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Electric shock from USB cable

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SeraphimLabs:
That sounds like a little more than a USB problem.

USB devices operate at 5V and no more than 1A per port. If you knock two bare wires together you might see a brief spark before the port shuts down due to overload, but it should not be anywhere near enough juice to be noticed- the ports by default only provide a few milliamps and devices have to negotiate with the controller if they want more than that.

If it is making a visible arc or actually shocking you for real, you definitely have some type of electrical problem.

40hz:
 FYI

This chart is based on an AC line in the USA.
 

So if I=V/R and we estimate the resistance of your  finger is about 2k Ohms and guess that one of the 5V rails is supplying the voltage then: 5/2000 = 0.0025 or 2.5 milliamps. More than enough to feel a tingle. And potentially more dangerous as the voltage goes up if it's not being driven by a 5 volt line. (Considering it's visibly sparking I'm guessing it's more likely to be closer to the 19 or so volts actually coming off the power brick since you usually need around 10 volts minimum to see visible sparking with DC. However, if it's somehow bleeding directly from the AC mains it's already at lethal potential. And the fact that it has a bite at such low voltages makes me think there is some AC bleeding in there. Not good!)

 :tellme:

f0dder:
Watt, you don't need more milliamps than that? O_o

Iirc USB is limited to 100 milliamps before negotiation... that's not at a very comfortable spot in that graph.

Shades:
Yep, according to my old electrical engineering teacher, the Nazi´s were responsible for these electrical insights by their experiments (read: torture). If I remeber correctly, there is also documentation about the exposure times to these various amounts of milli-amps and when these become fatal.

SeraphimLabs:
Yep, according to my old electrical engineering teacher, the Nazi´s were responsible for these electrical insights by their experiments (read: torture). If I remeber correctly, there is also documentation about the exposure times to these various amounts of milli-amps and when these become fatal.


-Shades (January 11, 2013, 02:00 PM)
--- End quote ---

It also depends on the total energy discharged. 50 joules in a brief period of time is the threshold of lethality. The body is resistant enough to electric shock that most people have to go out of their way to cause it or be caught in a freak accident to get anything more than a reminder out of a system running below 12V.

But the higher the voltage gets, the faster it can become dangerous. 50 joule discharges are not at all hard to create.

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