Why do you get shocked touching a live wire?

Thread Starter

thebriguy

Joined Aug 18, 2016
6
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?

I've searched and asked on other forums, but I've been unable to find a reasonable answer to this.
 

MaxHeadRoom

Joined Jul 18, 2013
25,469
Because of the resistive and capacitive coupling to earth ground, the same reason a neon test screwdriver lights when you touch the blade to AC power and you are wearing rubber soled shoes, there is a minute current that flows, because of this small leakage path.
In your case it is sufficient that you can sense it.
Max.
 

Thread Starter

thebriguy

Joined Aug 18, 2016
6
Because of the resistive and capacitive coupling to earth ground, the same reason a neon test screwdriver lights when you touch the blade to AC power and you are wearing rubber soled shoes, there is a minute current that flows, because of this small leakage path.
In your case it is sufficient that you can sense it.
Max.
Yes - I get that (that there is RC coupling to ground) - and was trying to express that in my original post. What I'm asking is why the numbers don't add up. For example, is my capacitance to ground number (100 pF) way off? If so, it would appear to have to be 100 times that (10 nF) to be of enough significance to feel a shock.

Likewise, when a bird sits on a wire, they have some R and C to ground. It seems clear that the resistance is extremely high. And the capacitance may be low as well. However, they are sitting on a line that is 100s or 1000s of times household voltage. So even if their capacitance to ground was 100x less (which it may be), they would feel a shock, same as I do (assuming their pain threshold is the same). Of course, it could be birds don't sit on extremely high voltage lines.

I'm trying to see where the numbers add up...
 

panic mode

Joined Oct 10, 2011
2,177
you may be particularly sensitive individual or more likely not realizing that you are completing circuit somewhere else.
i have touched live wires of various potentials and can tell with confidence that i don't feel a thing when touching only live conductor ( 120Vac/60Hz or 230VAC/50Hz).
also resistance you measure with common multimeter is fine for low voltage circuits. for higher voltages you need to use insulation tester and you will find that measured values are often significantly lower than those measured with DMM. try your math with 2kOhm for human body and forget capacitance.
 

Thread Starter

thebriguy

Joined Aug 18, 2016
6
Thanks - I think this answers my question (both of them), by giving an independent verification that you can touch a live wire and feel nothing.

The part that I touched was the retaining screw of a switch that was live. I suspect that what I did was touch the grounded front casing of the switch, but I did didn't realize it. That would of course explain why it was felt just in my fingertips.

Its been a number of years since I've shocked myself, and I can't remember the exact circumstances of the other times. (I do recall the first time it happened when I was a teenager, and I had put my finger in a lightbulb socket I thought was off - likely touched both sides of the socket in that case). But I've long though there was some capacitive effect to ground, and only decided now to do the math.

Thanks!
 

Thread Starter

thebriguy

Joined Aug 18, 2016
6
Thanks - I think this answers my question (both of them), by giving an independent verification that you can touch a live wire and feel nothing.

The part that I touched was the retaining screw of a switch that was live. I suspect that what I did was touch the grounded front casing of the switch, but I did didn't realize it. That would of course explain why it was felt just in my fingertips.

Its been a number of years since I've shocked myself, and I can't remember the exact circumstances of the other times. (I do recall the first time it happened when I was a teenager, and I had put my finger in a lightbulb socket I thought was off - likely touched both sides of the socket in that case). But I've long though there was some capacitive effect to ground, and only decided now to do the math.

Thanks!
 

tranzz4md

Joined Apr 10, 2015
310
I do it all the time, no prob, no tingle. I however, am a very dry person: I just barely sweat, and so I don't even couple well with humid air, or with the energized conducive surface. At least that's the conclusion I've come to over the years. I've been shocked when moist, or other factors that I could identify.
 

Thread Starter

thebriguy

Joined Aug 18, 2016
6
High voltage linemen ride on 500,000V regularly.

I've seen that video (quite awesome, thanks for the repost).

There's some distinct differences though: they wear a Faraday suit, and they are a far distance from ground (so capacitance to ground would be less). Clearly from the video, there is significant arcing as he comes close to the line - I wouldn't want to feel that without the Faraday suit.
 

tranzz4md

Joined Apr 10, 2015
310
I just can't believe how lax some people are around mains or other high voltages, coming from a building & rail back ground it's just something you don't take lightly or f%$^ around with.
I might agree with you, as I see people seemingly entirely unaware of what voltages are "high", or what voltage thresholds they tolerate, or how grounded they are in most situations and what other factors and hazards are present to them in an active, real time way, as they work and act. Unfortunately this site has become a place for "DIY" people, including those who are clueless about the actual hazards present, or their mechanisms of harm, or our bodies mechanisms of breakdown.

I say " unfortunately" because some of us will inevitably and unknowingly enable or encourage some others to do things which may well put them in harm's way. I have mostly just lurked here for years, but should probably just leave. This is less and less a site for discussion amongst professionals, and incidental hazards aren't readily recognized. A little burn from a soldering pen used to be the common hazard.
 
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