Todays' Service Call (Solve this service call)

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Ok, I'll bite, conduit to meter has insulated section or joint, phase is shorted to conduit, current is flowing from conduit locknut to grounded panel can? Scratch that, you said arc stopped with main off.
 
Well, we know we have a short somewhere in the house then sending power back to the panel, and yet not tripping the breaker. You turned off the AC unit to get that fixed, I assume, so if you still have an arc when you turn on the main breaker, then you have to check circuit by circuit... shutting each breaker down until the arc stops... that should tell you which group has the short...

I would say it has to be the set that the conduit belongs to, but, I know from experience that the arcing could be caused by another neutral in the panel being compromised... Over here in UK we would be able to id every circuit and ground to every breaker if it was wired correctly, by position, so could do our continuity tests to try to find it. Harder there, but still a process of elimination.
My thinking is that the loose grounding of the conduit nut allowed it to arc faster than another spot inside the breaker panel, but it is still a flow of fault current... again, I have been wrong more than I am right but, I am trying to learn by thinking... not just learn by watching others answer, so if I am being totally wrong, sorry...
 
Just to properly characterize the visible fault:
I would call that a glowing connection rather than an arc, based on what is visible in the short clip.

That means, among other things, that an AFCI device would not see anything to trigger on except possibly simple ground fault.
 
I can't think of anything that hasn't been ruled out except...

-Open neutral between the meter and the main panel (causing the neutral current to return to the neutral bonded at/near the meter via the conduit instead of the conductor).
-Some jury-rigged neutral or EGC creating a path back to the nipple we see in the video.

480sparky, can you at least confirm if your friend with the IR camera helped you find the issue?
 
jaggedben said:
I can't think of anything that hasn't been ruled out except...

-Open neutral between the meter and the main panel (causing the neutral current to return to the neutral bonded at/near the meter via the conduit instead of the conductor).
-Some jury-rigged neutral or EGC creating a path back to the nipple we see in the video.

480sparky, can you at least confirm if your friend with the IR camera helped you find the issue?
Click to expand...
Question.. bear with me ... but if no arc when main turned off, wouldn’t that rule this idea out? I mean, unless a main conductor has burned as well, so that when it gets warm enough through needs it then arc itself to the conduit? I know that can happen but have not seen it happen in real life so do not know if it would be like video...

in fact.. only way I know to check that is to turn up the load, and shut it down after arc is observed, with someone recording the times at the meter as well... to see if any current kept flowing when breaker shut...
 
jaggedben said:
I can't think of anything that hasn't been ruled out except...

-Open neutral between the meter and the main panel (causing the neutral current to return to the neutral bonded at/near the meter via the conduit instead of the conductor).
-Some jury-rigged neutral or EGC creating a path back to the nipple we see in the video.

480sparky, can you at least confirm if your friend with the IR camera helped you find the issue?
Click to expand...

The IR photos helped narrow it down, although he hasn't sent me copies of the images yet. But we noticed there were no GECs into the panel, either to any rods or pipes. There was, however, a single rod right below the panel (yes, indoors!), so while I was landing a #4 bare on the neutral bar, I noticed some arcing between the neutral lug and the bus for the neutral bars.

A loose neutral was my first impression, so I had originally tugged on it to see if the lug needed tightened. Turns out the lug itself was tight on the conductor, but not to the bus. So I turned off the main and pulled the neutral conductor out of the lug. By pure luck, the lug was bolted to the bus with a slotted bolt that was easily accessible through the lug. A good crank (about 1/8 turn) and relanding the neutral wire solved the arcing locknut.
 
And here's the irony of the whole fiasco.

1. The building originally had a 1-phase service that had been abandoned, but never removed.
2. The 'new' 3-phase panel was a cobbled-together mess of various Square D parts. Literally, a Frankenstien monster. A 40-space panelboard with only 30 spaces of busbars. And a 42-space cover.
3. It was not grounded to any rod or pipe.
4. And the most hilarious part is........... there were no 3-pole breakers. So why put in a new 3-phase service and not have any 3-phase loads?
 
__dan said:
EMT to a die cast set screw connector, coupling, then a die cast offset nip. Coupling doesn't look right, looks too big (thick) for steel threaded. Looks like it could be a PVC coupling.

THW conductors with the fiber bushing is old school, like 1960's or before. Die cast fittings are, I am guessing, from a later era. So the panel was taken apart and moved a little then put back reusing old parts (?).

Is the coupling PVC slipped over the EMT set screw connector, so they could neatly join to the die cast offset nip (?). I don't see the fourth THW, the neutral, but assuming it is there and bonded to the can. So they damaged the THW when they took it apart and put it back together and it is faulting to the offset nip, and since the coupling is (maybe) PVC there's no useful N to G bond at the meter socket and it has to flow through the loose locknut to the panel bonding screw (???).
Click to expand...
Look closely, looks to me like an offset nipple is threaded into a coupling at the end of the raceway run.

480sparky said:
The IR photos helped narrow it down, although he hasn't sent me copies of the images yet. But we noticed there were no GECs into the panel, either to any rods or pipes. There was, however, a single rod right below the panel (yes, indoors!), so while I was landing a #4 bare on the neutral bar, I noticed some arcing between the neutral lug and the bus for the neutral bars.

A loose neutral was my first impression, so I had originally tugged on it to see if the lug needed tightened. Turns out the lug itself was tight on the conductor, but not to the bus. So I turned off the main and pulled the neutral conductor out of the lug. By pure luck, the lug was bolted to the bus with a slotted bolt that was easily accessible through the lug. A good crank (about 1/8 turn) and relanding the neutral wire solved the arcing locknut.
Click to expand...
That is still a neutral connection issue and was an answer long ago, I have seen that happen this way before, as well as seen loose connection between different bars of the neutral assembly. One doesn't always catch such things while there is something glowing, but there is often signs of overheating or at least some arcing indication.
 
Yes, the red leg delta was odd also. Was it?, 240 V line to line?. With the reg leg delta and single phase loads, you can only load up to something like 50% of the kVA rating of the transformer.

You are really limited in loading up the red leg delta unless you have mostly 3 phase loads.
 
__dan said:
Yes, the red leg delta was odd also. Was it?, 240 V line to line?. With the reg leg delta and single phase loads, you can only load up to something like 50% of the kVA rating of the transformer.

You are really limited in loading up the red leg delta unless you have mostly 3 phase loads.
Click to expand...
Used to be common to mark the high leg with red, and often was on the right position instead of center position. POCO's meter needed high leg to be on right position to work properly is part of reasoning.

How much 120 volt loading you can have entirely depends on how the transformer (bank) is built. Single core transformer is often three same sized coils and is "fixed". A bank made of individual single phase transformers, can be built to suit the needs of the loads. If majority of load is going to be three phase loads they may make them all the same size, if the majority of load is going to be 120/240 single phase then they may use a large unit for that side and smaller units for the others, or even just one smaller unit and build an open delta configuration.

I've seen 50 or 75 KVA lighting pots with only a 10 or 15 kVA stinger pot before where the expected load on third phase was limited.
 
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