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Comment by bananas | Hacker News Reader

Until you connect your Arduino's USB cable to your computer which is on a faulty earth and then realise that you have 100-240v going through your mains, to your computer case, up your USB cable, through your meter, up your arm, across your chest, down your desk leg and back to earth...

The potential difference between the 5v and GND on your Ardunio might be 5v but between your Arduino GND and real earth might be 180v floating.

Believe me that shit is scary and will kill you.

I'd rather people blew $250 on a meter and actually read up on electrical safety to be honest. If that's prohibitive, you're welcome to kill yourself through ignorance.

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Why stop at multimeters, though? I mean, what would happen if you connected a USB drive "to your computer which is on a faulty earth and then realise that you have 100-240v going through your mains, to your computer case, up your USB cable, through your [hand], up your arm, across your chest, down your desk leg and back to earth". Clearly anyone selling $15 USB drives is endangering their customers and we should all buy $250 safety-rated ones.

In that case your computer is a death trap certain to cause fatal electrical shock the next time you connect your printer, flash drive, or USB nerf missile launcher. Your expensive multimeter probably won't save you in time (seriously though, do you test the grounding of your computer case every time you use it?)

> your computer which is on a faulty earth ... to your computer case

This can certainly happen and is indeed dangerous, but if the first time you find out is while experimenting with a USB peripheral on a test bench (as opposed to say resting your bare legs on the case), you should consider yourself lucky.

If you really are worried about getting killed while working on your electrical projects? spend $20 on a GFCI outlet or pdu. that's a life-saver. If your work bench doesn't have a gfci in the wall, go to home depot or whatever, and you can get a power strip that has GFCI functionality.

Note, from my understanding, the GFCI will provide some protection even if you only have two prongs; it monitors the hot and the neutral.

http://home.howstuffworks.com/question117.htm

They're a last line of defence to prevent persistent electrocution, burns and lock on.

They still take 130-500ms to trip which is more than enough to knock you into VF (that only takes 3ms).

Prevention is always the best approach.

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