for single phase loads. It is common to see drops at poles where only one cable goes down the pipe. This is because it feeds 6.9KV single phase transformers and the concentric neutral wire carries the return. This cable not limited though to single phase distribution. It is run to three phase transformers as well. In that case it serves as a shield for the cables and also a bonding/grounding conductor for the system. If you look at over head distribution you will see allot of poles that have 4 conductors. (3 phases and a grounded neutral) The concentric wires of underground cable always connects to that 4th neutral wire. So, to answer your question, it is both.

PS: Getting pretty slow in 441 huh.

 

[jamesguy10]

Yeah it is used for a neutral on underground residential primary lines. On higher voltage lines it is used to prevent stray voltage. Thats pretty much it. If it breaks on a 7200v line than your going to have some bad stray voltage problems.