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Ground and Electrocution questions

  1. Nov 15, 2015 #1
    << Mentor Note -- There is good information about shock hazards in this thread, and explanations about why some situations may not present a shock hazard compared to other situations that do. Please keep in mind that if you do not know much about electricity, you should always consider AC Mains voltage and other high voltages to be dangerous, and should not be touching any live wires. >>

    Hello again guys. Got another several questions from electric stuff.
    I was told that if Someone touched a cable and at the same time he touched ground or more precisely, earth, he would get electrocuted. Hope I'm not mistaken but I know earth has lots of positive charges that attract electrons but I was wondering if electricity would actually find it's way to its source with earth as its path. It sounds really crazy to me. I don't get why it would prefer to get through a human body and also earth to go back home rather than its usual path. I read in this forum something about earth's potential and I don't get it either, I just can guess its 0V but I may be confusing the concepts of electric potential energy and electric potential difference.

    If you shed some light on this I will really appreciate it. Thank you
     
    Last edited by a moderator: Nov 18, 2015
  2. jcsd
  3. Nov 16, 2015 #2
    The person who touches the cable while standing on the ground will be electrocuted IF by touching the wire, he forms a complete circuit from the cable via his body, through the ground and back to the voltage source.

    It is common for one side of a power system to be electrically connected to the earth, or at voltage reference point 0 V. So if someone touches the grounded side of the cable, nothing will happen. But if he touches the non-grounded side, then he completes a circuit between the higher voltage cable back to the original source through the dirt in the earth.

    This is of course a high impedance (resistance) path for the electricity, but think of V = IR (or the current I = V/R, meaning that even though R is large, V for a typical power cable is very, very high). Most of the electricity will travel along the original path back to the EMF source, but enough will go through the human's body and the ground to cause electrocution.

    On the other hand, if one side of the cable is not grounded, he will not be electrocuted by touching the higher voltage cable, since there is no complete circuit through the ground back to the voltage source.

    These diagrams illustrate what I mean (and are taken from the following website, with which I am unaffiliated - I suggest you read this for a better understanding)
    http://www.allaboutcircuits.com/textbook/direct-current/chpt-3/shock-current-path/

    Diagram 1: Person gets electrocuted if one side of the power system is grounded.

    00056.png

    Diagram 2: If neither side is grounded, then nothing happens.

    00058.png

    Of course, there is a such thing as accidental grounding, whereby a tree or other object falls against a power system, producing an unintentional connection to ground. The lesson is, there is no good reason whatsoever for one to be grabbing power cables.
     
  4. Nov 17, 2015 #3
    Thanks a los for the info! It really cleared most of my questions. However, we are talking about ALternate current which makes electrons go back and forth. It may sound silly but how can we know were the lowest electric potential is when the current is not actually going in a single direction?
    The bird does not get electrocuted because there is no difference of voltage between its "terminals"? I don't really get it. It sounds more like a parallel connection to me were current prefers circulating through the path of less resistance rather than the birds body. Am I wrong?
    This is a bit out of the topic but I remember a teacher told me that thunders induce high voltages in cables. Does earth play some kind of safety role in here? Most devices have the live, neutral and ground terminal in their sources. What's exactly the role of the ground connection in this case?
    Thanks a lot mate!
     
  5. Nov 17, 2015 #4

    davenn

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    Gold Member

    Yes, the bird's feet, legs, and body are making a parallel connection across a tiny section of the wire, but the resistance is very high compared to that through that same section of wire. Yes, there will be a current flow through the bird, but VERY small. This current will increase as the voltage in the wire circuit increases.
    Have you noticed that birds don't sit of the EHT transmission lines ?

    There is another reason as well ... wont go into that at the moment :smile:

    The thunder doesn't, the lightning does. Lightning induces voltages in 2 ways ....

    1) direct contact strike
    2) a nearby strike where the power lines, radio/TV antennas are within the strong electric field of the lightning strike

    Does the earth play a role .... only in that it is where the lightning is discharging to/from, between a cloud to ground or ground to cloud strike


    cheers
    Dave
     
    Last edited: Nov 17, 2015
  6. Nov 17, 2015 #5
    upload_2015-11-18_7-25-55.png

    You cannot touch a live conductor -even in an ungrounded system.The natural grounding impedance of the system-capacitance between conductor and ground or insulation resistance-it is still enough to facilitate return the current to the source.

    If the enclosure would be grounded, even in a case of a fault a live conductor touch the enclosure, then the potential of enclosure decrease up to close to ground potential and the person would get only this very low voltage between his touching organ and ground -or a grounded object.

    The "potential" it is the voltage difference between two points: one considered "0" or reference.Usually the remote "infinite" point is considered "0"- sometime the "clear ground".
     
  7. Nov 18, 2015 #6

    berkeman

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    Thread closed for Moderation...
     
  8. Nov 18, 2015 #7

    berkeman

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    Staff: Mentor

    Thread will remain closed, as the OP's questions have been addressed. Please be sure to read my Mentor Note in the first post of this thread. Thanks. :smile:
     
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