III phase 480V Delta Issue/Question

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Sarge590

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Hi All, new to this forum and I could use some help. I work in a large building that has a 3ph 480 delta system coming in. We had a new tenant come in with CNC machines etc. Electrician came in ran all his machines and we have not powered anything up yet. Our power leg to leg reads 480 ish although when we test phase to ground 2 are at 480 and one is at 12V to ground. None of the 3 phase units in the building were having issues but we didn't know why the one leg was reading 12V to ground, I isolated the problem by shutting down all main disconnects to different areas and traced it down to a roof top heater/ac unit, when I shut that disconnect going to the roof top unit that phase now reads 480 to ground and all phases read 480 to ground. Does anyone have an ideas what could cause this? I really appreciate anyone's thoughts on this. Thank you in advance.
 
Do you know what the grounding configuration is supposed to be? Your first readings indicate either corner grounded or ungrounded with a ground fault. Your second readings don't make sense: you can't have 480 phase to phases and 480 All phases to ground. Does the service have a main bonding jumper?
 
I agree with @electrofelon , the all phases 480V to ground doesn't make sense.

My best guess is that you have a 480V ungrounded system, and that after you fixed the ground fault you got essentially meaningless line-ground measurements with a high impedance volt meter. The high apparent voltages could be caused by harmonics coupling to ground.

For the CNC shop I'd recommend an isolation transformer to supply a properly grounded 480/277 wye system.

Jon
 
Sarge590 said:
... I isolated the problem by shutting down all main disconnects to different areas and traced it down to a roof top heater/ac unit, when I shut that disconnect going to the roof top unit that phase now reads 480 to ground and all phases read 480 to ground.
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Was this measurement made only at the disconnect for the roof top unit, or did you also check voltages at other parts of the system after shutting off that disconnect?
 
I thought the voltage to ground on a 480 volt ungrounded system ( floating) would read line to ground 480 on all phases untill one was grounded either on accident or intentionally.

This one one advantage of an ungrounded system. It reduces incidental arc energy from the first fault.

If so then then NEC requires ground fault detection equipment.

It's important to know what type of system your dealing with. See if listed at the service or MDP etc.
 
Tulsa Electrician said:
I thought the voltage to ground on a 480 volt ungrounded system ( floating) would read line to ground 480 on all phases untill one was grounded either on accident or intentionally.
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Granted my sample size is rather low, but I have found ungrounded systems float pretty damn close to "neutral" , that is 277 for 480 and 347 for 600. I had a very old 600v ungrounded Delta once and it floated perfectly centered - after I fixed the ground fault 😉
 
Thank you for all the replies, I will try to bullet the issues. One thing though we had Consumers power come in as we thought at first there was an issue with one of the legs being it was showing 12V to ground. Well consumers indicated it's a Delta system and they do not provide a ground coming into the building. So:

1.) Voltage coming into the building I have to ground 480 480 12V
2.) Voltage crossing each leg is 480 and the puzzling part to me was the one leg being 12V to ground.
3.) We have two main business in this building and from the 480 coming in that is split to each business to separate disconnects also there are various transformers bucking down to 230v etc. I did a process of elimination trying to figure out why (1) leg had 12V to ground. I found after shutting down both side separately I still had 12V on that one leg, BUT I had (1) separate IIIph disconnect coming out of the main building disconnect to that separate disconnect which runs to a roof top large AC/Heater for 1/2 of the building. When I shut that disconnect off going to the heater then I had 480 at the main disconnect coming in on all 3 legs hot leg to ground. If I turn that disconnect back on, I end up back with the one leg reading 12v to ground. That is where I'm stumped my friends. Sorry I took so long to reply back. I had one electrician telling me the building needs grounded better but I wouldn't think that is the issue with just that one heater seeming to cause issue?

Thank you in advance. Jeff
 
I also have a phase indicator light which when that heater disconnect is on, one light goes out. When I turn off that disconnect the light goes back on.
 
I agree with electrofelon in Post #2... A 480v ungounded system is likely to show various voltages to ground but I know of no condition that would result in 480v L to L and 480 L to ground on all 3 phases.
 
augie47 said:
I agree with electrofelon in Post #2... A 480v ungounded system is likely to show various voltages to ground but I know of no condition that would result in 480v L to L and 480 L to ground on all 3 phases.
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Remember that with an ungrounded system, the L to ground voltage is very poorly defined, set by things such as capacitive coupling and minor leakage currents. With a high impedance volt meter the L-G voltage can be just about anything, including > L-L voltage.

I bet if the OP measured L-G voltage using something like a 'wiggy' the voltage would be pretty near to zero.

The 480V number that they measured is not, and cannot be, a true solid 60Hz 480V. But it could be a real number caused by things like harmonic spikes.

-Jon
 
Could he have lost a hot leg and the heater is completing the grounded leg thru its load?
 
tortuga said:
Could he have lost a hot leg and the heater is completing the grounded leg thru its load?
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Not likely. The three phase loads (step down transformers) are functioning correctly (or no other problems have been reported), and the ground fault indicator light turns on when the heater is in circuit. Sounds to me that a classic ungrounded 480V system with ground fault indicating lights and a ground fault.

-Jon
 
winnie said:
Remember that with an ungrounded system, the L to ground voltage is very poorly defined, set by things such as capacitive coupling and minor leakage currents. With a high impedance volt meter the L-G voltage can be just about anything, including > L-L voltage.

I bet if the OP measured L-G voltage using something like a 'wiggy' the voltage would be pretty near to zero.

The 480V number that they measured is not, and cannot be, a true solid 60Hz 480V. But it could be a real number caused by things like harmonic spikes.

-Jon
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agree.. I see the possibility.. thanks.
(I've encountered the ungrounded systems often and have not seen 480 L to Gr on all three but do see the possibility)
 
Sarge590 said:
I also have a phase indicator light which when that heater disconnect is on, one light goes out. When I turn off that disconnect the light goes back on.
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That is exactly what you would see with an ungrounded system that has a ground fault. The phase indicator lights are not really phase indicators, but ground fault indicators.
 
Am i crazy in thinking that if you have a phase shorted to ground in an ungrounded delta that that you would read 480 to the other 2 phases to ground because that 3rd phase is connected to ground? I've always been a bit confused how this couldn't be a hazardous condition but I'm sure its just something I don't understand about the system.
 
Soyo102 said:
Am i crazy in thinking that if you have a phase shorted to ground in an ungrounded delta that that you would read 480 to the other 2 phases to ground because that 3rd phase is connected to ground? I've always been a bit confused how this couldn't be a hazardous condition but I'm sure its just something I don't understand about the system.
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The best way to think about this is that it is really like a 120/240 volt system. In that system, you have one conductor directly connected to earth. Same is true for a 3 phase 480 volt system. A connection to earth, either intentional, or accidental makes the voltage on the conductor connected to earth zero as measured to earth. The line to line voltages do not change. There is no hazard here, but since it is so different than the systems we normally see, you really have to take some time to understand it.
 
This is why when it is grounded. It becomes an grounded conductor.
Do not confuse with a netural.

Now that it iis grounded in this case most likely by accident. You can measure with a voltage drop to ground and or captive coupling.

And now can be a major hazard if not fixed on an ungrounded system.


This is one reason to find and fix the fault. It's usually a high impedance.
Where when we ground intentionally it is a low impedance.

One major difference between an ungrounded and a grounded system.
Edit
One can become a melt down on second fault and one may just trip out a breaker on the first fault.
 
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