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#110733 - 06/16/06 08:54 PM Opened a Can of Worms  
electure  Offline


Member
Joined: Dec 2000
Posts: 4,262
Fullerton, CA USA
I went to check out the power to an AC unit that had been blowing fuses repeatedly. Its disconnect had a burnt fuse clip. I also got some nasty voltage readings to ground. 235, 234, and 0. Not good for an ungrounded 240 delta. Rooftop EMT was kicked apart all over the place, and liberal doses of tar had been applied. Lovely [Linked Image]
I got the keys from a tenant, and headed for the electrical room.


[Linked Image]


[Linked Image]


[Linked Image]

I was fortunate to find that the circuit I was after was in the C-H board in pic 3.


Now the dilemna would be to find the ground in the system. The only way I can think of is to shut everything off, take voltage readings, and turn services and feeders on one by one until the ground is located, then follow it up the line. I don't think that those old circuit breakers stand a very good chance of turning back on once they're shut off
Any ideas besides the obvious? (Bulldozer, Gas & Match, or maybe just wait for a ground on another phase)


(i forgot a "/")

[This message has been edited by electure (edited 06-16-2006).]


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#110734 - 06/16/06 08:58 PM Re: Opened a Can of Worms  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
I have not worked on ungrounded delta but I though the normal way was to turn feeders off one at a time till the readings are right than work that feeder back toward the branch circuits.

Of course that assumes you only have one same phase ground.

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#110735 - 06/17/06 03:29 AM Re: Opened a Can of Worms  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,708
Anaheim, CA. USA
Dang, Scott - you sure get some Hellish things to shoot trouble on / Job walk fun!
But this is the "Glamorous Life" of the Project Manager, so we grin and bear it (ugggh!).

Anyhow, you have the right idea on tracing out the Ground Fault(s).
Hunt and Peck away on de-energized Branch Circuits until your Continuity Tester yells "Bingo", then search for the Fault, or Faults
(might need the Mounties to assist on this one, so call for Dudley Do-Right [Linked Image]...)

You will definitely want to isolate the Circuitry from any PoCo Line Side stuff, or anything connected to Customer Owned Transformers (like the one in the second image), as these items may inaccurately show L-G connections.
(if this sounds odd, post a "Whaddaheck???" message for explanations).

Seeing how many older roof top conduit runs are found in an "Accordion Fashion", it's an easy bet that you will find a few Ground Faults up there.

Those Ground Faults on Grounded Systems are really fun - especially when the Equipment Bonding means back to the Panelboard has an "Unintentional Impedance" placed in series (poor connections), and when the OCPDs for Branch Circuitry has an Inverse-Time Trip Characteristic which allows blazing Faults to continue "much too long"

Got to see this a few times, and "Funny" was not the overall general feeling everyone had!
They were quite interesting, though!

So, to sum things up - Ring the Branch Circuitry out until no _Solid_ Ground connections exist (on Circuits which should not have a solid ground connection), then do a "Smoke Test".

BTW, are you sure the system is an Ungrounded Delta?
Although the Ungrounded flavors do exist in a 240 VAC version, I have seen a high majority of the 240 VAC Deltas being Corner Grounded, and nearly all the 480 & 600 Volt Deltas to be Ungrounded.

Just a thought.

Scott35

edited tew git shed of baad speeellinn [Linked Image]

[This message has been edited by Scott35 (edited 06-17-2006).]


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#110736 - 06/17/06 03:00 PM Re: Opened a Can of Worms  
Rewired  Offline
Member
Joined: Jan 2006
Posts: 558
Hamilton, Ontario, Canada
Ungrounded delta system and NO "Ground indicator lights"???

A.D


#110737 - 06/18/06 12:18 AM Re: Opened a Can of Worms  
electure  Offline


Member
Joined: Dec 2000
Posts: 4,262
Fullerton, CA USA
Bob, your idea is a good one. If it is shut off until the ground "disappears", it might result in less trouble.

Scott, there's no evidence of intentional grounding at the service. All 3 legs are fused and the conductors are black, red & blue (no white) [Linked Image] It might be an even worse chore to correct it if it were a corner grounded delta.
Oh, I don't want to do this [Linked Image]

BTW, all that junk in the 2nd pic except for the starter in the center is hot. I didn't touch any of it. [Linked Image] 1993 newspaper and pile of beercans in front of it added nice touch.


#110738 - 06/18/06 12:50 PM Re: Opened a Can of Worms  
Alan Nadon  Offline
Member
Joined: Mar 2005
Posts: 399
Elkhart, IN. USA
Double check that it is not a corner grounded delta. Some electricians have been known to fuse the grounded leg, even though it shouldn't be fused.
Alan--


Alan--
If it was easy, anyone could do it.

#110739 - 06/20/06 05:19 AM Re: Opened a Can of Worms  
jes  Offline
Member
Joined: Jul 2002
Posts: 103
CT
If the phase to phase voltages were OK then it sure appears corner grounded to me. Might not have the corner ground connection in the service equipment. Have seen one where it was done on the utility transformer connections but not in the building.


#110740 - 06/20/06 09:39 PM Re: Opened a Can of Worms  
electure  Offline


Member
Joined: Dec 2000
Posts: 4,262
Fullerton, CA USA
That's kinda why I thought I'd turn everything off and check the voltage ahead of the service disconnects first [Linked Image]


#110741 - 06/21/06 08:45 PM Re: Opened a Can of Worms  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,223
SI,New Zealand
Crikey Scott,
You really struck oil here, mate!.
What a mess from start to finish. [Linked Image]

[This message has been edited by Trumpy (edited 06-21-2006).]


#110742 - 07/12/06 11:27 PM Re: Opened a Can of Worms  
Theelectrikid  Offline
Member
Joined: Apr 2004
Posts: 810
Levittown, PA
One word, "DYNAMITE!!!"
(More power! More power! More power!)

Ian A.


Is there anyone on board who knows how to fly a plane?

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