Shock Current Path Question

Discussion in 'General Electronics Chat' started by mako, Jul 4, 2008.

  1. mako

    Thread Starter New Member

    Jul 4, 2008
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    :confused: I have a question about the section 'Shock Current Path' (http://www.allaboutcircuits.com/vol_1/chpt_3/3.html). On the fourth picture, I didn't understand why the person was not shocked. If a person touches a wire connected to a source and is standing on the ground, wouldn't that form a circuit itself (i.e. generate voltage between the source and ground)? Does that mean that the ground always has an equal potential to wherever it is connected to? I'm confused.:eek:
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Did you read the accompanying text?
     
  3. mako

    Thread Starter New Member

    Jul 4, 2008
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    Yes, I read it but I didn't understand why there can't be a way for current to be established through the person. Wouldn't the connection from one side of the source to ground form another circuit regardless of the fact that the other circuit is not being completed?

    I think the accompanying text you mentioned is:

    "Since there is no complete path (circuit) formed through the person's body from the bottom side of the voltage source to the top, there is no way for a current to be established through the person."
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Because the voltage source in question was floating. At no point was it contacting earth (ground). As the text says, the reason this is not desirable is it is all too easy to ground it somewhere you didn't anticipate, creating other unexpected hazards elsewhere.
     
  5. Winston

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    Dec 25, 2007
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    What do you mean by, "voltage source is floating?" I can't find that up to that point in the book.
     
  6. leftyretro

    Active Member

    Nov 25, 2008
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    In those drawings a dotted line shows the current path that will cause the shock. The fourth drawing shows a voltage source that does not have either side connected to ground, therefore these is no current path to the person regardless that he is standing on ground and the reason they didn't draw a dotted line. For a shock to occur there has to be a complete circuit from one polarity of the voltage source through the person and returning to the opposite polarity of the voltage source.

    Ground it's self is not a path to any voltage source unless that source has one side of it wired to ground.

    That make sense?

    PS: A "floating" voltage source would be one that doesn't have either side wired to ground. A 500 volt battery that is just sitting on a desk with no wires attached can be safetly touched as long as you only touch one terminal. Touch them both and you will not be happy.


    Lefty
     
  7. Winston

    Member

    Dec 25, 2007
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    Much obliged.

    I posted a question about that page, but I think the mod believes it's been asked before, but I looked and my particular one hasn't. I didn't understand why touching a different part of the circuit, say, just scooting along further toward the other side, made a difference in whether you were shocked or not. Either way, you're standing on the ground.

    I think a managed since then to figure out (I always do right after I break down and ask on a forum) that the current is going from one side to another and that if it's grounded at some point "behind me" (it's easier for me to think of it that way than "top of") then there's no shock because that would be like asking the current to double back on itself.

    I can hear someone saying, now, but isn't the current in ac going in both directions alternately? But, that must be getting way ahead of myself, considering where I am in the book.
     
  8. Winston

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    Dec 25, 2007
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    A "circuit" implies a circle, which implies a <b>return path</b>. In that diagram, there is a path from the wire, through the person, to the earth and... nowhere. Just space. No return path. So it's not a "circuit".

    Unless the voltage were high enough to ionize the air...:D
     
  9. leftyretro

    Active Member

    Nov 25, 2008
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    Well I can't tell from above if you have a handle on it or not, but do try and sort it out if you plan on playing or working with electricity.

    Lefty
     
  10. Winston

    Member

    Dec 25, 2007
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    Well, no. I can't make sense of those diagrams. I get if the ground is on top, there's no return path, but when the ground point is at the bottom, why is it that if you're on one side of a component, you're safe, but if you're on the other side, now there's a return path?
     
  11. leftyretro

    Active Member

    Nov 25, 2008
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    There is no top and there is no bottom, just paths in which current can flow. For current to flow through your body (and shock you) there must be a voltage potential difference between two contact points of your body. Did my battery example tell you anything?

    Lefty
     
  12. Winston

    Member

    Dec 25, 2007
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    The page in question describes two parts of the circuit as top and bottom.

    I get that you must be between two potentials. And if you touch the wire that is grounded at the zero end, the first pic shows that you get shocked because there is a path way through you, the ground and to the (positive) terminal. The second pic shows you down further along the circuit, and now you don't get a shock.

    I guess, for some reason, something is happening with that one component such that being on one or the other side of it means something, but it doesn't say what. There is still a voltage on one side of you, and the ground leading to the grounding rod to the zero potential terminal on the other side of you.

