Electrical (?) house fire

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Old 09-02-20, 05:38 PM
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Electrical (?) house fire

I have an 85 yo single family house. It was completely rehabbed, including wiring 3 years ago.
Four months ago there was a house fire in the attic where the roof meets the wall. The insurance company sent a fire origin and electrical engineer investigator who determined that a too tight wire staple was most likely the cause. The insurance company subrogated to the general contractor's insurance.
Right after repairs were made there was another fire about 3 feet from the previous one, again where the roof meets the wall. However, this time there were no wires in the immediate area. There is a wire mesh lath in stucco wall under the siding and I'm wondering if this became energized. I know this can happen. It seems too much for coincidence that there are different causes.
The county fire investigator also mentioned this.
The insurance company will again send investors so we'll see what happens.

Any thoughts?
 

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09-03-20, 01:02 PM
cwbuff
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A very interesting topic. Anyone who has installed staples has probably missed the staple and hit the cable and left wondering if you caused any damage. After reading some conflicting stuff on the internet I decided to do an unscientific experiment. I took a piece of NM-B 14-3 and stapled it to a 2X4. I used a plastic coated staple and a metal one. I hammered the staples as hard as I could and far as they would go. I also gave the cable itself 5 or 6 really good whacks with the hammer. Then I stripped the jacket and examined the conductors. The wires under the plastic coated staple were barely marked as were the wires I pounded where there was no staple. The jacket under the metal staple was noticeably deformed but not split. The insulation on the three insulated wires was also noticeably deformed but not split. There was no bare conductor exposed. The ground wire was unmarked.

Here's a semi wishy washy article on the topic https://www.researchgate.net/publication/288833845_Fires_Originating_in_Branch-Circuit_NM_Cables_due_to_Installation_Damage


 
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Old 09-02-20, 07:22 PM
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The staple would have to be installed so tight that it damages both the outside jacket and the inside wire insulation. This could only be done with metal wire staples with no plastic. The damage would also have to be so perfect that it would not cause a short and allow arcing. Was this circuit on an AFCI breaker?

I can see a siding nail hitting a cable and energizing the mesh depending on the length of the nails used.
 
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Old 09-03-20, 08:10 AM
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Any thoughts?
Yes, I have a few thoughts. I notice that the investigators "determined that a too tight wire staple was most likely the cause." That statement tells me the investigators really weren't sure what caused the first fire and were just speculating because they didn't know. This really isn't so unusual. Unfortunately investigators frequently look to the electrical system when they cannot definitively determine the cause of a fire.
 
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Old 09-03-20, 09:07 AM
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I kinda hate to get into this conversation because I'd rather be there to look at it. I was a fire fighter for 12 years & I took some fire causes/arson investigation courses through LSU back in the 70's & 80's.
But, the first thing I would look for once my leads took me to that specific electrical issue is; was there discoloration &/arcing on the wires. If so, that's just another clue to send me in specific direction in my investigation. If not, again a clue as to which direction my investigation went. If a staple shorted the wire, or caused a heat source, there would be evidence... I'd look for discoloration, scratches etc, for other clues. The fire investigator should have been able to definitely rule in or out the wiring issue/cause.

I suspect there was sufficient evidence in the end or the contractors attorney(s) would have countered with their expert..... at least I hope they had their expert to examine the evidence.
 
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Old 09-03-20, 12:12 PM
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Thanks for the replies. I should say "most likely " are my words. I haven't seen a report yet. After the fire in april this year the investigator said there was evidence of arcing when we were at the house. He took the wire away for analysis.
The arcing was on top of a beam where a staple might have been, but was not found..... maybe cleaned up with debris.
The other staples were not plastic coated.
I was told by the electrician that there were no arc fault breakrs and there shoul have been since the hose wiring was done in 2017. It passed code inspection then. I'm in New Jersey.
 
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Old 09-03-20, 01:02 PM
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A very interesting topic. Anyone who has installed staples has probably missed the staple and hit the cable and left wondering if you caused any damage. After reading some conflicting stuff on the internet I decided to do an unscientific experiment. I took a piece of NM-B 14-3 and stapled it to a 2X4. I used a plastic coated staple and a metal one. I hammered the staples as hard as I could and far as they would go. I also gave the cable itself 5 or 6 really good whacks with the hammer. Then I stripped the jacket and examined the conductors. The wires under the plastic coated staple were barely marked as were the wires I pounded where there was no staple. The jacket under the metal staple was noticeably deformed but not split. The insulation on the three insulated wires was also noticeably deformed but not split. There was no bare conductor exposed. The ground wire was unmarked.

