Static electricity

[Wisest_Novel]

scr4, you’ll have to further educate my over-simple mind. What is the difference between

Originally posted by Wisest Novel
…wouldn’t it then be most accurate to say that power (or wattage, if you prefer) is what constitutes the hazard? If I read you all correctly, you need the presence of both adequate current to propagate the energy and sufficient potential …

and

*Originally posted by scr4 *
**So it’s a high-voltage source capable of delivering high current which constitutes a hazard. **

other than the substitution of “high” for “adequate” and “sufficient”?

Also, although I wasn’t limiting my point to 12v car batteries, I don’t know what you define as “a huge amount of power.” Regardless, the point I was trying to clarify was that although it’s commonly said “current kills,” it’s not exactly complete. It’s also governed by the presence of adequate voltage to complete a circuit, and further mitigated by the resistance of the preson involved. Though I will admit I’d be hard pressed to come up with a practical example of a high-current, very-low-voltage application.

ZenBeam, thanks for the analysis of internal impedance and resistance. I was looking for average skin resistance measures earlier in the thread, and couldn’t find any. Today I came up with This thing from a UDel course. I’m not crazy about its credibility, since it has a disclaimer at the top that some of the original source material is lost. Please feel free to supplant this with better data if you have it. Also, scr4 will be happy to note that it contains the words, “CURRENT KILLS (not voltage)” prominently at the top of the page.

Dry skin resistance is typically ~500,000 ohms.
Wet skin resistance falls to ~1000 ohms.
Internal resistance of body is only 100 to 500 ohms.

I also found some interesting measures of perception and “let go” thresholds both both DC and AC circuits in The Merck Manual of Medical Information–Home Edition.

The body can perceive direct current entering the hand at about 5 to 10 milliamperes; it can perceive common household current, which is 60-hertz alternating current, at about 1 to 10 milliamperes. The maximum current that causes arm muscles to contract but still allows the hand to let go of the electric source is appropriately called the let-go current. This value is about 75 milliamperes for direct current and about 2 to 5 milliamperes in children, 5 to 7 milliamperes in women, and 7 to 9 milliamperes in men for alternating current, depending on the muscle mass of a person’s arm.

Here’s where I need help. If the resistance numbers are correct, a 12V battery would only pass on the order of 1mA through wet hands, which Merck says is below perception threshold. But most of us know through experience that ain’t so. Is the error in my computation, or the resistance figures?

One last thing, just for completeness (and yes, I know this isn’t on OP; I hope SenorBeef isn’t offended by the direction this thread has taken). In terms of life-threatening damage, Merck says

At currents as low as 60 to 100 milliamperes, a low-voltage (110 to 220 volts), 60-hertz alternating current traveling through the chest for a split second can cause life-threatening irregular heart rhythms. About 300 to 500 milliamperes of direct current is required to have the same effect. If the current travels directly to the heart, for instance through a pacemaker, a much lower current (less than 1 milliampere) can cause irregular heart rhythms.

Sheesh. And just before I hit Post, I see Attrayant has hit on most of my points, and probably more clearly. Dadgummit.

[bbeaty]

*Originally posted by Wisest Novel *
If the resistance numbers are correct, a 12V battery would only pass on the order of 1mA through wet hands, which Merck says is below perception threshold. But most of us know through experience that ain’t so. Is the error in my computation, or the resistance figures?

Maybe the car’s alternator is putting out pulses? I know that the average voltage for a recharging car battery is around 14.5V. But maybe that average is made from narrow pulses of a much higher voltage. When the engine is stopped, the battery would go back to 12V. Also, if your hands are wet and have a tiny bit of battery acid on them, then the skin resistance is abnormally low. In that case even a 9V battery might give you a zap.

Also, it’s not voltage nor current nor wattage that’s dangerous. It’s CURRENT THROUGH YOUR CHEST that causes problems. For example, if you stick a couple of wires from a 500V battery against your hand, and if 100mA flows through your muscles there, it will hurt. But it’s far from lethal. Touch those two wires to your nipples, and THEN you have a problem.

[Wisest_Novel]

*Originally posted by bbeaty *
**Also, it’s not voltage nor current nor wattage that’s dangerous. It’s CURRENT THROUGH YOUR CHEST that causes problems. **

Again, I must respectfully submit that this, while true, is very incomplete. There are six methodologies of electrical injury: electroysis (which leads to ulceration), burns, tetanus (involuntary muscle contraction), respiratory arrest, cardiac arrest, and ventricular fibrillation. Which mode(s) occur(s) is dependent on the properties and path of the circuit.

**Touch those two wires to your nipples, and THEN you have a problem. **

You first.

[bbeaty]

*Originally posted by Wisest Novel *
Again, I must respectfully submit that this, while true, is very incomplete.

You’re right of course. As a “rule of thumb”, the first danger you encounter with voltages higher than 40V is heart fibrillation. If you MUST get shocked, try to keep the current path away from your chest area, and don’t stick wires into open sores. As conditions get worse, many other effects become important. For example, if you’re standing in water and cannot let go of a live 120V line, you’d better worry about asphixiation. Or if you get yourself between a 10KV power line and a ground, then internal steam explosions and massive tissue charring are going to be a bit more worrisome than fibrillations.

See: successful suicide (not for the squeamish!)
http://www.drmegavolt.com/underpages/gallery/pcp_on_pole.asp

[Gary_T]

Originally posted by bbeaty ***
Maybe the car’s alternator is putting out pulses? I know that the average voltage for a recharging car battery is around 14.5V. But maybe that average is made from narrow pulses of a much higher voltage. When the engine is stopped, the battery would go back to 12V.*

The overwhelming majority of vehicle charging current is steady DC, it makes a flat line on an oscilloscope. However, alternators also leak a small amount of AC (they work by generating AC which is rectified into DC). In a properly working system the AC is usually 20mA or less, but it some cases a faulty but still functioning alternator will give out several hundred mA of AC.

While it is nominally a 12V system, actual rest voltage of a fully charged battery is 12.6.

[ZenBeam]

Dry skin resistance is typically ~500,000 ohms.
Wet skin resistance falls to ~1000 ohms.
Internal resistance of body is only 100 to 500 ohms.

I just tried measuring my resistance. Holding the leads in opposite dry hands, about 500,000 ohms. With wet fingers, (surface wet, not wrinkled-finger wet) it dropped to 100,000 ohms. I tried putting the leads on oppostie sides of my tongue, and I still got 60,000 ohms.