# Very odd ground fault on 3 phase delta



## Stew985

Hi Guy, I have managed to confuse many electricians and hydro guys. I have a 3 phase 240v delta system. 3 weeks ago I had a major ground fault. All 3 phase lights went bright and one went super bright. Voltage to ground was about 350v 350v 430v!! Normally it's about 130v. It did it quite a bit that day. Now it has toned down but there are microsecond blips that trip my CNC mill. I have turned everyone's main fuse panel on the entire grid off at the hydro side and I can still see it on their system. None can figure out what's happening. Currently I'm running my mill off a generator and it's fine but expensive. Wet , dry, hot ,cold does it anytime completely random. May not do it for an hour but may do several times in a minute.

Help!!

Thanks,
Stephen


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## AK_sparky

Am I understanding this correctly: Those are the voltage blips you are seeing straight from the POCO? I would be chasing them about this.

Unless of course you are the POCO.

Either way, interesting issue. I'm looking forward to hearing if anyone here has seen it before.

When you say it is normally 130V, you mean each individual phase is normally 130V to ground? I thought in an ungrounded delta (which I assuming based on having phase lights) you shouldn't have a voltage to ground, unless there is a fault.


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## Stew985

Yes I have had the power company here many times. They are completely stumped. I have always had voltage to ground and they do say it's an ungrounded system so I'm not sure how it gets power. The phase lights are normally dim and they are linked from phase to ground. A shop down the road many years ago had a motor shorting, when that happened my 3 dim lights went to 2 brighter and 1 out and that's what the hydro guys say should happen. Phase to phase voltage has alway been fine.


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## Stew985

Link to some pictures and vid's of the problem

https://goo.gl/photos/Xa4rhxwyExiqhVBL9


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## inetdog

One of two things is happening:
1. Your ungrounded delta is getting crossed to the phase voltage of a higher voltage system, or
2. A restriking arc fault is pumping your system up to a high DC offset from ground.
Make sure to check for DC next time it happens.
Do your service conductors (and POCO's secondary conductors) run close to any higher voltage system? 
The fault could also be inside the service transformer.


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## RePhase277

Yes, I think that there is a grounded system that has shorted to the ungrounded system somewhere. The brightest lamp on the detector is likely the phase with the short. Is there another system feeding the building, or are there transformers within the building fed from the same main?


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## Stew985

There is a second 120/240 system. I did check it on one long fault but came up with exact voltages there should have been between hots and ground/neutral. The system come in on it's own but I will double check to see if any of the 2 systems get close.


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## AK_sparky

InPhase277 said:


> Yes, I think that there is a grounded system that has shorted to the ungrounded system somewhere. The brightest lamp on the detector is likely the phase with the short. Is there another system feeding the building, or are there transformers within the building fed from the same main?


This is what I was thinking too, but I tried to model that fault with some math, but the numbers didn't add up. I was thinking maybe a 120/240V system was connected to it somewhere.

Maybe someone with some more experience modelling 3 phase systems could figure out where and what kind of fault would cause those numbers.


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## Stew985

Recently the faults have been very quick, fraction of a second. But that's enough to trip my computer on my CNC. Also working at my shop on the weekend, when it's quiet, near the phase lights I can hear quick little surges. Sometime none for a while then a bunch at a time, then nothing then a one or a few.


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## Stew985

Just an observation, On my street the newer building are on 208 wye. That system neutral is connected my neutral/ground system. Any chance that has a fault?


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## Jack Leg

I believe there should be a current transformer or a potential transformer that those lights are hooked up to. check these transformers. This post may be to late.sorry


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## Stew985

Well, The issue isn't resolved yet. I have put in an isolation transformer and it's working for my CNC mill but the power just fired my $3000 plasma cutter. Blew up most of the input capacitors inside it. When I walked into my shop and looked at it looked the smoke looked like a movie special effects. 

I have a hydro engineer trying to figure it out. They recorded 400 V ground faults on a 230 V system. This guy is totally confused and is going to talk to other engineers. 

Stephen


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## frenchelectrican

Have the hyrdo check their voltage regulator transformer.. When those transformer adjust the voltage it can make a tempory spike if it was switched under the loads...

And have hyrdo check other customers which they may have mulfi voltage in their place..


