Medium voltage metering - Primary PT terminations

[BigMike]

A customer has requested that we provide a meter for metering a 13.2kV switch. I have everything figured out with the exception of the primary connection of the PTs to the MV bus.

I have selected the PTs to use, but I was curious as the proper way to terminate the conductors on the primary side of the transformer. I've seen many 12.47kV (7.2kV L-G) meter installations that use a single unshielded conductor. I'm not sure this is right based on the 310.10(E) requirement for shielded conductors. With 13.2kV being 7.6kV to ground, I believe my only option is to use a shielded cable. Do I need stress cones? Any help is appreciated. Thanks.

 

[jdsmith]

This type of PT wiring is normally provided as part of an equipment assembly. It is not uncommon to do this type of work in the field, but it is much less common than installing MV cable in tray or conduit where shielded conductors are required. The equipment standards that the manufacturers build to (UL, NEMA, or IEEE depending on the equipment) do not require shielded conductors. This is first and foremost a question of practicality - often the conductors are too short to allow stress cones to be installed. Secondly within the confines of engineered equipment it is possible to route 15 kV or even 35 kV class conductors in a manner that minimizes the likelihood of partial discharge occurring.

I have done this sort of work in the past where there have not been inspections or where the inspectors were very practical and I have not had a problem using unshielded conductors. In some cases the unshielded conductors were readily available through a supply house, but for certain sizes we have gone back to the equipment manufacturer and purchased the cable through them if we didn't have another source. The right answer for you depends on your local AHJ. In some locations AHJs have required a NRTL field inspection for equipment modifications of this sort. Some AHJs will allow the manufacturer's field service group to do the work without requiring a NRTL field inspection. Some inspectors want to make any equipment modifications subject to the NEC, which can lead to an interesting discussion about the scope of the NEC and the scope and use of UL and other product standards.

I won't offer an opinion on which of the above practices is best. If unshielded conductors are routed incorrectly inside 15 kV equipment partial discharge and premature failure can almost be guaranteed. For a line connected PT, depending on the protection scheme, this can represent a major fault with a fairly high fault current and high arc flash incident energy. If you are familiar with PD and equipment standards then a talk with the AHJ may be in order. If not then you could hire a field services group, either the equipment manufacturer or an independent, to advise you on this installation.




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