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

Discussion in 'General Electronics Chat' started by pitanjaiodgovori, Apr 8, 2018.

  1. pitanjaiodgovori

    Thread Starter New Member

    Apr 8, 2018
    6
    1
    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:

    [​IMG]
    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:

    [​IMG]

    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:

    [​IMG]

    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.

    [​IMG]
     
  2. MrChips

    Moderator

    Oct 2, 2009
    19,111
    6,143
    Vac from mains is not floating. It is referenced to ground.
    Touch one side of Vac and you complete the path to ground via your body.
    This is a stupid and dangerous thing to do. Don't attempt to test this.

    You do not need to be standing on earth to be electrocuted. There is enough leakage current that can prove to be fatal.
     
  3. pitanjaiodgovori

    Thread Starter New Member

    Apr 8, 2018
    6
    1
    I'm not going to test anything. I just want to understand how things works. Why one contact is not floating? If my model is correct, from last picture we can see that one contact is floating...
     
  4. MrChips

    Moderator

    Oct 2, 2009
    19,111
    6,143
    That is the model in your picture. In real life that is not the model. In real life, one line (NEUTRAL) is referenced to earth. Hence LINE is not floating.
     
  5. jpanhalt

    Expert

    Jan 18, 2008
    7,635
    1,857
    pitanjaiodgovori likes this.
  6. ericgibbs

    Moderator

    Jan 29, 2010
    8,505
    1,711
    hi,
    Maybe a image will explain.
    E
    elin4.png
     
    -live wire- likes this.
  7. pitanjaiodgovori

    Thread Starter New Member

    Apr 8, 2018
    6
    1
    So my model is not good. According to you, valid model would be:

    [​IMG]

    Right?
     
  8. pitanjaiodgovori

    Thread Starter New Member

    Apr 8, 2018
    6
    1
    While I was writing message, ericgibbs already wrote message. I see now that he draw exactly what I did.
     
  9. pitanjaiodgovori

    Thread Starter New Member

    Apr 8, 2018
    6
    1
    Let say we have some generator which generates exactly same voltage waveform as we have in AC wall outlet ( vac(t) = Vm * sin( w * t ) = 220 * sqrt(2) * sin ( 2 * pi * 50 * t)). There are two contacts on generator for loads to be conected and NO contact is connected to earth. Then, it would be safe to touch only one contact at time of that generator because other one will be floating and there is no closed path for current to flow, right?
     
    -live wire- likes this.
  10. MrChips

    Moderator

    Oct 2, 2009
    19,111
    6,143
    Yes and no.
    There is something called leakage current. You do not require ohmic circuitry for leakage current to occur. AC current can cross a capacitor. Hence capacitive coupling between the body and the AC source can cause leakage current to flow.

    You need not even have to touch the cables from the generator. I have experienced a tingling sensation while standing directly below high voltage power lines.
     
  11. ericgibbs

    Moderator

    Jan 29, 2010
    8,505
    1,711
    hi,
    If there is no Earth return, touching either wire would not be a problem, both at the same time Yes!
     
    pitanjaiodgovori likes this.
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
    18,805
    5,858
    If you really want to learn the history and means of earth grounding, pick up a copy of the IAEI book by Eustace Soares.
    Here is a relevent page using N.A. service as an example.
    Max.
     
    cmartinez and pitanjaiodgovori like this.
  13. Chris_Aker

    New Member

    Dec 3, 2017
    2
    3
     
  14. Chris_Aker

    New Member

    Dec 3, 2017
    2
    3
    As Mrchips states, the Neutral side of the incoming main supply is bonded to earth. That provides the current path back to Neutral. Only a few milliamps can be fatal.
     
  15. Tonyr1084

    Distinguished Member

    Sep 24, 2015
    3,437
    937
    Ask yourself why birds can land on high voltage transmission lines. The answer to that question is that no circuit is completed. Their bodies are at the same potential as the power line, even if it's at 25,000 VAC. I can't speak from experience, but I'd imagine they MUST feel something as the charge changes from positive to negative (as the sine wave goes). I've seen video of high tension lines being repaired by a helicopter and a linesman sitting on a side platform. First thing he does is electrically bond the aircraft to the line using a long pole. You can see the voltage jumping between the pole and the wire. Once bonded he can safely touch the line and perform repairs to the cable as needed. Because there is no current passing through him to a different potential his life is not in danger. However, if he were to touch the wire without bonding to the line then he'd see a significant charge and likely, like any decent electronic component exposed to an over-voltage condition, his smoke may leak out.

