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  #16 (permalink)  
Old 07-30-2004, 02:20 PM
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this is why you dissconnect the negative battery cable.

grossly graphic images from s15layinrubba

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Old 07-30-2004, 09:13 PM
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Finger Licking Electrical Burns...

Ever notice that people who work with electricity (not AA batteries either) never ever wear jewelery (wrings, watches or loose dangly chains around the neck etc).

To use the into one hand and out the other - as a standard pathway and resistance parameters.

Here we go:

I am a qualified first class weldor, and there is a thing that (on average) clean dry cool skin has about 45V resistance to current flow and calloused skin (only bits of the hand) have more resistance.

And welding is a very hot and sweaty job...

Most welder machines have an open circuit voltages of either 45V or 75V - so that people either get little or no electrical shocks..... unless thier hand is sweaty, and they lean agains the boiler being welded..... and they stick the electrical rod underneath their sweaty armpit (in coveralls).... while they light their cigarette - and then die.

This is why you can grab or at least touch both terminals of a 12V battery and not get any electrical current flowing through your body...

(2 x 45V) - 12 = 68V above current flow.

BUTT - when JEWELERY covers the skin, the moisture (persiration = water + salts) builds up underneath the jewelery, significantly dropping the voltaic resistance to SFA (= virtually nothing or close to it)

You have metal (good conductor) -> wet salty skin (very low resistance / fairly good conduction) and this one piece of jewelery has efectively dropped the electrical resistance to about half of what it was formerly 90V - 45V = 45V...

And if you wear a ring or rings on both hands... or a watch and a ring on either hand etc.... what you have done is set up a fairly lethal situation as far as electricity being able to go through your body.....

You get other fun things like people who wear steel cap boots., and the leather covering the steel cap has worn through.. and the boots are filled with hot and sweaty feet.... and some one has their welder set to 75V and they are kneeling "on the job" and they pick up a conductive electrode with bare sweaty hands.......

As one old tradesman once said - "Well apprentices are cheap... if one kills himself, they just get me another one".

Then you get the classical "dead short (circuit)" where the jewelery conducts electrictity between part A and part B...

Time for pretend time.

Say 1 PSI = 1V....

Electricity is like a garden hose and a fire hose.... if you can have a small hose feeding water at high pressure and it delivers say 10 gallons a minute at 100 psi... (100V) and then you have the fire hose feeding 10 gallons a minute at 12 psi....(12V)

The idea is the VOLUME of water delivered in a period of time...

The fun thing about 12 batteries is that while the VOLTAGE is LOW, they are designed to SUPPLY huge amounts of power for short periods.....

All 12 V batterys deliver stacks of power and I can't compare a motorcycle battery to a pick up with 2 BIG batteries in parrallel, but I recall an average car battery will put out about 250~300 amps in a dead short...

And in dead shorts... they will supply enough electricity (power = heat) to incinerate a ring on your finger or a watch band around your wrist... in the blink of an eye......

And dead shorts tend to "weld" the ring in place while your finger gets burnt off.....

The old maxim of an ounce of prevention is better than a pound of cure - tends to hold much truth....

Depending upon the job.... which means that you take precautions to stop shorting from occouring or that you disconnect the battery....

And unless the job required live electricity - either isolate the actual circuit and or disconnect the battery at the negative terminal (on neg earth systems)

I also note that all the SMART weldors, autoelectricians and people / professionals who work with serious amounts of power - well NON of them, wear jewelery - when working with electricity.

The DUMB ones can't count to ten.

Take the hint....

Last edited by ButtWeldor; 07-30-2004 at 09:31 PM.
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Old 07-31-2004, 02:43 PM
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I understand the reasons but but still fell for protection of the electrical equipment, any arc can cause problems. I have never arced a wrench and many years but I am carefull. After seing all the evidence though I will change my practices because I am thinking it is better to protect me.

As to jewelry have any of you seen the motorcycle show on SPIKE TV? this freak is supposed to be rebuilding a motor and he has a ring on every finger a long chain around his neck and such. YEAH he has a clue!

I figure every project that I ever started has been started and ended with 10 fingers and I plan on keeping it that way
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Old 08-03-2004, 02:04 PM
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I didn't read every post in this thread.
I'm not a master electrictian but I am a mechanic in the Army, we take a small and short electrical course.
So my understanding is quite simple. Electricty travels from negative to posative, not the other way around:-)
And about the jewlery thing...
Yeah wearing jewlery is stupid. You gotta understand that gold is the best conductor as far as metals go. Yes water is better:-)
But yeah gold is highly conductive. In the Army we have to use tape around our rings if we choose to wear them. I do not even wear my ring when performing any sort of mechanical/electrical work. Or when I'm on duty at all actually.:-)
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Old 08-03-2004, 03:38 PM
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Hate to tell you but water is resistive not conductive, it is the junk in the water that is conductive. We have actualy used purified water to insulate things in special projects.
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Old 08-04-2004, 06:18 AM
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A friend of mine (another electrical engineer) used to charge the battery in his ski van (outside the ski shack) with a lab type power supply..(this is no problem - it was free - he already owned it). He used to disconnect the battery cables before connecting the power supply (why i cannot imagine - made no sense to me and i've got a masters in EE too). He would disconnect the positive cable only. I asked why once..got no satisfactory response so i droppeed it..was his truck after all.

Anyway.. one day while reconnecting the positve cable he'd disconnected..he touched the sheet metal with the wrench (half inch open end) he was using. Since the battery had been on charge for a couple of hours, it was full of hydrogen (and oxygen). The resulting explosion removed ALL plastic from the battery above the metal battery tray (last 1/2 inch of case). Only the posts and plates were left - along with the fiber seperators between the plates. (i'd never seen a naked battery before). Plastic pieces went a measured 45 feet 6 inches. A noticable dent was produced in the hood - which was open at the time (Twas a Dodge sportvan - with the short hood..engine partially inside passenger compartment). Fortunately, altho drenched in acid, he was not hit by any large plastic pieces - since unlike me..he did not need to wear glasses (eyes were unprotected). I missed the festivities, arriving shortly afterwards from the ski lift. His wife had presence of mind enough to strip off his new Icelandic Xmas sweater and soak in baking soda.

He doesn't do that any more.. *G*..And i'm REALLY careful about sparks around batteries myself.. partic if i've been charging them..

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