480 open delta service

[texie]

Open delta 480 volt service. No neutral from POCO bank to service. When I look at the bank on the pole I see 2 primary and 1 primary neutral conductors and 2 transformers. I can see 1 phase of the secondary is grounded to the common primary neutral. Is this a corner grounded delta secondary as it would be if it was a closed delta?

 

[kwired]

Open delta 480 volt service. No neutral from POCO bank to service. When I look at the bank on the pole I see 2 primary and 1 primary neutral conductors and 2 transformers. I can see 1 phase of the secondary is grounded to the common primary neutral. Is this a corner grounded delta secondary as it would be if it was a closed delta?

Yes, only difference is it doesn't have same capacity as a closed delta with same sized transformers.

 

[texie]

Yes, only difference is it doesn't have same capacity as a closed delta with same sized transformers.

Agreed. I guess what confused me was that I have never encountered an open delta 480 volt corner grounded service. All my previous 480 volt corner grounded experience was with a full bank. Open delta 120/240 3 phase 4 wire is very common where I'm from so I have done many. I guess you must have a lot of 480 volt corner grounded open delta for center pivots?

 

[templdl]

Just think that the use of an open delta is at less cost that a closed delta. Could it be that the addition cost for the third transformer may provide some belt and suspenders should one transformer fail. If a transformer other than the one with the lighting tap fails at least you can run on the other two but at a reduced capacity.

 

[kwired]

Agreed. I guess what confused me was that I have never encountered an open delta 480 volt corner grounded service. All my previous 480 volt corner grounded experience was with a full bank. Open delta 120/240 3 phase 4 wire is very common where I'm from so I have done many. I guess you must have a lot of 480 volt corner grounded open delta for center pivots?

Corner grounded systems have pretty much disappeared for irrigation services, too many untrained people putting their fingers where they don't belong and the fact that it is 480 volts to ground is the primary reason - I think. But yes when they were more common you could find some open delta supplied systems that were corner grounded. Remember you can ground any point you wish on the secondary side and the system will not care, it is codes that require specific conductors to be grounded in certain cases.

 

[kwired]

Just think that the use of an open delta is at less cost that a closed delta. Could it be that the addition cost for the third transformer may provide some belt and suspenders should one transformer fail. If a transformer other than the one with the lighting tap fails at least you can run on the other two but at a reduced capacity.

There is no "lighting tap" on a corner grounded system.

 

[templdl]

There is no "lighting tap" on a corner grounded system.

Interesting. Texie just stated the he has an open delta with 120/240. When one side of a delta is center tapped what is it referred to?
Usually the correct designating would be 240/120 delta 3ph 4w using an open delta in this case if I understand texie correctly.
On texie's open delta he can't get 120 because he has an open delta?

 

[GoldDigger]

Interesting. Texie just stated the he has an open delta with 120/240. When one side of a delta is center tapped what is it referred to?
Usually the correct designating would be 240/120 delta 3ph 4w using an open delta in this case if I understand texie correctly.
On texie's open delta he can't get 120 because he has an open delta?

I do not see any overwhelming reason not to put a 120/240 center tapped winding on one side of the delta and ground the 120 center tap. That would be exactly the same as a high leg in terms of line voltages, just with one winding missing.
What you cannot do is ground any point other than the 120 center point in that system.
So the contradiction in the embellished original description was the combination of 120V and corner grounded. Not between 120V and open delta.
Although I suppose with enough warnings, etc. you could still derive a single 120V to ground line from a corner grounded delta. But it would not be a standard voltage system under the code and might not be allowed without using a separate transformer to supply the single sided 120V (as an SDS?).

BTW, the 120/240 is not allowed to be supplied by the missing transformer in the open delta.
By the laws of physics, not the code.

 

[texie]

GoldDigger and templdl-
Hold on here. I'm well aware of 240/120 3 phase 4 wire open delta. That is not what my OP was about. I was just inquiring about 480 open delta corner grounded VS 480 volt delta corner grounded with 3 transformers. Take another look at my OP.

 

[GoldDigger]

GoldDigger and templdl-
Hold on here. I'm well aware of 240/120 3 phase 4 wire open delta. That is not what my OP was about. I was just inquiring about 480 open delta corner grounded VS 480 volt delta corner grounded with 3 transformers. Take another look at my OP.

I did not say that you were the one who added the 120/240 into it.
And any open delta, with few exceptions, needs to be grounded somewhere. I have a hard time thinking of a good reason (some reasons, yes) for having an ungrounded open delta.
For that matter, having one end of the open delta grounded instead of the common point seems like a pretty poor idea too, but it would also work.

 

[texie]

I did not say that you were the one who added the 120/240 into it.
And any open delta, with few exceptions, needs to be grounded somewhere. I have a hard time thinking of a good reason (some reasons, yes) for having an ungrounded open delta.
For that matter, having one end of the open delta grounded instead of the common point seems like a pretty poor idea too, but it would also work.

Well, I guess I might as well tell why I made my original post:
Existing, old, 100 amp 3 phase 480 volt corner grounded system. (It just happened to be an open delta-hence my original post) Meter socket and 100 amp main fusible disconnect get replaced due to age and failure by EC. He does the new meter and disco as if it is a 480 ungrounded system-he did not realize that it is grounded at the POCO bank. I come along a cry foul as you can't supply an ungrounded premises system from a grounded source-period, do not pass go, etc.

 

[kwired]

GoldDigger and templdl-
Hold on here. I'm well aware of 240/120 3 phase 4 wire open delta. That is not what my OP was about. I was just inquiring about 480 open delta corner grounded VS 480 volt delta corner grounded with 3 transformers. Take another look at my OP.

I re-read your OP three times, and found nothing in there about 120/240. I would also like to ask GoldDigger and Templdl to read the title of the thread.hmy: