Ask Me Help Desk - Trouble shooting a 3phase system

Hi All,
I have a 480 VAC 3 phase ungrounded system. We monitor the system via a set of lights for each phase. If one phase goes to ground then that light should glow bright. In my case I have the B phase light out completely. While trouble shooting the problem I found which disconnect switch was feeding the problem. It fed a light panel. When I shut everything down on that Light Panel... the phase light did not come back on. The only thing still in the system is a Transformer. So I must have a problem with that Transformer. Here is my question. Why, when I read the voltages between phases it said 480VAC... and Phase to ground read 240VAC. Should I of not been getting weird readings, or my 3 phase motors starting to
Act up ?If not why? Can you explain this to me so I can better troubleshoot my 3 phase system.
Thanks
Cory

You probably have a 480 volt "Y" transformer and they grounded the center tap so all three windings will read 240 volts to ground.

Quote:

Originally Posted by ZCCG

Hi All,
I have a 480 VAC 3 phase ungrounded system. We monitor the system via a set of lights for each phase. If one phase goes to ground then that light should glow bright. In my case I have the B phase light out completely. While trouble shooting the problem I found which disconnect switch was feeding the problem. It fed a light panel. When I shut everything down on that Light Panel.... the phase light did not come back on. The only thing still in the system is a Transformer. So I must have a problem with that Transformer. Here is my question. Why, when I read the voltages between phases it said 480VAC.... and Phase to ground read 240VAC. Should I of not been getting weird readings, or my 3 phase motors starting to
act up ?If not why? Can you explain this to me so I can better troubleshoot my 3 phase system.
Thanks
Cory

Jack,
Thanks for answering my question.
You are 100% correct... Lets assume it is a center point Y transformer. Then my reading of 240 from Phase to ground are correct. So are the readings of 480 between phases.
Maybe I presented my question a bit wrong. It has more to do with the way the phase lights are working.
I have troubleshot D.C. systems with ground lights for years. I have never dealt with ground lights on a 3 phase A.C. system. I am making the assumption that ground lights work about the same either on a.C. or a D.C. system.
With this kind of light monitoring system... the 3 lights glow dim when all is working well. When 1 phase goes to ground then that phase light will glow brightly. In my case, I have the B Phase light completely out. I am wondering what a completely out light is telling me. Don't ask if the light bulb is good or not... I already know it is. When I kill the power to the Light Panel that the disconnect is feeding... all my phase light come back on correctly. I already know that this is telling me that I have a problem with the transformer that the switch is feeding. I am curious about how this system works... is it the same as ground lights on a D.C. System and what in the world does a bulb completely out tell me. I would have expected that if I had a problem with a one phase I would have noticed it either in my voltage readings or even noticed some of my 3 phase motors acting up... Either getting Hot or the speed varying.
If you or anybody can explain to me about these phase monitoring lights then maybe next time I have a problem... I can save myself some time in troubleshooting the problem.
Thanks again,
Cory

You got me on this one. Its been a few years since I've dealt with ground fault lights. I sure you've checked to see if you have lost one of the phases completely but that's the only thing I can think of that may cause one light to go out completely and the other two showing a fault. I'll think about it and let you know if I come up with anything. Good luck

Jack,
I do Thank-you for your answers though.
At least I have found a place to try to pick somebodys brain.
When mine goes dead.
Thanks... Have a great day

That is not how the lights worked in the factory I managed. And I know just how they were wired up because I personally supervised the installation. Each phase had a wire running to an inline fuse, and then to a 480/6 volt transformer with a bulb socket and to ground. The transformer and sockets were the common AB things you see next to start/stop switches. (Well most of those are 120 control voltage.) As long as none of the phases were grounded, the lights glowed green. If a phase was grounded, its light no longer had enough voltage between it and ground to light the light. The 480 was a floating, ungrounded voltage, so everything was fine until something got between what the 480 was shorted to and ground. The lights were our only indication of such a severe hazard. This is why I went to the bother of having them installed.

