corner ground delta

[Pierre C Belarge]

I also am interested to see how this installation is finished.

Is it possible for you to draw a picture of the wiring diagram? Maybe take a picture of the drawing and upload it.
I am curious about the grounding electrode selection and conductor location.

 

[mull982]

With X0 connected to neutral, the wye side of the transformer will be behaving like any wye connected multi-wire branch circuit. If X0 is bonded to ground as well as to the neutral then you will have current flowing on the ground, the same as you would with any improper downstream neutral-ground connection.

If a neutral is not connected on the primary and only the wye point is bonded to ground then I would expect all of the unbalance current to return on ground.

What is the point of connecting a neutral wire on the primary XO bushing? Would it striclty be to deal with any current inbalance on the primary?

 

[hillbilly1]

If a neutral is not connected on the primary and only the wye point is bonded to ground then I would expect all of the unbalance current to return on ground.

What is the point of connecting a neutral wire on the primary XO bushing? Would it striclty be to deal with any current inbalance on the primary?

Their is no neutral on the secondary,(keep in mind that this backfeed, and the secondary will be 480 volts) but will have one phase grounded to provide a path for any faults to ground from the other two phases. On the primary side "XO" will not be grounded or connected to the incoming power neutral, Since the incoming power is also a wye connection, this is a somewhat unstable wye to wye connection, thus is not used often. The better scenario would be to have a 208 three phase delta connected primary with a 277/480 secondary even though the load does not require a neutral.

 

[craig65]

That's sounding better How are your your equipment grounding conductors installed / connected ? Where and how are you going to corner ground ?

#4 bare to grounding electrode/frame,#6 THHN from frame to Wye side panel,
ground lug bolted to a welded stud.
I was planning to take "A" phase to ground, @ the same ground lug.
Is that correct?

 

[jim dungar]

If a neutral is not connected on the primary and only the wye point is bonded to ground then I would expect all of the unbalance current to return on ground.

This is the correct result, and an improper installation. The grounding path is not to be used as a normal current carrying conductor.

What is the point of connecting a neutral wire on the primary XO bushing? Would it strictly be to deal with any current inbalance on the primary?

A neutral connection to the X0 bushing will cause problems. There are only disadvantages to making this connection on a 208Y used as a step-up transformer.

 

[Cold Fusion]

---A neutral connection to the X0 bushing will cause problems. There are only disadvantages to making this connection on a 208Y used as a step-up transformer.

Jim -
I'm not questioning your comment - I'm just curious. Don't the utility companies often use a wye primary for their distribution? Do you know why that is? I sure don't.

cf

 

[jim dungar]

Jim -
I'm not questioning your comment - I'm just curious. Don't the utility companies often use a wye primary for their distribution? Do you know why that is? I sure don't.

cf

A utility MV multi-grounded neutral distribution system, is an entirely different animal than what the NEC requires us to install, this is why I was specific in addressing a 208Y to delta step-up. Also, single phase transformers, with their individual cores, behave slightly different than three phase units on a single three-legged core.

 

[Cold Fusion]

A utility MV multi-grounded neutral distribution system, is an entirely different animal than what the NEC requires us to install, this is why I was specific in addressing a 208Y to delta step-up. Also, single phase transformers, with their individual cores, behave slightly different than three phase units on a single three-legged core.

Tried to edit but got a phone call and had to work - the nerve of those people.:roll:
I'm not concerned with single phase house stuff. And I don't see how MGN primary would affect a three phase distribution xfm.

But after thinking about it for a bit, I did further recall the utility transformers are Y-Y and that is not the same as the OPs Y-D.

No need to go further with this - I'd be hijacking. (bad dog)

cf

 

[mull982]

That's sounding better How are your your equipment grounding conductors installed / connected ? Where and how are you going to corner ground ?

I agree that corner grounded is the way to go with the Delta secondary.

I have a few cases with "wye" primarys where this wye point on the primary was grounded and the equipment grounds (EGC's) for the loads coming off the delta secondary were connected to this same point. Never thought much about this until this thread. What would happen for a ground fault in this connection?

