3phase 3wire grounded service

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sandsnow

Senior Member
Well finally ran into something interesting at work. not the same old MC and PVC.
This vintage 1960's service is supplied by POCO at 240V 3phase 3wire corner delta. And from a pad mount XFMR!! I always thought those went back to pole tops.
Anyway, the contractor has Sq D service equipment. I think it's a QED type, this is by phone call. According to Sq D bulletin QED boards are listed for use on corner grounded systems.
Has anyone hooked one of these up recently or at all???
I'm curious about the bonding provisions for the grounded phase.
Are they right there just as in a 4 wire board???
Is there an extra part or kit you need to order.
I want to be able to tell this guy some good info.
I am going to look at it tomorrow.
Thanks guys
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
Re: 3phase 3wire grounded service

Larry,
As long as the breakers and panel as listed for use on a corner grounded system you can use single phase panels. It will look just like a standard 3 wire systme. You can also you a three phase panel and land the grounded conductor on one pole of a three pole breaker. A main bonding jumper is required at the service disconnect. Also note that the rules in Article 200 apply to this grounded condutor.
Don
 

sandsnow

Senior Member
Re: 3phase 3wire grounded service

Don
Thanks.
I have not seen a service suplied corner grounded system. Seen a few premises delta's both grounded and ungrounded.
So I am more concerned with actual "how to" of bonding the grounded phase in the service equipment.
Will Sq-D provide a main bonding jumper or will one have to be created in the field?? A length of conductor for example.
 

don_resqcapt19

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Location
Illinois
Occupation
retired electrician
Re: 3phase 3wire grounded service

Wayne,
Yes the breakers must be straight and not slash rated for this application.

Larry,
The panel should have provisions for the main bonding jumper if the panel is service rated. As I said you can do this with a "single phase" panel and that is the easiest and cheapest. If so the installation will look just like a normal single phase service. If a three phase panel is used and the grounded conductor is landed on the main breaker, I don't know how the bonding is accopmlished.
Don
 

sandsnow

Senior Member
Re: 3phase 3wire grounded service

Don
When I talked to the EC on the phone he told me he already has a Sq-D 3phase service switchboard. Sq-D has in there bulletin the QED boards are set up for this. I'll post what I find today.
Here's a link to what I found on Sq-D's site. Interesting reading. It has a little on why they were used, code references, UL standard references, advantages and disadvantages.

Corner Grounded Delta

Good diagrams here:

Corner Grounded Schematics
 

jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
Re: 3phase 3wire grounded service

IS the switchboard going to have QMB fusible distribution or I-Line circuit breakers (QED is not enough of a description)?

Most probably the switchboard will look more like "single phase" than three phase, after all it only needs 2 hots and a grounded conductor. I don't believe they will supply (3) phase bus bars without special request.

If it has a Suitable for Use as Service Entrance (SUSE) label from UL, then it needs to have the correct bonding provisions and instructions. IF it has a Suitable only as Service Entrance lable, then it must have the bonding installed by the factory.
 

sandsnow

Senior Member
Re: 3phase 3wire grounded service

Jim
Since I haven't been there yet, I'm hoping he has the right equipment. He told me on the phone he did not specify grounded B phase. So there is a chance the equipment is right.

edit: I was told the POCO wants CT's in each phase. I need to verify this. I hope that does not create a conflict.

[ January 13, 2006, 10:24 AM: Message edited by: sandsnow ]
 

sandsnow

Senior Member
Re: 3phase 3wire grounded service

The sw/board turned out to be labled for 3phase 3wire, but no had provisions for bonding B phase. It appeared to have been ordered incorrectly. Sq-D's bulletin on this(see above post) specifically mentions to specify grounded B phase when ordering.
I was also dismayed to see a glaring violation of 408.3(A)(2) on Sq-D's part. Equipment ground buss located at the top mere inches away from unguarded service buss bars.
The contractor is trying to convince POCO to supply as ungrounded delta. I don't know if POCO is allowed to do this or will do it. That would solve one problem.
 

jim dungar

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Location
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PE (Retired) - Power Systems
Re: 3phase 3wire grounded service

Interesting your comment about the ground bus location. I have never seen a QED switchboard that did not have the ground bus on the bottom.

Was this switchboard order with a SUSE label?
 

sandsnow

Senior Member
Re: 3phase 3wire grounded service

I don't know how it was ordered.
It was labeled as SUSE.
It was two sections and correctly labeled 1 of 2 and 2 of 2. 1st section was a combo UGPS on bottom with metering compartment above. 2nd section was bussed over from the 1st. Equipment ground buss across top on both in the back at the top of the enclosure over the uppermost ungrounded buss. In the 2nd section the ground buss turned along the side and had factory lugs for termination. Factory cables connected the Type MG c/b to the busses. The load side of the MG c/b was connected to vertical I-line bussing. All c/b's had straight voltage ratings of at least 250V.
I took pictures, but glare from the flash makes the labels unreadable.
 

sandsnow

Senior Member
I hope this image comes out ok.
This project is finally finished. The 3 phase 3 wire gear was kept and a MBJ was installed. Barely visible in photo (middle top behind main).
A little hard to get used to seeing the white conductors on the c/b's.

newservicecloseup2.jpg
[/img]
 

iwire

Moderator
Staff member
Location
Massachusetts
sandsnow said:
The electrician thought I was nuts when I first told him that B-Phase had to be taped white.

No doubt. :lol:

I think that comes from us calling white Neutral often incorrectly, instead of calling it the grounded conductor

I have never worked on or seen a corner grounded system.

Here all I see is 480Y/277, 208Y/120 and 240/120.
 
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