If they were snow chains, wouldn't they design them so they didn't drag on the ground? If these ones were hitting the pavement, it makes me think they are grounding chains.

 

They're not supposed to drag. They come with very long chains to accomodate different sized vehicles. Once they are installed, the chains are to be cut so they just barely stay off the pavement. Sometimes they're installed wrong, sometimes the pivot arms work loose, etc. Either way, you don't ground anything (especially a vehicle) by using something that creates random sparks near the fuel source.

 

If you go to the site I mentioned earlier insta-chain.com , this will show you what the traction chains look like.

If they do not look like this, you saw something else.

 

nope.... For one.... we don't get enough snow for this city to invest in the automatic snow chains....
It has to be static drop chains
http://www.wilsonantenna.com/generalq.htm read number 5

 

I don't buy the idea that any Fire Truck has anything installed incorrectly. Those guys take enormous pride in their rigs.

 

Just to set the record straight, dragging a chain under a truck does nothing for grounding or static dissipation. As for automatic snow chains, I have no idea.

 

YES we do!!! However, some depts (especially municipal depts) do not have the luxury of having "top notch" mechanics work on the apparatus. So some "minor" things may not work as well as they should. Also, whos to say it wasn't something that recently happened (like the swing out of adjustment) that they haven't had fixed yet. I'm not trying to be a smart *** here, but I'm almost positive it was a insta-chain that just happened to be dragging for whatever reason.

Valid point. I must say though that I have never seen a chain used as a grounder. What I have seen are thick rubber straps with very thin strips of metal made into them that hang from the rear bumper or DOT bars of trucks. They do not create sparks either.

 

The best part about this thread is that only itisedt knows for sure what he saw, and the rest of us are just guessing.

That being said, I'll bet a shiny new quarter that they were for static.

 

I'm absolutely 100% sure that they are onspot chains. I'm a FF/EMT as well as a salt truck driver. We have them on every ambulance, firetruck, and dumptruck. We don't get a lot of snow here either(3-5 snows of usually 4 inches or less. Maybe one big one of 5-10 inches if we're lucky), but when lives are on the line you want to be able to get around no matter what.

They are air operated and when they are in the up position they tuck up into the frame, in the center of the vehicle, almost or actually in front of the wheels depending on the tire size. Depending on the tire size and if you have DRW or not, the chains will just barely touch the ground when not moving. When you are moving, they often do touch the ground. That comes from a department that has way too much money and has top notch service.

It's not a grounding chain or anything like that. We've never needed one (nor has any of the other slew of departments around here) and if we did, I'm sure that static whips like those found on airplanes whould be used. We would never use anything that intentionally drags the ground as the sparks could set a fire at an auto accident or something. Also, our big ladder truck that weighs 80K empty does not have onspots as it's so heavy that it either doesn't need them or can't use them. It has nothing else touching the ground other than the 10 radial tires.

Mike

Edit: On the cost of onspots, a new ambulance is about 100-160K. A new firetruck is about 250K(for hella stripped)-1 million for decked out ariel truck. Onspots are pocket change. If it saves one collision or wreck of a truck and the lives of the men on board, it's 126.2% worth the cost.

Edit2: Check out this site Go to Onspot's FAQ. They are only like 1500 per set for a truck with airbrakes, and 2K without airbrakes. They do work in reverse from the site and from personal experience.

 

All of our fire trucks and ambulances have drop chains, and they look nothing like the insta-chain system. They all drag on the ground. I find it hard to believe that ALL the chain systems are broken at one time in the department. I believe its for static discharge.

 

Guys it is not for static discharge. Time for an electrical lesson.

The body and frame are the ground plane for a vehicle and is isolated from the ground by the tires. This means that ground on today cars are never zero volts. It will always have some pontential on it. This is not static but a known potential. With this known potential on ground the engineer is able to design the electrical systems around this.

All ground really means is something that has less voltage than the source. I have seen grounds at almost true 0 and grounds as high as 3 volts. As long as the source is higher you will have current flow. The absolute worse thing you could do is have a ground that is not stable.

 

Quote:
The absolute worse thing you could do is have a ground that is not stable.
~~~~~~~~~~~~~~~~~~~~~~~

Terry, Don't you live in an earthquake zone ?

 

Good one Dennis

 

The idea of grounding chains/straps is to drain off any potential between the truck and the earth/ground - this can build up because, as was stated before, the rubber tires are insulators. If the tires are the only part of the truck which touches the ground, then a static potential can build up and you can draw a nice arc when you do connect the truck to ground using a conductor. Ask anyone who delivers oil or gasoline - those folks have to be very careful!

That said, I'd bet my money that what was seen was spot chains...

 

Most fire trucks/squads come from the factory with the onspots, most of our newer apparatus have onspots. So I am almost possitive thats what it was.