High voltage and electric shock

Question

I have some silly question that I hope someone can answer.

If a coord, connected to the wall socket, is damaged exposing the wires inside, and one accidentally touches these wires, would the current going through the body to the ground be lethal. I would assume it is, but I have read that the resistance of the human skin is 100,000 ohms, which means the current through the body would be 240V/100,000ohms = 2.4 mA . I also read that lethal current through the body is around 15 mA, so 2.5 mA quite low?
If I am insulated from ground (for example standing on a chair), what would happen if I shove a fork in to the power outlet (or some other metal so that the power is short circuited). Would I be unharmed because the current would not go through me because I am not connected to ground? (Of course the fuse would break immediately, but lets assume that there are no safety-systems).
The article on a website mentions that in this circuit

the guy does not get electrocuted, but in this image

he does get electrocuted. Why is this? In the first Image the person is connected to ground (0) and is also holding the the high voltage line (240V), so the electrons/current would want to flow through him I would assume?

Can only have two links in my post. Here is the website i am talking mentioned https://www.allaboutcircuits.com/textbook/direct-current/chpt-3/shock-current-path/ — lijas, Aug 07 '17 at 18:07
When you put the fork into the socket L & N the voltage on the fork will be 240 / 2 = 120 V due to the equal(ish) resistance of the live and neutral wires. A short while later the circuit breaker should trip. It might be too late for you. — Transistor, Aug 07 '17 at 18:25

score 3 · Accepted Answer · edited Aug 07 '17 at 18:44

3

I have read that the resistance of the human skin is 100,000 ohms, which means the current through the body would be 240V/100,000ohms = 2.4 mA .

That's mostly true, but if your skin is damp, or worse, damp and salty, its resistance will be lower.

I also read that lethal current through the body is around 15 mA, so 2.5 mA quite low?

Also mostly true, but if you have a heart condition, or wear a pacemaker, or just get unlucky, the lethal current for you could be lower than the average one.

I am insulated from ground (for example standing on a chair), what would happen if I shove a fork in to the power outlet (or some other metal so that the power is short circuited).

Typically this will start to melt and or vaporize the shorting wire, with a fairly dramatic show of light and heat.

A small current will also flow through your body due to the capacitance between you and the ground.

Usually you're on your butt before you realize what's happened. (Note: I've only encountered this with 110 V mains. Don't try it at home)

In the first Image the person is connected to ground (0) and is also holding the the high voltage line (240V), so the electrons/current would want to flow through him i would assume?

Notice that in the first picture, the power source is not connected to ground, so the unlucky guy's foot connected to ground doesn't produce any path for current to flow back to the power source.

In the second picture, both the guy and the power source are (indirectly) connected to ground, so there is a complete circuit formed through the guy and the ground, and the guy is not going to have a good day.

edited Aug 07 '17 at 18:44

Neil_UK

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answered Aug 07 '17 at 18:27

The Photon

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Still dont fully understand the third question... Am I wrong in thinking that there is a potential difference in the first picture (240V to 0V), leading to current being able to to flow through the person? Would it be different if it was DC? – lijas Aug 07 '17 at 18:36
No, the power source only produces a potential difference between the two terminals it's connected to. It can't force a potential between either terminal and earth without being connected to earth. (But see the scenarios in the linked page involving trees for cases where either of the two terminals might be grounded by a fault causing the other one to become hot relative to ground) – The Photon Aug 07 '17 at 18:46
So I made the circuit in the images with a battary as the power source, a LED as the guy, and a resistor as the load. I did not know what to use as ground, but I used my right and left hand as some sort of ground (what else could i use?). The diod lit up (very dimly) if I used the circuit in image 2 and was dark if I used the circuit in image 1 (as predicted). What would happen if the ground the tree touches and the ground the guy touches (in image 2) were very very far apart (which I guess can be normal)? Does the two ground need to be connected to each other in some way? – lijas Aug 07 '17 at 20:04
The earth has poor conductivity but high cross-sectional area, so it can be used to carry substantial currents. See [Single-wire earth return](https://en.wikipedia.org/wiki/Single-wire_earth_return), for example. – The Photon Aug 07 '17 at 20:17
RE: "Does the two ground need to be connected to each other in some way?". Yes, see [Can a circuit be powered from Earth with a return ground on Mars?](https://electronics.stackexchange.com/questions/79447/) – The Photon Aug 07 '17 at 20:22

score 0 · Answer 2 · answered Aug 07 '17 at 18:22

Still, 2.5mA will give you a decent tingle.
For starters, you'd be stupid. But if you short it touching neutral first, with one hand, you'd theoretically be fine. Since the fuse or circuit breaker will trip.
If you do not, you'd have half mains voltage on you due to resistor divider, which will find it's way through capacitive coupling. Probably still giving you a tingle. I would not try it.
In the first image, the source is not grounded. There isn't a complete circuit.
The source is floating, or in other words, isolated.

score -1 · Answer 3 · answered Aug 07 '17 at 18:41

You should know that the current always flows in a closed path, since you know what fuse is. The current leaving +ve of battery has to somehow return back to the negative, then only the circuit be complete and current would flow. In the first image the circuit is not complete as the "High Voltage" doesn't have a common ground with the person. The current doesn't flow through him. If the current would want to flow though him then from where will it return back to the battery? There is no path for the current to flow back to the battery. Its like saying the circuit is insulated from ground. As you become insulated by sitting on a wooden chair. You will just hurt yourself from the fork getting heated up. Human body offers about 2000 oh

High voltage and electric shock

3 Answers3