Corner grounded delta connections

[goems]

Hi,
I need some help determining what is the prefered connection for a wye/delta 480v. 3/0 service. In the past we've used 3 different connections, I've listed them, can anyone explain which one would be considered the safest and meets NEC. Could you also explain the pros and cons to each connection:

Connection #1 is a 3 wire corner grounded service.

Connection #2 is a 3 wire ungrounded service.

Connection #3 is a 4 wire service with a center point ground which is tied into the pole ground and meter base.

Thanks for any help you can offer.

 

[charlie]

Re: Corner grounded delta connections

I have to assume that you are talking about a utility provided service and that you are not trying to deal with a particular problem. Any of the service configurations you mentioned is OK as far as the Code is concerned but most electric utilities will only give you a limited choice unless you have a real need for something unusual. For instance, we will only offer a 480Y/277, 3?, 4w service in the 480 volt configuration. A 480 volt delta is virtually nonexistent on our system but we do have some.

Connection #1 takes full voltage overcurrent devices, you can't use any circuit breaker with a slash rating like 480/240 volt or 480/277 volt devices. All faults will be ? to ? faults. The grounding is also required at the pole ground and at the meter fitting.

Connection #2 is the same as #1 except you may also build up static charges on the system. You should have ground detector so you know when a phase is grounded. It needs to be repaired as soon as practical so you don't take out a circuit or two if a second one were to develop.

Connection #3 is the only one we offer and is the best for safety in my opinion. The faults are more likely to be ? to grounded part and lesser voltage. You may use a slash rated 480/277 volt overcurrent device.

I am sure others will add to these comments.

 

[jim dungar]

Re: Corner grounded delta connections

goems,

Is your option #3 a 4 wire wye? Then Charlie's comments are accurate.

If your option # is a 4 wire delta, then Charlie's comments are not applicable. I have never seen (or heard of) a 277/480 4 wire delta.

Also, if you chose option #1 (which is my favorite) you must use circuit breakers rated for "Grounded B Phase" applications. This is rating is more restrictive than simply full rating the breaker.

[ April 12, 2005, 08:30 AM: Message edited by: jim dungar ]

 

[goems]

Re: Corner grounded delta connections

Thanks guys for the responses, I'll try to clarify a few things. This is a utility provided service using 240/480 transformers. In the past we always used to use con. #1, then we were told to use con.#3 and now were being told to use con. #2 because it is the only one that meets NEC. I'm trying to gather information on what everyone else out there is doing or what the prefered way in the industry is? Thanks again for your help.

 

[charlie]

Re: Corner grounded delta connections

Sorry, a 240/480V, 3?, 4w service is so rare that I didn't think you really meant that one. It is built the same way as a 120/240V, 3?, 4w service with a high leg and you use 240/480V, 1? transformers instead of 277V, 1? transformers to build the bank. I have seen a couple of these but they are not common and I don't know anything about them.

As far as the NEC and the NESC, all four of the connections I have talked about are valid and may be used.

 

[coulter]

Re: Corner grounded delta connections

Originally posted by jim dungar:
... Also, if you chose option #1 (which is my favorite) ...

Jim -

How come do you like 480 corner grounded delta over say 480 wye impedance grounded (my favorite).

I'm assuming the application is industrial - cause if we are discussing commercial buildings, I'm out of my element.

carl

 

[jim dungar]

Re: Corner grounded delta connections

Carl,

You did not list an impedance grounded wye connection as one of your options.

For a dedicated supply to a single machine I usually specify a 480V corner grounded system. For complete industrial plants (multiple machines) I am partial to High Resistance Ground systems.

 

[coulter]

Re: Corner grounded delta connections

Originally posted by jim dungar:
... You did not list an impedance grounded wye connection as one of your options. ... For complete industrial plants (multiple machines) I am partial to High Resistance Ground systems.

Ah - mis-translation on my part. I should have translated to: "Of the options listed, corner grounded delta would be my choice."

That makes sense. Thanks

carl

 

[charlie]

Re: Corner grounded delta connections

An impedance grounded system would not be one of your options. This was a utility provided system and I know of no electric utility that would provide and maintain a resister nor would they violate the NESC by not grounding the wye at their transformer.

 

[kiloamp7]

Re: Corner grounded delta connections

I dont know, kinda puzzled.
What is all this corner-grounded delta stuff?

I assume that most of these are semi-old, existing systems. Why would a corner-grounded delta be selected for a new application?

The solidly-grounded wye secondary is the modern way to go for 600V. & below. There is of course the resistance-grounded wye, if needed, for industrial.

I am in north Texas, where usually when I see a corner-grounded delta, it is really a pretty old ungrounded delta that has a ground fault or multiple faults on the same leg.

 

[charlie]

Re: Corner grounded delta connections

It is just like any other configuration only different.

 

[kiloamp7]

Re: Corner grounded delta connections

The economy of the open-delta or closed-delta system, with the corner ground & use of 2-pole bkrs. makes sense for some applications such as runs to remote pump stations.

I guess I should have asked "why would a corner-grounded system be selected for a new application for an interior premises wiring system ?"