Small shock from mains

Okay so I was replacing some sockets the other day and accidentally got a small shock (for some reason one upstairs socket was not connected to the upstairs sockets breaker…)
Anyway I only felt a small tingle in my finger and I’m trying to discern what happened. I was sat directly on top of carpet, and the breaker did not break upon being shocked. Which of the following is correct:

  1. If I only touched one wire it must have been the live wire. The only way I could have felt a shock from the neutral is if there was a significant current draw elsewhere, but the property is empty with next to no appliances plugged in. So it must have been the live. In that case why did I only feel it in my finger? The current would flow down my arm to my legs and to ground. I get carpet is a great insulator but even so surely I should have felt it somewhere other then my finger? Also shouldn’t the breaker have switched upon current flowing down live but not detected back on neutral (even if this current was tiny)?
  2. I somehow bridged live and neutral with my finger. The current only flows between a small section of finger and thanks to the high resistance of my skin only a small current flows. The breaker doesn’t switch because this is perfectly normal behavior. What I don’t understand is from what I have read I would expect my finger to at least burn slightly, rather than a small tingle and no discernable mark on my finger. Also since I would have to have touched one wire first (law of probability) I suppose it would have had to be neutral then live (else we’re back to 1)?

Electrical Engineering Asked by Mr_Random_Guy on January 1, 2021

3 Answers

3 Answers

You could have felt the shock only in your finger because that small point was where it all entered and therefore had the highest current density, and once it was inside you, the charge spread out while flowing through you thereby reducing the current density. I would think if you were standing on just a tippy toe you would also have felt something there too.

The closest I ever got to be shocked was when I was holding two wires on some ~50V (I think?). I felt a tingling in both my hands where the current entered and exited.

Correct answer by DKNguyen on January 1, 2021

Some electricians in Canada and the US di not use a light or a meter to test if a circuit has 120VAC. Instead they use their fingers.

Answered by Audioguru on January 1, 2021

Assume capacitance, as you sat, created by 30cm by 30cm area, thru 1cm of carpet, into underlying conductive underflooring/concrete.

Using C = E0 * Er * Area/Distance, with E0 = 8.9e-12 farad/meter and Er = 5, we now have

C = 45e-12farad/meter * Area/Distance

C = 45-12 * (30cm * 30cm / 1cm ) * 1meter/100 cm

C = 45e-12 * 900/1 * 1/100 == 45 * 9 pF = 405 pF. Call it 500pF

Now, from Q = C * V, and differentiating with constant dC/dT using chain rule,

I = C * dV/dt

and for clean power line sin, you have dV/dT = 160v peak * 377 radians/second ~ 60,000 volt/second.

I = 0.5e-9Farad * 6e+4 volt/sec= 3e(-9 + 4) = 3e-5 = 30 microAmps

which is well below the danger (heart pulsations, or muscle lockup), but enough to feel.

Answered by analogsystemsrf on January 1, 2021

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