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Joined: Dec 2007
Posts: 35
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adroga Offline OP
Member
Today we went back to a building where we installed an MCC, transfer switch and some emerg panels and while doing some small finishing work the building electrician told us a phase is showing a ground fault.

Its a delta and phase to phase we get about 600 volts, but one of them shows 0 volts to ground.

I was thinking it looks like a corner grounded delta, but our work was up and running 2 months ago and the problems started last week, coincidently while some major work is being done on another floor.

I still havent learnt enough about delta-wye systems, especially when they have an open or grounded configuration.

What should I be looking at? We arent diagnosing the problem, but I would like to understand it for myself.

No real question here, just looking for some feedback.


Joined: Jul 2004
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With no other guidance about how the system is grounded I would go look in the service disconnect enclosure and see what was going on there. A corner ground should make itself fairly obvious. To start with one of the phases should be identified with white. Someone should be able to tell you what kind of system it is tho.


Greg Fretwell
Joined: Dec 2007
Posts: 35
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adroga Offline OP
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Just to clarify on my background... I am a 5th month apprentice. I have some basic 3 phase theory that I learned in school but not enough understanding for my liking. We aren't troubleshooting this problem yet, but I wanted to understand it better.

I need some reading material and so far havent found exactly what I would like to find online concerning delta/wye systems.

Sorry if this seems like a vague question, but maybe you guys can point me in the right direction.

Joined: Mar 2004
Posts: 947
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twh Offline
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It might be an ungrounded system with ground fault indicators.

Joined: Aug 2005
Posts: 214
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Originally Posted by twh
It might be an ungrounded system with ground fault indicators.


This makes a lot of sense with a 600V Delta, and "problems" arising recently.

Last edited by Elviscat; 09/18/08 12:35 AM.
Joined: Mar 2005
Posts: 1,213
S
Member
If the delta isn't corner-grounded at the transformer, than it's not intentionally corner-grounded, and you have a ground fault somewhere.

This is actually one of the benefits of a floating delta- you can short a phase to ground, and still operate. Beware that this can do very bad things to TVSSs which now see increased voltage to the other lines!

Joined: Dec 2007
Posts: 35
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adroga Offline OP
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Whats TVSS?

edit... just looked it up and basically it surge protection?

Supposedly the day before(maybe day of) the grounded phase there was an issue with an undersized neutral of a subpanel which was # 10 and they measured 65 amps.. the neutral blew and subsquently damaged the ups of the computer rooms.

Could the phase to ground have caused this and not the neutral?


Last edited by adroga; 09/18/08 04:15 PM.
Joined: Mar 2005
Posts: 1,213
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Yes, it's surge suppression, usually an MOV that will shunt the protected wire straight to ground; voltage drop does the rest. It's designed to kick in at a certain point above the normal line voltage so as to minimize any transients. Corner-grounding a delta will push the other two legs dangerously close to the threshold for TVSS activation though, at which point it will become a direct short circuit and draw fault current until either a breaker closes or something explodes.

All the high-end UPS systems have this internally. In fact, Powerware engineers cautioned me explicitly about not using a floating delta on one of their 500kVA units lest the delta drift off center and we blow the UPS up.

Although 65A on an undersized neutral can't be good either.

Joined: Dec 2007
Posts: 35
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adroga Offline OP
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Hey guys,

Supposedly the in house electrician did a shutdown of certain areas and discovered the short to ground came from a newly installed liebert whose # 10 gauge armored cable was forced into a connector rated for 12 gauge and crimped tightly.

I relieved it wasn't our installation...



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