Ungrounded Delta anomaly

[rsnell22]

While investigating a chiller which was indicating a phase reversal fault, I discovered that there is voltage to ground, approximately 280 volts, on all three legs, measured with both a DMM and a "wiggy" type tester.

I know for a fact that the three phase source is a three wire ungrounded delta.
The only stepdown transformer on the system has a 208Y120 secondary.

I have worked with ungrounded delta systems in multiple locations and have yet to see something like this.

 

[Hv&Lv]

While investigating a chiller which was indicating a phase reversal fault, I discovered that there is voltage to ground, approximately 280 volts, on all three legs, measured with both a DMM and a "wiggy" type tester.

I know for a fact that the three phase source is a three wire ungrounded delta.
The only stepdown transformer on the system has a 208Y120 secondary.

I have worked with ungrounded delta systems in multiple locations and have yet to see something like this.

Could be the POCO has the bank built with a neutral, it was never used for the equipment.
Many times a 277/480 bank is built, and the electrician doesn't use the bare conductor on the 4 wire ACSR feeding the building.

 

[gar]

130413-1631 EDT

280 * 1.732 = 485 V. What does this tell you?

.

 

[don_resqcapt19]

While not common, I have seen ungrounded systems that had enough capacitive coupling that there was enough current to pull in a solenoid voltage tester when testing voltage to ground.

 

[texie]

Could be the POCO has the bank built with a neutral, it was never used for the equipment.
Many times a 277/480 bank is built, and the electrician doesn't use the bare conductor on the 4 wire ACSR feeding the building.

Could be, but you can't legally or safely supply an ungrounded system from a grounded source. Maybe he has an ungrounded source and system with a phase unintentionally grounded. Does the system have a ground monitor?

 

[rsnell22]

Yeah, 485 is the measured phase to phase voltage. You got that right. Good 'Ol 1.732...

My concern is to protect the equipment I am responsible for, the chiller.
The circuit board involved in phase sequence and voltage protection is expensive, and I just want to be sure of what I'm seeing before I throw somebody else's money at a new board.

No ground detection of any kind in the delta service, a clear NEC violation there.

I've seen all kinds of weird stuff on ungrounded systems, just never such a consistent voltage to ground on all three legs like this.

 

[gar]

130413-1925 EDT

If the source transformer is truly known to be an ungrounded delta, then equal capacitive loads in combination with equal resistive leakage loads to ground from all the phases will produce the result seen.

The next test might be to put a somewhat larger resistive load than just the meter from one leg to ground and observe the voltage change from that leg to ground. Basically checking the source impedance. If there is some ground fault sensing of some sort stay below that current level.

.

 

[don_resqcapt19]

...
No ground detection of any kind in the delta service, a clear NEC violation there. ...

When was the service installed? The requirement for ground fault indication is fairly new...2005 code.

 

[Hv&Lv]

Could be, but you can't legally or safely supply an ungrounded system from a grounded source.

Not quite sure I follow...
Some POCO's do not build 3 wire deltas anymore. You only get a four wire bank. Still if you get three wires, the bank is liable to be center tapped to one pot for lightning protection.

 

[rsnell22]

The original install was in the 70's, but the transformer and service gear are replacements within the last several years. In WA, the AHJ isn't always using the most recent code. Every other ungrounded system I've had anything to do with always had ground detection, if only light bulbs.

My concern, other than safety, of course, is that there is something here that may have goofed up a multi thousand dollar circuit board, and might do the same with a replacement. Other than that, I'll have to back off and let the on-site guys take care of it.

 

[GoldDigger]

Not quite sure I follow...
Some POCO's do not build 3 wire deltas anymore. You only get a four wire bank. Still if you get three wires, the bank is liable to be center tapped to one pot for lightning protection.

So if you want to operate an ungrounded delta inside the plant, with ground detectors, etc, you have to use your own isolation transformer now?

 

[texie]

Not quite sure I follow...
Some POCO's do not build 3 wire deltas anymore. You only get a four wire bank. Still if you get three wires, the bank is liable to be center tapped to one pot for lightning protection.

If the POCO is furnishing a grounded Y bank it is not an ungrounded service. Just because you don't bring the neutral to the service disconnect and ground it at the service disconnect does not make it an ungrounded system. Art 250.24(C) tells you this specifically. I agree that most POCO's are trying to get rid of ungrounded service but if they change to a grounded bank the customer must convert to a grounded system or it may kill somebody or burn the place down. Think of the return paths possible back to the grounded POCO bank.

