control panel wiring damage photo - gueses about cause? 6

[electricpete]

I was asked to look at a photo of some damage found in some wiring in a control panel (my understanding is that nothing tripped, blew or alarmed, the damage was just visually found). I'm remote with very limited data and communications and was asked to give an opinion about the cause.

I haven't tracked down the circuit of involved wires yet. But the interesting thing is the physical characteristics of several different vertically-adjacent wires all damaged at this location but apparently not further away. It leads me to believe the problem originated here and propagated from one wire to the others thermally at this location. Most likely a loose crimp on a lug landed at terminal strip point 573, 671 or 672. I don't think a short occurred because there is no report of blown fuse or tripped breaker and I don't really see a anything that points towards a short although the visibility is limited in some places. At the same time I don't really see a slam dunk location where the high resistance would've been.

I recognize this is woefully small amount of info. (I will get more if I can about what are the associated circuits along with more detailed inspection results.) I'm just curious if anyone has ideas about what observations about the photo that might shed light on the possible origins of the damage?

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(2B)+(2B)' ?

 

[electricpete]

I was finally able to talk to the craft that inspected this. This is a motor control circuit, all the affected leads carry motor current. They tugged on all the leads and no crimps appeared loose. They consider that it appears more likely an arc than an overheating event based on the defined shape of the pattern (rather than gradual change from thermal) and also the presence of soot. Since no recent blown fuses / breaker trips or alarms occurred (this has since been confirmed) and the weather is currently dry, this arc likely occurred in the past. There may have been some poor insulation / separation of the lugs.

That's his diagnosis. It sounds pretty reasonable to me (I agree it looks more like an arc pattern in the photo than a heat pattern). Although personally I've not really encountered arcing on 480 volt systems before though (outside of unusual switching of inductive load currents). Just out of curiosity, is it plausible to you that arcing could have occurred?


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(2B)+(2B)' ?

 

[Compositepro]

I agree that the damage is caused by an electrical arc between wires and not due to over-current. At 480 vac, arcs are self-sustaining. This was a small arc, so the current was quite limited by something like the coil of a relay or contactor. The arc may have initiated due to a loose strand of wire at crimp terminal.

I have worked in facilities that handle carbon fiber and it was not unusual for carbon filaments floating in the air to land and cause shorts. At 120 vac the filaments would burn-up with a barely noticeable flash. At 480 vac there was a bang and the circuit breaker would trip, as well as the feeder breaker.

 

[jraef]

2 additional things to look into:

1) Since those are motor leads, (5T1, 5T2, 5T3 etc.) I would be checking the Motor Thermal Overload Protection. It's possible that the motor was experiencing overloads, so someone "cranked 'er up" (or replaced the heater elements if it is an older type) to avoid the "nuisance" of having to keep resetting the overload relay. I have seen that a lot over the years. Consequences be damned, just don't make me have to come down here to figure out why the motor is overloading...

2) The motor is wired up with "portable cord", likely SO cord, and the conductors in there are finely stranded to be extra flexible. The problem is, it technically requires special crimp lugs that are rated for use with fine stranded conductors, called Class G, H, or K stranding, as opposed to standard building wire that is Class B or C stranding. The problem arises because the finer strands tend to spread out when crimped, instead of compressing into a more solid mass, and that results in less conductor material carrying the current, so it over heats, increasing the resistance and setting up a thermal runaway.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[che12345]

Dear Mr. electricpete
"...I was asked to look at a photo of some damage found in some wiring in a control panel (my understanding is that nothing tripped, blew or alarmed, the damage was just visually found). I'm remote with very limited data and communications and was asked to give an opinion about the cause..."
My preliminary opinion as following
1. No sign of [arcing or tracking] (cross terminals).
2. No sign of (over-hating) on the [left-hand side wires and all cable lugs].
3. (Over-heating) after the [cable lug], along the wire conductor and insulation.
4. (Cable lug insulation) on Terminal 673 shows [early sign of over-heating spot] (see darkened area/spot).
A. Wrong size cable lug.
B. Poor quality crimping tool/workmanship. BTW: a quality crimping tool (can release) only [when fully crimped].
C. Crimping tool with wrong selection of tool and die set.
i) High resistance at crimping point. Heat conducted to the wire conductor and insulation, which may be 70/90 deg grade. Cable lug insulation is of higher temperature grade, showing only partial damage.
Che Kuan Yau (Singapore)

 

[itsmoked]

Arcing.
No gradual overheating or those lugs would've been greatly color changed as would the terminal block metal.

This wiring looks old. I recently pulled out a hundred foot roll of SO 10AWG on a wooden spool. I wired the spool end to a machine bench. Then I rolled out about 35 feet and went to install a plug on it. After I unjacketed the end and started working on the individual wires I found a crack in the white conductor. I thought maybe I'd nicked it. So I cut off a foot. Then I started again. Same thing happened. I knew I hadn't nicked it that time. So as I worked my way back down the cable the white insulator fell off in 1 inch chunks. This continued for enough feet back down the cable that I condemned the entire roll.

One or more of those wires could've spontaneously cracked leading to an inductive caused arc. I notice the white one is likely where it started.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

This is 2 motor circuits 5T and 6T?

It looks thermal to me. Heat starting at a crimp, or possibly a few of them. I've seen similar damage a number of times.

3/8" is enough clearance for 480V to not arc, and it appears there is about that much space between wires in the failure area. Fuses on small motor circuits like that would not allow a single arcing event to be sustained long enough to cause that much damage. And I'd think someone would have noticed if it flashed over multiple times between fuse changes.

Isn't the wire from 6T2 burned off and touching the 6T1 wire? Is that what resulted in finding the failure?

 

[prc]

It looks like an arcing from 5T3. Is it a large motor with VCB? Was there any tripping during switching on from VCB induced surges?