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Let say we have some DC voltage source which produce some voltage which I will label as V. Let also say that we have some real bulb which is supposed to work when voltage V is applied across its terminals. That bulb has some internal resistance R_internal. I will model real bulb as ideal bulb in series with its internal resistance R_internal. We know that for current to flow we need closed path between source and load so if we connect our voltage source and our bulb like on picture below, we will see light from bulb because current will flow: enter image description here I draw two equivalent schematics for this situation.

Let us now see another situation. We know that in AC wall outlet there are two "holes". Mathematical expression for voltage waveform between these two "holes" is shown on picture below:

enter image description here

If we represent voltage between these two "holes" as voltage generator like on picture below and if we connect to it some real bulb which is supposed to work with this AC voltage we will see light from bulb because we have completed circuit and alternating current can flow:

enter image description here

Why is it dangerous for human being to touch one of contacts in "holes" in AC wall outlet? I mean, according to previously discussed situations, there should be no closed path for current to flow If I stand on earth and touch one of contacts in AC wall outlet. I should touch both contacts in wall outlet for current to flow through me and that will be very dangerous. Thanks in advance.

enter image description here

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    \$\begingroup\$ It's very simple to see from your last picture. One of the two conductors is also connected to earth in the transformer before it reaches your house. That one is safe to touch, but if you touch the other one (the "live") you close the circuit through earth \$\endgroup\$ – Andrés Apr 7 '18 at 22:02
  • \$\begingroup\$ The current will flow(current will circulate) to neutral through the earth(because earth and the neutral are connected). So there is loop. Look at this: epanorama.net/documents/groundloop/feed_1phase.gif In your case earth is that ground wire. But if you are totally isolated and touch the live it won't harm you. If your foot is wet or naked and has contact to earth then it is fatal. \$\endgroup\$ – user16307 Apr 7 '18 at 22:04
  • \$\begingroup\$ If there is isolation transformer between the mains and you, it is also safe: i.stack.imgur.com/AYytn.gif But with capacitive coupling you will feel shock. \$\endgroup\$ – user16307 Apr 7 '18 at 22:11
  • \$\begingroup\$ See also electronics.stackexchange.com/q/215868/4512. \$\endgroup\$ – Olin Lathrop Apr 7 '18 at 22:18
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    \$\begingroup\$ @MITURAJ Here: electronics.stackexchange.com/questions/349854/… asking about the loop.. The answer is great. \$\endgroup\$ – user16307 Apr 7 '18 at 22:24
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This is a fallacious question, meaning bad logic from false assumptions.

is it dangerous for human being to touch one of contacts in AC wall outlet?

What is dangerous to be ignorant (unaware) of the reason how it "could" be dangerous.

If you touch AC line AND also have ANY stray or direct contact with Neutral or Earthing ground then current may flow.

Earth leakage can be a humid weather wearing damp socks on a dusty floor which may be uncomforatble leakage but not lethal. Or you may have leakage resistance or stray return current path "conductance" or "dielectric impedance" to conduct to the earth return path, which is also shared with the NEUTRAL wire.

The "ground" may appear on the surface of an appliance or plumbing or peripheral PC case that is also "earth" bonded or grounded meaning 0V connected to earth bond wire.

Voltage just has the "potential" to be dangerous but it is current that you feel or contracts muscles or becomes lethal. This depends on V/R or the leakage resistance and insulation breakdown voltage that has the risk to conduct current that is be dangerous above a certain threshold.

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  • \$\begingroup\$ Sorry but these are the facts (1) (proven by laws of Physics and Aristotles laws of logic, so idiots need not reply (-1) \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 7 '18 at 22:13
  • \$\begingroup\$ For example I touch white neutral and with with hot wires for electrical changes "live" with power on and that is safe for me. But the question says "humans" and I am human so the question is fallacious. and so are the idiots who said -1 \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 7 '18 at 22:46

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