12kv MV Utility System

[Mcooke]

I'm working on a project and we are about to start a MV tie-in, 15kv copper tape shielded cable. One of the facility electricians is saying that the MV Feeds both A/B originating from an existing vault need an equipment grounding conductor run in the conduit with the MV Cable. The MV Feeds consist of 3, 4/0 cables, no grounded conductor. The duct bank has one bare #2 copper grounding electrode (could be EGC in this case?) in it that will be bonded to the grounding ring/switch rack and enclosure at the new vacuum disconnect switch and to the grounding configuration (ground rod) at the existing vault.

Does the utility side of system like this typically require a equipment grounding conductor with each feed A/B of the MV Cables? The drawings do not indicate a ground as one would typically expect to see it occuring on the secondary side of the TX rather than primary.

Thanks in advance!

 

[Hv&Lv]

Utilities generally use concentric neutral cable, so there is a grounded conductor included.
on delta utility systems I have also noticed it used, although I’m sure somewhere out there a utility is using tape shield cable.

 

[jim dungar]

Is this an extension to a utility installation or is it a customer owned MV distribution?

 

[electrofelon]

Utilities generally use concentric neutral cable, so there is a grounded conductor included.
on delta utility systems I have also noticed it used, although I’m sure somewhere out there a utility is using tape shield cable.

Lots of 4800 Delta here in upstate NY, and all CN cable as well.

 

[Mcooke]

Is this an extension to a utility installation or is it a customer owned MV distribution?

Hey Jim,

It's an extension of a Campus owned MV System. I worked on a different project on the campus a few years ago and we also completed a MV Tie-In for it. That project we did not run a grounding conductor in the bundle of MV Cables. For this reason I am not sure why this facility electrician is asking for a grounding conductor or an EGC. He is quoting the NEC but wouldn't this be considered an engineered system and generally regulated by the NESC?

 

[jim dungar]

If it is customer owned the NEC has precedence over the NESC.

 

[Mcooke]

If it is customer owned the NEC has precedence over the NESC.

Thanks Jim, I appreciate the feedback. We'll dive into it a bit further. Happy Holidays!

 

[Mcooke]

Jim,

This maybe a silly question but do you happen to know if MV Distribution is considered a circuit prior to connecting to any step down transformer or service point? for the sake of applying 110.54 (B)

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[electrofelon]

I'm working on a project and we are about to start a MV tie-in, 15kv copper tape shielded cable. One of the facility electricians is saying that the MV Feeds both A/B originating from an existing vault need an equipment grounding conductor run in the conduit with the MV Cable. The MV Feeds consist of 3, 4/0 cables, no grounded conductor. The duct bank has one bare #2 copper grounding electrode (could be EGC in this case?) in it that will be bonded to the grounding ring/switch rack and enclosure at the new vacuum disconnect switch and to the grounding configuration (ground rod) at the existing vault.

Does the utility side of system like this typically require a equipment grounding conductor with each feed A/B of the MV Cables? The drawings do not indicate a ground as one would typically expect to see it occuring on the secondary side of the TX rather than primary.

Thanks in advance!

I think we would need to know whether the utility system is grounded or not. 250.190(C)(2) states:

2) Shielded Cables. The metallic insulation shield encircling
the current carrying conductors shall be permitted to
be used as an equipment grounding conductor, if it is rated
for clearing time of ground fault current protective device
operation without damaging the metallic shield. The metallic
tape insulation shield and drain wire insulation shield
shall not be used as an equipment grounding conductor for
solidly grounded systems.

If this is a delta system, the tape shield MAY qualify as the EGC. If this is a grounded system, then you can never use a tape shield as the EGC.

 

[Mcooke]

I think we would need to know whether the utility system is grounded or not. 250.190(C)(2) states:



If this is a delta system, the tape shield MAY qualify as the EGC. If this is a grounded system, then you can never use a tape shield as the EGC.

Hey Electrofelon thanks for the response, take a look at these plan pages noting feeder size as well as a #2 bare copper conductor in the duct bank which extends from manhole to manhole and to the switch pad. This is a 12kv delta feed as there is no grounded conductor in the feed, the facility electrician stated that the system is a "solidly grounded system". My understanding was that a Solidly Grounded system or grounded system consisted of the utility feeders that are intentionally connected to ground at some point, although my understanding was that is typical for a feed with a grounded conductor (Neutral) which is bonded to ground. All metal components, switch racks, enclosures, etc are bonded to ground rods and rings at manholes and switch pads respectively. Is it typical to utilize. a solidly grounded delta system for mv distribution? I know this is getting a little drawn out....

 

[electrofelon]

Hey Electrofelon thanks for the response, take a look at these plan pages noting feeder size as well as a #2 bare copper conductor in the duct bank which extends from manhole to manhole and to the switch pad. This is a 12kv delta feed as there is no grounded conductor in the feed, the facility electrician stated that the system is a "solidly grounded system". My understanding was that a Solidly Grounded system or grounded system consisted of the utility feeders that are intentionally connected to ground at some point, although my understanding was that is typical for a feed with a grounded conductor (Neutral) which is bonded to ground. All metal components, switch racks, enclosures, etc are bonded to ground rods and rings at manholes and switch pads respectively. Is it typical to utilize. a solidly grounded delta system for mv distribution? I know this is getting a little drawn out....

First let me give the disclaimer I dont do much MV work and not very familiar with it other than running small MGN systems with CN cables. There are not too many guy on here that do non-utility MV work. But I'll take a stab at it....

There is some confusion here with the term "delta". Often it is used sort of informally to refer to a three wire supply without a neutral. There may be an assumption that if there is no neutral than the system is ungrounded, but the source could be a grounded wye and just that the neutral is not run with the phase conductors, and 250.184(B)(6) confirms not running the neutral is allowed (I have never heard of utilities using a corner grounded delta so Im just going to assume that is not a thing). In this case, you still need to run an EGC, as indicated in 250.184(B) and it would get connected to the POCO neutral which Im guessing should exist at you service point if its an MGN system. Its also possible that the POCO distribution system could be unigrounded and only grounded at the sub station with out a neutral available at the service point. IT would help to know exactly what you have at the MV service point with the utility.