Line to ground readings on ungrounded delta secondary

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powerpete69

powerpete69

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Professional Electrical Engineer
Ungrounded delta secondary. 480V three phase.

Electrician took readings on factory equipment..... and on line side of main breaker. Utility company also got the same exact readings on secondary of transformer.
480V to ground on phase A
480V to ground on phase C
0 V to ground on phase B.

We all get 480V line to line as expected.

The electric company says the B phase to zero is not surprising, as there literally is no ground reference. They also say their transformer reads line to line 480V on all three phases so they are good. They don't care about line to ground simply because its ungrounded with no ground reference.

Incidentally, all electrical equipment in factory works perfect. No hiccups.

Do we agree with this or do you think there is something faulty with their transformer?
What have other people found out in this situation?
 
powerpete69 said:
Ungrounded delta secondary. 480V three phase.

Electrician took readings on factory equipment..... and on line side of main breaker. Utility company also got the same exact readings on secondary of transformer.
480V to ground on phase A
480V to ground on phase C
0 V to ground on phase B.

We all get 480V line to line as expected.

The electric company says the B phase to zero is not surprising, as there literally is no ground reference. They also say their transformer reads line to line 480V on all three phases so they are good. They don't care about line to ground simply because its ungrounded with no ground reference.

Incidentally, all electrical equipment in factory works perfect. No hiccups.

Do we agree with this or do you think there is something faulty with their transformer?
What have other people found out in this situation?
Click to expand...
You have a B phase ground fault. That’s why you get 0volts from it to ground because it’s at the same potential. Also explains your 480v to ground readings.

You are now operating on a corner grounded delta unintentionally.

Now that I’ve wrote all this, if it’s truly an ungrounded system there should be ground fault monitoring somewhere to alarm of such ground fault.
 
Agree with everything here:
Dsg319 said:
You have a B phase ground fault. That’s why you get 0volts from it to ground because it’s at the same potential. Also explains your 480v to ground readings.

You are now operating on a corner grounded delta unintentionally.

Now that I’ve wrote all this, if it’s truly an ungrounded system there should be ground fault monitoring somewhere to alarm of such ground fault.
Click to expand...

You have a corner-grounded delta, whether intentional or not.
 
powerpete69 said:
The electric company says the B phase to zero is not surprising, as there literally is no ground reference. They also say their transformer reads line to line 480V on all three phases so they are good. They don't care about line to ground simply because its ungrounded with no ground reference.
Click to expand...

I agree with Dsg319.

B phase = 0 to ground implies that B is connected to the ground reference, but this _might_ be a very high impedance connection and not actually a problem.

Remember that there is no true ungrounded system; the systems that we call 'ungrounded' are actually grounded by capacitance and leakage.

In an ungrounded system, I'd expect voltages to be all over the place, and depending upon things such as leakage or current flow through the meter you might see something that looks grounded. If you measured with a low impedance meter I'd expect all phases to measure 0V to ground.

-Jon
 
It's my understanding that something can blow up if there is an additional ground in the future.
I wrote a letter to elec company rep explaining we likely have a problem in transformer they need to fix.
 
powerpete69 said:
It's my understanding that something can blow up if there is an additional ground in the future.
Click to expand...
Well, a fault on another phase will result in a short circuit.

powerpete69 said:
I wrote a letter to elec company rep explaining we likely have a problem in transformer they need to fix.
Click to expand...
If the 0v remains with your main disconnect open, yes.
 
winnie said:
I agree with Dsg319.

B phase = 0 to ground implies that B is connected to the ground reference, but this _might_ be a very high impedance connection and not actually a problem.

.

-Jon
Click to expand...
Yeah I agree Jon. Maybe everything is fine, maybe there is a first ground fault. I will say that I have worked on one ungrounded system, a 600v at an old mill in Providence RI, and despite being old, I got L+G readings all real close to 347 using a DMM (I know it wasn't a grounded wye as I found a first fault, and after I fixed that it came back to center).

Pete, did you try readings with the service disconnect off?
 
powerpete69 said:
Ungrounded delta secondary. 480V three phase.

Electrician took readings on factory equipment..... and on line side of main breaker. Utility company also got the same exact readings on secondary of transformer.
480V to ground on phase A
480V to ground on phase C
0 V to ground on phase B.
Click to expand...

winnie said:
B phase = 0 to ground implies that B is connected to the ground reference, but this _might_ be a very high impedance connection and not actually a problem.
...
Click to expand...

