When is a ground fault a problem?

Prog

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I'm getting some bites from some of my machines. Nothing near mains voltage (I've touched enough hot outlets to know), but still some definite tingling. One should be grounded, which I plan to investigate. One is getting a new cord with an untampered ground pin.

Is grounding the machine just a bandaid though? Shouldn't there be no* potential on the ground lines anyway? If so, where should I start investigating this leakage? PSU, I assume?

*Pedantic physics nerds: by "no potential" I mean "a negligible amount of potential, far below the threshold of detection in any reasonably dry person"
 
It's not that uncommon to have some potential build up due to the stray capacitance between the AC typically in the PS/monitor and the chassis. Grounding should eliminate this. If it's floating it can charge to a relatively high potential, I've seen as much as 60VAC on the ground if left floating. (enough to feel it) Because it's due to stray capacitance and/or very high resistance the current is very small so when you ground it there should be very little current flowing. If it is a significant current when you ground it then there IS a problem you should investigate. Try grounding with a 1K resistor then measure the voltage across the resistor. 1V = 1mA, you shouldn't see more than 10mA I would think. Typical GFI breakers will trip between 10-100mA of ground current (or mismatch between neutral and hot)
 
I agree with most of what you said. But a GFCI should trip at a much lower level than that and very fast (within 30-100 milliseconds at 5mA or so) otherwise severe injuries can occur. For instance a sustained 75mA is very often fatal at 60 Hz.

Some rules of thumb that I remember:
  • 1mA = that's tingly
  • 5mA = that hurts
  • 15mA = can't...let go!
  • 75mA = can't...........breathe........-> respiratory arrest
  • 100mA+ = oooh....my heart! -> cardiac fibrillation

  • 200mA~250ma = dang it! my muscles just snapped my bones!
(Gamers and other weaklings obviously have some resistance on the last one! Due to nerd muscles. :p)

Of course it takes a few seconds to die, but it happens at much lower currents than is commonly assumed.
 
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