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Old Feb 7th 2017, 12:18 PM   #1
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Electrical circuit closing through body

Hello. I have a couple of questions.

1.) Hypothetical situation: If I touch a live mains wire (not a closed circuit, just one wire sticking from the socket) electricity is not going to flow through my body which is not grounding because I'm on the 4th floor on my building. So I would not get a shock, right? Then why can I use a neon bulb screwdriver for testing a live wire? It still requires an electric flow through the body...

2.) Imagine a barbecue meat fork with two prongs. When I was little I stuck one of those in a socket but I held it by the plastic handle, so I'm good. But what if I was holding the metal itself? Theoretically the circuit will close at the point where the prongs meet, we would have a short circuit and there will be no flow above that point. Is that correct? Also, if I was holding it by the exact place of the meeting prongs, only my fingers would get damaged and not the entire body, right?

I'm not an engineer soo... sorry for the stupid questions, but I'm curious Thanks.
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Old Feb 8th 2017, 07:03 AM   #2
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1) If your feet are perfectly insulated, and no part of your body is touching anything that is connected to ground or another electrical circuit, then you are correct - you would not get a shock. You would be like a bird on a wire - notice how a bird can sit on an electric high voltage wire without harm? That's because there is no electrical circuit through his body. But if the bird's wing span is large enough so that his wing tip touches one of the other wires, he would be electrocuted.

2) The voltage at the "hot" end of the prong is equal to the supply voltage (i.e. 110 or 220VAC), and the other prong is has voltage equal to zero (theoretically that is - actually the neutral lead in the outlet isn't exactly 0, but it's close). When you short these together the actual voltage at the fork is somewhere in between those values. If you touched the exposed fork prongs with your finger you would have the same situation as in (1) - either nothing would happen (if you are perfectly insulated) or you would get a shock that would travel through your finger, hand, arm, shoulder and the rest of the path through your body to whatever the point of grounding is (your feet perhaps).
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Old Feb 8th 2017, 07:03 AM   #3
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basically yes, but...

"Ground" basically means something large enough (with a sufficiently different electrical potential) which will accommodate the excess electrons (or supply electrons to satisfy the deficit) which is present at the end of the live wire.

Being on the forth floor won't necessarily save you, it depends on the capacity of your building to supply or accept electrons.

The neon bulb tester lights up on a very small flow of electrons, but critically a small flow of high energy electrons.
High energy electrons mean they come from a high voltage source, but the neon tester is designed to restrict the flow to a non-dangerously small number of electrons per second.

With the fork, it is generally said that the electricity takes the path of least resistance,
but that is a simplification, or approximation, for when the difference in the resistance of the two paths is very great.
If the ratio of resistance of path A to path B is 9 to 1 then the electricity will split in the same 9:1 ratio.
For a human verses a metal fork the ratio will be thousands to 1 so the amount of electricity taking the human body route will be small, but not zero.
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Old Feb 8th 2017, 09:46 AM   #4
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Huge thanks to both of you, I get it now
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Old Feb 8th 2017, 10:36 AM   #5
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However you will feel it if you are drunk and hold two wires to your tongue that are supposed to be connected in a phone jack. Not that I know this from personal experience or anything.

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Old Feb 9th 2017, 06:56 AM   #6
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Originally Posted by topsquark View Post
However you will feel it if you are drunk and hold two wires to your tongue that are supposed to be connected in a phone jack. Not that I know this from personal experience or anything.

-Dan
Now THAT would hurt! I can't standing touching a 9 volt battery to my tongue (an old trick for testing whether it's alive or not). A copper telephone line is normally at -48V DC, but during ringing the voltage is around 90 VAC!

I have a slightly less embarrassing story about my first trip to visit a telephone company central office back when everyone had copper land lines. I was in a group that was touring the CO, and we stopped in front of the Main Distributing Frame, which is where the outside plant pairs come into the building to be connected to the switching equipment. The connections are wires that are connected to exposed metal terminals, using either solder, wire wrap, or insulation displacement terminals. As the group stopped to talk I casually put my hand up and leaned against the bare face of some of the terminals. No problem for about 30 seconds, but then I experienced ringing voltage. As my hand jerked back I of course tried to act nonchalant, like I knew to expect it. But it was pretty obvious that I'd been caught unawares. My colleagues all thought it pretty funny.
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