Ground Fault On A Delta

Eddy Current

Three days in a row I've got a late night service call for a retirement high rise. They have a small emergency panel that's loosing a phase. When I get there I find a blown 15amp-600volt fuse on the "A" phase. Put in a new fuse and all is good. Next day, same call, same problem.

I went back during the day and tested the circuit with a megger. Everything Megs clear?

Got the call again Friday morning, threw in a fuse but it blows right away. Take some readings on the main 3phase 600v and this is what I find.

Any phase to any other phase----600volts
phase "A" to ground ----600volts
phase "B" to ground ----600volts
phase "C" to ground ---- 0volts

Also, there are no ground fault indicating lights on this service.

MSSI

OK, so what does voltage have to do with a blown fuse?

Show me amperage on each leg.

Is 600V a standard in canada??

Eddy Current

Yes, 600v is standard but phase to ground should be 347volts.

The fuse that blows is a 15 amp 600volt fuse for a small 10kva transformer (600v ---> 120/208v) which runs a small 120/208v panel.

When operating normally the primary side(600v 3phase) draws 7amps per phase. The small 120/240volt panel has all single pole 15 amp circuits for emerg lights, fire panel, front door access, enterphone etc.

When the main service is normal I get 347volts phase to ground,600v phase to phase. When the ground fault is there (seems to be only in the evening/overnight) I get 600volts to ground on two of the phases and zero volts to ground on the other. And this little 15amp fuse blows?
Keep in mind that the phase that reads zero to ground still has 600 volts phase to phase. The rest of the building still works, 600v elevators, mechanical stuff etc.
This reading is everywhere on the 600volt system when the fault is there.

mrmike

Just wondering what you mean by "meggars clear". Does this mean infinity readings? IS there a disconnecting means or did you just remove each fuse & meggar on the load side? Did you take the wires off each leg on the load side & meggar them to ground, if you didn't remove the fuses? Did you check the voltage on the line side coming into the emergency panel? Sometimes we tend to miss things.

Eddy Current

Disconnected the wires feeding the tranny at both ends and disconnected the secondary wires feeding the panel at both ends. Megged everything and got infinite readings. All wires to each other and to ground, The disconnect fuse holders to ground, the lugs on the tranny to ground.

There are four disco's coming off of the main 600volt splitter. When the fault is happening the line side of all of them read zero to ground on the "C" phase. I had the poco come out and check things. All is good from their end. They supply 600v delta to the building(3 wires, no neutral)
Also turning off the small disconnect (that keeps blowing the fuse) makes no difference on the odd voltage readings.

Right now everything is fine, of course, but I expect it to happen again tonite.

mrmike

Guess you can disregard some of my questions as I posted it before I saw your last post. Another Question is it 4-wire coming into your main panel. From what it sounds your somehow loosing a ground-hence I suspect that the 600 volt readings might be just going back around to phase to phase, making it an open Delta. What are you checking the ground thru?.

Eddy Current

Only three wires coming from the vault in a metal conduit. 3 phase Delta, 600volt phase to phase. When ground fault is gone, I read around 350v to ground on each phase.

The main service disconnect is bonded to a bare copper wire leading into the slab.

(If it were a 3 phase four wire "Y" I would get 347volts to ground/neutral, 600 volts phase to phase.) Standard in Canada Eh!

mdshunk

Is this really what you think it is, or is this a 600V corner grounded delta that has a bond wire intermittently going open? If so, a DLRO might help. The diagram below more closely mirrors the installation you describe, even if you normally anticipate seeing a 347/600. If this feeds transformers, it may well be just plain delta with no neutral.

Eddy Current

Just plain delta 3-wire, no neutral. Feeds elevators, one small tranny for the emerg panel and a few other mechanical things that are all 3-phase 600v. The building has another 120/208 volt service coming from the vault that feeds the apartments, lights, etc.

This type of system usually has three ground fault indicating lights to let you know if there is a ground fault. For some reason this one was missed, I already have a quote made up to put one in after I fix the fault.

InPhase277

If this is a regular 600 V delta, with no neutral and ground indicators, you will never read 347 to ground. It sounds like C phase is grounding somewhere or through something. You will then read 600 V p-to-p an 600 V p-to-g.

347 V will only appear with reference to a neutral. This system sounds like it is floating, hence the ground indicators. When C phase grounds, you essentially have a corner grounded delta. Find out where C is faulting and you will most likely solve the whole problem.

precisionbuild

Do you have a Ground Fault Analyzer? One like this? http://www.multi-tech-industries.com/gfault.html

Eddy Current

Yes, but I do get aprox 150volts or so to the ground that bonds all the disconnects together-when everything is normal. I know that there is no ground or grounded conductor in this type of system.