Here's a semi wishy washy article on the topic https://www.researchgate.net/publication/288833845_Fires_Originating_in_Branch-Circuit_NM_Cables_due_to_Installation_Damage


 
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Old 09-03-20, 05:36 PM
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Was there any electrical cable or anything else electric where the second fire started?
 
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Old 09-03-20, 07:39 PM
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I find CWBuff's post in line with my thinking. I would think the electrician would have to really wail on the staple with the hammer in order to damage Romex with a staple.
 
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Old 09-04-20, 10:48 AM
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"Was there any electrical cable or anything else electric where the second fire started?"
The second fire was in the attic above a newly installed drywall ceiling. There are no outlets there. The only electric usage was on the second floor by a contractor, saw and compressor.
 
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Old 09-05-20, 09:15 AM
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The arcing was on top of a beam where a staple might have been, but was not found..... maybe cleaned up with debris.
Sounds as if the damage was quite extensive for the beam containing the alleged staple that caused the arcing to be totally gone. If the damage was this extensive I question how arcing cables were not totally destroyed beyond the point of showing an earlier arcing condition. If the beam remained, I question what happened to the staple. Staples driven that far into a wood beam or joist do not simply fall out and disappear.
 
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Old 09-05-20, 09:50 AM
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Here are some pictures of the wire that was under the metal staple. I pounded these staples about as hard as I could with a 16 oz hammer. What appears to be exposed conductor on the white wire is actually pinched insulation. It was not breached.

 
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  #12  
Old 09-06-20, 05:25 AM
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Since you said there were no wires or cables where the second fire started, there is no point talking about an alleged staple.

Did the fire start within 24 hours of when the contractor packed up his electrical gear and departed?

The insurance adjuster already made his report. If you still want to pursue it then you might investigate how the metal lath screen got energized if it did. I will leave the methodology to you and/or any other experts you wish to engage or who wish to chime in here. If something energized came in loose contact with something grounded, a small current flow is enough to cause heating that could ignite something else.



 
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Old 09-06-20, 02:29 PM
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I was helping a friend wire his new home. His dad was also helping. His dad actually created a few shorts by over driving staples. It's dependent on many things. This wiring was in a cold house so I'm sure the insulation cracked. These were actual dead shorts..... not iffy.

I was working on a "flip". On old home that was being rehabbed. Added a large dormer to the second floor. I noticed the "crew" using lots of airgun nails. Didn't notice any over length nails. We wired the entire second floor. After we wired the second floor they added the outside siding. I went back after the insulation installation just to check up and found that they had used nails that were longer than 1/2 of a stud. I'm guessing 3-1/2" nails for the siding.

I flipped out on the GC. He didn't seem to care much. I was anticipating shorts. I checked each circuit for a short but no way to check to ground. All checked ok. All those circuits were AFI protected. Luckily we had no problems but could have with a nail that long driven thru a wire inside a stud.
 
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Old 09-06-20, 02:49 PM
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"Since you said there were no wires or cables where the second fire started, there is no point talking about an alleged staple.

Did the fire start within 24 hours of when the contractor packed up his electrical gear and departed?"

The discussion about overdriven staples came from the first fire where there was a wire with evidence of arcing on top of a beam. I didn't see any wires were the second fire started but that was only a few days ago and the insurance investors haven't seen it yet.
The second fire started while the contractors were there.


 
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Old 09-06-20, 02:53 PM
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"Here are some pictures of the wire that was under the metal staple. I pounded these staples about as hard as I could with a 16 oz hammer. What appears to be exposed conductor on the white wire is actually pinched insulation. It was not breached."
I read somewhere or was told that compression of the wire changed the insulating property of the wire. This gets worse over time and can lead to arcing. Even years later.
 
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Old 09-06-20, 03:07 PM
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Sure. If the wiring stays compressed the insulation will thin out.
 
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