3 phase transformer grounding

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dave121

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A couple grounding questions. The power co. supply is a corner grounded delta. 3 wires only. B phase is grounded at the service disconnect. This feeds a 3 phase 480 v. primary, 120 v./ 208 v. secondary delta/Y transformer.

First part. Correct me if I'm wrong, but the secondary, 0 term. is to be bonded to the trans. frame, and connected to the grounding system, ( building structure or water pipe ) and electrode. With the 0 term also being the neutral terminal for the 120 v. load if any.

Second. Feeding the primary side from the service disconnect, 3 wires only to the transformer, no bonding to the frame, and no 4 th. wire from the service disconnect? In the disconnect slugs in the B phase instead of fuses.
 
dave121 said:
A couple grounding questions. The power co. supply is a corner grounded delta. 3 wires only. B phase is grounded at the service disconnect. This feeds a 3 phase 480 v. primary, 120 v./ 208 v. secondary delta/Y transformer.
Click to expand...

This sounds OK.

dave121 said:
First part. Correct me if I'm wrong, but the secondary, 0 term. is to be bonded to the trans. frame, and connected to the grounding system, ( building structure or water pipe ) and electrode. With the 0 term also being the neutral terminal for the 120 v. load if any.
Click to expand...

This sounds good too.

dave121 said:
Second. Feeding the primary side from the service disconnect, 3 wires only to the transformer, no bonding to the frame, and no 4 th. wire from the service disconnect?
Click to expand...

So you're saying the service enterance isn't grounded?

dave121 said:
In the disconnect slugs in the B phase instead of fuses.
Click to expand...

I'm having a hard time understanding what you mean here.
 
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For the first part you are correct, BTW the terminal is XO.:smile:

For the second part, I would believe that you need a circuit breaker, see 2005 NEC art 230.90 (B). Since you have a phase conductor that is grounded you must provide the means to open all conductors simultaneously.

I have never seen a 480v service with a corner ground, however the code only address this requirement {250.90(B)} not the voltage.

Don't forget that your raceways on the feeder to the transformer must comply with 250.97 for 250v and above.
 
dave121 said:
A couple grounding questions. The power co. supply is a corner grounded delta. 3 wires only. B phase is grounded at the service disconnect. This feeds a 3 phase 480 v. primary, 120 v./ 208 v. secondary delta/Y transformer.
Click to expand...

This sounds OK.

dave121 said:
First part. Correct me if I'm wrong, but the secondary, 0 term. is to be bonded to the trans. frame, and connected to the grounding system, ( building structure or water pipe ) and electrode. With the 0 term also being the neutral terminal for the 120 v. load if any.
Click to expand...

This sounds good too.

dave121 said:
Second. Feeding the primary side from the service disconnect, 3 wires only to the transformer, no bonding to the frame, and no 4 th. wire from the service disconnect?

So you're saying the service enterance isn't grounded?

In the disconnect slugs in the B phase instead of fuses.
Click to expand...

I'm having a hard time understanding what you mean here.
 
dave121 said:
Thanks for the info. We have some of these services in our area, I think they came about as a way for the power co. to get away with only burying 3 wires.
Click to expand...

The power companies have to ground their stuff just like we do.
 
physis said:
dave121 said:
In the disconnect slugs in the B phase instead of fuses.
Click to expand...
I'm having a hard time understanding what you mean here.
Click to expand...


If the service is grounded delta it would be a violation to place a fuse on the grounded conductor so if all three phases are run through a fused disconnect the grounded phase position could not have a fuse it would have to have a slug or 'dummy fuse'.


Neutral_photo1.jpg


http://host1.publiquik.com/bussmann_web/family.cgi?familyUid=737

Don't also forget that the grounded conductor of delta is required to be white or gray just like any other grounded conductor.
 
I'll try to clarify, the b phase goes to the center conn. at the top of the the disc, and also with a non reversable connector bonds to the box frame via the neutral lug. The grouding electrode conductor is also attached at this point. Main bonding jumper installed in the neut. lug. The reason for the slug in the b ph. is to have a guarented connection back to the source because the b ph. is your ground.
 
dave121 said:
Second. Feeding the primary side from the service disconnect, 3 wires only to the transformer, no bonding to the frame, and no 4 th. wire from the service disconnect? In the disconnect slugs in the B phase instead of fuses.
Click to expand...
An EGC of some type is required from the service to the transformer.
 
don_resqcapt19 said:
An EGC of some type is required from the service to the transformer.
Click to expand...
agree. If your raceway system meets the requirements, it can be your equipment grounding conductor, if not, you will need the 4th wire.
As far as grounding, you treat your corner grounded system the same as any other. You just happemn to have a grounded phase.
 
iwire said:
If the service is grounded delta it would be a violation to place a fuse on the grounded conductor so if all three phases are run through a fused disconnect the grounded phase position could not have a fuse it would have to have a slug or 'dummy fuse'.


Neutral_photo1.jpg


http://host1.publiquik.com/bussmann_web/family.cgi?familyUid=737
Click to expand...

Thank you Bob, I don't get to work with three phase much. It seems like such an odd concept I'd have never guessed.
 
iwire said:
Don't also forget that the grounded conductor of delta is required to be white or gray just like any other grounded conductor.
Click to expand...


I wonder if an aluminum plug would meet that requirement?

I do know it would make it less safe.

Edit: Or at least reliable.
 
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