Note, our 480 was 277 volts above ground. I did my best to keep my workers above ground too.

A center tapped secondary shouldn't gound a primary.

Ground fault indication for an ungrounded delta is a bit complicated to understand, even more so to explain.

Here is a link to a decent explanation with diagrams
http://xnet.rrc.mb.ca/janaj/Hydro%20.../grounding.DOC

I think you've lost a phase in the transformer and power in equals power out right? So check the (I bet it's a ) fuse in the disco for the transformer, one phase is blown?

Quote:

Originally Posted by ZCCG

Hi All,
I have a 480 VAC 3 phase ungrounded system. We monitor the system via a set of lights for each phase. If one phase goes to ground then that light should glow bright. In my case I have the B phase light out completely. While trouble shooting the problem I found which disconnect switch was feeding the problem. It fed a light panel. When I shut everything down on that Light Panel.... the phase light did not come back on. The only thing still in the system is a Transformer. So I must have a problem with that Transformer. Here is my question. Why, when I read the voltages between phases it said 480VAC.... and Phase to ground read 240VAC. Should I of not been getting weird readings, or my 3 phase motors starting to
act up ?If not why? Can you explain this to me so I can better troubleshoot my 3 phase system.
Thanks
Cory

Hi,
Labman, TKRussell and Shuntripper, thank-you for your answers.

Labman, The system that I am using sounds about the same as the one that you mention. The only difference that I can see is that on my system, the leg that is grounded will glow bright. Otherwise the 3 lights glow dim when all is good. Believe me, I am doing my best to keep myself and my co-workers above ground too. Thank-you for your answer

Shuntripper, I know that there are no blown fuses. Everything inside the lighting panel that the transformer feed... works fine. When I shut down all the breakers inside that LP, my phase lights still are not correct. The B Phase is still out. Thank-you form your answer.

TKRussell, Thank-you for the link. I am on my way there right after I get done typing to check it out. Judging from some of your responses to other questions on this site, I am sure that this link will be informative. I will report back after I go to that link. I will also report back when I find out what is wrong with my system at work. Just to share the information in case it can help anybody else out.

Again, Thanks everybody, it is great to have a place to ask questions like this and have so many folks reply with answers to try to help. I hope that I can return the favor o someone else that is in need.

C'ya

Quote:

Originally Posted by ZCCG

Hi All,
I have a 480 VAC 3 phase ungrounded system. We monitor the system via a set of lights for each phase. If one phase goes to ground then that light should glow bright. In my case I have the B phase light out completely. While trouble shooting the problem I found which disconnect switch was feeding the problem. It fed a light panel. When I shut everything down on that Light Panel.... the phase light did not come back on. The only thing still in the system is a Transformer. So I must have a problem with that Transformer. Here is my question. Why, when I read the voltages between phases it said 480VAC.... and Phase to ground read 240VAC. Should I of not been getting weird readings, or my 3 phase motors starting to
act up ?If not why? Can you explain this to me so I can better troubleshoot my 3 phase system.
Thanks
Cory

Shuntripper,
As noted in a earlier post... I did shut down all breakers in the LP and I still had that light out. The only other thing in the system is the transformer that feeds the LP.
Next time I can kill the power to the LP, I will go and check the Transformer... Hopefully that will be this morning.
Thanks,

Labman and TK,
First Tk... That link is a good one. It did not really answer my question but it is a good bit of information. I saved it and will use it for future reference.
Labman... Lo and behold, you were exactly correct about the way that the lights work. The information that I was given about the system was not right. When 1 phase goes to ground... the light for that phase does indeed go out. Not glow bright like I was led to believe. I guess that you can not always listen to the people that are suppose to know the system and have worked on it for years.
The problem turned out to be a grounded leg going to my transformer. I pulled in a new feed of #4 wire and all is well now.
Thanks for all of the help with this issue.
Cory