Your body's resistance, which can vary widely, is around 10Kohms. Much lower if you're in the bathtub taking a bath and touch a wire with wet hands. GFI's are set to trip at about 7ma of imbalance. 7ma of course will not trip a regular 15 amp circuit breaker or a 400 amp breaker. A major variable if you live or not is the path the current takes thru your body. One hand touching the 120V and the other hand touching a ground will cause the current to pass thru the area of your body where your heart is. That ain't good, might goof up the heart beat.

So the answer is: the fault current is the killer, but because of Ohm's law what voltage source you happen to touch while part of you is also touching another voltage source or ground will determine what fault current you'll pass thru you. A 12V car battery won't create enough current to harm you (but do be careful of metallic rings on fingers don't create a dead short from the + terminal to the car's frame and get very hot and burn your finger off).

Higher voltages will create higher fault currents thru your body given the same resistance. Also a poor enough contact that would let you survive a contact of 120V to ground can kill you at 277V.

If you touch 120V or even 277V and nothing else (because you are standing on insulation) you are an open circuit and no current flows. That's the reason for the ham radio safety rule of placing the other hand in your pocket while poking around high voltage in a radio transmitter. Not that you purposely touch high voltage, but if you slip with the voltmeter probe this precaution offers redundancy to prevent shock.

[This message has been edited by wa2ise (edited 04-13-2005).]