I think that I have two phases grounding. At one time I had the fault (C grounded-600v to ground) and when I turned one of the disconnects off the fault went away-almost. It kind of half went away, I was reading "A" to ground 400v, "B" to ground 400v and "C" 200v?

When only one phase is grounded the system still operates but in the evening/overnight a second phase grounds and for some reason the smallest fuse in the system blows......I think. Or mabey it's not a very good ground and then overnight it becomes a better connection. I'm actually excited to get the call tonight so I can start exploring. Man I'm such a geek!

acrwc10

So if I understand you correctly you are saying this system, has no ground detectors and is not a corner grounded system.
I find this a little odd that it would have passed an inspection like this. With out seeing it I would expect that it would have been a corner grounded system by what you described, cheaper and safer then ground detection.

acrwc10

http://www.mikeholt.com/safety.php?i...ems%20(2-21-2K

You may want to check out this also.

nap

am I missing something here or what?

If you read 600 to ground on 2 phases and 0 to ground on 1, (and this is an ungrounded delta) that one phase has a ground fault. You had the chance to track it when you got those readings but apparently didn't do it.

I don't know how you are getting 347 to ground when all is well since you state this is not a grounded system so there would be no actual ground reference. This reading would be meaningless. Not sure where you would even get any consitant reading and why it would even be 347.

what I see as happening is, C phase is either grounded all the time or it is imtermittantly grounding. Not a terrible deal until A phase also went to ground. Then you had a phase to phase short via some ground connection.
when you megged this, did you meg all phases? If they were all clear at the time, the the first situation is apparently the case.

you need to hunt the ground short. I would suspect both A and C have insulation worn through in the same place. I would suspect the short burns itself clear when the two phases both ground and then eventually, the weather, some machine movement, or other reason causes them both to touch ground again.

Eddy Current

Sorry for the "ground" confusion. There is no ground. When I say voltage to ground I mean one phase to the copper bond that bonds all the equipment together. It is not 347volts, and never will be....there is no reference to ground.....in theory. I do get a reading of around 350volts phase to bond. From now on I will call it a bond instead of a ground so not to confuse.

I've narrowed it down to two different ground faults. I got the call again tonite, same readings 0 volts to bond on "C". I turned off the disco for the emerg panel and the "C" phase voltage to bond changed to 200volts. I then turned of another small disco and the ground fault cleared completely. This other small disco is for the post lights around the building and is controlled by a photo-cell, contactor etc. (This explains why it was only happening at night) I turned the emerg panel back on and turned one circuit off at a time until the ground fault on the "C" phase cleared again. It was on a small lighting circuit for the lobby. I left both circuits off for the night.

Met on site with the inspector and my supervisor on Thursday and I asked him the same question. He said he will send the building owner a notice that says the indicating lights will have to be installed.

nap

You did say it was 347 volts to whatever.

If your equipment is grounded, then to bond or ground would be the same. In either case, since this is a floating delta, there is no reference to either point so any voltage you read to bond or ground is merely a capacitive coupling and not an honest voltage, at least when things are correct.

Eddy Current

Then how do you tell which phase is grounded? Keep in mind there are no ground fault indicating lights. Actually, ground fault lights are wired from phase to bond so me measuring phase to bond with a meter is the same thing the lights are doing. Different voltages = brighter or dimmer bulbs.

What was the purpose of your last post? If you read my posts you'll see that I've already said that.
Have you ever measured voltage on the main service when there is a ground fault on the system? How did you do it? How would you go about trouble shooting it? Now answers to those questions would be a helpfull post.

nap

In a floating delta, none of them are grounded so any that read voltage to ground are not grounded and those that read 0 to ground yet read to the other phases are grounded.

are the tubs grounded (not just bonded but actually bonded to earth? If so, bond points and ground are one and the same.

in one post you said you should, and did read 347 to bond and in the other one, you said there was no proper reference to ground (presume you meant bond). Just pointing out you disagreed with yourself.


if the fuse or breaker did not trip (as in a floating delta) yes.

with a voltmeter. How else?

just kidding. from the phase to ground. I explained above what you will read.

First, you turn off the power. Since you are having 2 phases going to ground or at least some common point (supported by the 0 volt to whatever and then a blown fuse after presumably the second phase hit the same connection), having something blow up i nyour face is not a good thing.

So, now, you check for continuity from each phase to a ground or bond point, whichever it is shorting to. You then simply track that short by continued removal of subsystems until the fault goes away. You then search that subsytem for the short.

So, do those answers qualify?

Eddy Current

I only explained myself better because you are confusing things. Your not helping at all. Do us all a favor and read all my posts again before responding.

I only put that in so YOU can understand. Instead of pointing out meaningless and non-helpfull comments, you should concentrate on trying to help.