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#210583 07/11/13 09:03 PM
Joined: Jul 2013
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My co-worker and I were replacing some old wallpacks today with new. I was assembling fixtures while he put them up. He is working on the last fixture while I am cleaning up and I hear the building back-up genny fire up. Thought is was a weird time for a genny run at 3 in the afternoon. Next thing I know a women is asking me what is wrong. She explained the building lost power. My co-worker walks over and has slight arc burn on his arm. Said he was holding the fixture against the wall when it slipped down a little and the hot leg started crackling. Next thing he knew genny behind him was running. I went down to the elec room and the 1200a main is tripped. I reset the trip mechanism than reset the main switch. Everything is fine...genny cycles down and all is well. My thinking is that the lighting circuit has no, or a bad earth ground and caused the trip. Am I correct in this assessment? Theories?

Joined: Apr 2002
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Apparently, the arc at the lighting circuit tripped the GF protection,

You neglected to describe the 'arc'; hot to ground, hot to neutral, hot-hot?

What you describe as 'earth ground' is the equipment grounding conductor (EGC) which is the fault path for OCP.

The GF can trip on phase to phase, phase to ground inbalances.

I had a 3000 amp 277/480 main trip on GF traced back to a fluorescent (old magnetic) ballast. Thru the 277 LP, thru a 277/480 MDP and to the main, without any feeder or branch trips. Hence, 'selective coordination' comes to play in critical ops.




John
Joined: Jul 2013
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I am very aware earth ground is the grounding conductor.

Not sure exactly what the hot wire shorted on, I am assuming the metal box. He had the neutral wire (grounded conductor) wire nutted.

Joined: Jul 2001
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J
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[quote=SafetyWiredMy thinking is that the lighting circuit has no, or a bad earth ground and caused the trip.[/quote]

Actually it sounds like all of the bonding paths and grounded conductors are working correctly.

What you have is clearly a case of poor protective device coordination, as was previously mentioned.

Most manufacturers will ship their GF devices set to the absolute minimum. If these settings are not changed the main GF can very easily trip before a 20A branch device operates.

I would guess that fixture and ballast changes result in the predominant number of main GF nuisance trips.

Joined: Dec 2000
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Had it shorted to the neutral, the GF protection would has seen it as just another load, and it wouldn't have tripped.

If you had turned off the power beforehand, as you should have, this never would have happened.

He's lucky that he just had a slight arc burn. Things could have been much worse.

Joined: Jul 2013
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Originally Posted by electure


If you had turned off the power beforehand, as you should have, this never would have happened.



You are absolutely right! If I had been the one working on it I would have shut it off.

Joined: Nov 2005
Posts: 826
J
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The main breaker should have had an indicator showing the nature of the trip, most likely GF. Sometimes the first breaker to have ground fault sensing is the main. You should look at the trip setting and time dial of the main Vs downstream trip units. The downstream units should always win the race. Main: "A", ".3S" Branch: "A", ".1S"--> Branch should trip first under ground fault conditions.
Main: "A", ".1S" Branch: "A", ".1S" --> It's anybody's guess.
We try to check our Digitrip units every 2 to 3 years, to verify that the pickups and times are as set. Sometimes that's not the case until those little rotary switches get exercised.
Joe

Joined: Jul 2013
Posts: 44
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Originally Posted by JoeTestingEngr
The main breaker should have had an indicator showing the nature of the trip, most likely GF. Sometimes the first breaker to have ground fault sensing is the main. You should look at the trip setting and time dial of the main Vs downstream trip units. The downstream units should always win the race. Main: "A", ".3S" Branch: "A", ".1S"--> Branch should trip first under ground fault conditions.
Main: "A", ".1S" Branch: "A", ".1S" --> It's anybody's guess.
We try to check our Digitrip units every 2 to 3 years, to verify that the pickups and times are as set. Sometimes that's not the case until those little rotary switches get exercised.
Joe

I believe there is only one trip mechanism and it is just above the Main switch. I had to remove the clear plastic cover to reset. To the right are the branch disconnects and there are no trip mechanisms.

Joined: Apr 2002
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What Joe is saying is "selective coordination" in basic terms. If you have GF on the 'Main', and on panels further downstream, the downstream panels should be at a lower setting then the 'Main'.

It sounds like the facility you were in only has GF at the 'Main', which is fairly common.

Testing and adjustments to GF trip settings and times should be based on the factory specs, or engineering studies.


John
Joined: Apr 2002
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Bump:
Any updates on this??


John
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