Electric shock at high voltage but low charge

Discussion in 'General Electronics Chat' started by Benjamin Buemann, Apr 17, 2015.

  1. Benjamin Buemann

    Thread Starter New Member

    Apr 17, 2015
    1
    0
    My question relates to the statement "The electric shock applied by an electric fence is not letal despite the high voltage due to the fact that the current is very low. However ohms law must apply at the moment you touch the fence and the high voltage should therefore result in a high current through your body. As I understand it the electric pulse in the fence is very short and the amount of energy released during the shock corresponding low. But nevertheless, if only ohms law prevail for a very short time you must be exposed to a high current but how can the heart tolerate that? A similar situation could be that you connect to a heavely charged condensator with at low capacity or a generator creating a high voltage but at a very low power output.
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,775
    4,804
    There are a few factors at play. One is that the source has a fairly high impedance and so much of the voltage is dropped across the source as soon as current starts to flow. Second is the low energy available for the current and so the voltage drops very quickly as soon as current starts to flow. Your body's response comes in several ways, but a big one is related to the total charge that flows through sensitive areas as well as the time over which it flows. So even a fairly large current can be tolerated if it is sufficiently brief.
     
  3. BR-549

    Well-Known Member

    Sep 22, 2013
    2,004
    394
    Ohms law is what saves your life.

    The high voltage source has impedance. When you touch the fence, your body impedance is put in series with the source impedance.

    That total impedance and the short duration......is what limits the current.
     
  4. BR-549

    Well-Known Member

    Sep 22, 2013
    2,004
    394
    I never scoped an electric fence. At first, I would think like Wbahn, that the voltage would drop long before end of pulse.

    But with a thick hide large animal standing on insulated hooves, maybe it's full voltage thru-out the pulse.

    Any farmers out there?
     
  5. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,692
    2,756
    I lived on a farm for a bit when I was a kid. I *loved* touching the electric fence.

    Hmmm....now that I think about it, this might explain a few things...
     
    shortbus likes this.
  6. crutschow

    Expert

    Mar 14, 2008
    13,050
    3,244
    The heart is quite tolerant of DC shocks. For example, a defibrillator actually applies a DC (unidirectional) pulse through the heart to start it beating when it stops. So the voltage or current applied by the electric fence is not of particular concern since its a unipolar pulse, unless perhaps it's enough to cause loss of consciousness. Certainly many old TV repair guys have been zapped by the CRT anode voltage without dying (although they may have felt like they did), and that packs a lot more punch then any electric fence controller.

    It's AC current through the heart of sufficient amount (something greater than 50mA I think) that can cause fibrillation and consequent death (if a defibrillator isn't quickly used).
    That was the basis for the great AC versus DC war between Edison and Tesla.

    (It's rather interesting that the natural heart pacemaker system seems to be a quasi-stable feedback loop oscillator. When disrupted by an AC current it becomes unstable and has to be reset by a DC current pulse to become operational again.)
     
    Last edited: Apr 17, 2015
Loading...