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## Stew985

It's not line to line it's line to ground and it's an ungrounded delta system so that why they are so confused. There shouldn't be much power line to ground. Normally sits around 160V.


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## RePhase277

Is there another service that feeds this building, or is it just the 240 volt ungrounded delta?


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## Stew985

There is a 120 system but it come in on the other side of the electricle room


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## Vintage Sounds

In an ungrounded system there will always be some induced voltage to ground due to capacitive coupling. It's not "real" voltage coming from the transformer windings. There might however be something that is causing higher voltages to be induced. Maybe a new installation somewhere on the street?


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## emtnut

Has anyone done any Meg tests to see if there is in fact a 'ground fault' ?

Also, given the surge you get upwards of 430V ... do you know of any 480V services near you ?


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## Stew985

They did a meg and found no problems. There isn't a 480V system but the mains are 600V, Think that's a Canadian thing.


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## telsa

I have seen the same baffling transient faults -- on a much smaller scale -- but the behavior matches.

The system was an old, old, computer system ( 1981) that was fed through isolation transformers. These were so big that they had their own room, closet, really, The daughter cards were immense that far back.

The tech was stumped all day long. (Saturday, off hours)

He'd swapped every single daughter board time and again and couldn't find where the 'blips' were coming from. Each 'blip' crashed the software. The design didn't have enough capacitance to carry energy through even such brief excursions.

I finally pitched in: it's your isolation transformers. One is so marginal that its experiencing corona discharge -- to the chassis -- on a very sporadic basis.

Upon swaping transformers, the 'blips' jumped to the twin machine. It WAS the isolation transformer.

Transformer breakdowns are so rare that techs are stumped when one is at the edge.

Your FIRST hit -- the major ground fault -- did the damage to the transformer -- opened up the insulation.

You must immediately meg any suspect transformers, ditto for the Poco.

ANY in your system can be 'blipping' you out.


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## telsa

Stew985 said:


> They did a meg and found no problems. There isn't a 480V system but the mains are 600V, Think that's a Canadian thing.


Do you have OTHER transformers -- even single phase -- within your building ?

Socal Edison -- in 1945 -- found that autotransformers -- failing on the load side -- right at various power tools -- were more than enough to damage everything in sight.

Yes, THAT system was an ungrounded delta.

It caused Socal Edison to establish EUSERC.

Yes, it was that famous back in the day. Lots of print on it. Tons of EEs jumped in on the matter... for weeks. ( It was a big war plant, hence, ungrounded delta. )


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## emtnut

Stew985 said:


> They did a meg and found no problems. There isn't a 480V system but the mains are 600V, Think that's a Canadian thing.


Given it's an intermittent fault, Megger will read fine most of the time.

Yep, mostly 600y347 up here, but 480 deltas are around.

Given this some thought ... And I really have no doubt that Hydro's transformer is grounding out on you .... whether it's a bad Tx, or someone on the same Tx spiking the load, would be the puzzle to figure out.

Normal for you to have ~120 V to ground (this should be checked for real or ghost power if you haven't already)
1 phase of the Tx ground faults, and adds ~~300v ... bingo, your mysterious 430V to ground voltage.


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## Big John

To be 1000% clear, your phase-to-phase voltages are rock solid and do not fluctuate?

Because I'd be very hard pressed to explain why good nominal P-P voltages would be damaging equipment with no ground reference. I've seen UG systems hover near 1kV-to-earth and as long as there is no insulation failure, everything works fat, dumb, and happy.

I have never seen a restriking fault as it happened: My assumption was that they were fast and obvious. I wouldn't expect you to see sustained overvoltage for as long as that video showed, but I'm guessing.

While they will definitely cause equipment damage, the results are almost always on multiple phases causing short-circuit trips.

Assuming your second service comes from the same set of prinaries, you can rule out primary problems by seeing that all L-L and L-G voltages on that service stay stable.


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## Stew985

I have never seen a spike between phases. The hydro engineer had a recorder on everything for several weeks. 

My CNC mill was only blowing fuses and varisters. As the power come in it check all voltages P to P and P to G. thats where it's blowing. I'm not sure what was up with the plasma machine. I don't normally have it hooked up but when my employee was using it we had a major ground spike and it blew most of the power input capacitors in a great cloud of smoke.


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