 

[gar]

130413-2207 EDT

rsnell22:

If you don't expect 280 V from each leg to ground, then what do you expect?

.

 

[texie]

So if you want to operate an ungrounded delta inside the plant, with ground detectors, etc, you have to use your own isolation transformer now?

If the POCO puts in a grounded service you would have no other choice to be safe and compliant. Or convert your system to grounded-likely not a practical fix.

 

[hurk27]

If the POCO puts in a grounded service you would have no other choice to be safe and compliant. Or convert your system to grounded-likely not a practical fix.

I agree because the utility has referance the X0 to Earth if you now have a first fault you will have a 480 volt potential between all the grounding in the plant and Earth without any OCPD's opening, very dangerous, my first thought would be to get the utility to loose the X0 bond as they have put people at risk, without the X0 bond you would just have an ungrounded system like a delta without the Earth referance at the pole.

 

[hurk27]

While investigating a chiller which was indicating a phase reversal fault, I discovered that there is voltage to ground, approximately 280 volts, on all three legs, measured with both a DMM and a "wiggy" type tester.

I know for a fact that the three phase source is a three wire ungrounded delta.
The only stepdown transformer on the system has a 208Y120 secondary.

I have worked with ungrounded delta systems in multiple locations and have yet to see something like this.

I have seen an ungrounded 480 volt system that just happen to have an equal capacitive coupling that it would read 277 volts to ground, but its rare, but I would check that 480/208/120 transformer to see if it has a WYE primary and the primary X0 got bonded which it shouldn't be.

A WYE primary will act as a zig zag or auto transformer on the primary side and derive a neutral point for the 480 ungrounded which could set up an unreliable fault current path which could be dangerous.

Also if the chiller has VFD drives, many do not like ungrounded systems and may have to be fed through its own SDS to derive a grounded system?

 

[hurk27]

Not quite sure I follow...
Some POCO's do not build 3 wire deltas anymore. You only get a four wire bank. Still if you get three wires, the bank is liable to be center tapped to one pot for lightning protection.

If they did that on an ungrounded system without requiring other changes to the system they would be setting themselves up for a very large law suit, as stated above grounding a system at the pole and leaving the system without a fault current path except Earth will pose a very dangerous set up if a first fault were to ever happen, because on a first fault all the grounding of this building will have a 480 volt or in your example 277 volts potential to earth, concrete, or anything else at reference to Earth but not the grounding system.

Some think that this would be no different then a high impedance grounded system, but there is a difference, in the high impedance system you are deriving a high impedance ground through a grounding transformer X0 from a system that has no reference to earth, a first fault only turns the system into a corner grounded system, it still will not have a reference to Earth so there is no potential to Earth from the first fault.

 

[hurk27]

When was the service installed? The requirement for ground fault indication is fairly new...2005 code.

I never understood why it was only required for high impedance systems back then or SDS with a primary of less then 1kv but only on the control part.

Of course as I said in another thread when I first got into industrial one of our first jobs at the beginning of each shift was to test for a ground fault, we later installed a detector.

To me anyone not using a ground detector totally defeats the whole reason for running an ungrounded system since if you don't know you have a ground fault how can you repair it in a timely manner in a controlled shut down?

 

[Hv&Lv]

a first fault only turns the system into a corner grounded system, it still will not have a reference to Earth so there is no potential to Earth from the first fault.

Duly noted. However, if the first fault is never cleared, as sometimes happens, what happens later on when there is a second fault?
Ungrounded deltas are not safer and should have a ground detection system.

There will be capacitive coupling without the solid ground, so reading 277 (or thereabouts) isn't unheard of and is to be expected.

RUS has no specs on ungrounded deltas.

 

[Hv&Lv]

If the POCO is furnishing a grounded Y bank it is not an ungrounded service. Just because you don't bring the neutral to the service disconnect and ground it at the service disconnect does not make it an ungrounded system. Art 250.24(C) tells you this specifically. I agree that most POCO's are trying to get rid of ungrounded service but if they change to a grounded bank the customer must convert to a grounded system or it may kill somebody or burn the place down. Think of the return paths possible back to the grounded POCO bank.

Usually corner grounded delta for a 480 power bank, not a wye.
Although sometimes the center of one delta leg is tapped to ground for lightning.