Wierd Plasma cutter effect

[ajjelly]

So I was using a plasma cutter to cut off some shock mounts on a rear D44 axle. I had the ground lead attached to the frame, but I was cutting on the axle. The space was kind of tight so I was had to make a couple passes with the plasma torch to get it all cut off. When I was finished I took off my gloves and reached across the frame to pick up a tool. In the process my arm brushed accross my braided stainless steel brake hose that connects the hard brake line on the frame to the "t" block on the axle. When I did this it was so hot I got a bad burn. The strange thing is that none of the hard steel brake lines on the axle were hot and they were even closer to where I was cutting. The axle housing wasn't that hot, and the "t" block that the hose hookes into wasn't hot. I have 2 questions:


1. Why would the stainless brake line get so hot when nothing else was that hot? (could it have been from current passing from the ground lead on the frame over the stainless brake line to where I was cutting on the axle)

2. Is my brake line hosed (pun intended)?

[ChicksDigWagons]

There is a reason you should ground as close to your work area as possible...

[Bruce T]

Yes, current was passing through it. Braided brake lines are like having a shitload of tiny wires trying to carry current and the indivual wires were probably overloaded. You would think it would have chose a differend path but that was probably closest/easiest. You probably boiled the brake fluid in it, although I dont know if thats a big deal or not.

[Travis Waldher]

well.. look at it like this.

you have rubber/poly or some other nonconductive bushing between your links/springs. Correct?

If coil springs - I bet there was a rubber/poly/whatever bushing between the spring and the spring bucket.

so electrically, your axle is somewhat isolated... except for that steel braided brake line.

[crashinaz]

The story is this... Your axle was completely isolated through bushings. The brakeline was the only conductor between the frame and the axle... The reason why the braided line was hot and the hardlines were not is this... Stainless usually has high concentrations of chromium and nickel, BOTH of which substantially increase the impedance or electrical resistance of the material... This resistance againt the flow of electrons generates heat, so there you have it :D.

[Travis Waldher]

well... I would sooner bet his hardlines weren't hot because his stainless brakeline attached to a hardline that had a hard and conductive mount to the axle housing.

The electricity never passed through the majority of the hardlines to cause them to heat up.

[crashinaz]

Doesn't really matter... Hardlines were steel, brackets were steel, braided line was stainless... Neither of the mild steels have an issue conducting electricity. You could throw your welder ground on a hard brakeline and it wouldn't heat up... Not that it even matters, as most hardlines are bolted to and laying against the chassis or axle anyway.

[FJ_Dave]

It's the same reason the tongstun (sp) in a light bulb heats up and glows white and your supply/ground wires don't...... resistance = heat.

It sounds like to avoid further worm bites (I.E. self inflicted injuries) you should pack up your plazma cutter and ship it to me.

[ajjelly]

Thanks, for the replies guys. Guess we have that mistery solved. What do you all think about the brake lines. Are they toast or what. I don't know what the effective heat range of the fexible brake lines is.

[Jeepnford]

I would probably replace the flex hose. I'm sure it's not designed for that much heat.
Don't feel too bad. My welding instructor told stories of people welding on truck frames after attaching the ground clamp to a wheel lug. Welded the bearings together.

[OCM]

This happened to me a few years back. Grounded my welder to the frame and welded up some stuff, then finished up something on the axle housing without moving the ground. By the time I HEARD the stainless brake line crackling, it was RED hot. The lines are teflon lined and I would guess anything like this would damage them.
Karl.

[Trailer Guy]

This happened to a buddy of mine a few years ago. It happened to a front brake line, actually got hot enough that it caught on fire. Man, what a mess, but it was really damn funny.

[Goat]

Fawk, the brake lines...I would be more worried about the plasma cutter. I don't know how long you were cutting but I bet you put a nice beating on your pilot arc resistor.

[rusted]

It *is* a straight up resistance issue. Cross sectional area primarily. Braided line probably has 1/20th the CSA of the hard line. And a plasma runs for that about what 20amps?

Even between the last hard mount and the connection btw the hardline and the braided, the hardline wouldn't get hot off of that.