MV Flexible cable

[philly]

What type of cable is it that you typically see internal to Dry Type transformers and some other MV equipment? I'm referring to usually orange color cable that looks like flexible cable or S.O. cord.

I've seen this internal to MV dry type transformers (between cable termination point and coils) as well as for MV Switchgear PT circuits.

Is this usually a specific type of cable? Is this cable typically fully insulated? Are there bending radius limitations similar to typical insulated cable?

 

[Jraef]

Inside of gear, we would use an unshielded cable to get the extra flexibility (referred toin MV cable as "rope stranding"). We bought it from Anixter, it was called MV "Jumper Cable". This type of cable can only be used where contact with people and electrical grounds is controlled and limited to authorized personnel. But the thing is, the entire ASSEMBLY is then subjected to testing and listing, so the lack of shielding becomes less relevant. You can't really use that in the field though.

 

[paulengr]

It is red. That was sort of a trade mark for one manufacturer but now everybody makes it in red.

It is technically “temporary hookup cable”. It can only be used (NEC/UL) for temporary MV jumpers. This is the only type of unshielded wire over 2 kV NEC allows. Inside enclosures like SIS it is a “component” so legal for use. The assembly is Listed, not the components. It is unshielded so bend radius is 6x OD. This is acceptable because everything is lashed down, spaced with glastic spacers and lots of cable ties. The air space around the cable is important too. So a 5 kV rated cable might operate at 15 kV. If it touches itself the rating is NOT 5 kV. I have tons of photos of manufacturers that screwed up installations by failing to follow MV installation practices and laid it against grounded metal or each other. It takes 5-10 years for 4160 to eat itself, only months at 13.5 kV.

If you are trying to do this yourself in an NEC environment the best trick we have found is cut the jacket/shielding/semicon way back. The termination kits specify a minimum overall length, not a maximum. Give yourself plenty of room. Nothing wrong with 24” of insulated (but not shielded) cable before ending at the lug. If there is no vertical space most switchgear offers a “high hat” accessory that is just an extended enclosure on top. Also tape terminations are 6-9” long. Shrink kits are 12-15” or more. The bend radius of the unshielded cable is same as what you are seeing. Only thing is it’s unshielded so you MUST take steps to ensure the cables don’t get under the minimum allowable spacing or they will track and fail prematurely.

When ordering if the manufacturer offers a “termination” or “wiring” cabinet, buy it. You won’t be sorry. We are distributors for a popular MV company and I’ve personally learned this lesson the hard way myself.

 

[philly]

It is technically “temporary hookup cable”. It can only be used (NEC/UL) for temporary MV jumpers. This is the only type of unshielded wire over 2 kV NEC allows. Inside enclosures like SIS it is a “component” so legal for use. The assembly is Listed, not the components.

Not clear on what you mean with assembly listing vs components? If these cables were added to a new switch to make a connection to a close coupled transformer (through an air terminal chamber) would this be a legal use assuming clearances etc.. you mentioned below are maintained?

It is unshielded so bend radius is 6x OD.

Is this a specific NEC reference?

This is acceptable because everything is lashed down, spaced with glastic spacers and lots of cable ties. The air space around the cable is important too. So a 5 kV rated cable might operate at 15 kV. If it touches itself the rating is NOT 5 kV. I have tons of photos of manufacturers that screwed up installations by failing to follow MV installation practices and laid it against grounded metal or each other. It takes 5-10 years for 4160 to eat itself, only months at 13.5 kV.

Is there an NEC or other reference that show the required clearances (PH-PH & Ph-Gnd) for exposed bus (bus, terminals, etc...)?

Specific to the cable I'm not 100% clear on how a 5kV rated cable could carry 15kV if not rated? Does it have to do with the fact that there is no shielding and the spacing requirements?

 

[paulengr]

Not clear on what you mean with assembly listing vs components? If these cables were added to a new switch to make a connection to a close coupled transformer (through an air terminal chamber) would this be a legal use assuming clearances etc.. you mentioned below are maintained?

“Listed” means tested and approved by a Listing agency. But the Listing agencies differentiate between assemblies and components. UL for instance stamps components with “RU” instead of “UL”. Component listings are third party testing for manufacturers to use as a part. It’s not necessarily required but manufacturers prefer to purchase third party tested parts. Plus it speeds up testing since the components don’t have to be retested. This is generally captured in UL standards. Google RU vs UL.

Is this a specific NEC reference?

No. Components are outside NEC.

Is there an NEC or other reference that show the required clearances (PH-PH & Ph-Gnd) for exposed bus (bus, terminals, etc...)?

No. It is part of acceptance testing but there are general rules of thumb and charts. Most companies use 2.5” at 2300 V, 4” at 4160, 7-8” at 7200, etc.

Specific to the cable I'm not 100% clear on how a 5kV rated cable could carry 15kV if not rated? Does it have to do with the fact that there is no shielding and the spacing requirements?

The insulation rating is between two conductors. Generally you use the minimum of both creep (surface) and direct paths since surface (creep) is usually very different especially considering contamination. So on an uninsulated system with say cable rated for 5 kV and say 10” which gives 10 kV in a high pollution environment yields 15 kV overall.

 

[philly]

The insulation rating is between two conductors. Generally you use the minimum of both creep (surface) and direct paths since surface (creep) is usually very different especially considering contamination. So on an uninsulated system with say cable rated for 5 kV and say 10” which gives 10 kV in a high pollution environment yields 15 kV overall.

Circling back on this to make sure I understand and perhaps an example may help...

Lets say I have 15kV rated unshielded cable routed inside of an equipment between a transformer and close coupled piece of MV Switchgear with the system operating at 15kV. Is there a minimum clearance in this case that this cable needs to maintain to grounded parts as well as each other? I'm thinking there is not and that they can technically touch each other since they are rated for 15kV and operating at 15kV?

Now lets say i have an application where 5kV rated unshielded cable is being used inside of an enclosure operating at 15kV (close coupled connection, PT connection, etc...). I believe this 5kV unshielded cable can be used on a 15kV system if proper clearances to grounded parts of enclosure and between each phase is met?

So another words in general if you have the right spacing you can apply a lower rated unshielded cable at a higher rated voltage given the spacing reequipments to ground and phases are met? IE operating a 15kV unshielded cable at 69kV with proper spacing?