Log Truck vs 7200 V

[cranky108]

What was the estimated operating time for this fault?

 

[jraef]

I found out about tire rubber conductivity a few years ago when working on a 2300V motor controller for a rubber mixer at a retreading plant. The ignorant workers in the plant were having trouble with the controller overheating on hot days, so they propped the door open and put a fan up to blow air into the cabinet. The carbon black of course followed the air, and although it was a vacuum starter so there was no immediate ignition source, eventually the dust built up on the conductive surfaces and KABOOM! The place lit up like a fireworks factory for a second or two. Luckily nobody was killed.

Still, the resistance across the tires must have been fairly high because the fault would have had to continue for a while it seems to cause that much damage. Unless a brief spark ignited dry grass or something else. It almost looks as though the fire started at the cab and worked backwards. Maybe a surge through the winch motor, igniting the wire insulation, then rubber hoses etc?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[dpc]

cranky108,

I don't know, but I'm guessing several minutes, at least. A 100 ohm fault impedance would be 72 amps, which probably isn't going to trip a distribution feeder ground relay.

72 A at 7200 V is about 500 kW just to put it in perspective.

I'd guess the fault impedance started pretty high, then went down as the fire progressed. But there would still be the earth return to factor in.

 

[davidbeach]

If dpc's 72A is in the right ball park, I would expect that the fault was cleared when enough smoke created a phase-phase flash over in the lines above.

 

[Compositepro]

There was a fuel fire. The fuel tanks are completely gone. They must have been aluminum.

 

[cranky108]

I just wanted to know as I have a standard debate with the distribution guys. They want the ground set higher, and I say lower.
There isen't a best answer, just the debate.

 

[davidbeach]

There isen't a best answer...

That's the heart of a lot of protection work. That's why the answer to all protection questions is "It depends."

 

[jraef]

When the truck melts, should have been lower. When the entire county gets cut off because a bird crapped on an insulator, should have been higher.

You're right, there is no answer, only the debate.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

Guys have you never seen a tire fire? All you would need is a one inch section of a tire ignited and the rest will burn. It would probably take that current all of 1 second to have that one inch section burning. And on all 14 tires. Tire fires are hot. Often fire departments cannot put out a pile of them at all. So a few duals get cranking, the fuel tanks boil over, and that's it for Mr Truck.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Lest we've forgotten this is an image about 2 seconds into an incident.

Keith Cress
kcress -

http://www.flaminsystems.com