Re:Why Three Prongs? (1996)
by kragen on Monday July 08, 2019 @03:43PM
(#20386066)
> does the capacitance internal to the generator matter, and what level might this typically be?
It might, but it can only make the situation safer. For example, if the "neutral" wire's only connection to the chassis is through its capacitive coupling to that chassis, then the impedance from that capacitance is in series with whatever the chassis-puddle capacitance is. Typically such capacitances are going to be much smaller, because the chassis and hypothetical puddle have much larger surface areas than anything else in the generator. Some can be larger, but their impedance isn't going to go below zero. (A sufficiently large tuned inductance could go below zero, but it would have to be very carefully tuned to lower the impedance rather than increase it.)
> Also, how did you estimate the 1KΩ resistance to ground? Mostly as an aside, I presume you are familiar with the possibly apocryphal story of electrocution with a 9V battery?
Mostly by sucking on ohmmeter leads in the past, although it's also common in textbooks. Yes, I've heard the story; if I had heard it earlier, I might have been more reluctant to suck on ohmmeter leads, although modern digital VOMs use smaller currents that are probably not dangerous.
It might, but it can only make the situation safer. For example, if the "neutral" wire's only connection to the chassis is through its capacitive coupling to that chassis, then the impedance from that capacitance is in series with whatever the chassis-puddle capacitance is. Typically such capacitances are going to be much smaller, because the chassis and hypothetical puddle have much larger surface areas than anything else in the generator. Some can be larger, but their impedance isn't going to go below zero. (A sufficiently large tuned inductance could go below zero, but it would have to be very carefully tuned to lower the impedance rather than increase it.)
> Also, how did you estimate the 1KΩ resistance to ground? Mostly as an aside, I presume you are familiar with the possibly apocryphal story of electrocution with a 9V battery?
Mostly by sucking on ohmmeter leads in the past, although it's also common in textbooks. Yes, I've heard the story; if I had heard it earlier, I might have been more reluctant to suck on ohmmeter leads, although modern digital VOMs use smaller currents that are probably not dangerous.