# Ungrounded system?



## Ray Cyr (Nov 21, 2007)

The plant I work in has three large, three phase transformers. Each one feeds its’ own switchgear. The serving utility tells us that all three are Delta secondary configurations. I am told that each is an ungrounded system.
On system “A” at disconnects fed from its’ switchgear, when I measure voltage I read 480 volts from line A-B, line B-C and line A-C. When I measure voltage between line A and the disconnect cabinet, I read 480 volts. Same thing for line B. Between line C and the can I read 4 volts (nope, not a typo).
I get the same results on system “C”.
On system “B” I read 480 volts line to line and 277 volts from each phase to the can.
I know what my thoughts are on this and they don’t agree with what I am being told…
I will appreciate your thoughts on this.
Thanks,
Ray


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## seo (Oct 28, 2008)

Sounds like system A&C are corner grounded delta and system B is a wye system.


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## micromind (Aug 11, 2007)

It's also possible that A and C are indeed corner grounded systems, but system B is not grounded at all. 

Instead it uses a ground detection relay. These relays require that there be no intentional grounds anywhere in the system (from the secondary of the transformer and downstream), and some of them have 3 small potential transformers inside. These transformers are wye connected, and the center of the wye is grounded. In the event of a ground fault, one of these transformers will go to zero (or very nearly zero) voltage, and this will cause the relay to trip the feeder breaker. 

You can read 277 to ground because the impedance of the meter isn't low enough to affect the voltage of the relay transformers. If you install an actual 277 volt load on the system (like lighting), the ground detector will see it as a fault. 

Then again, it could be a wye connected transformer.

Rob


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## Ray Cyr (Nov 21, 2007)

Thank you very much for the replies. My first thoughts agreed with seo, but there is so much that I still don't know that I didn't want to close my mind to other possibilities simply because I don't know of their existance. Also, I want to trust the people that are telling me what these systems are even though my limited experience can't explain the difference between what they are telling me and what I think my meter is telling me.


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## Hhighwings (Feb 5, 2009)

Ray Cyr said:


> The plant I work in has three large, three phase transformers. Each one feeds its’ own switchgear. The serving utility tells us that all three are Delta secondary configurations. I am told that each is an ungrounded system.
> On system “A” at disconnects fed from its’ switchgear, when I measure voltage I read 480 volts from line A-B, line B-C and line A-C. When I measure voltage between line A and the disconnect cabinet, I read 480 volts. Same thing for line B. Between line C and the can I read 4 volts (nope, not a typo).
> I get the same results on system “C”.
> On system “B” I read 480 volts line to line and 277 volts from each phase to the can.
> ...


 Wow, the reason I came to this site is because I came across the same situation and was looking for feedback. I have 19 volts A to ground ,460 volts B to ground, and 460 volts C to ground. Before I found out it was a delta-delta ungrounded system, I thought the building lost A phase, my thinking now is that A phase is grounded somewhere (unintentionally), but the building manager, and other electricians I work with disagree. I'm thinking if everything was fine on an ungrounded sytem, all my voltages should be around zero to ground, but if A phase has a ground fault somewhere, it would make the potential difference between B and ground or C and ground close to 480 volts. This wouldn't cause a problem with the system unless another phase goes to ground, which would cause some serious fireworks. Is there anyone with more experience with ungrounded delta sytems that agree with me?


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## te12co2w (Jun 3, 2007)

This wouldn't cause a problem with the system unless another phase goes to ground, which would cause some serious fireworks. Is there anyone with more experience with ungrounded delta sytems that agree with me?[/quote]
I too have little experience with ungrounded systems, but I do know that if you do have a ground fault on one phase, the system will continue to function. As you wrote, there can be serious consequences if another phase goes to ground. I'm sure someone with more experience will respond sometime.


