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"It is not the voltage that kills you, it is the current" is an electricity safety tip I've heard many times.

For example, here is an experienced electrian making the claim:

The real danger with electrical shock is amperage, not voltage. Although people have been killed by as little as 120-volt AC and low volatge DC, the silent killer is the amount of amperage in milliampers that flow through a person's body.

and a claim by New Jersey State Council of Electrical Contractors Associations:

It's The Current That Kills

Offhand it would seem that a shock of 10,000 volts would be more deadly than 100 volts. But this is not so! Individuals have been electrocuted by appliances using ordinary house currents of 110 volts and by electrical apparatus in industry using as little as 42 volts direct current. The real measure of shock's intensity lies in the amount of current (amperes) forced though the body, and not the voltage. Any electrical device used on a house wiring circuit can, under certain conditions, transmit a fatal current.

and a typical forum post:

No its current thats dangerous. You can generate 20,000 volts of static electricity walking across a carpet and get zapped a littel but its not going to hurt you. Take a low voltage, High current, and good conductor path and you're dead meat.

Technically, it seems to be a correct statement. But is it any use for my safety, or is it just misleading?

I'm looking for examples of working with low voltages where you would expect to be safe, when in fact you are not. This seems to be the point of the phrase. The only example I have heard is that touching a 9V battery to your tongue is dangerous, which I think is nonsense. Luke gave a good example of burns from shorting a car battery - that's one I had not thought of.

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Well, 110V is not really high voltage, just more than a 9V battery. 10,000 volts acts more like "high voltage" but 100,000 volts is even better (for arcs, lighting gas tube lights at a distance, etc). Not that you would really try the experiment, but metal shorting a power outlet will give you a burn or a fire depending on how quickly the breaker blows. Don't do it! –  Paul Apr 2 '11 at 1:21
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I recall reading---but can provide no citation---that frequency plays a important part in the biological effects for cases where the power dissipation is not the primary damaging agent. (DC may be considered as the limit of low frequency.) –  dmckee Apr 2 '11 at 5:26
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I asked a question on the physics stackexchange to clarify the issues with Ohm's law. The short answer is: 1. Some batteries have less internal resistance. 2. When the current gets too high, some batteries won't be able to maintain the rated voltage –  Casebash Apr 3 '11 at 1:09
    
@Casebash Thanks for tying that in your answer from physics stackexchange. That's a great explanation of how Ohm's law can be deceiving! If you want to see it at work, you can put a multimeter at the terminals of a car battery that is driving a high powered audio amplifier. When the bass hits hard, you'll see the voltage dive (as the current draw goes up). –  Luke Apr 6 '11 at 1:10
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Ever touched a doorknob and gotten an electric shock? If so, that's a minimum of 20,000 volts. At least 40,000 if you could see, hear and feel it. That alone should be enough to answer the question. ;) –  Mason Wheeler May 13 '11 at 11:49
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2 Answers

I'll attempt to illustrate here with an example.

If you take one of the little 12V garage door opener batteries and short out (directly connect) the two terminals with a piece of wire or something else. You'll get a light current flow through the wire or metal. It may get a little warm. This battery is only capable of supplying a small amount of current.

If you take a 12V car battery and short out the two terminals (don't do it, it's not fun), you will be met with a huge current arc that will likely leave a burn mark on whatever was used to short it. This is because the car battery is capable of discharging a large amount of current in a very short period of time.

One time when working under the hood of a car I shorted out a wrench from the positive terminal of the battery to my watch, and my watch was touching the frame. In the brief instant the current was flowing through my watch it became glowing red hot and the watch left a burn mark (still scarred to this day) on my wrist. A 12V garage door opener battery would not have done this.

However, this is not the entire story. As suggested already by others, Ohm's Law describes the relationship between Voltage(V) and Current (I) with V=IR.

There is a great All About Circuits article that actually addresses your question here. They make a good point that if Voltage wasn't dangerous, nobody would ever make signs that said DANGER -- HIGH VOLTAGE!

They provide a great summary themselves at the end, which I'll provide here:

  1. Harm to the body is a function of the amount of shock current. Higher voltage allows for the production of higher, more dangerous currents. Resistance opposes current, making high resistance a good protective measure against shock.

  2. Any voltage above 30 is generally considered to be capable of delivering dangerous shock currents.

  3. Metal jewelry is definitely bad to wear when working around electric circuits. Rings, watchbands, necklaces, bracelets, and other such adornments provide excellent electrical contact with your body, and can conduct current themselves enough to produce skin burns, even with low voltages.

