Well, it sounds like the consensus is that 12 volts just doesn’t provide enough juice for a person to feel under ordinary circumstances. So I guess those little plastic “insulators” on the handles of the cables were there just to give the uninformed a sense of security.
Still, it sounds like when I’m holding that last connection* in my hand, there is current passing through my body, just not enough to feel.
All in all, I guess I’m feeling a little better about the idea of jumping cars, but now I’m scared sh*tless about these primary and secondary ignition systems. Where are those evil things located and what exactly should I NOT be touching when working on a car?
*Actually maybe at the first connection. When I touch the positive terminal, there will be at least some current flowing through me to the ground, right?
No, there is no return path through the ground. The only way would be if the negative lead were still connected and part of you were in contact with an exposed piece of bare metal on the car.
OK, I’m back. The battery in my truck is a brand new Optima red top. My tuck uses side terminal cables, so I have two bright shinny top posts available to grab. This battery is rated at 800 cold cranking amps, and 1000 cranking amps. At todays temps it could probably deliver in excess of 1500 amps for a short burst. As they say that is a metric buttload. Experiment #1
Clean dry hand.
Hook one my little finger around one terminal and my thumb around the other. Attempt to pull the two terminals toward each other to achieve a good mechanical contact.
Result? Nothing was felt. Experiment #2
Clean dry hands.
Grab one post in one hand, the other post in the other. Hold posts tightly.
Result? Nothing was felt. Experiment #3
Wet one hand with water. Repeat Experiment #1
Result? Nothing was felt. Experiment #4
Wet both hands with water. Repeat Experiment #2
Result? Nothing was felt. Experiment #5
Repeat Experiment #4 with the engine running, this brings up the voltage by about 1.3V to about 14V
Result? Nothing was felt.
So at this point, I had pretty much given up on being able to shock myself off of a 12V battery. So in the interest of fighting ignorance, I wondered what the resistance of my skin measured. Experiment #6
Set my fluke meter to read resistance and repeated Experiment #1 using Fluke leads instead of the battery terminals.
Results? 2.2 M ohms resistance Experiment #7
I wet my hand, and repeated Experiment #2 again using the fluke meter instead of the battery terminals.
Results? 1.0 M ohms.
To be truthful both of the above reading bounced around a bit, the values given were pretty much the lower end of what was observed.
While I realize this doesn’t approach pan fried semen, I think I can say that this myth is busted.
Rick, Let me first say that I have never managed to shock myself with 12 VDC either, but this quoted part is not quite correct. Both hands on the pipe will be a parallel circuit. The current path is not from hand to hand, but from each hand to ground. Placing both hands on the pipe is analogous to placing two wires under a single terminal, as the pipe is at the same voltage potential everywhere along it’s length. So the resistance is halved.
As for your second post regarding your experiments, three things strike me. First, water is not as good a conductor as sweat, which is the case in the anecdote. Second, your fingertips don’t have much contact area, so will have a higher resistance. Third, the skin on your fingertips and hands is considerably thicker than elsewhere on your body and have a correspondingly higher resistance.
If you try the experiment again, work up a good sweat and check the resistance on the underside of your arm - or across your forehead. That’s where I got zapped. I was changing a compressor and leaned forward into the 24 volt wires. It was not a tingle - it HURT!
I think your experiment illustrates a vital point - shocking yourself with 12 VDC is not something to be concerned about.
If memory serves, AC current results in…umm…physiologically noticeable effects at lower amperage than DC current does. See this chart for example:
BODILY EFFECT DIRECT CURRENT (DC) 60 Hz AC 10 kHz AC
---------------------------------------------------------------
Slight sensation Men = 1.0 mA 0.4 mA 7 mA
felt at hand(s) Women = 0.6 mA 0.3 mA 5 mA
---------------------------------------------------------------
Threshold of Men = 5.2 mA 1.1 mA 12 mA
perception Women = 3.5 mA 0.7 mA 8 mA
---------------------------------------------------------------
Painful, but Men = 62 mA 9 mA 55 mA
voluntary muscle Women = 41 mA 6 mA 37 mA
control maintained
---------------------------------------------------------------
Painful, unable Men = 76 mA 16 mA 75 mA
to let go of wires Women = 51 mA 10.5 mA 50 mA
---------------------------------------------------------------
Severe pain, Men = 90 mA 23 mA 94 mA
difficulty Women = 60 mA 15 mA 63 mA
breathing
---------------------------------------------------------------
Possible heart Men = 500 mA 100 mA
fibrillation Women = 500 mA 100 mA
after 3 seconds
---------------------------------------------------------------
link
So your 24VAC transformer (assuming you were dealing with 50/60Hz) will result in a painful shock at roughly 1/7 the current, and starts out at twice the voltage, meaning you’d have to have 14x less resistance to get the same effect with a 12VDC system. Roughly. I doubt you’re going to achieve that without a nice big open wound pressed hard against a highly conductive surface. I certainly won’t say it’s impossible to seriously injure or kill someone with 12VDC, but you’d have to put some serious effort into achieving sufficient conductivity.
