newbie shortcircuit question

Discussion in 'General Electronics Chat' started by J0ker, Apr 18, 2012.

  1. J0ker

    Thread Starter New Member

    Apr 18, 2012
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    Hello! I'm super excited to have found this forum, been looking for something like this for ages (I'm spanish and one day I thought...try it in english, duh!)

    Ok here's my question. Why does a ground connection prevent electrocution in case of shortcircuit? Shouldn't I (i.e. the victim) be just another path for the current to go to?

    I assume that I offer more resistance than the cable that goes down the ground but... some electricity should go through me to the ground as well, right?

    Thanks!!
     
  2. vpoko

    Member

    Jan 5, 2012
    258
    47
    Yes, but if the connection to ground offers low enough resistance, the current going through you will be negligible. Also, if you have a fuse or circuit breaker on the circuit, the low resistance path to ground should blow/trip it, while the amount of current going through just your body may not be enough to do that (since your body will look like just another load to it).
     
  3. J0ker

    Thread Starter New Member

    Apr 18, 2012
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    So even if you have a ground connection, you could get a little of electroshock? Maybe so little you can't even feel it?

    Thank you! It was really bugging my mind
     
  4. @android

    Member

    Dec 15, 2011
    178
    9
    Always remember two things about current viz.
    1. Current always takes lowest resistance path.
    2. The amount of current will be decided by load not the source.

    Hope that helped!
     
  5. vpoko

    Member

    Jan 5, 2012
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    47
    You know, I'm starting to doubt the first part of my answer. That is, if your house had no circuit breaker and could deliver any amount of current, I'm not so sure that having a path from the chassis to ground would reduce the amount of current a person touching the chassis would suffer. Since the hot-to-person-to-ground path is parallel to the hot-to-chassis-to-ground path, both paths would have the same voltage across them, which is the same as the voltage between hot and ground. As long as mains could deliver as much current as needed, I believe a person would still get the same amount of current through them whether or not there was a safety ground. What I believe the safety ground accomplishes is tripping the circuit breaker.

    Could someone with more knowledge than me (that would be almost anyone) chime in?
     
  6. P-MONKE

    Member

    Mar 14, 2012
    83
    5
    That's how I believe it works too - the much lower resistance path from the mains directly to ground compared to the person to ground path enables the fuse/circuit breaker to become open circuit before you get fried. I think that time is an important factor.

    Residual current devices (RCDs) achieve the same result without a ground connection by detecting any imbalance between the current flowing into a device from the live side and that returning on the neutral side on the premise that the difference is flowing to ground where it shouldn't be!
     
    Last edited: Apr 19, 2012
  7. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    The difference in resistance between a human body and a chassis ground is on the order of 1000:1 or even 10,000:1 so when you do the math treating them as parallel resistances connected to the same voltage source (whichever mains peak value you choose), it becomes apparent that the low resistance path of the chassis ground prevents the current through the human reaching harmful levels.

    Consider the case of high voltage line workers wearing faraday suits.
     
  8. P-MONKE

    Member

    Mar 14, 2012
    83
    5
    It would depend on the impedance of the source emf, surely? If that was low enough, as vpoko points out indirectly, the voltage across the body would be that same as that across the grounding circuit and hence the current would related to the body's impedance, irrespective of the lower resistance path to ground.
     
    Last edited: Apr 19, 2012
  9. KJ6EAD

    Senior Member

    Apr 30, 2011
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    That's true, but even without a breaker or fuse, the conductors would quickly melt open. The human just hopes the supply opens before the chassis ground does.
     
  10. P-MONKE

    Member

    Mar 14, 2012
    83
    5
    Haha - I'm a pessimistic bugger - I think I'll stick to my breakers and earth leakage devices! ;)
     
  11. J0ker

    Thread Starter New Member

    Apr 18, 2012
    17
    0
    I'm more confused than before then :p

    From what you said, I'm guessing then that the usefulness of the ground connection is to make the fuse go off and cut the voltage before you get fried? If that's the case, then the ground connection doesn't make the current go down the earth SO it doesn't go through you. Current will also go through you depending on your impedance, because it's in parallel.

    Mmm?

    (so i guess it's useless to have a ground connection without a safety fuse in the house/wherever?)
     
  12. vpoko

    Member

    Jan 5, 2012
    258
    47
    More than that, it's probably useless to have a house without fuses or breakers because it'll burn down sooner or later. As KJ6EAD said, a conductor (wire) somewhere would melt open if nothing else limited the current, and hopefully it wouldn't be near anything flammable.
     
  13. P-MONKE

    Member

    Mar 14, 2012
    83
    5
    This is why circuit breaking safety devices are installed where the current enters the building, thereby cutting the power to the whole of the circuit.
     
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