MV Cable Shield requirements 7

[JBUDA54]

I am currently on a design/build project writing some specifications on Medium voltage motor control centers/Cable/Motors. There was a recent discussion within my group about when to use shielded cable for medium voltage loads. According to an article written by John A. DeDad Editorial Director for EC&M, he makes the following statement: "As we all know, the metallic shielding in a medium voltage (MV) cable provides the necessary uniform electric field within the cable's insulation." See the following website for a write up of his article:

http://www.ecmweb.com/ar/electric_groundfault_coordination_include/.

It can also be found in the EC&M Feb 1, 1995 Magazine Issue Titled "Ground-fault coordination should include MV cable shielding." My question is what dictates when shielded cable should be used? I realize that cost dictates some projects as to how stout an electrical system should be installed as far as the implementation of good engineering practices, but is there a standard that governs this area? A senior electrical engineer told me that shielding should be considered for loads voltage levels above 2400V. Any assistance with this subject would be greatly appreciated.


Jason Buda, PE
Greenville, SC
(864) 676-6458 PH
Jason.Buda@Jacobs.com

 

[jghrist]

See NEC 310.6. "Solid dielectric insulated conductors operated above 2000 volts in permanent installations shall have ozone-resistant insulation and shall be shielded." There are some exceptions.

 

[busbar]

It may be simple economics. Above 5kV it does not seem practical or cost-effective to produce non-shielded cable that can be expected to survive over the life of associated gear.

 

[sdlloyd]

The use of a cable shields is dependant upon the electrical stress of the cable and whether or not any air gaps will discharge (spark).

I have extracted the following from the "Electric Cables Handbook" which covers the subject. Note: screens [UK] = shields [US].

“At 6kV and above, as a means of containing the electric field within the insulation, semiconducting screens are applied over the conductor and insulation. By this means it is possible to eliminate any electrical discharges arising from air gaps adjacent to the insulation. The coefficient of thermal expansion of polyethylene and EPR is approximately ten times greater than that of either aluminium or copper, and when the conductor is at its maximum operating temperature of 90 C, a sufficiently large gap is formed between the insulation and the conductor to enable electrical discharges to occur. This discharge site and any others which are formed around a conductor when the cable is bent can be eliminated by applying a semiconducting layer over the conductor.

During the 1960’s semiconducting tape screens were applied over the conductor but these have since been superseded by an extruded layer. This has the advantage of providing a smoother finish and, as it can fill the interstices between the wires, a circular envelope around the conductor. By reducing the concentration of flux lines around the individual wires, the electrical stress around the conductor is reduced by about 10% and 15%. The semiconducting layer is compatible with, and bonds to, the insulation. A nominal thickness of 0.4 to 1.0mm is typical."

Shield materials are relatively expensive and thus the thickness of both the conductor and insulation shields is constantly being reduced in the drive to save cost. The screens are however a vital part of the cable design and if too thin a layer is extruded there is a risk that voids and/or defects (comprising of pre-cured lumps "hang-ups&quot will appear during extrusion.

Hope this helps.

http://www.cableconsulting.net

 

[DanDel]

For short MV runs (up to 15kV) that are not enclosed by metal conduit(such as connections inside switchgear from an MV switch to a transformer) shielded cable is typically not used.
When there are grounded metal surfaces close to the conductor, the voltage stresses on the insulation at those points are greater than at other points in the cable. Shielded cable causes the stress to be uniform at all points in the cable insulation(as already mentioned).

 

[JBUDA54]

sdlloyd,

It appears you are an expert in the subject matter. Thank you for that information. I did come across a good reference book published by Southwire called Power Cable Manual. Chapter 4 covers MV shielding requirements. It can be found at the following web address:

http://www.southwire.com/tech/library/pc/pwrcab04.htm

Are there any other good papers or standards on the subject? Thanks for everyone's response. It really shed some light on a subject that I haven't had much experience with.

Jason Buda, PE
Greenville, SC
(864) 676-6458 PH
Jason.Buda@Jacobs.com

 

[jbartos]

Suggestion: Visit

http://www.usawire-cable.com/techpapers/effects_shield_loss.htm

etc. for shields

 

[JBUDA54]

jbartos,

You always have great references! That was a very interesting paper! Thanks for that resource!


Jason Buda, PE
Greenville, SC
(864) 676-6458 PH
Jason.Buda@Jacobs.com