 

[augie47]

On this particular installation, frankly I've gotten lost. I'm with Pierre, I'd like to see a drawing.
As far as the XO on a wye primary, I was of the opinion that you never connect it to a neutral,ground or anything, but I will bow to those far smarter than me to confirm or rectify that thought.

 

[craig65]

here is what I gather from all this so far
to fix what I have I need to take it all apart, run 3 phases and a ground, no neutral from the wye side panel to X1 X2 X3, bond the case of the transformer with the grounding conductor from the wye side,the grounding electrode conductor and the grounding conductor from the delta panel. Run H1 H2 H3 to the delta panel and corner ground from "A" phase to ground and them the transformer will be wired correctly and safely. Then if there is a ground fault on any one phase it will trip a breaker and the rest is easy........

 

[jim dungar]

here is what I gather from all this so far
to fix what I have I need to take it all apart, run 3 phases and a ground, no neutral from the wye side panel to X1 X2 X3, bond the case of the transformer with the grounding conductor from the wye side,the grounding electrode conductor and the grounding conductor from the delta panel. Run H1 H2 H3 to the delta panel and corner ground from "A" phase to ground and them the transformer will be wired correctly and safely. Then if there is a ground fault on any one phase it will trip a breaker and the rest is easy........

Almost.
On new installations, the corner to ground is the "B" phase.
Your protective devices, if they are breakers, will need to be rated for a "corner-grounded" delta.

 

[augie47]

Almost.
On new installations, the corner to ground is the "B" phase.
Your protective devices, if they are breakers, will need to be rated for a "corner-grounded" delta.

Craig, To expound a little on what Jim notes (in case you aren't familiar with that) the Code reference is 240.85. You breakers for this installation would not be "slash rated" ie: 480/277 but would be straight rated.
Do you know what cat# breakers are being used ?

 

[craig65]

Craig, To expound a little on what Jim notes (in case you aren't familiar with that) the Code reference is 240.85. You breakers for this installation would not be "slash rated" ie: 480/277 but would be straight rated.
Do you know what cat# breakers are being used ?

no sir; I do not, but I am familiar with 240.85. I have been in touch with the owners, thanks to you all I sounded like I knew what I was talking about...
and informed him of his problem..as soon as he gives me the nod I will get some before and after pics to post

 

[craig65]

Almost.
On new installations, the corner to ground is the "B" phase.
Your protective devices, if they are breakers, will need to be rated for a "corner-grounded" delta.

is this the standard? I havent got the Ok from the owner to fix it yet,he is thinking it over.....:roll: I bet if some kid get killed on his light pole he would want it fixed,why is it that people take electrical saftey for granted.....until some one gets hurt. then its the electricians fault?

 

[augie47]

don't you feel fortunate to have inherited this job ? :grin:

 

[blueheels2]

Anybody got a picture of the connections inside the transformer of a corner grounded delta. I understand what you guys are saying kind of but never seen one and in my mind it sounds like it would be a dead short (phase to ground).

 

[jim dungar]

is this the standard?

Yes B-phase is the standard. In fact, many people will call these systems "Grounded-B Phase" instead of the more generic 'corner-grounded'. Equipment for these systems is available from every major manufacturer. Breakers for corner grounded systems are kind-of special in that only 1-pole may actually interrupt a phase-ground fault. See Square D Supplemental Digest (or your favorite manufacturer's catalog) for more information. And remember, your grounded conductor needs to be white or gray.

This system may seem strange to 97% of electricians, according to Brian, but in reality while they are uncommon they are not rare, in many parts of the country.

 

[hillbilly1]

here is what I gather from all this so far
to fix what I have I need to take it all apart, run 3 phases and a ground, no neutral from the wye side panel to X1 X2 X3, bond the case of the transformer with the grounding conductor from the wye side,the grounding electrode conductor and the grounding conductor from the delta panel.

You are meaning "equipment ground", not grounded conductor? Correct?

 

[craig65]

don't you feel fortunate to have inherited this job ? :grin:

I have more faith in me than anyone else, what I am thankful for is this forum.
this aint the first time you all have saved me ass