    You can touch an isolated power line as long as you don't provide a path to the rest of the circuit. I've seen guys grab 120 VAC lines that were energized. As long as they didn't touch ground or neutral they didn't suffer any ill effects. I wouldn't try it. But I've seen it. And I wouldn't advise it either. So kids - DON'T TRY THAT AT HOME! Being insulated from ground via shoes, carpeting, wooden floor substrates and such make for a fairly good insulator. But if there's moisture in the carpet you're asking for a zapping. Concrete can also conduct electricity. But typically it has to be some higher voltage than 120 VAC. And yet, even 120 VAC can be dangerous if you're on it bare feet. So again - DON'T TRY THAT AT HOME!

    In your first circuit there was no path to "Return" (or ground as you might be thinking of it). Every circuit has a source and a return. Otherwise it's just a wire. (Antenna not included) But when you complete a circuit between source and return - you may be "Returning" to dust.

    I realize this is hypothetical speak. Still, safety is ALWAYS #1 when dealing with high voltages. Let me rephrase that - Safety is ALWAYS #1 when NOT working with extremely low voltages. It's been my experience that potentials as low as 65 VAC can be felt. One afternoon on a job site the question was being asked why a certain component tested good but when installed into the module it was blown. The reason was because of a faulty ground presenting a potential voltage through capacitive inductance (if that is the correct terminology for voltage cross talk between lines) into the module. The soldering iron used to install it was properly grounded while the module itself was not. Touching it with the soldering iron presented a return path for the 68 volts. It was discovered when I happened to be touching the power strip that was plugged into a properly grounded outlet with one hand while the other hand was on the module, which was under test via a computer system that was NOT properly grounded. I cold feel the tingle of the voltage. When I made comment about it suddenly the whole problem made sense. The soldering iron was providing the "Return" path and the component, sensitive to voltages as low as 5 VDC was being blown out.

    Hypothetically speaking, your circuit does not provide path to ground. The later circuits WITH grounding DOES complete the path to Return.
     
  16. dendad

    Distinguished Member

    Feb 20, 2016
    2,921
    810
    And why risk it?
    I have had a bite of the mains (240VAC here) and it hurts!
    There are many things in life that are not worth trying. Walking blindfold across a freeway, drinking unknown liquids out of random bottles, touching the live mains on purpose....
    Some people are ignorant, and that can be fixed with education, and some people are stupid, and not much can be done with that.
    You are no longer ignorant as the reasons have been explained to you. Please now, do not be stupid.
     
    pitanjaiodgovori likes this.
  17. MaxHeadRoom

    Expert

    Jul 18, 2013
    18,805
    5,858
    One way of indicating a current path through the body is when you use one of the 'Electricians' screwdriver/testers that has a neon lamp in the handle, when you connect the screwdriver to the live side of the outlet, the neon lights due to a very small current passing through the body to earth.
    You can even stand on a milk crate and it will still light!.;)
    Max.
     
  18. dendad

    Distinguished Member

    Feb 20, 2016
    2,921
    810
    Another point, on your simulations circuits, you need to add capacitance from each side of the mains to earth anyway. That is always there.
    And this capacitance can carry significant current.
     
  19. pitanjaiodgovori

    Thread Starter New Member

    Apr 8, 2018
    6
    1
    This message looks to me as you think that I'm going to make some experiments and to touch AC mains contact. I have NEVER intention to do that. I'm not crazy. Before I started this thread, I knew very well that AC mains voltage is lethal. I draw even waveform of that voltage and gave amplitude of that voltage (220*sqrt(2) in my country). I just didn't know that neutral side of main supply is bonded to earth. Now, because I know that fact, I unterstand how human being completes circuit when touches phase line of AC wall outlet.
     
  20. MaxHeadRoom

    Expert

    Jul 18, 2013
    18,805
    5,858
    This is the basis of the operation of RCD/ELT , Earth Leakage Trip device.
    It carries the current for both feeder lines of the service, e.g. L1 & N.
    If there is any slight unbalance between the two, it is usually an indicator that the L1 side has leakage to ground and therefore is passing more current than the N side.
    In this case the device open circuits.
    Max.
     
    pitanjaiodgovori likes this.
Loading...