Grounding a delta/delta step-up XFMR

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The Chicken

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Hello all.

Over the years, I have found a lot of good info here-thanks!

Now I have a question/problem that for some reason I just can't seem to wrap my head around. After hours of research, I think I have only managed to confuse myself more...

So here is my question.

I have a delta/delta transformer. I need to feed it with 208 3 phase (3 current carring conductors and a ground).

I need 480 3 phase from it (3 current carring conductors and a ground)

So my confusion comes in surrounding the ground. There is no center-phase(high-leg) tap.

I have read and looked at several different options-now I am just more confused. Some of the info I come across seems to be geared more towards a utility.

The transformer is to feed 480 to a trio of disconnects that will in turn feed 480 machines for temporaray connection for testing purposes before they are shipped out to their final owners. No neutral needed. But I am concerned about personal saftey in the event of a phase short circuit.

Any help here would be greatly appreacated.

Thanks!
 
You can corner ground it or leave it ungrounded and provide ground detectors. If you leave it ungrounded, the first ground fault is not a hazard, but a second one on a different phase is really a phase to phase short.
 
Well, from what I have researched, I was leaning towards a corner-grounded system. Having never encountered one before, I don't really know much about them. Most of the info I could find on them seemed to revolve around service drops in older installations.

Are there any pitfalls/hazards I should be aware of if I corner ground this limited-use system(the customer plans to use it perhaps a couple days a month for testing-the rest of the time it will be powered down by the disconnect on the primary 208VAC side of the transformer)?

Also, should we decide to leave it ungrounded, any ideas/links to ground fault dectors?

Like I said, my main concern is to prevent someone from getting hurt should a ground fault occur. In a normal Wye system, a ground fault would trip an overcurrent device, unless I have really misunderstood something here, an un grounded delta will not trip an overcurrent device in the event of a ground fault-or have I led myself astray here?

Also, if I corner ground B phase, and on the equipment B phase shorts to the machine chassis, will an overcurrent device trip on the B phase (doesn't seem likely) or will one of the other phases trip? I have read that with this set up, only two pole overcurrent devises are required, but I have not been able to find any wiring diagrams(except for just the transformer wiring) to help me sort this out as far as overcurrent protection is concerned.

Again, thanks for any input.
 
Some VFDs don't like corner grounded systems.
The only overcurrent device that you can use with the grounded phase is a common trip breaker. You can't use a fuse. In general you install the corner grounded system like you would a 120/240 volt single phase system. Sometimes, if the listing permits, you can use single phase disconnects and or breaker panels. Note that all breakers must be straight rated, 480 volts, you are not permitted to use slash rated (480/277) breakers on the corner grounded system.
Also the grounded conductor has to comply with the rules in Article 200 just like any other grounded conductor.
 
after some more research last night regarding what you have mentioned here in how grounded conductors have to be treated as well as its means of disconnect, and of course the possible clash with vfds , I have come to realize that a corner grounded system is not going to work for me . I have already installed all the disconnects and other parts and I have to have this powered up today .

So that leaves me with finding a ground detector.

I was trying to find a source for 1 last night but then my computer crashed( when it rains, it pours !) does anybody have an idea where I can get a price on 1 of these units or where to source 1 from I do not want to build 1 from scratch ?

Thanks again for all the help!
 
There is another pitfall for your delta/delta. The voltage to ground is 480 not 277. Much more destructive in a fire and from a personel standpoint. While you don't need a neutral, why not use a 208 to 277/480 V step up transformer, install a system bonding jumper to the XO, a GES and then you are done.
 
Where did you find a delta-delta transfromer with 480V and 208V windings?

Temporary connections to temporary loads, VFDs, unfamilar power systems. this screams for a 208-480Y/77 unit like Tom suggested.
 
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