Why do you get shocked touching a live wire?

Question

A number of times (over many years) I've accidentally touched a live AC mains wire (120V, 60Hz) while working on home wiring. The feeling is always similar: uncomfortable tingling feeling at the point of contact (generally the tip of my finger). In all cases, I most certainly have not had "good" contact with ground (for example, when it occurred today, I was standing on a plastic stool, with rubber soled shoes).

My question is: why do I feel a shock?

More specifically:

• My resistance to ground is too high to have any non-negligible current.
• My capacitance to ground seems not high enough to have a noticeable shock (see below).

I see some resources online suggesting a capacitance to ground of a few 100 pF. If true, then (assuming 100 pF capacitance):

Z = 1/(2*Pi*f*C) =~ 26 MOhms
I = V/Z = 120V / 26 MOhms =~ 5 uA

The threshold of sensation is generally specified to be around 1mA (somewhat less at 60Hz). So, if my body capacitance is correct, then I'd expect to feel a sensation only at voltage 100 times higher (10kV). Of course, my body capacitance could be wrong. Is it, or is there a different effect going on?

Question part 2: Why is the sensation only in my fingertip?

Answer 1

A perceptible shock suggests that you actually experienced >500 µA for at least 10 ms.

The human body for ESD has 100 pF in parallel with a resistance ranging from 1.5 kΩ to 100 kΩ. If your skin is damp or broken, it's much lower. When receiving a shock at 60 Hz, the capacitance is equivalent to the ~26 MΩ impedance in parallel with <100 kΩ resistance. So we have two parallel impedances, one much smaller than the other – so capacitance is negligible for a 60 Hz shock (unless the shock is very high frequency or impulse-like).

Using 100 kΩ, you experienced ~1.2 mA of current depending what part of the 60 Hz was in effect, inside the AC-2 (perceptible) region.

Electric shocks kill when your resistance is much lower or when the path of least resistance includes your heart. If your resistance was 1.5 kΩ the shock would be 80 mA, potentially fatal if it lasted long enough.

Why only your finger? Most of the electrical resistance in the human body is the outer layers of skin. Inside the body your tissue is saturated with blood, and connected to the bloodstream. It's wet and conductive. Most of the energy from the shock is deposited in the larger electrical resistance.

Suggestion: get voltage sensing tools that light up when held near live voltage. They could save your life.