transformer in reverse

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mickeyrench

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edison, n.j.
we have a trans that is wired in reverse the prim. is 208v and the sec. is 480v. it is suppling power to 2 - 30amp. 480v recept. when checking output i find that there is 480 between each phase lines but to ground there is only 15volts. i know there is a problem with the grounding. there is no connection to the building steel , the trans is just sitting on floor. when the trans was wired up the on the sec. side the elect. installed ground wire from the recept. to the case of the trans. no other ground is there. should he have wired xo on trans.? how do i make this safe ? i plan on installing a #6 from the trans to building steel after that i'm not sure. thanks for any help
 
For the average electrician trying to avoid present and future problems IF I WERE YOU, I would buy a 208 delta to 480/277 Wye transformer few bucks more, but available.
 
Even if the secondary side of a transformer is 480Y/277 volts, it is not required to ground the secondary side if the neutral is not used as a conductor. After the three phases leave the transformer, you will not be able to tell that the secondary is a wye instead of a delta regardless of the instrumentation you use.
 
mickeyrench said:
should the trans still must be grounded to the building steel/
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Are you talking about one of the output lines being grounded, or the case?

I am ambivalent about grounded delta systems. There are some advantages to them, but maybe it is better to leave it ungrounded. If you do so, you need to add ground detector circuits.

I am used to plants that would have a fit if a single ground fault shut down their plant, so am used to mostly ungrounded delta systems. Over the last few years, that has started to change some though.
 
Delta Grounding

Delta Grounding

mickeyrench said:
should the trans still must be grounded to the building steel/
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If you are not going to have a corner grounded delta system then take a look at NEC 2005 250.30(B) Ungrounded Systems and 250.30(A)(7) Grounding Electrode.
 
I don't deal with transformer GECs enough to have that info on the tip of my tongue, and I am going to focus on the transformer coils themselves. You can't answer the 'should I bond to building steel' question until you first answer 'what part of the transformer should I bond' question.

As others have mentioned, the _secondary_ of the transformer is not grounded. This means the actual current carrying wires have no connection to bonded metal. This is entirely separate from any use of equipment grounding conductors, building steel, ground rods, etc.

You asked if X0 of the transformer should be bonded. That is on the right track, but the problem is that X0 is on the _primary_ of this reversed transformer. Bonding X0 would actually be quite incorrect, and would create objectionable current on your bonded metal/EGC/GEC system. You would want to bond the 'H0' of your transformer...except that since the transformer secondary is a delta, you don't have an 'H0' to bond.

If you elect to keep the delta secondary, you have several options:
1) leave the secondary ungrounded but install proper ground fault detection
2) corner ground the secondary, making one of the phases a _grounded_ conductor
3) using a grounding transformer to derive a neutral, and grounding that
4) getting the proper transformer with a wye secondary

-Jon
 
brian john said:
For the average electrician trying to avoid present and future problems IF I WERE YOU, I would buy a 208 delta to 480/277 Wye transformer few bucks more, but available.
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It's already there and installed, it is not a perfect world.

But if your buying I bet they will do it. :grin:
 
Pierre C Belarge said:
The method of installation of the OP'a transformer leads one to ask...how is a ground fault situation going to create a path where the fault current will flow back to the source to open the overcurrent device???
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Its' not, but that is acceptable with the addition of ground fault indicators.
 
winnie said:
You asked if X0 of the transformer should be bonded. That is on the right track, but the problem is that X0 is on the _primary_ of this reversed transformer. Bonding X0 would actually be quite incorrect, and would create objectionable current on your bonded metal/EGC/GEC system. -Jon
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You wouldn't bond X0, but a grounded neutral conductor should be run to the transformer and connected to X0 to stabilize the neutral.
 
jghrist said:
You wouldn't bond X0, but a grounded neutral conductor should be run to the transformer and connected to X0 to stabilize the neutral.
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Running a neutral to the XO will end up bonding it and it is a bad idea.

Used in reverse XO should remain floating.
 
I have reversed a transformer in the pass for a large grocery store chain to temporarily use street lights. I would not recommend this as permanent installation.
 
Before when this has come up I have said that unless the transformer is listed for a step up its a violation.
I used a step up once and its a delta - Y. when you reverse a transformer it becomers a Y - delta. Its important to ground the secondary of the 480 delta to stablize the voltage. In the 1940s there were 480 ungrounded delta systems and the motors were burning out (weakest point of the system) due to ground causing resonate voltage up to 2200 volts.
The nec does not require a delta to be grounded but its recommended.
 
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