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Comment by mw67 | Hacker News Reader

But as far as dangerousness, only 0.1A can kill a human, whereas voltage is not so important.

Voltage is what drives current so they're directly proportional and their proportion depends on electrical resistance. Given that human body's resistance to electricity is more or less constant, that allows calculating a dangerous voltage just as easily.

Except, you also need to keep in mind the internal resistance of your power source. If that one is higher enough than the human body resistance, you could have a very high voltage which is actually not very dangerous.

I assume this is the reason I was able to safely use a 5000V power source in school, despite being shocked multiple times (IIRC, the maximum current it would give, even if you shorted the output terminals, was 3mA. Our teacher at the time did initially insist on wearing latex gloves, but they were discarded once we realised the current would just arc through the gloves and leave holes in them).

You do start to see some odd effects at those voltages though - I recall holding one of the output plates for ten minutes or so, without realising the supply was on. Didn't really think much of it, until I realised I could hold my hand over a piece of tin foil (roughly 1cmx1cm) and it would be attracted to my hand from 2-3cm away (that persisted for five minutes or so).

The body's resistance is generally not what matters, it's generally the resistance of the connection between the body and the power source. If the connection is made via an air-gap spark you can need thousands of volts to ionize the air to allow the spark to form which allows the current to enter the body (corresponding to tens of thousands of ohms at a killing current, although that resistance is highly non-linear and drops suddenly after the initial spark connection ionizes the air). If the connection is made via EEG or EKG leads attached with conductive paste, the connection resistance can be a fraction of an ohm and the killing voltage can be well under 100 volts. The highly variable nature of the connection resistance is just one reason why the killing threshold is generally described in terms of amps rather than volts. If you're worried about whether a circuit could kill a patient and you must think in terms of voltage, you're not worried about whether you know the body's resistance, you're worried about whether you know the connection resistance.

In practice pretty much everyone who designs safety critical systems where electrocution is a concern focuses on current because it's a much better way to model and analyze the system risks than voltage.

Through your body, yes. With 1V you can't drive much of a current through ones body.

Even 48 volts won't drive any appreciable current through dry skin, but you can string four car batteries together and weld with them. It's frightening that people are so ignorant about electricity.

Eh, as far as the list of frightening things that people don't know about goes, electricity is pretty far down the list for me. Most people just don't come into contact with it often enough, so simply saying things like "don't get near a downed powerline" is probably good enough.

Stuff like radon or CO are way higher on that list, in my personal opinion. Hell, even people's general ignorance of their countries' political process and what their (elected or not) officials are doing is way more frightening to me than their ignorance about electricity.

Sorry to be so damn pedantic.

I think it's fair to say "invisible" things are generally all pretty high on the list of frightening things people don't know about.

Including but not limited to: software, nuclear radiation, CO and chemistry in general, electricity, microbiology, politics in practice, macro-economics and banking, ... (it's a pretty long list)

Thoughts. Thoughts are the worst.

You're right, ignorance of those things is more dangerous. But people do interact with electricity every day, so I would have thought they'd know the basics a little better.

Not through dry skin, no. But if the thought of it makes you sweat...

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