Sustained 240V arc (w/ video)

[Rampage_Rick]

I've got an office-type building that's been vacant for the past 6 months. I guess they'd been using one of those 4800W construction heaters late last year. The heater is connected to about 50 feet of cabtire of appropriate size. Today it was discovered that there had been a fire of some sort at the plug/receptacle. The event was caught indirectly on a security camera about a week ago.

My observations:
- The heater is fairly new, standard 4800W 240V with factory 6-30P connector. The switch was in the "off" position.
- The receptacle is a standard rectangular Leviton 6-30R with a standard metal clamp in a 1" KO.
- About 50 feet of 6/3 cabtire run into the panel. (Hot+Hot+Ground) Cable had been coiled up for storage, but was still energized.
- A 30A 2-pole QBH-type bolt-on breaker (I believe it's C.E.B brand, possibly C-H, not Sylvania) The breaker had not tripped when I looked at it.

I can't figure out why it failed after such an extended period of time... The only possible answer I can come up with is perhaps some critter crawled inside the 6-30R and shorted the hots? The biggest hole is maybe 1/4"...

 

[the blur]

I'm guessing the resistance of the arc itself was high enough not to trip the breaker.... that combined with 50 feet of SJ.

Still doesn't make sense, as there was no load.

I'm curious as to what the others will say.

 

[ptonsparky]

That sure would have been a good test for an afci. Interesting.

 

[Strife]

I've got an office-type building that's been vacant for the past 6 months. I guess they'd been using one of those 4800W construction heaters late last year. The heater is connected to about 50 feet of cabtire of appropriate size. Today it was discovered that there had been a fire of some sort at the plug/receptacle. The event was caught indirectly on a security camera about a week ago.

My observations:
- The heater is fairly new, standard 4800W 240V with factory 6-30P connector. The switch was in the "off" position.
- The receptacle is a standard rectangular Leviton 6-30R with a standard metal clamp in a 1" KO.
- About 50 feet of 6/3 cabtire run into the panel. (Hot+Hot+Ground) Cable had been coiled up for storage, but was still energized.
- A 30A 2-pole QBH-type bolt-on breaker (I believe it's C.E.B brand, possibly C-H, not Sylvania) The breaker had not tripped when I looked at it.

I can't figure out why it failed after such an extended period of time... The only possible answer I can come up with is perhaps some critter crawled inside the 6-30R and shorted the hots? The biggest hole is maybe 1/4"...

Looking at the video footage, I'm looking at about 12 minutes of constant arcing.
It doesn't make sense, the damage shown (before and after) is NOT CONSISTENT with a sustained arc of over 10 minutes. If that strobing in the video was actually an arc, I seriously doubt you'd have a building to come to.
And sorry, from the damaged to the cord, the plug, and the carpet, I can't see the ash spread you described either.

 

[K8MHZ]

It looks like the sheath has been cut off one of the cables about a foot from the fire.

What's up with that, I wonder?

 

[ActionDave]

Looking at the video footage, I'm looking at about 12 minutes of constant arcing.
It doesn't make sense, the damage shown (before and after) is NOT CONSISTENT with a sustained arc of over 10 minutes. If that strobing in the video was actually an arc, I seriously doubt you'd have a building to come to.
And sorry, from the damaged to the cord, the plug, and the carpet, I can't see the ash spread you described either.

I would beg to differ. Arcs can go on and on and not set anything on fire.

I have watched a lot of fire videos. They always show the fire starting and then spreading to some near by combustibles. In the still on this video there are no easily combustible materials near by.

 

[stevebea]

That sure would have been a good test for an afci. Interesting.

Yeah, my thoughts exactly! Back in December we had a house fire that started with a brand new but faulty electric blanket and i couldnt help but wonder if that branch circuit was afci protected the outcome would have been different.

 

[Rampage_Rick]

I snapped some pics with a real camera:

It looks like the sheath has been cut off one of the cables about a foot from the fire.

What's up with that, I wonder?

I think that's just a scuff on the cable, plus the fact those pictures were taken with my phone. The only pre-existing damage I noted was a minor nick in the cabtire jacket, wrapped in electrical tape, about 6 feet before the receptacle. The insulation on the conductors wasn't damaged.

And sorry, from the damaged to the cord, the plug, and the carpet, I can't see the ash spread you described either.

Ash, and little spheres of molten copper embedded in the carpet:

 

[Jraef]

Looks to me as though the arc started in the receptacle, the damage to the cord cap appears to be incindental, i.e. from the heat of the arc in the receptacle. If I had to guess, I'd say there was a loose strand in the receptacle and maybe a little moisture inside (maybe the heater was there trying to dry out wet carpet?). An arc started on that loose strand touching ground or neutral, that set off that initial flash seen in the video. Then because there was a carbon trace in that box and probably still had moisture, it conducted, slowly at first but eventually into a full blown but high resistence arcing ground fault. No load necessary, the arc WAS the load. But just because it was arcing to ground and making a lot of light and some heat, it wasn't enough current flowing to trip the breaker or to make enough heat to ignite the thermoplastcs in the cord cap, the receptacle box, or even the carpet.

It's actually also a testament to UL94V, the testing requirements for plastics used in electrical components.

Cabtire... never heard that term in my life before, I've seen it used twice today! I had to look it up to satisfy my curiosity. Origianally a Canadian term, "Cabtyre" because a company originally developed the highly abrasion resistant rubber for the wheels (tyres) of horse drawn carriges (cabs) to make them quieter, but someone discovered it was the solution to the intense abrasion they were experiencing in mining cables that were using older vulcanized rubber for sheathing.