I just have a question I'm having trouble finding an answer to, and really its just because I'm curious. Why is it that a 12 volt battery, that say is capable of something like 240 amps, not be as dangerous as 120 volts AC at say 15 amps? The wattage is higher in the case of the battery. Somewhere I read, said that low voltages aren't as dangerous due to the resistance of your skin. But I always thought that it was amps that are dangerous, as a static shock is something like 10,000 volts (can't remember) but no current. Am I just cookoo, or is there something to this?

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Originally Posted by KingsX

But I always thought that it was amps that are dangerous,

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This is a complete wives tale. How this keeps getting perpetuated is beyond me.

The voltage is the dangerous part. The amps of a circuit or battery are completely irrelevant. When someone says "It's the amps that kill" they mean it is the flow of current through the body that kills. As little as 10mA of current through the body can do harm. This 10mA can come from a 1A fuse in a piece of electronics, or a 2000A commercial electrical service, or a 12v 600AH battery.

Current passing through the heart muscle. It's in the 10 mA range, I believe is lethal. Skin resistance between two points varies I measured me and got about 100K from hand to hand.

Using V=I*R;i=10e-3 and R=100K. You get 1000 V as lethal.

I think skin resistance is much lower than that.

12 V Batteries are dangerous too, but for a different reason. A screwdriver can be propelled into your head. It can also get really hot and fuse to the terminals.

Static electricity is very brief and even at high voltages, it's not going to stop the heart long enough to matter.

Does this help?

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Originally Posted by KingsX

Why is it that a 12 volt battery, that say is capable of something like 240 amps, not be as dangerous as 120 volts AC at say 15 amps?

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Don't know how you measure "dangerous", but having been shocked by both, both are well respected. First, read Electric Shock. Then applying Ohm's Law, where Current = Voltage / Resistance, for any given resistance of an electrical path (you), the current that will be present at 120V AC is ten times greater than that which will be present at 12V DC regardless of the maximum current that can possibly be drawn from the current source and any effects specific to the type of current AC or DC. For the sake of example, let's say the resistance from your finger to toe is 500 ohms (I have no idea what it is). For 120V AC, current = 120 Volts / 500 ohms = 0.24 amps. For 12V DC, current = 12 Volts / 500 ohms = 0.024 Amps. 0.24 amps vs. 0.024 amps, ten-fold difference.

Resources:
R. Fish & L. Geddes, Medical and Bioengineering Aspects of Electrical Injuries

So the amps thing is a misnomer. So current (amps?) is calculated as volts /ohms. I think I see. So basically if a device needs to draw "x" amount of amps for a required voltage, it will as long as the breaker will provide it. If it needs 20 amps, and you have a 15 amp circuit, it'll flip. With a battery, same goes (not the breaker part). Depending on the device, the battery will drain faster or slower, by the amp usage. So a person will "use" x amount of amps because current will pass through that person, and the resistance is what tells what amps the person will draw from the electrical source, whether from battery or AC circuit.

EDIT: So lets say an electrical device requires only 2 amps, that is in essence the same as creating a "resistance" by only drawing 2 amps. Just because the battery can provide way more, or a 15 amp circuit can provide 15 doesn't mean that that is what will be drawn. A person won't draw all 15 amps, or all 240 amps (or whatever a 12 volt battery can provide).

In essence, yes.

Resistance of the load determines voltage required.

Simple example:

Your designing a heater:

You are given a voltage
You can vary R all over the map based on available materials, so R gets constrained by the diameter of the wire and the amps available. So you could draw 10-20 amps, You can also constran yourself based on watts. I'd like my heater to dissapate 1200 Watts.

So, therfore I need 10 amps at 120 V. Now I can find R. Now I can find a wire size that gives me a given R.

Right, and you have a predetermined R value depending on the situation. If your skin is wet, or other factors like that will lower your resistance, as wearing rubber gloves would raise it. Thanks guys for the info again!

Take a look here. It does a better job.

http://www.cmia.org/images/est.pdf

Rubber gloves or any kind of protective media has an insulation resistance and a dialectric breakdown or volts/mil rating. At say 10 KV. The rubber gloves may not be able to protect you, but at 120 VAC they are fine.

Below are extreme examples of work on live high-voltage power lines using the air itself to provide a high resistance (insulation) from ground. Not sure if this is relevant, but it's cool as hell, and I always think of this whenever resistance is mentioned.

YouTube - Helicopter Power Line Maintenance
YouTube - Agrotors / Haverfield Helicopter Power Line Maintenance

I've seen similar videos, always amazes me.