  4. Low voltages can still be dangerous even if they're too low to directly cause shock injury. They may be enough to startle the victim, causing them to jerk back and contact something more dangerous in the near vicinity.

  5. When necessary to work on a "live" circuit, it is best to perform the work with one hand so as to prevent a deadly hand-to-hand (through the chest) shock current path.

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+1 Good example of how high current, low voltage can be dangerous. –  Joe Daley Apr 2 '11 at 1:28
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You have to keep in mind that if you short a car battery with a metal object, you're going to get a pretty spectacular result since the resistance of that object will be low, causing a high current. That said, large batteries/capacitors are not toys, and they are specifically designed to put out a lot of current in a very short amount of time. –  Michael Apr 2 '11 at 10:06
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voltage doesn't kill you, voltage difference kills you. I can safely grab with both hands a wire with a 100,000V potential, as long as I also happen to have a 100,000V potential. Getting back to ground would be tricky though. –  Carson Myers Apr 2 '11 at 18:00
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@Carson Myers I see what you're trying to say, but actually saying that a wire alone has 100,000V of potential doesn't mean anything. Potential is always a difference. Though I think you mean 100,000V with respect to ground. –  Luke Apr 2 '11 at 20:16
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@Luke right, I tried to correct it, but I waited too long :( –  Carson Myers Apr 2 '11 at 22:01
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No, this is a misleading oversimplification. Humans are high resistance. Current can't flow through a person without a high voltage to drive it.

Yet when you scuff your feet across a carpet on a dry day, you are charging yourself up to thousands of volts. When you then touch a grounded metal object, the discharge can send several amps of current through your body. This is more than 25 V and 70 mA, so if those can each kill you, why doesn't a shock? Because the duration of the discharge is a fraction of a microsecond, and the total energy release is only a few millijoules. It doesn't have enough time to cause fibrillation or to heat up lots of tissue to the point of burning.

"The effects of electrical current passing through the human body are covered at length in the International Electro Technical Commission document IEC 479-2:1987. In this document it indicates that a transient or capacitive discharge, as is the case with static electricity, requires energy in excess of 5 Joules (5000mJ) to produce a direct serious risk to health." — Static electricity in modern buildings

The problem with this saying is that it misleads people into thinking that high-current power sources are more dangerous than high-voltage power sources. Most power sources are voltage sources, not current sources. This means they output a constant voltage, and the current in the circuit depends on the resistance of the load (the human body, in this case). This is true for power lines, batteries, etc. Most people don't understand that the current listed on a power supply is just a maximum rating, and won't actually go through their body if they touch it.

If you connect a 1 kΩ resistor across a 12 V supply, the same 12 mA of current will flow whether the supply says 100 mA or 100 A. The amperage rating of a power supply just tells the current it could source, if connected to a small enough resistance. It doesn't force that amount of current through anything it touches, or it would be constantly arcing through the air.

Yes, car batteries can source a lot of current (hundreds of amps), but this only happens when they're connected across a small resistance. If you connect a screwdriver across the terminals, a huge current will flow and the screwdriver will melt, battery will explode, etc. If you put your hands across the terminals, nothing will happen. This is because your skin's resistance is much higher than the screwdriver's resistance. So the 12 V 600 A car battery will not harm you because the voltage is not high enough.

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Spot on, except for "Several amps of current" - even at the instant of connection, given 20kV static charge and a body resistance of 100K the instantaneous current would be only 200mA . . and that's in the artificial case where there is no other resistance. –  peterG Feb 9 at 23:15
    
@peterG that's a link to the source of that statement. "relatively high current which maybe several amperes dissipated within a fraction of a microsecond". Remember that body resistance is not linear and varies with voltage, internal body resistance can be as low as 300 ohm, etc. –  endolith Feb 10 at 3:01
    
I did read the link, and that's where I got the 20kV. But tbh I fell into the trap of finding the idea of several amps -even for that tiny moment at the top of the curve - so counter-intuitive that I thought it can't possibly be right. But yes, if we define 'several' = 2, then 20kV would require a body resistance of 10k, which isn't at all unlikely. So my only recourse is to question the model - can we really model this as 300pF at 20kV discharging via 10k, or is that an oversimplification? –  peterG Feb 10 at 13:03
    
@peterG: Yes, it's counterintuitive, which is the point of my answer: It's not the current or the voltage. It's the energy, and whether it's enough energy to destroy tissue or stop your heart. I don't know what the correct model for the human body is, but at high voltages insulators break down and their resistance drops a lot. –  endolith Feb 10 at 15:50
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