What are the chains I see hanging underneath fire trucks (maybe other trucks as well?) near the rear axle. They’re short lengths of chain, hanging free, well short of touching the roadway. They’re not positioned close enough to the wheels to be snow chains, even in storage.
I believe it’s so you can hear them coming…but I knwo at night they put on quite a show with how much they spark!
Obviously hear them coming without the sirens.
I think The Master anwered this one in his books…
It is to dissipate static electricity.
Hmmm, if they were long enough to touch the road (at least some of the time), then I’d guess they were there to electrically ground the truck, so there’s no nasty static electricity build-up.
I’ve seen static discharge chains on a variety of utility and other trucks.
Seem possible for what you saw?
There can be enough static charge on the tanker to blow the whole thing to “kingome come” if the operator tries to empty his tanker into the service station,or depot,storage tanks.
Touching the grounded filler pipes on the storage tank with the discharge nozzle
could create an arc at the point of flow.
The chain-or,often, just rubber strap prevents that from happening!
EZ
I thought they looked too short to touch the roadway, or else I would have suspected the static discharge explanation. But it’s not like I’ve crawled under one to get a close look, so maybe they can touch the road after all.
Certainly sounds like the logical explanation.
I’m not convinced of the effectiveness of grounding a vehicle by a conductive chain or strap, even when it’s in contact with the road, which the chains in the OP weren’t. Tyres aren’t a particularly good insulator (must be the carbon black content or something) and any static built up by the car relative to the road will be dissipated through the roadwheels. If your petrol (gasoline) powered vehicle really was a travelling mass of static charge, then you’d be in serious trouble when you came to refuel. Contact with an earthed pump nozzle to a charged car is going to produce a spark.
In Britain in the 1980s there was a craze for hanging conductive rubber straps from the bumpers (fenders) of cars to “prevent static buildup and travel-sickness.” You can still buy them if you really want.
That said, I used to suffer horribly with static electricity with one particular Citroen BX I had. This would usually manifest itself as a painful little lightning bolt from finger to door handle as I got out of the car, and was worse in the summer, or when I was stressed. It was making me nervous every time I expected the inevitable shock. Eventually I worked it out - it wasn’t the car that was charged up, it was me. I was sitting in an insulating seat, with rubber soled shoes and pedal covers and a plastic steering wheel. And I had the car’s fan blowing full over my face (no aircon), charging my dry hair up like a van der Graaf generator. I’d then get out and touch a metal car at earth potential. Ouch! I got just as big a shock if I touched an earthed lamp post, so it was definitely me, not the car.
I eventually implemented a workaround. I found that, even when fully charged, I could close the car door safely if I put a leather key fob between my finger and the door. The moisture content of the leather was enough to leach away any static charge in my body, but slowly enough such that I didn’t feel a shock.
According to the guys at cartalk, one of the reasons people get shocked a lot more getting out of their cars these days is that many tires are in fact a lot better insulators than they used to be. I’m not sure if this applies only to tires typically sold in the US or if this is a worldwide trend.
Most people with static buildup on their cars dissipate the charge through themselves when they get out of the car and shut the door. Also, spark alone isn’t enough to cause a fire. You need spark, fuel, and air, all at the same time, and all in roughly the right amount. However, there are still somewhere in the neighborhood of a half dozen pump fires per year (in the US) caused by static discharge. It does happen, though some of these are (like you and the Citroen) caused by the driver scooting across the seat.
Other possibilities for the OP:
My pickup truck’s spare tire is attached to a chain which is winched up underneath the truck. Were the chains in an area big enough to possibly contain spare tires?
I’m also wondering if the chains were on winches that could be used to pull vehicles out of a ditch.
I understand why a fuel truck needs grounding, but a fire truck? Unless you’re re-creating Farenheit 451, there’s nothing inflammable in a fire truck other than its own engine fuel. Just a lot of water.
And, in the case of a ladder truck, grounding could be dangerous or at least rather festive if the ladder came in contact with live power lines.
This site also mentions the use of “automatic tire chains” for ice and snow:
http://answers.google.com/answers/threadview?id=548666
Ummmm…that would seem, to me, to reduce the amount people get shocked. Granted, my physics and circuits classes were a few years ago, but IIRC, the reason you get shocked is because you have a build up of potential, and it can’t get to ground through you, but when you touch the car, it completes a connection to ground and arcs to the car, thus causing the shock. If the car was as well, or better, insulated than you, you wouldn’t get a shock, because the electricity would not go to the car.
WAG1: maybe pumping tons of water can create a static build-up, like an economy-sized Van De Graf generator.
slightly less wild AG : fire trucks are often going to be taken into emergency/disaster type situations, where there might be, say, a ruptured gas line or volatile spilled chemicals. A large spark could be particularly bad in that situation. And since fire trucks are in that situation relatively more often than a standard passenger car, it’s worth grounding the fire trucks.
Regarding getting shocked when getting out of a car: My solution is to touch the frame before my feet hit the ground. Usually accomplished by wrapping my hand around the frame. Much less yelping results, unless I forget to do that.
Another guess, getting away from the speculation about static electricity…even if the chains were not on winches, perhaps they were attached to anchor-points on the chassis to provide a convenient point to connect a tow-cable, if and when the truck needs to move a large load.
This is correct. They’re almost certainly On Spot automatic tire chains. They’re activated by a dashboard switch and it spins the lengths of chain under the tires. You get the same effect as tire chains, but you don’t need to stop to take them on/off. They’re very common on large emergency vehicles- ambulances, fire engines, etc.
St. Urho
Paramedic
The water should be at the same earthy potential as the fire truck, so it should be doubly good at damping down static.
But I do recall some very nasty problems with static discharge caused by CO2 fire extinguishers - they would sometimes cause more fires than they put out. But the nozzles are designed not to create static now, so don’t let that put you off using one when necessary…
There are two ways that you can get a static shock from a car.
First, is basically what you describe. You build up a charge on your body, which is then discharged through the car to ground when you get out of the car. If the car is better insulated you still might get a shock as the charges equalize between your body and the car, but it probably would be reduced.
Second, is that the car builds up a charge between itself and ground. When you get out of the car, when you go to shut the door you are completing the path between the car and ground, so all of the charge in the car dissipates through you. This is the type of shock that is greatly reduced or elminated through chains, braid wire, etc. hanging off of the car.
Oh yeah, they retract out of the way when not in use, that’s why it appears they’re not positioned close enough the the wheel to be snow chains. I’ve never heard of an engine dragging chains to be grounded.
Ahhhh, I didn’t think of the car discharging static through a person. :smack: