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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?

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  • \$\begingroup\$ SCNR: What was your other hand doing at the time? \$\endgroup\$ – Turbo J Nov 13 '16 at 1:23
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    \$\begingroup\$ Had to look up SCNR. \$\endgroup\$ – Brian Nov 13 '16 at 1:26
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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.

Diagram of electric shocks as experienced by humans

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.

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    \$\begingroup\$ I missing what path you are suggesting the current is flowing through my body (at 1.5kohms to 100 kohms) being that I was not grounded... \$\endgroup\$ – Brian Nov 13 '16 at 2:46
  • \$\begingroup\$ How about the displacement current which flows only momentarily? \$\endgroup\$ – soosai steven Nov 13 '16 at 3:03
  • \$\begingroup\$ @Brian you were grounded. Your body was at roughly the same electrical potential as earth, with a high impedance connection to it. Even if you were "floating" with different electrical potential, you would probably experience a shock anyway. Linesmen can work on live wires if lowered from a helicopter or raised on a non-conducting platform but they usually experience an arc while approaching, until they bond to the live wire and match its electrical potential. \$\endgroup\$ – jbarlow Nov 13 '16 at 21:20
  • \$\begingroup\$ @jbarlow But that implies that standing on a plastic stool with rubber soled shoes is a much lower impedance than my body's internal resistance - that doesn't seem correct. \$\endgroup\$ – Brian Nov 13 '16 at 21:39

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