    I get that "electrically common" means the wire could be there, or it could be shorter, or the component could be directly on the terminals. But I can't get from there to seeing why being on one side of a component matters. Am I not still between one side of the source and the other? Many thanks.
     
  13. leftyretro

    Active Member

    Nov 25, 2008
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    "But I can't get from there to seeing why being on one side of a component matters"

    Because that component is 'consuming' all the voltage, it's called the load and the common side of the load is at the same potential as the common side of the voltage source and if your hand and your feet are both in contact with circuit common then there is no potential difference to allow current to flow through you.

    Lefty
     
  14. Winston

    Member

    Dec 25, 2007
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    "Consuming all the voltage" - I'm not sure what that means. Is that mentioned in the book before this chapter?

    I'm not familiar with terminology like "common side" of this and that, but I think what you mean is, I can hang from a wire from my hands and feet and not get a shock, but I can't hang from a wire on one side of a component and have my feet hanging from another wire on the other side of the component.

    What I can hear someone having trouble with is that while there is no voltage between the point where they're grabbing the wire with hand and the point where they've reached up with a foot to hang from the same wire, but a few feet further along that same wire, are we still not between the neg. terminal and the pos. terminal, between that voltage?

    Is it the case, then, that a horse shoe attached to two common points on a wire will not conduct a current, even though current is moving through the wire it's touching?

    Many thanks.
     
  15. beenthere

    Retired Moderator

    Apr 20, 2004
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    Somehow, the concept of hanging from wires or performing gymnastics upon those wires is taking attention away from the necessary point - that coming in cintact with a voltage point may cause a current through your body.

    Take it as a given that any voltage point you are likely to encounter - power line, exposed terminal, etc - will be referenced to the surface you are standing on (ground), and so may realistically be seen as shock hazards.

    Viewed in that way, you will take proper precautions like not coming in contact with a potential shock hazard. If you wish to verify the hazard, using a voltmeter is the only smart thing to do.

    About floating voltages. Think of an operating transformer whose secondary terminals have a difference of 500 volts. Touching both of those terminals will result in a shock. But, if neither terminal is ties to ground, you may touch either terminal with no shock hazard. The voltages on the transformer secondary are relative to each other, but have to connection to ground. In the absence of a circuit making a current path, no potential difference exists, and so a shock hazard is not present.

    Remember to treat every exposed wire as a live wire.
     
  16. Winston

    Member

    Dec 25, 2007
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    I know, I don't plan to hang from any wires. Just using certain images to try and understand the theory being explained on that page. Still, it must be the case that there is such a thing as a safe side of a circuit, otherwise why have this chapter at all? I want to know how that works, because apparently it doesn't work at all the way I thought it did, and which I suspect a lot of people think it does.

    leftretro already explained floating voltage as that of an ungrounded circuit. But what I'm really trying to understand is this business of "electrically common" (which I can't find anywhere else on the internet or in the books I have). It would appear that electrically common refers to a wire, a path, that is uninterrupted by a component and has so little resistance that it can be ignored and regarded as no resistance. But that page puts a number at one 90 deg. angle, and another, and another and it makes one think there is something special about those particular "points".

    So, is there going to be a current through that horse shoe?
     
  17. Wendy

    Moderator

    Mar 24, 2008
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    The safe side is ground itself. Neutral is also grounded, but this is done at the pole.

    The reason one side is grounded is if you do have an accident, such as a hot wire touching a tree (which is what the illustration shows) then you blow a fuse or a breaker rather than present a shock hazard.

    If neither side is grounded and one side touches a tree (also shown in the illustration, trees are conductive) then you have a wire you might think as safe, but is deadly.
     
  18. Winston

    Member

    Dec 25, 2007
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    That part is easy to understand. :)
     
  19. Winston

    Member

    Dec 25, 2007
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    So, the earth, and the grounded wire, and you touching the grounded wire, can be thought of as one entire object that is at zero potential... even though the diagram makes it look like there is a path?
     
  20. Wendy

    Moderator

    Mar 24, 2008
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    It is part of the path, or a potential one at least. The reason it exists is a outlet has 3 wires, neutral and ground being similar potentials. If there is a short inside a piece of equipment where the hot (dangerous) wire touches the case a breaker or fuse blows, removing the threat.

    The ground wire on an outlet never, ever carries current. This allows for GFI plugs in the bathroom, where there is a lot of water connected to metal plumbing that is througly grounded. If it senses any current in ground it trips.
     
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