A question about 'shock current path' in "Vol I - DC > Chapter 3: Electrical safety"

Discussion in 'General Electronics Chat' started by injtsvetkov, Dec 12, 2015.

  1. injtsvetkov

    Thread Starter New Member

    Dec 12, 2015
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    Hello, I got a bit confused while reading this page. Even after reading it several times I could not get it clear. Please don't laugh at me too much if my questions are stupid but I see there are other people also posting threads about that page and it seems that it needs some clarification.

    Confusion 1: On one hand, in order to have electron flow there has to be a complete path from one pole of the source to the other pole. On the other hand, electrons flow whenever there is a potential difference between two points. On the picture the voltage source is DC (up side + and down side -). Lets say it's 12V DC for example. It is known that the ground potential is 0V, doesn't that mean that there will be a 6V potential difference between one side of the source and the ground? If so wouldn't there be some flow of electrons from the ground (as an infinite source of electrons) towards the positive pole?

    [​IMG]

    Confusion 2: It is said that if two people standing on the ground touch different points in the circuit they will both get shocked. Now on this picture current is flowing between the second person and the tree, which means the tree also gets shocked. I can't understand what stops it to flow between the two people instead? In my opinion there would be current between the two people too, which means the bird is wiser than the tree and the two people altogether because it's the only one who wouldn't get shocked in this scenario.

    [​IMG]

    Please help me to understand this grounding concept because the more I read it the more confused I get and even more questions arise. I really hope some of the moderators to pay attention because it seems that I'm not the only one confused.

    Thank you!
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    In the first illustration there is no ground, so no path through the person exists.

    In the second the tree is a conductor, and grounds on side of the voltage source. The person on the grounded side is safe, while the person on the hot side is not.
     
  3. injtsvetkov

    Thread Starter New Member

    Dec 12, 2015
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    Thank you for the prompt reply!

    Unfortunately the confusion gets bigger, somehow it does not make sense. You say that tree is a conductor but isn't the human body also a conductor?
    Lets see the next picture. There is no tree here and current flows between the two people because there is a potential difference and a path for electrons to flow. Now if we add a tree touching the upper wire there would be current between it and the person touching the lower wire but that cannot stop the current between the two people because the potential difference between them still exists. It is like two parallel branches of a circuit. The way I see it, is that no matter how many trees we add on the upper wire the two people still will get shocked because it's just a separate branch of the circuit.

    [​IMG]

    Lets make the scenario even clearer, we substitute the tree with a third person. So here we have one branch between Person 1 and Person 2 and another branch between Person 2 and Person 3.

    000600.png

    Is there any truth in this picture or I got it all wrong?
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Really, where is the battery grounded? It isn't, so those pictures do not make sense.

    If you hold a 9V battery (and not touch either terminal) what is your potential to the battery.

    Hint: There isn't any, you are not part of the circuit.

    Visualize wires instead of the ground symbol, all that symbol does is show common points.

    Since this has turned into a basic question of electronic theory I am moving this thread.
     
  5. Veracohr

    Well-Known Member

    Jan 3, 2011
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    First remember that a voltage measured at a point is always in reference to some other point.

    Now, I've redrawn your three scenarios inserting resistors for the people. I drew red lines for (potentially) lethal current through people, and orange lines for circuit current. Arrows are drawn in the direction of electron flow.

    Scenario 1:

    Ground is only equal to 0V if we define it to be so. The top circuit shows a 12V source which has a 0V reference in the middle which we call "ground", and is the same "ground" as that which the person is standing on. The person gets shocked because the point we're calling "ground" is common to both the source and the person, enabling a current to flow through the person (also because the person is represented as a resistance). In this case Person 1 indeed has 6V across them as you mentioned, but only because there's a common point (direct connection) between them and the source.

    The bottom circuit shows a 12V source which has no external reference, such as a battery. The person is standing on "ground", but since the 12V source has no reference to this "ground", there is no current that can flow through the person. The important thing to realize here is that we haven't made "ground" equivalent to any point in the 12V source. Therefore, the point we're calling "ground" is not in any circuit which connects the two poles of the 12V source.


    Scenario 1.png


    Scenario 2:

    This scenario only makes sense if you view the tree as a conductor with no resistance. If that is the case, both sides of Person 1 are at the same potential (they both connect to +12V without going through any other resistance), so no current flows through Person 1. On the other hand, the bottom side of Person 2 is connected to +12V while the top side of Person 2 is connected to 0V, so current can flow through him/her and thus deliver a shock. In this scenario we have defined "ground" as +12V.

    In reality a tree will probably have some resistance.


    Scenario2 .png


    Scenario 3:

    In the third scenario as you have defined it, the tree is replaced by another person, represented by another resistance. Current was already flowing through this branch because of Person 2, so Person 3 now receives a shock as well. In addition, the new resistance of Person 3 means that the two sides of Person 1 are now at different potential, thus enabling a current (and resulting shock) through Person 1. In this scenario we have defined "ground" as a point somewhere between the two poles of the source (where depends on the relative resistances).


    Scenario 3.png
     
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  6. injtsvetkov

    Thread Starter New Member

    Dec 12, 2015
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    0
    Thank you guys for your time and effort!
    Now the things began to make some sense.
    Generally the reason for my confusion was the misunderstanding that there is not much difference between the resistance of a tree and that of a human body. So I thought that an accidental grounding by a tree has pretty much the same effect as when a person (lets say barefooted) touches the wire. Now I understand that "Grounding" means connection without any resistance to earth ground, because if the tree has some resistance than this is not grounding.

    Thanks again and happy holidays!
     
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