I know it’s the amps and not the voltage that zaps you, but the chart shows probable death at 500 mA whereas car batteries are rated at, what, 800 to 1000 Amps! I don’t know enough about the relationship between volts and amps, but it sure looks like a car battery has the potential to blow your arm off.
Actually, potential is exactly what the car battery lacks, since potential is another term for voltage. As Rick demonstrated, your skin’s normal resistance is too high for 12 volts to push current through. All I’ve been saying, is that if you really tried hard enough, you could, probably, get a shock out of it. But I think your arms are safe.
And Gorsnak, your point is well taken, but I handle 24 VAC barehanded all of the time and have rarely gotten as much as a tingle. But on that occasion, I literally saw stars. So my point is that salty sweat and wide open pores on a hot day makes a significant difference. As Will Repair pointed out, a 9 VDC battery will give you a tingle if you touch it to your tongue. :eek: :smack:
With sweaty hands grabbing one probe of my multimeter in each hand I generally read about 100-200k ohms. With dry hands it’s more like 1-2M ohms.
Amps = Volts / Ohms
So, with sweaty hands grabbing one battery terminal in each hand, we’re looking at about 12/100000 = .12 milliamps - not anything you’ll feel. Actually it should be a bit higher, because battery terminals have more surface area than multimeter probes. The car battery can deliver a lot of amps, but only with a very low resistance load - like, for example, an electric motor that isn’t spinning very fast. Ooooh, isn’t that an amazing coincidence!
Now, here’s the kicker (as Rhubarb knows firsthand) - the resistance across your body can actually drop substantially lower than 100k ohms. Looking at our chart we see that for 60Hz AC, it takes about 10mA to cause significant pain. With a 24V source, that would have required a resistance of only 2.5k ohms. But now if we translate that back to our 12VDC automotive electrical system, we’re looking at 5mA, which when dealing with DC is “at the limits of perception”. So if we make as good contact at Rhubarb did in getting a nasty shock off a 24VAC transformer while working on a car, we’ll feel a slight tingle, much like when you lick a 9V battery (very much like, since we’re dealing with exactly 1.33x the electrical potential).
Yes but there has to be a ground somewhere. If the person in the picture is not grounded, it does not matter what the total circuit resistance is as there is not a complete circuit. One of the master’s minions dealt with that question here.
If the person is grounded, you have to account for the resistance of that ground path. Unless you jab a copper ground spike though his chest, there will be a resistance in the ground circuit. If you do jab said spike though his chest, the resistance of his hands becomes secondary in IMHO.
Ass-suming that the ground path has the same resistance as his hands (It would probably be greater as the electricity would probably have to travel though clothes or shoes or some other barrier)
But anyway assuming the exact same resistance for the ground path we have a series /parallel circuit.
Total Circuit resistance = (2000/2) + (2000) = 3000 ohms. Their answer is only off by 300% :rolleyes:
::: Grumble:::
::: Rick wanders off to the kitchen mumbling about the things he does in the fight against ignorance:::
OK, I ran the kitchen sink until the water was as hot as it would get (125F is what my water heater is set at) I dissolved salt into the glass until no more would dissolve. This has to be a stronger salt solution than sweat. I would have used sweat, but I have a hard time working up a good sweat in 50F weather.
I went out to the truck and repeated Experiment #3 with salt water, and the engine running.
Result? I could fell a tingle. So off I went to get my trusty Fluke meter. Set it up for amperage and did the test again. 3mA was the reading. Now don’t forget that this is with a stronger salt solution than sweat. With a weaker solution I would expect a lower conductivity.
I agree that the skin on my hand is thicker and has a higher resistance than the skin of many other parts of my body. However since I don’t have a prehensile Johnson, I don’t work on batteries with Mr. Happy and therefore I did not measure the resistance of Mr. Happy.
So in conclusion, let me amend my answer. It is possible to get a detectable shock from a 12 V battery, but you have to have proper conditions. It would be easier to feel the shock if you are touching things with your parts of your body other than your hands.
::: Shrug::: I agree it does rise to the level of pan fried semen, but then again what does?
As a younger person, we used to magnetize screwdrivers by wrapping them tightly with wire then touching each end of that wire to a car’s battery posts. I’ve never gotten shocked. When I worked at a filling station as a kid, we would short out batteries with a piece of metal to deaden it for the dude that came and picked up our dead batteries.
:eek: :smack:
