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Death by electricity

Discussion in 'General Electronics Discussion' started by boogyman19946, Nov 11, 2011.

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  1. boogyman19946


    May 2, 2011
    My dad and I were arguing today over which property of electricity has to be high enough to kill a person. My dad argues that it is voltage that kills because an arc welder generates high current yet it fails to shock a person because of low voltage; I fought that idea by saying that the human body has a significantly higher resistance than the path that his hands would come in contact with with and so there would be a miniscule current through his body (if any at all) while most will still flow through the wires. When asked to show how I know that current kills, I fell a little short on arguments XD.

    Now, I figure it's not exactly a clear cut equation considering that voltage and current are closely related and inevitably having generating a high voltage, one should also get higher current, all other factors being equal (including resistance). But if we had two humans with bodies of different resistances (let us supposed one has 2kOhm and the other has 1.5kOhm), what kind of electricity (what would be it's voltage vs current) would be required to kill either of them.

    Also, how is it possible that a welding machine can generate current that is higher than it's voltage? The only way that I can explain it is that we're taking the voltage after all the voltage drops to that point and going from there, since current stays the same throughout the entire circuit. I figure that voltage is not really a measure of some kind of instantaneous property but rather the difference in "charge" between two points.
    Last edited: Nov 12, 2011
  2. davenn

    davenn Moderator

    Sep 5, 2009
    Hi there

    There's a very old saying that has withstood the test of time....

    Volts Jolts, Current Kills

    it takes as little 30mA through the heart to put it into fibrillation
    But it is a combination of things that determine the outcome, the Voltage, current and resistance.

    take a car battery 12V capable of supplying 400A or so of current. You can hang on to a terminal in each hand you are are not going to get zapped. But as you increase the Voltage... 20V --> 40V ---> 60V --> 80V you are finally going to reach a Voltage that the resistance of your skin will break down and start to conduct. Without looking it up to confirm, its from memory somewhere ~ 50V. and as you go higher in voltage you will get more current flowing.
    Again... its NOT the Voltage that kills you, its the current that flows. The increasing Voltage provides the "push".

    Yes they are related, but No higher voltage doesnt assume higher current. You can have a starter motor in a car drawing 400Amps out of a 12V battery. or conversley you can have 26kV (26,000V) on the the back of an old style TV picture tube and there is only a few ... less than 10mA ( 0.01A) flowing.

    this is very similar to a question I have just done a long writeup answer for in another forum.

    Firstly... the majority of the resistance of the human body is in the thin outer layer of skin. Its a number of MegOhms
    ie. very high resistance. Theres of course 1 major factor that varies that resistance ... how damp the skin is.
    very dry = many MOhms to very wet = only a few Ohms

    The next major factor as to if you will get electrocuted or just a severe shock is the path taken by the current through the body.
    in 1 hand throguh chest and out the other hand is most likely to be fatal. Its straight through the heart.
    In left hand down left side of chest and down legs and out could well be fatal again a reasonable path close/through the heart.
    In right hand and down right side of body may be fatal but your probably more likely to survive, unless the current is flowing for a signif time.

    There's another old saying that i need to remind you of ... it also has withstood the test of time

    A Technician always has one hand in his pocket when measuring hi voltage circuits

    it avoids that - through hand through chest and out other hand usually deadly scenario

    Last edited: Nov 11, 2011
  3. boogyman19946


    May 2, 2011
    Awesome post Dave, thanks for all the info! That makes a lot more sense now!
  4. Merlin3189


    Aug 4, 2011
    Just wanted to say, the way I heard it was,
    "It's the Volts that jolts, but the mills what kills."
    mills being milliamps of course.

    I always think when I hear this sort of question (Volts vs Amps) that, if there's no current, nothing happens. It's always the current that counts: Volts, or preferably PD, is just one of the factors determining the current.
    But this does ignore a third factor - time. When you get a shock from a 'static' charge, I guess the Voltage could be in the thousands or more (blue sparks in air tend to run at kV/mm ), but I've never felt as much effect as from a few hundred Volts from a touching a power rail. If the body's resistance is the same in each case, the current must be higher from the higher static Voltage. But for static we are discharging (or charging) the fairly small capacitance of the body. So the Voltage and therefore the current falls off very rapidly. We do get a high current, but for a very short time. I think this is the basis of RCD/ELCB protection: they break the circuit within 100mS. (A Masterplug device claims, "The .. RCD adapter ... cuts off the power within 40 milliseconds if an earth current fault is detected. This is fast enough to prevent a fatal electric shock." )
    Perhaps we should be asking, is it the Coulombs, Joules or Watts which kill?
  5. davelectronic


    Dec 13, 2010
    Hi there, playing about modding and testing an atx psu today i became a human scope, i could feel every bit of that sine wave at a potential of 240 volts, and it hurts, i got careless, volts amps jerks yer the works, and up my left arm, not funny, scary stuff.
    Dave. :)
  6. jackorocko


    Apr 4, 2010
    Recently I been rewiring main disconnects and running new wires from the meter box to the main disconnect while the incoming wires are still hot. There is no better advice when you are working around un-fused mains.

    Yes, 240V does hurt like hell. I got nailed being a novice working on an electric dryer one time. Careful dave, we enjoy your post!
    Last edited: Nov 12, 2011
  7. jackorocko


    Apr 4, 2010
    coulombs is the electrical charge present, which we measure as the Ampere. Joules is coulombs of charge over a time period. Watts is unrelated as it is the measure of power which is volt * amps.

    Again. it is the current that fibrillates the heart. It is this action that will stop the heart, just like a defibrillator will restart the heart the same machine could just as easily stop the heart. Your bodies resistance to current and the amount of voltage present determines how much current is passing through the body.
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Yes, let's keep the "death by electricity" in the purely theoretical domain.

    Thanks for your efforts.
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