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## Dembones (Mar 24, 2007)

Loads of ungrounded system information:
http://www.i-gard.com/company.htm


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## jonr (Oct 15, 2010)

the system you have is a ungrounded system , the reason you are reading 4 volts on c phase on a system is that there is a ground fault on that phase , as a example take b system and go to a source bucket from a small switchgear and you can (as crazy as it sounds) short out from ground to a phase and it will not trip circuit then check b system again and you will read 480 on 2 phase's to ground and 1 to 50 volts on the other. if all systems are good it will read 277 to ground on all phases


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## Zog (Apr 15, 2009)

An ungrounded system has no intentional connection to ground with respect to its current-carrying conductors. Ungrounded systems are commonly distributed in a 3-phase, 3-wire delta configuration. 

On low voltage systems, typical system voltages are three-phase 240, 480, or 600 volts. These systems have no intentional reference to ground but the system’s conductors are capacitively coupled to ground through the distributed capacitance of coil windings, cables, and other components. ​ 
The advantage of this system is that when a ground-fault occurs in one of the circuits, little fault current would be able to flow in the grounding path since the system has no direct ground reference. For many industrial applications such as automotive plants or paper mills with many motor loads, this provides a significant advantage. A ground-fault can occur on one circuit without causing damage or shutting down the system. Often times ground-fault detection schemes are employed that allow the faulted circuit to be identified and manually shut down for correction at the facility’s earliest convenience. Ground fault detection and alarm equipment is strongly recommended where ungrounded distribution systems are applied.​ 
There are some disadvantages to this system. When a fault does occur in one phase, the other two phases now have an elevated voltage to ground established. For example, on an un-faulted 480 V system, the voltage level of ground might float around 200 to 300 V depending on the system capacitive coupling. Once one phase has been grounded, the other two phases are elevated to 480 V above ground. If the fault is of an arcing nature where it continually re-strikes, the voltage experienced by the other two phases can reach magnitudes several times the normal voltage. If this condition is not remedied quickly, the over voltage stressing of the insulation can break it down causing one of the other phases to fault to ground. At this point, with two phases grounded the system experiences a phase-to-ground-to-phase fault (short circuit through ground).​


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## walter stewart (Aug 10, 2013)

*sleepy or sleeping leg??*

I just came from a business where there air conditioning system was not working, I knew last night the transformer on the pole outside the building blew up and was replaced. I found that the or a main 200 amp 250 volt fuse was blown, I thought all the equipment might have single phased and one leg drew more current and blew the fuse so I replaced it. when I turned the fusible main disconnect on it started emitting sparks and humming unbelievably at the point where the 3\0s came thru a 2 inch offset nipple. I realized then thought maybe a wire was pinched going thru the nipple but when I inspected them the insulation was fine I then realized the locknut on the nipple was loose causing a bad path to ground. I started ringing wires out with my ohm meter and found one leg B phase was grounded out.
the B phase 3/0 connected on the meter socket to the line side with probably a #8 copper and the 3/0 on B phase didn't pass thru the meter from the main disconnect and went directly to the breaker panel. the load side of the B phase in the meter socket had a jumper directly to ground and it had a plastic orange notice on it that read if not wired to an ungrounded system remove jumper before powering or something to that effect maybe I got it backwards?? ive been licensed sinced 1981 and never came across anything like that before I recall in high school the teachers taught about the BASTARD leg on a delta system but it looks like N STAR put in the opposite type of transformer you think they have come across this before and would notify their customers effected that they could have this problem because the amount of sparks emitted when I turned that disconnect on for a second or two at the most was incredible. my boss called this a sleepijg leg or sleeply leg I have never heard the term but he has a whole slew of crazy made up lingo. I put in a 200 amp under ground service a month ago he wanted me to use 4 inch p v c and then I buried it 3 feet into the ground with the meter socket 3 feet above ground and he says to me make sure you pitch the p v c from the meter to the pole so the pipe doesn't fill with water and have it back up into the meter socket. Well who will have a 4 inch pipe fill with water and back up six feet high and fill a meter socket I told him it will spill out the drainage hole I punched out in the bottom of the socket!!!!!!! I told him to shut up because he was making a fool of himself in front of the customer


