Corner grounded delta

[Brand New]

I am installing a service disconnect on an old industrial building. When I first looked @ the job I noticed it was 3 phase 480V. After looking into it it is apparently a corner grounded delta system. The grounded "B" phase isn't bonded anywhere as far as I can tell. It goes through 2 three phase fused disconnects then to a transformer. I have never dealt with this before and I am having a hard time wrapping my mind around it.

Should the grounded "B" phase be treated as a grounded conductor or an ungrounded phase conductor?

Should it be fused in the 2 disconnects it goes through.

Should it be bonded anywhere? @ the service? @ the transformer?

Thanks to anyone who can bring me out of the dark.

 

[Brand New]

Anyone have ant tips or advice? Reference?

 

[LarryFine]

First, you have to determine whether the B phase is grounded intentionally or it's an accidental fault. There should be a bonding of the grounded phase in the main disconnect enclosure, just with like "normal" services.

There should also be no fusing in the grounded conductor, and it should be white or gray, and otherwise treated like any circuit conductor, just like we do with neutrals in "normal" services. It's just a grounded conductor.

 

[LarryFine]

Anyone have ant tips or advice? Reference?

Please be patient, Mr. New. We're here when we have the time, and not everyone who reads the posts has something constructive to add to every topic.

 

[augie47]

Please be patient, Mr. New. We're here when we have the time, and not everyone who reads the posts has something constructive to add to every topic.

that never stooped me from posting :grin:

New, if it is not intensionally grounded, you might look at 250.32(B). You are probably not obligated to "upgrade" to meet that section of the Code, but it's not a bad idea

 

[Brand New]

My apologies. I get a little anxious. I guess I need to wait til the POCO comes to shut down the power to get into the meter base to see how it is terminared.Thanks Larry for taking the time.

 

[augie47]

It does not hurt to look at the meter, but as Larry pointed out in his post, the connections/fusing in the service discponnect(s) will most likely be the key.

 

[iwire]

I am installing a service disconnect on an old industrial building. When I first looked @ the job I noticed it was 3 phase 480V. After looking into it it is apparently a corner grounded delta system.

What is leading you in that direction?

Given the lack of obvious bonding and that the fact it is also fused on all legs it appears to be ungrounded delta.

 

[Brand New]

I have never heard of a corner grounded system before but there is a transformer inside that I can barley see a transformer symbol that I looked up and it is a corner grounded delta.

Conductors come from a sealed meter to a disconnect inside the building. In that disconnect the B phase is landed on the B phase terminal that is fused.

If it were a ungrounded delta and it lost the B phase wouldn't I see something wrong on the transformer secondary?

 

[LarryFine]

Given the lack of obvious bonding and that the fact it is also fused on all legs it appears to be ungrounded delta.

Exactly, which is why I started my first response the way I did. This isn't something one should make a guess about.

 

[LarryFine]

I have never heard of a corner grounded system before but there is a transformer inside that I can barley see a transformer symbol that I looked up and it is a corner grounded delta.

That dashed line isn't a sure sign; it's more like a suggestion. The bond, if there is one, can be anywhere between the transformer secondary terminals (inside the housing) and a terminal in the enclosure it runs to.

 

[jim dungar]

That dashed line isn't a sure sign...

The dashed lines on transformer diagrams are not intended to indicate a ground. They are there to indicate the phase shift of the primary and secondaries. Any relation to the grounded conductor is pure coincidence.

 

[Pierre C Belarge]

Take a look at this:


http://www.ecmag.com/index.cfm?fa=article&articleID=10247

 

[Brand New]

So if the B phase isn't a neutral but it is a grounded conductor should it remain fused in the disconnect? Treat it as a Phase conductor or a grounded conductor? Thanks to all for the help.

 

[roger]

Can you check to see if there is a "ground fault detector" somewhere? It should either be in or close to the Main Distribution gear, it sounds as though you are dealing with an Ungrounded Delta.

Roger

 

[Brand New]

We opened the meter and B phase is not grounded. It passes straight through to the fused disconnect. It does seem like it is an ungrounded system but if we just lost B phase we wouldn't have the correct voltage on the secondary side of the transformer. Right?

 

[Brand New]

Old Building. No ground fault detector.

 

[roger]

Okay, let me get this straight, from the POCO there is a three wire service going to a disconect(s) then directly into a transformer, this three wire service entrance feeds nothing else, is this correct?

Now, is the transformer a Delta/Wye?

How is the grounding done on the secondary side of the transformer?

Roger

 

[Brand New]

Correct- feeds nothing else
I believe it is a delta/wye. Can barley see info on the nameplate.
secondary neutral and grounds bonded

 

[roger]

Is the service disconnect you are installing "ahead of" or "downstream" of this transformer?

Roger