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Not electrocuted?

  1. May 9, 2009 #1
    Today, a friend was moving a cable with a little piece of metal. He happened to cause a short circuit and nothing happened to him. The circuit was holding around 10 white lamps, 120V and I can guess its about 40W each. The little piece of metal even got perforated, and yet nothing happened to him. Thanks God.
    Can anyone explain me how can this happens?
     
  2. jcsd
  3. May 9, 2009 #2

    mgb_phys

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    The resistance between the live+neutral cables through the short was a lot less than the resistance through him to the ground
     
  4. May 9, 2009 #3
    The neutral wire is at ground potential. The metal probably touched the neutral wire first and then shorted to the hot wire. If it had touched the hot wire first your friend would have at least received a shock.
     
  5. May 10, 2009 #4
    Why? This is crazy! Do you guys know how to shut the circuit off before working on it and how to verify that it is indeed off? 200 milli-amps (1/5 of one amp) is enough to be deadly. If this was a circuit in a house, it's probably a 15 amp line. Note the difference.
     
    Last edited: May 10, 2009
  6. May 11, 2009 #5
    He broke the switch itself and was trying to put the plastic together with a little piece of metal he found, so the teacher woulnd't punish him for breaking it. Yes I forgot to say it was in school.
    After that I told him "You know that could've killed you", and as many people say, he replied "Nah, 120V won't kill me".
     
  7. May 11, 2009 #6

    RonL

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    You might ask the teacher to discuss that in class.
     
  8. May 11, 2009 #7

    turbo

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    Your friend is badly misinformed. Hand-to-hand or hand-to-foot transmission paths through the core of your body can kill you with very little current.
     
  9. May 11, 2009 #8
    Many people think that it is the voltage that kills you, which is a large misconception. In fact, I have a friend who had to receive a 5000 V jolt to pass one of his biomedical technology courses. It's the amps that are dangerous, and it doesn't take much to do the job. Human skin resistance is typically about 1 mega-ohm (10^6) for dry skin, but for moist skin the resistance can be reduced by a factor of 100.
     
  10. May 11, 2009 #9

    mgb_phys

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  11. May 11, 2009 #10

    turbo

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    I have restored and repaired and rebuilt a lot of tube-operated guitar amplifiers. The very first thing I do with every amp after disconnecting the power and removing the chassis is to connect the B+ rail to ground and do some voltage checks to make sure that there are not some capacitors lurking to bite you.
     
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