Avoiding electrocution

Originally published 2002 in Atomic: Maximum Power Computing
Last modified 11-Feb-2014.


People don't understand electricity.

They know that there are volts and watts, and they probably know there are amps too, and the concept of ohms may have crossed their mind at some point. The relationship between these things, however, seems to be difficult to grasp.

You'd think an electrician would have some idea about this field of endeavour, though, wouldn't you?

Well, the one that e-mailed me and, in the course of a brief exchange of mail, made clear that he could not get a clue even if provided with a large amount of money, very clear directions to the clue red-light district, and aftershave irresistible to clues, didn't.

He thought that it wasn't the volts that killed you, it was the amps.

Well, that's true. As long as there's no significant current flowing through you, you're safe. You can stand there all day with one hand pulling sparks from the dome of an electrostatic generator and the other hand grasping a cold water pipe. You may do a fair bit of twitching and end up with a lot of little spark-punctures in the end of your finger, but you won't die.

Static's a special case, though; the actual number of electrons involved is minuscule, compared with what's used by practically any real electric device. In other words, the current capacity of a classroom-sized static generator is really, really small.

If, on the other hand, you cut the fence-wire around a major suburban power substation and then charge in and swing a crowbar at anything that's sitting on a four-foot stack of insulators, your life will become exceedingly nasty, brutish and short.

This is because current and voltage are, usually, connected. More volts into a given resistance means more current. And a substation busbar has no trouble delivering really impressive amounts of current to someone who's decided to pursue a career as a ground stake.

If you're going to be fooling with your PC, it helps to understand the difference between these situations, and why low voltage isn't an electrocution risk, unless you really try very hard to make it one.

Alternating current (AC) circuits can get complicated, but for direct current (DC) circuits like the power plugs inside computers, the basic rule is dead easy. Voltage (in volts) equals current (in amps) times resistance (in ohms). This is Ohm's Law, V=IR. Or I=V/R, or R=V/I, depending on what values you know and what value you're trying to figure out. And power, in watts, equals volts times amps.

For really impressive low voltage currents, let's think about starting a car.

As anybody who's managed to jam his stainless steel watch bracelet between the positive terminal of his car battery and the negative-ground frame will be able to tell you, car batteries can deliver quite a lot of current.

Automotive starter motors run from 12 volts DC, and can draw hundreds of amps for the (with any luck) brief period when the engine's cranking. Let's assume the battery delivers exactly 12 volts, and exactly 240 amps is being drawn. With that information, you can use Ohm's law to figure out the resistance of the circuit (including the starter motor, its wiring, the internal resistance of the battery, et cetera). R equals 12/240, or only 0.05 ohms. And the circuit power, mostly accounted for by the motor, is 12 times 240 - an imposing 2880 watts.

If 240 amps passes through any significant amount of a human body for any significant period of time, investigators may have to employ DNA analysis to determine who that stuff they found all over the place used to be. You really don't want to make yourself part of a 240-amp circuit. Actually, only about 30 milliamps (0.03 amps) across the human heart has a good chance of stopping it.

Where people go wrong, here, is by thinking that if you disconnect one battery lead in your car, hold the end of that lead in one hand, touch the battery terminal with the other hand, shout "Je ne regrette rien!", and then get a partner in scientific exploration to turn the ignition key, you'll be deader, faster, than someone who's already started his skydive when he discovers that the thing on his back actually contains some chocolate and a sleeping bag.

What will actually happen in the above situation - the car situation, not the skydiving one - is nothing.

The reason for this is that the human body has quite a lot of resistance of its own. If you've got a good contact - the terminals are big enough that the whole palm of each hand can touch them - then the resistance across your body is likely to be between about 2,000 and 50,000 ohms. The thinner and wetter your skin, the lower the resistance will be.

Even if your resistance is only 2000 ohms - which it probably won't be - that's enough to drop the circuit current to about 6mA. You may be able to feel that. You probably won't.

Since your resistance is actually likely to be much higher, the current through your body is likely to be trivially low. And since your body is part of the circuit, the current through the whole circuit will also be trivially low. Hence, the starter motor won't even twitch, the car will not start, and you will not die.

nozap300.jpg (23803 bytes)

To emphasise this point, I sent my misguided electrician correspondent the above picture, of me holding wires connected to the terminals of the rather hefty battery I used for my do-it-yourself UPS piece.

I don't think it helped.

Anyway, this is why it's perfectly safe, electrically at least, to work on your PC with the thing plugged in and turned on. Inside the power supply are mains voltages that can hurt you. So don't go sticking screwdrivers in there. But outside the power supply, 24 volts is the most you can see, across the -12 and +12 rails.

You may smash a fingernail with a fan, you may cut yourself on your cheap pressed-metal case, you may destroy components in any of a number of amusing ways, but that's about the end of it. Unless your PSU has a horrible electrical fault or you've plugged it into a badly miswired outlet, electrocution is not on the agenda.

If you burn your skin off or stab through it - which can happen, in higher-than-mains electrocution cases and other mishaps, including certain naval incidents - then the resistance across the whole body under the skin is only about 500 ohms; even less, if you stab deep enough. So if you take a couple of sharp probes, connect them to 12 volts, and stab them into your chest, you'll be in a bad way.

Fortunately, though, most of us have skin everywhere we're likely to touch electrical contacts.

I've got a funny story about that, having to do with licking the mains and rebooting my brain. But it'll have to wait for another time.

More electronics info

If you're in the mood for an online electronics primer, I used to recommend this site, except it now appears to be toast (archive.org semi-mirror here).

Try, instead, here, here (or maybe here...) and here.

Or here.

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