If there was a ground fault detector and it was disconnected from phases A and C, then you could get the measured readings as a result of the detector's L-G impedance pulling down the voltage on phase B. But that would be unlikely because it requires two defects (in this case open circuits) to be present.
 
powerpete69 said:
Hmmmmm elec company rep says that will send someone out right now to test transformer.
Click to expand...
If this system is in fact an ungrounded delta, as others have stated, this likely do to a phase becoming grounded unintentionally. This raises the question as to whether you have ground detection installed as required. But note that this likely has nothing to do with the POCO and their transformer but rather a grounded conductor in the customer premises.
And, yes, unintentional grounds in an ungrounded system need to be located ASAP as if you get another one on another phase you will have a direct short. You mentioned that everything works fine in the plant. That is one of the few advantages to an ungrounded system in that it gives you time to locate the issue without losing power.
 
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Thanks for comments guys.

Yes, when they took voltage on secondary of transformer, the main disconnect switch was correctly in the open position removing the rest of the system. And all the insulators in the main breaker gear were all just replaced as well today before the readings.
The main disconnect is roughly 15 feet from the transformer. This ground fault is likely in the phase B conduit heading into the building or there is some sort of leakage in one of the transformer insulators....or perhaps something else.

I now need to find out who owns the conduits from transformer to gear. The transformer is definitely utility owned.
If utility company owns wires and conduit it will be on their dime. If we own wires, it will be on our dime.

It should also be mentioned that this ground fault may have been there the last 25 years.....just noticing it now. And yes, we do need to add ground fault detector system.

And a little more background, this all started because we heard a electrical crackling in the main switch gear. That being said, electricians had noticed the strange phase B to ground low voltage. over past few months. Upon inspection of gear, no arcing could be seen, but we all definitely heard it intermindantly over past few weeks.. Breakers all tested out well. Low resistance when contacts closed and high resistance when contacts open. Again, all insulators have been replaced as we believe there was flashover. Unfortunately, this remote gear had slipped under the radar of the normal preventative maintenance over the past 20 years or so....hence its existing problems. All our other subs get full maintenance every 3 years as they should.
 
texie said:
This raises the question as to whether you have ground detection installed as required.
Click to expand...
Ground detectors are a relatively recent NEC requirement. I would not be surprised if there are more installed ungrounded systems without than with them.
 
When you have a Delta system installed within the last 40 some odd years, it is USUALLY configured as either a corner grounded Delta, or you install Ground Fault Monitoring equipment. For many utilities this is a REQUIREMENT. Most people will opt for the corner grounding because it's cheaper. In a corner grounded Delta system, it is usually the B phase that is grounded, but there is actually no Code requirement dictating which corner is to be grounded, only how and what to use.

 
This could be on of those situations where the POCO has always had a phase grounded intentionally at there end as they intended it to be a corner grounded system all along but the customer assumed it was ungrounded.
 
jim dungar said:
Ground detectors are a relatively recent NEC requirement. I would not be surprised if there are more installed ungrounded systems without than with them.
Click to expand...
I think that requirement first appeared in the 2005 code, but all of the older ones around here had ground detector lights.

Typically 3 sets of 240 lamps connected in series. Each set of two lamps was wired in a wye, with the center of the wye connected to ground and the each of the ends connected to the phase conductors.
 
220517-1732 EDT

I have equipment in many different auto or supplier plants, and every one operated with ungrounded delta as distribution at 480 V or in the machines. Always had lights to monitor that the system was properly floating. Thus, a single ground fault or partial fault did not shut the line down. Someone could investigate the problem and decide whether to wait for a break or other normal time to work on the problem. Or one could even look for the problem when the line was running.

Over 20 years ago if a part supplier shut down a final assembly line they might be back charged in the 200,000 to 500,000 dollars per hour range. I knew an assistant plant manager at one plant that had to write one of those checks.

.
 
don_resqcapt19 said:
I think that requirement first appeared in the 2005 code, but all of the older ones around here had ground detector lights.

Typically 3 sets of 240 lamps connected in series. Each set of two lamps was wired in a wye, with the center of the wye connected to ground and the each of the ends connected to the phase conductors.
Click to expand...
Of course if was a 480V service there would have been 480V lamps, or more typically transformers or fuses used as voltage dividers.
 
jim dungar said:
Of course if was a 480V service there would have been 480V lamps, or more typically transformers or fuses used as voltage dividers.
Click to expand...
Never seen 480 volt lamps ....two 240 volt lamps in series on each leg of the wye...total of six lamps...these were all on 480 volt systems. Most every older industrial with ungrounded systems did this around here.
 
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