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Electric shock from USB cable

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40hz:
^True. The current is a function of voltage and resistance and is continuously variable. Besides. the human body's resistance can run from 1K ohms to around 120K ohms depending on how much body mass is in the circuit path. So it's not easy to estimate an individual's true electrical resistance in an actual situation with any degree of accuracy. The critical factor is if the circuit path crosses the heart area.

Interestingly each type of current (DC and AC) presents its own dangers. DC makes you more likely to "stick" to a load since it paralyzes muscles including the heart. AC's back and forth keeps you from locking up as hard, so (theoretically) it's easier to pull free. But the alternating current makes your heart to go haywire since it confuses the electrical signals and causes fibrillation, whereas with DC there's a good chance the heart may 'auto-reset' after the DC jolt and resume beating normally. This is why medical defibrillators use DC current.

Although both forms of electrical current are dangerous, serious AC shocks are considered (by many medical people) to pose a greater threat since the human heart can remain confused even after the person is disconnected from the circuit. Often with fatal results.

f0dder:
I was always under the impression that you needed relatively high amounts of energy to be toast - that you needed either voltage or amperage to be pretty nasty. Like, getting main-currents (what do you have in the states, 110v? We've got 220/230v in .dk). But still got the idea that the amps was the worst part. Never really grokked what ratio between the two you needed before it was dangerous, so I've always kept cautious and switched off the mains when I needed to do any kind of electric works.

Guess I got a scare early on, though. Sometime before third grade, some of the older boys at the school I attended back then teased a younger kid to jump into a transformer station to fetch a DKK20 (or summat) coin - 20+ years ago, adjust coin currency etc. (though I guess you don't time-adjust a human life in the same way). So the 1 minute mourning and the story of the tiny little exit holes in his toe and his temple kept me somewhat fearful of electrical installations ever since.

Carol Haynes:
Guess I got a scare early on, though. Sometime before third grade, some of the older boys at the school I attended back then teased a younger kid to jump into a transformer station to fetch a DKK20 (or summat) coin - 20+ years ago, adjust coin currency etc. (though I guess you don't time-adjust a human life in the same way). So the 1 minute mourning and the story of the tiny little exit holes in his toe and his temple kept me somewhat fearful of electrical installations ever since.
-f0dder (January 11, 2013, 05:16 PM)
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Yikes! Nasty story.

Worrying too as I have a workshop with a power station transformer in a fenced enclosure immediately outside the window. I asked the electric board who was going to paint and repair my window since there are big signs saying "High Voltage - Danger of Death" all round the fence. The electric board told me to climb over the fence and paint the window myself!

40hz:
Never really grokked what ratio between the two you needed before it was dangerous,
-f0dder (January 11, 2013, 05:16 PM)
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The formula is Ohm's Law:

Current in amps = Voltage in volts/Resistance in ohms

Clearest explanation I ever saw is here. Check out the VIR triangle at the link. That's the easiest way to remember the formula variations.

General rule of thumb: once you hit 1/10 of an amp (i.e. 100 milliamps) you're at risk with house current levels and frequencies.

barney:
No comments on the various formulations & guides - some old tech is still valid  ;), just a cautionary observation/experience.

When I worked at Hughes Aircraft in Tucson, AZ (USA), we had a lot of DC-powered test stations.  One night a tech was called in to work on a test station.  Unfortunately, his testing had to be done on a hot station, i.e., with the power on.  (It didn't have to be done hot, but the difference was several hours as opposed to a few minutes.)  He got across 400 volts DC.  It literally threw him halfway across the room - perhaps I should say his muscle spasm when he got across the contacts threw him halfway across the room.  I helped to recover him, get him on a gurney the shop nurse brought.  Significant, if not major, burns on this hands and arms,  and a partially dislocated shoulder from jumping away while trying to hold on.

Up until that time, in my ignorance and supposed invulnerability, I was wont to work on 120V AC circuits hot - home wiring and the lot - comfortable in my presumed immunity.  I don't do that any more.  (I've also gotten burned fingers working with 9V lantern batteries, but that's a different matter.)

Anytime you're working with current, you can be in harm's way, even though the amount of current seems minuscule.

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