I'm going nuts

[LHarrington]

Good afternoon,
Just finished up connecting a new piece of equipment in the plant that requires 240 volt single phase. We purchased a Square D 5kva trans to step down the 480 volt at the buss to 240 required. After all connections are made we energize the 480 all is good, check the secondary off the trans 235v all appears good. Try to energize the equipment nothing! Start troubleshooting find a burnt wire in the slip ring for this equipment. Go back to breaker after the trans find one leg to ground 120v the other is 300v to ground. Check it with a wiggy and nothing happens. Consulted Square D tech support they claim I'm reading phantom volt with my DMM and not to be concerned. Am I missing something. I compared my reading to an identical trans less than 20 feet away and I get 120 to ground both legs. Both trans are wire the same and neither one of the have any of their legs grounded.

LHarrington

 

[raider1]

Are you saying that both transformers have ungrounded secondaries?

Chris

 

[LHarrington]

yes both have ungrounded secondary

 

[Smart $]

yes both have ungrounded secondary

There are limited conditions under which a 240V system is not required to be grounded. What type(s) of equipment does this transformer supply?

 

[realolman]

I don't think you should get anything to ground, if the secondary of the transformer is not grounded.... unless something else IS grounded that maybe shouldn't be...

I wouldn't bet the rent on "phantom" voltage just yet... check your wiggy and make sure it works. From your description something seems weird... These slip rings go to maybe multiple heating elements, that may have one grounded somewhere?

The 120 and 300 really doesn't make sense to me.

 

[LHarrington]

This 240v is for a overhead gantry crane. The slip rings I had spoke of allows it to spin 360 degrees. I have checked the voltage again and still find 299-300v on on leg to ground. In this case would I want to tie X2 and X3 to ground? What is confusing to me is the identical trans very close by is wired the same and it reads what I would expect. 240 across legs 1 and 2, Leg one to ground is 120v and leg two to ground is 120v.

 

[steve66]

Maybe different wiring lengths (maybe caused by the burnt slip ring) is causing your different voltages to ground.

If its an ungrounded system, why would you worry about the voltages to ground? I would make sure you don't have continuity to ground, but otherwise, don't get too hung up on the unbalanced voltage to ground. If your wiggy says no voltage to ground, thats probably the reading to trust.

Sounds like your only problem might be the burnt slip ring.

Steve

 

[LarryFine]

I compared my reading to an identical trans less than 20 feet away and I get 120 to ground both legs. Both trans are wire the same and neither one of the have any of their legs grounded.

My guess: The one reading as if it's grounded either is, possibly inside the load, or you have enough loading on it, maybe simply in the amount of wire it its loads, that it's capacitively coupled enough to read grounded.

As for the other one, I'd start with checking the neutral (X2-X3) to ground voltage with the wiggy, and see if you can safely ground it. There could be a short inside the transformer itself. Try both transformers unloaded.

It's tough to troubleshoot when you're not there.

 

[Protek]

Try reading the x-former voltage to ground with the load disconnected. Then connected. The only way you can read volts to ground form the x-former is if the x-former is grounded at some point, inside or elsewhere on the secondary. Probably have a ground out on the circuit.