A 12V shock hazard?

 

A 12V shock hazard?

oh!
i was talking about a general case;
like mains supply.

 

Hello everyone, I didnt realize how old this post is...lol. Anyhow, I found here while looked up a question on line asking... "is it ok to use a toggle switch as a ground?", for a kill switch I ran to my car months ago. I thought instead of running a fuse switch and a toggle switch, I'll just run the switch to the ground source, and that it would just be safer in case the switch ever breaks etc.. I ran into this post and was glad to see "thingmaker"s reply and i'll take his word for it with 5k plus posts, and I just dont see why not to use a ground source instead of a power source on most auto toggle switches. I am not a elec. engineer etc... only the wiring ive always done is to my vehicles etc...

To add to this..... I had a toggle switch as a kill switch years ago on a hot rod that was wired to the live wire of the ignition. The toggle switch fell apart after years of use and while I was driving the car, it died on me and then I started smelling smoke and saw smoke! Fortunately I thought of the toggle switch and It Was the toggle switch falling apart, fortunately I was able to reach the wires and hold them with my fingers till I pulled over.

Thanks, Bill

 

At one time everything on an automobile was switched on the -ve side, the +ve was taken to chassis instead of -ve.
Max.

 

there actually is a reason for disconnecting the ground terminal of a car battery first, if you touch anything with your wrench while disconnecting the hot side of the battery first, there will be a very interesting, possably hazardous shower of sparks and molten metal. f you touch anything while disconnecting the ground side, nothing happens.

 

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just a little off topic, but speaking of car batteries...when jumpstarting, yes, it's supposedly safer to connect the ground wire to chassis last and remove first because the spark is further away from the battery, and on the rare occasion you might get some hydrogen gas from the battery, it won't light off... though, I can tell you because I've actually lit hydrogen a few times as training exercises at work, (i'm a fireman) you'd need alot. alot more than a car battery makes when charging. it dissipates VERY quickly.
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Certainly a small amount of hydrogen is not dangerous in a open space, but in a closed space, such as inside the battery where water is decomposed into hydrogen and oxygen (in the ratio to make a perfect explosive mixture), it can cause the battery to explode (and an explosion of a battery containing sulfuric acid is not something you want to be near). Here's a few pictures of such explosions.

One evening as I was leaving work, I hear a small explosive sound coming from the parking lot. It turns out a couple people where trying to jump-start a car and the battery exploded. Luckily, neither of them were injured by the explosion or the acid but it was scary.

 

So why does Ford Motor Company and Navistar place their switches on the ground side of the circuit?????

 

MOD Note: This thread should probably be closed because it has to do with automotive mods (not to mention being a nearly decade old thread). As long as the discussion remains conceptual regarding switch location and technical issues associated with it, I don't see a reason to close the thread (another mod might disagree).

Depending on the details of the circuit, putting the switch in the "hot" side or on the "ground" side might make a difference or might be irrelevant. It is often more convenient to switch the ground because you may only need to run one wire to the switch and then ground the other side locally. This is why it was done this way on much of the electrics on the F-15 and on many other aircraft as well. But you have to take care to consider sneak paths through the circuit that can find their way to ground via another circuit. This is also possible when the hot is switched, but is less likely. Another consideration is failure modes. On light aircraft the ignition (mag switch) grounds the magneto to kill the engine (you actually stop it by starving it of fuel before turning off the mags, but that's a separate issue). If the switch or the wiring fails then it will likely fail as an open and the engine will run (or, more to the point, keep running if the failure happens in flight). This is a good thing. If it fails on the ground while the engine is stopped, the consequence is a hot mag which means that if the prop is pulled through by hand the engine could fire off, at least momentarily (i.e., long enough to remove an arm or head). This can be mitigated by always treating a prop as though it is ready to come to life at all times, which far easier to do than dealing with an engine that dies because a wire frayed in two at a switch.

 

If it fails on the ground while the engine is stopped, the consequence is a hot mag which means that if the prop is pulled through by hand the engine could fire off, at least momentarily (i.e., long enough to remove an arm or head). This can be mitigated by always treating a prop as though it is ready to come to life at all times, which far easier to do than dealing with an engine that dies because a wire frayed in two at a switch.

Much like treating all wires as hot until proven otherwise.
This fail-safe approach has saved my life more than once.

 

So if you want to approach a 100% safe thing you should run wires close enough at a certain place and switch on/off both. Much less things could go wrong that way. Thinking of safety not economy..! (?)