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#81525 - 08/26/02 06:23 AM HV feeder in PVC ductbank (12470)
yooper Offline
Junior Member

Registered: 08/26/02
Posts: 8
Loc: Michigan
Two 12470 circuits come thur customer owned swithgear, then go thur a ductbank system (pvc) to transformers in verious locations all underground. Both circuits go to each building.The building are also tied together by common phone , gas ect. QUESTION is an equipment ground wire required to run in each pve conduit with the phase conductors??What if a neutral is present,Can the grounding then be done at each location if so sizing of equipment ground wire?? also would it have to be HV shilelded as the phase conductors,if run with the phase conductors I have talked to many so called experts in this field and found a big difference in opions. Cable manf. tell me there shield is NOT a gr. wire.

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#81526 - 08/26/02 04:56 PM Re: HV feeder in PVC ductbank (12470)
George Corron Offline

Registered: 05/16/01
Posts: 728
Loc: Lorton, Va USA
You're not getting any response because there probably 1/2 dozen different styles of MV cables, and you're not providing sufficient info.

Most MV cables I've worked with DO have a shield that is a full ground/neutral (in some cases), but I have worked with just a shield. That shield is grounded, but is not of sufficient size to be considered a ground in the true sense.

The shield in that case is there only to displace lateral stress on the cable, and if it is not grounded, your cable will NOT last long. You normally do not pull a ground in these conduits, but some designs absolutely require it. Where are you? do you have many thunderstorm days? Wide open area like an airport? Ground wire is indicated IF the shield is not at least 50%.

That's probably all I can help (confuse?) with the info provided.

#81527 - 08/26/02 05:33 PM Re: HV feeder in PVC ductbank (12470)
resqcapt19 Offline

Registered: 11/10/00
Posts: 2209
Loc: IL
In my opinion, you need some type of EGC conductor between the main switchgear and the transformers. This is needed to clear any faults on the primary side of the transformer. Each of these transformers would be separately derived systems and would need a grounding electrode system. If the secondary side has a neutral, they will also need a main bonding jumper.
Where I am currently working, we use a similar set up only with 4.16kV distribution. The 5kV cables have either wire or tape shield. These shields are not suitable for use as a grounding or neutral conductor. We pull in a bare or 600 volt insulated EGC in each raceway. This is bonded to the frame of the transformers. The size is based on the rating of the primary OCPD and Table 250.122.

#81528 - 08/26/02 08:30 PM Re: HV feeder in PVC ductbank (12470)
Bjarney Offline

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
To the original poster — Is NFPA 70 the prevailing document for the subject installation? What edition? Are the are the current-carrying conductors listed type MV90 or MV105 cable? Is it single or multiconductor assembly? Are there any phase-to-neutral-connected transformer windings supplied in the installation?

[Nota Bene: 15kV-class equipment and cables are termed by ANSI, IEEE, NFPA and UL to be medium voltage.]


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