electrical safety confused?

Discussion in 'General Electronics Chat' started by motorcyclestunts, Jan 10, 2015.

  1. motorcyclestunts

    Thread Starter New Member

    Apr 3, 2011
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    http://www.allaboutcircuits.com/vol_1/chpt_3/3.html

    Hello everyone ,

    I'm having a hard time understanding this section .

    In this first picture, the only reason i can see him not getting shocked is because electricity would take the path of the least resistance . I can't understand what hes trying to say ; if anyone can explain what he means I would appreciate it Capture.PNG
    In the second image, wouldnt the current flow from the circuit through his body and into ground? wouldnt that be a complete path with ground ?
    Capture1.PNG
    Capture2.PNG
     
  2. ian field

    Distinguished Member

    Oct 27, 2012
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    If you can't get it from those diagrams - take up woodwork!
     
  3. crutschow

    Expert

    Mar 14, 2008
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    In the first case the circuit does take the path of least resistance through the return wire. (Actually the current travels in proportion to the relative resistances, but since the person and ground have such a large resistance in comparison to the return wire, only a minuscule current flows through the man.

    In the second image the high voltage is not grounded (which is not the usual case), that's why the person does not get shocked.
    It's an abnormal condition to illustrate a point.
     
    djsfantasi likes this.
  4. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Harsh post when not offering an explanation.

    TS, trace the possible flow of current with your finger. The foot is grounded, through the body, through the top wire, through the tree, to ground. Ground to ground; no potential drop, no shock.
     
  5. ian field

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    When I was a lad I had an old valve (tube) B&W telly in my bedroom, and a long wire aerial out the window for the old regen radio someone gave me.

    One day I looked out the window and noticed loads of birds perched on the wire, so I pulled the aerial from the radio and stuck the lead on the TV EHT rectifier - a few of the birds had corona discharge around their beaks and various other pointy bits, but none of them died.
     
  6. wmodavis

    Well-Known Member

    Oct 23, 2010
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    Electric current does not just take the path of least resistance rather it flows through all paths that form a complete circuit. It's called parallel electric circuits. The current flowing in each path is determined by the voltage present and the resistance in each of the parallel paths.
     
  7. ian field

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    Nowhere near as harsh as what would happen if the OP stuck their finger in the gubbins while leaning against an earthed radiator!!!
     
  8. Steve R in FL

    New Member

    Jan 9, 2015
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    Part of the confusion might be that the negative post of the battery is NOT grounded in all of the diagrams.
     
  9. profbuxton

    Member

    Feb 21, 2014
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    Firstly lets keep this fairly simple and lets ignore any resistance in the wiring and earth paths.
    So:
    (Pic 1) The bird is not "shocked" since it does not complete a current path across the supply(even if bird is at high voltage there is nowhere for the current to complete a circuit). The human is not "shocked" because there is no potential difference the wire he is holding and the battery connection. Both are connected to "earth" (ground) at the same potential difference so no current will flow.

    (Pic 2)Again the bird is not shocked for the reasons stated above. The human is not shocked because although he is "earthed" there is no path for the current to flow back to the supply. There is only one earth point and that is the human.

    (Pic 3) Bird is not "shocked" for the same reason as above. Now, however we have a accidental connection from the high voltage side to earth via the tree. The human connecting to the high voltage side will not be "shocked" for the same reason as in Pic 1. The human now touching the negative side of the battery will be "shocked" because there is now a potential difference between the earth and the negative side. It is basically the opposite of the connection in Pic 1 except the connection through a fault(tree). One could just as easily make a proper connection from the battery positive to earth and the same would apply.

    (pic 4) Bird is not "shocked" as per above. This battery is isolated from earth, a "floating" supply.Both humans are "shocked" since they have made a connection "across" the load. One human connecting one side to earth will not be shocked but if the other human connects the other side of the load at the same time they will "shocked" since they are both connected across the full battery potential and form another "load" on the battery. Note that the complete current path for them is via "earth"(ground).

    Most supplies connect one side to Earth(ground) and the load is connected via some form of protection device(fuse, circuit breaker) so that if there is a accidental earth on the "positive side" a "short circuit" current to earth will blow(trip) the fuse(circuit breaker)
    Some systems use floating supplies for fail safe reasons where supply cannot be interrupted(railways etc.) but they have special devices to detect any single earth faults to generate an alarm so it can be fixed before a double earth fault occurs.


