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Old 09-19-2017, 06:34 PM   #1
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Default Weird Electrical Short

I'm an avionics systems technician so my main field is aircraft electrical systems. We had a breaker popping and once I isolated where the short was down to a single segment, I went along the wire bundle to see if I could find a break or the area shorting.

I narrowed it down to a short 2 foot section, I opened up the wire bundle only to find that the wire was very much intact. This is not a shielded wire and there is signs of a bit of abrasion on the entire wire bundle on the airframe. The problem is the suspect wire is not damaged as far as I can see after a thorough cleaning and inspection.

What I'm confused about is how the wire is shorting when the insulation is good and the wire shows no visible signs of damage? It's really got me thinking.... Just wondering if anyone else has come across such an issue.
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Old 09-19-2017, 06:50 PM   #2
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I'm curious to know how you isolated it down to a 2' section?
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Old 09-19-2017, 06:56 PM   #3
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If you did narrow it down to the 2' section, the problem is there and you are not seeing it.

I think you didn't actually narrow it down to that section. You just thought you did.

Maybe the fault was in the previous section and you cleared it when you disconnected the last section.

Troubleshooting involves some assumptions and sometimes an assumption is wrong.

Last edited by 220/221; 09-19-2017 at 06:58 PM.
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Old 09-19-2017, 07:15 PM   #4
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If you did narrow it down to the 2' section, the problem is there and you are not seeing it.

I think you didn't actually narrow it down to that section. You just thought you did.

Maybe the fault was in the previous section and you cleared it when you disconnected the last section.

Troubleshooting involves some assumptions and sometimes an assumption is wrong.
One would think that, yes, but while having the multimeter connected for resistance to ground. We proceeded to move that section of wire between two clamps and we were able to remove the short and then reproduce it by moving the bundle back to its original position. I'm very confident in troubleshooting. This isn't the first time I've kind of seen this. Saw it once where insulation was worn down on a shielded wire and it grounded itself through the insulation. Once I relieved pressure off the wire bundle the snag went away.

I did notices some metal flakes from previous metalwork done 4-5 years ago. The term is coined SWARF, and it's the only thing I can think of at the moment causing this issue. They were very fine though, like glitter. At This point I'm looking for something to else to explain this issue. I've determined that the wire is in good shape and I'm ready to proceed in securing the bundle and prevent any possibility for further chafe.
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Old 09-19-2017, 07:31 PM   #5
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AC or DC?
Is it possible insulation is worn so thin that voltage is jumping across but your eye can't see the insulation wear? What voltage is this?
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Old 09-19-2017, 07:36 PM   #6
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AC or DC?
Is it possible insulation is worn so thin that voltage is jumping across but your eye can't see the insulation wear? What voltage is this?
Most aircraft typically have 24 volts DC battery but for AC typically 208Y120 400 HZ ( useally run from main engine generator or APU )

so with high HZ it possiblty it can make a light flashover on it.
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Old 09-19-2017, 07:49 PM   #7
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It's not that unusual for insulation to be compromised in a way that you can't spot with a visual inspection.

I'd use an insulation resistance tester anyway just to confirm your theory. If you can disconnect and isolate the wires in question, you can test at 50v (double system's operating voltage) and see what you see. (You could test at a higher voltage and get a helper to run their fingers down the wire to pinpoint the compromised spot )
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Old 09-19-2017, 07:51 PM   #8
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So you're t-shooting an aircraft wiring short?

Here I thought you were an avionics tech, t-shooting home wiring.


You got to give us details in the beginning.
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Old 09-19-2017, 08:15 PM   #9
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Quote:
Originally Posted by FuriousSquirrel View Post

I did notices some metal flakes from previous metalwork done 4-5 years ago. The term is coined SWARF, and it's the only thing I can think of at the moment causing this issue. At This point I'm looking for something to else to explain this issue. I've determined that the wire is in good shape and I'm ready to proceed in securing the bundle and prevent any possibility for further chafe.
I'm no avionics tech ... but you can throw as much SWARF on pretty much any cable .... and all that will happen is there will be SWARF on the cable.... unless the jacket on the cable is compromised.


How did you determine that the wire is in good shape, when there is a short in it ???
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Old 09-19-2017, 08:27 PM   #10
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Time for either the megger or the tick tracer.
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Old 09-19-2017, 08:32 PM   #11
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Do you work for an airline or a company that services commercial aircraft? Is so, would you please share which airlines y'all service? Thanks.
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Old 09-19-2017, 08:33 PM   #12
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If it's in an airplane, throw the whole harness in the dumpster
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Old 09-19-2017, 08:35 PM   #13
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Do you work for an airline or a company that services commercial aircraft? Is so, would you please share which airlines y'all service? Thanks.
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Old 09-19-2017, 08:46 PM   #14
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If it's in an airplane, throw the whole harness in the dumpster
Ya that is correct and they do not want anything happened while in flight that is the last thing they need to dealt with it.

FAA is very strict on this and they have specific way to fix it.
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Old 09-19-2017, 09:55 PM   #15
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Originally Posted by FuriousSquirrel View Post
One would think that, yes, but while having the multimeter connected for resistance to ground. We proceeded to move that section of wire between two clamps and we were able to remove the short and then reproduce it by moving the bundle back to its original position. I'm very confident in troubleshooting. This isn't the first time I've kind of seen this. Saw it once where insulation was worn down on a shielded wire and it grounded itself through the insulation. Once I relieved pressure off the wire bundle the snag went away.

I did notices some metal flakes from previous metalwork done 4-5 years ago. The term is coined SWARF, and it's the only thing I can think of at the moment causing this issue. They were very fine though, like glitter. At This point I'm looking for something to else to explain this issue. I've determined that the wire is in good shape and I'm ready to proceed in securing the bundle and prevent any possibility for further chafe.
You determined that the wire is in good shape how ?
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Old 09-19-2017, 10:04 PM   #16
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Default Weird Electrical Short

If you could re-produce the problem just by moving the bundle, GET RID OF THAT BUNDLE!!


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Old 09-19-2017, 10:23 PM   #17
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I'll definitely do further testing tomorrow. These aircraft have been around for 50 years with zero support. It does amaze me that they still fly :s cars don't do that well without support. Either way I'll take the merger to it tomorrow.

As much as I would love to scrap the bundle, it's not feasible due to the design and support equipment I got to work with.

As for the plane and people I work for, i can't divulge that information. Let's just say the planes will be finding their retirement home soon enough. Hopefully no delays in that respect. :/
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Old 09-19-2017, 10:25 PM   #18
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Megger * not merger, damn phone.
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Old 09-19-2017, 10:31 PM   #19
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Do you work for an airline or a company that services commercial aircraft? Is so, would you please share which airlines y'all service? Thanks.
No worries I do not work for a commercial company or an airline.
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Old 09-19-2017, 10:33 PM   #20
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I will bet that the aircraft manufacturer would recommend replacing the wiring loom !


And the customers would be more at ease !
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