83 Voltage Throw Over Scheme

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SG-1

SG-1

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Ware Shoals, South Carolina
I am looking for the code references for violations in the following scheme.
Background:
Two transformers are within either metal-enclosed or metal-clad medium voltage switchgear. They are contained in metal structures. The transformers are grounded using the switchgear ground bus, usually a 1/4 x 2 copper bar at the transformer.
The output is typically used for control power, lights & space heaters.

Issue:
Only the ungrounded conductors of the two separately derived transformers are being switched. The grounded conductor is not switched.
 
Properly accounting for bonding only the secondary neutral, which is not counted as a connection to the primary neutral even though it is grounded at its source, there are only two choices:
1. Connect the secondary neutral to the incoming primary neutral and bond the transformer case to the incoming EGC, for non-separately-derived, or
2. Do not connect the secondary neutral to the primary neutral, but bond it to the GES for a separately derived source.

For a more precise statement, change neutral to grounded conductor everywhere above. :)
 
Ron, nearly all loads are 120VAC. A typical transformer would be 15kva with a ratio of 4160:240/120 or 13800:240/120...etc. Medium Voltage range.
Typically a neutral is never brought into the gear. Once or twice a year I see a neutral bus. The transformers are L-L & X0 is bonded to the ground bus & brought out for the 120VAC circuits.

GoldDigger, this grounded conductor is always connected to the neutral point.

What I need specifically are the code references that are violated when the throw-over is wired in this manner: Transformers are separately derived & the neutral is not switched. The only type of ground fault protection provided is GFCIs on the 120VAC receptacles.
 
SG-1: If there is a neutral point, then for most voltage systems NEC requires that it be grounded and feed the grounded conductor. But you can still have a grounded conductor even when there is no neutral point. One example is a single phase transformer with a 120V secondary.

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SG-1 said:
What I need specifically are the code references that are violated when the throw-over is wired in this manner: Transformers are separately derived & the neutral is not switched. The only type of ground fault protection provided is GFCIs on the 120VAC receptacles.
Click to expand...

Definition of Separately Derived System from article 100:
“An electrical source, other than a service, having no direct connection(s) to circuit conductors of any other electrical source other than those established by grounding and bonding connections.”

Therefore if a neutral connected to the two transformers is not switched then it is a direct connection, and so by the definition above the transformers do not constitute separately derived systems.

But I think I'm getting your concerns. So is the X0 of each transformer bonded to its own separate ground bus, and not one common ground bus? If so, where is the neutral conductor for the loads connected?
 
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Is this one piece of metal enclosed switchgear or two? If this is two pieces how are the ground busses bonded together.

You are using the term throw-over, is this a 'control power' transfer scheme in a Main-Tie-Main circuit? I don't think I have ever seen the control power neutral switched. in one of these.

Different separately derived transformer secondaries are allowed to have a common grounding point (GEC), in fact it is required.
 
Synchro, The H1 & H2 terminals are connected L-L, no primary neutral. The secondary is wired 240/120 with X0 bonded to the ground bus & taken out to serve 120VAC loads. Ground bus is continuous between the two transformers.

Jim, this is in a medium voltage M-T-M line up with an automatic transfer. This is control power, space heaters, lights,... etc. One piece of equipment. Sometimes it is wired off panel for a UPS or other customer equipment. I do not remember any switched neutrals either in the past.
 
SG-1 - I do not believe you will find it as a code violation.
You will have inappropriate current flow during a ground fault as indicated in the diagram I posted in the link, but since the ampacity is low and you don't have ground fault trip, you will not be affected in that way.
1580743275815.png
 
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