Now that AFCIs are required by NEC for most of the house, I've been wondering how 'required' they really are. I mean, how many people have put a nail through a cable? Well, 3 weeks ago, a neighbor helped me understand why they are needed.

At about 9pm, my neighbor who moved in the prior week, was outside screaming... couldn't figure out what was going on, a lost dog, something domestic. Finally, she got out the word 'Fire'. We called the fire department and helped her get out of the house along with her 2 year old and dog. The house had quite a bit of smoke, and visible flames from the basement. A fire extinguisher knocked down the the flames in the basement ceiling joists, and the fire department took care of anything remaining.

After the fire department called the 'all clear', it was obviously an electrical fire, started in the basement, catching the paper-faced insulation on fire, and the prior owner had burlap stretched across the ceiling for whatever reason. It probably looked nicer than just the old basement look. Some of the joists were quite charred, but it overall, not nearly as much damage as it could have been. The bundle of NM-B cables mid-basement were completely melted through, leaving only the copper wires intact. Upon first glance, everything seemed reasonably updated (especially for a 150 year old house)

A few days pass with insurance claims, Servpro cleanup, etc... it turns out the TV installer put a staple through a NM-B cable while installing the cable TV. The next time the clothes dryer was turned on, the minor short caused a small arc and heating... resulting in catching the paper insulation and burlap. Their insurance company is currently suing the TV company, and repairs have just begun.

Long story short, now I see the need for AFCIs. It's also a good reminder for everyone to ensure they have working smoke detectors! They luckily had one on the main floor, though it would have been better to have one downstairs too. They were all rattled, but everyone's safe.


I know it's not a question, but I thought the folks here might be interested in a bit of a story.

-Mike

 

Glad to hear everyone came out OK!

 

If this was an electric (240 volt) clothes dryer then it would not have been required to have an AFCI circuit breaker, even in new construction to the best of my knowledge.

 

That is correct! .................

 

Equally unclear if a AFCI would have even tripped. Now, there are current waveforms that will trip a AFCI and not a GFCI, from memory these are fast transient waveforms (arcs).

Curious that the fire started only after the load was switched on.

 

Probably hit the neutral, so no current until it was turned on.

 

I don't find that curious at all. No current would flow in any of the conductors till the dryer was turned on. I suspect the staple hit one of the hot legs and decreased the size of that conductor substantially, but didn't cut completey through the conductor. When the dryer was turned on and load of the heating element was applied, the area of the conductor that was reduced in size turned into a heating element and glowed red and started the fire. The breaker probably did eventually trip after the fire was started.

 

Yes, that's entirely plausible. When I wrote my comment I was imagining a RG6 coax shield taking some of the line current to ground, but not enough to trip the 30A breaker. From Mike's first post, it sounds like the fire inspector reported a definitive analysis on the cause.

 

My understanding is that it was a stackable washer/dryer on a 15/20A circuit. But alas, Furd is correct that it probably wouldn't have been AFCI protected on a 30A dryer circuit anyway.

It seems that the branch circuit breaker tripped first, but shortly thereafter the main tripped too.

I'm of course now kicking myself for not taking pictures while I was there... though I figured it a bit uncaring to ask to see the 'devastation' the day after

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Another member here PMed me to say though this was an 'obvious' installation mistake, it goes to show that you really do need to know what you're doing when you're working on the electrical system in your house. Just because you installed it and it works, doesn't mean it's correct. Be careful out there!

 

I think the "obvious installation mistake" was made by the cable TV installer. Those people are told to install in the quickest way possible and they take no extra steps beyond getting the signal to the television. I personally would not let one past the demarcation point on the outside of my house.