    PS Lesson: Be a bird!
     
  10. Steve R in FL

    New Member

    Jan 9, 2015
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    Something seems wrong with the second diagram. With the entire circuit floating over ground, isn't it ambiguous what either wire's potential might be compared to ground? Granted, there might be very little current flow, but if you were to reach up and grasp the skids of a helicopter hovering over you which had spent many hours in flight, you might find a pretty enormous potential difference between it and ground?
     
  11. bwilliams60

    Active Member

    Nov 18, 2012
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    I get the diagrams but what about woodwork? What is the point you are trying to make? The person asked a question on a forum looking for an answer and all you have is an insult? Towards him and woodworkers. If you have nothing good to say, save your fingertips.
     
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  12. profbuxton

    Member

    Feb 21, 2014
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    Steve R in FL, Since clouds built up a enormous STATIC charge between themselves(cloud to cloud discharge) and terra firma(earth, as the stuff we stand on) you have a point that if you reached up and touched a charged cloud(as per Ben Franklin) you would get a lethal(usually) shock.
    Probably you may get a STATIC discharge from a chopper because the rotors swishing around may have caused a built up of charge during interaction with air molecules. Can't say I have heard of it though but I stand to be corrected on that point.
    The circuit in question will not build up a static charge with respect to earth so there is no danger of getting a shock.
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
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    If you don't get the joke - forget woodwork and take up clay modelling!
     
  14. ian field

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    Oct 27, 2012
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    IMO; the diagrams would have been better if they'd focussed on the difference between direct mains and supply via an isolating transformer.

    Batteries with lethal voltage were pretty much in decline since post WW2, so the examples don't really strike at most peoples way of thinking nowadays.

    The diagrams are rather too simplistic, and assumes that just anyone would know that current has to flow round a completed circuit for anything to happen.

    You'd be astonished by just how many non-technically people aren't capable of connecting up a battery, bulb and switch!
     
  15. bwilliams60

    Active Member

    Nov 18, 2012
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    I really don't find any joke about it. In fact I find it an insult. I have a son who is a very good cabinetmaker and very intelligent otherwise. He chose his craft because it's what he Loves to do. I know many other "woodworkers" who are very intelligent people and I resent your comment about woodworkers or any other person "less than" an electronics genius such as yourself. This is a forum where people come for answers and far too often I see people insulted on here because they don't know something you consider simple. There are no stupid questions but every once in a while, there are stupid answers. There are a lot of people on here willing to help and if all you have is a "smart-ass" comment, best keep it to yourself. I was hoping you would take the high road, but.....
     
  16. ian field

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    Oct 27, 2012
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    But little chance of getting electrocuted unless they use dodgy power tools.

    Not so much about intelligence level - but if you can't grasp the concept of completing an electrical circuit from those diagrams, it might be wiser not to mess with it!
     
  17. bwilliams60

    Active Member

    Nov 18, 2012
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    Then perhaps you could state that a little more eloquently instead of making the OP feel stupid. I have no doubt you are probably intelligent when it comes to electronics, but people leave forums because of insulting comments such as yours. As a professor, I can tell you that not all people learn the same way. I find I have to explain concepts four or five different ways before my class of 30 all get what I am trying to say, and then I have to show them to back it up. Then they learn. Be patient with people and help them if you want to be a good teacher and that is what the forum is all about, helping people. I don't believe you were born a genius but I am sure you have had many good teachers along the way who were very patient with you when you didn't understand something. Pay it forward.
     
  18. motorcyclestunts

    Thread Starter New Member

    Apr 3, 2011
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    are you those type that insult people online but are too cowardly to ever say anything in person.
     
  19. motorcyclestunts

    Thread Starter New Member

    Apr 3, 2011
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    Thank you . I understand it now and thanx to everyone else who helped.
     
  20. ian field

    Distinguished Member

    Oct 27, 2012
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    When someone clearly lacks the aptitude to handle something that might kill them, it seems sensible to point out the danger.

    But hey ho - Darwinism rules.
     
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