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## micromind (Aug 11, 2007)

walter stewart said:


> I just came from a business where there air conditioning system was not working, I knew last night the transformer on the pole outside the building blew up and was replaced. I found that the or a main 200 amp 250 volt fuse was blown, I thought all the equipment might have single phased and one leg drew more current and blew the fuse so I replaced it. when I turned the fusible main disconnect on it started emitting sparks and humming unbelievably at the point where the 3\0s came thru a 2 inch offset nipple. I realized then thought maybe a wire was pinched going thru the nipple but when I inspected them the insulation was fine I then realized the locknut on the nipple was loose causing a bad path to ground. I started ringing wires out with my ohm meter and found one leg B phase was grounded out.
> the B phase 3/0 connected on the meter socket to the line side with probably a #8 copper and the 3/0 on B phase didn't pass thru the meter from the main disconnect and went directly to the breaker panel. the load side of the B phase in the meter socket had a jumper directly to ground and it had a plastic orange notice on it that read if not wired to an ungrounded system remove jumper before powering or something to that effect maybe I got it backwards?? ive been licensed sinced 1981 and never came across anything like that before I recall in high school the teachers taught about the BASTARD leg on a delta system but it looks like N STAR put in the opposite type of transformer you think they have come across this before and would notify their customers effected that they could have this problem because the amount of sparks emitted when I turned that disconnect on for a second or two at the most was incredible. my boss called this a sleepijg leg or sleeply leg I have never heard the term but he has a whole slew of crazy made up lingo. I put in a 200 amp under ground service a month ago he wanted me to use 4 inch p v c and then I buried it 3 feet into the ground with the meter socket 3 feet above ground and he says to me make sure you pitch the p v c from the meter to the pole so the pipe doesn't fill with water and have it back up into the meter socket. Well who will have a 4 inch pipe fill with water and back up six feet high and fill a meter socket I told him it will spill out the drainage hole I punched out in the bottom of the socket!!!!!!! I told him to shut up because he was making a fool of himself in front of the customer


This may very well be a grounded B system, and when the POCO replaced the transformer, they connected it as a 4 wire ∆. This would result in a bolted fault from phase B to neutral/ground.


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## macmikeman (Jan 23, 2007)

Walter- Long , long time ago , way before it became common for people to be walking around with a video recording device in their pockets, I sat waiting for the poco to arrive at a dwelling at the end of a long driveway. The night before it rained like a Mother-----. That house was probably 500 ft set back from the street , where there was a 2' x 4' x 30" deep concrete splice vault on the side of the pavement that was now sitting under a stream. Down at the guys meter water was gushing out like a firehose. "Hot" water. Utility breakers apparently were not getting enough fault current to open up , and you could not even walk close to this thing without getting the tingle. The owner's were stranded up in the pole house they lived in. I wish I now had a video of that meter socket . Anyway, Hawaiian Electric Company dealt with it, not me. That incident stays with me, I always take elevation drop into account when running underground work.


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## vk saxena (Oct 12, 2013)

In three phase ungrounded system motors were on load suddenly one phase of a motor got earted then at the same time other motors which are connected through the same bus bar also got burst with cracking sound i learnt that in this case phase voltage of other motor increased considerably and may be 1.73times but i could not understand mathamatically if any body could help me then i will be very thankful to him.


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## Big John (May 23, 2010)

Saxena: I can't explain the math vectorally, but before the ground fault, all the phase probably had a line-to-ground measurement of about 57% the line-to-line voltage. 

When one phase went to ground it means the other two phases suddenly went to a full 100% of the line-to-line voltage when measured to ground. It's not unheard of for this to cause weak insulation to fail. 

But since you had failures in a number of motors, my guess is that you had intermittent ground in the first failing motor. This restriking fault can cause transients many times higher than the normal voltage level, and will cause multiple failures. I've seen that happen first hand where it destroyed a lot of windings.


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