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Electrocution

Discussion in 'Electronic Basics' started by Peter, Sep 26, 2003.

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  1. Peter

    Peter Guest

    I'm having trouble working out the formula for electrocution. (Yeah,
    ok, "Stick your finger in a power point".)

    Here in Oz we run 240v which is much nastier than 110v. From what I
    read 100mA is very fatal and even 20-30mA can be. But doing the sums
    it just doesn't add up.

    I measure my resistance to ground with a multimeter and it reads
    anywhere from 2M ohms and up depending on shoes, floor covering etc.
    Using V=IR on 2M and 240v gives a current of 0.1mA. Is that enough to
    kill you?
     
  2. Don Kelly

    Don Kelly Guest

    -----------
    Your multimeter may measure 2Mohms but it uses a little 1 to 3V battery. Try
    it with milliammeter and a 240V supply. A thin high resistance layer which
    can withstand the low voltage supplied by a multimeter can break down at a
    higher voltage and be quite conductive. Don't trust a multimeter for
    resistance measurements in circumstances such as these. That is why
    "Meggers" are used for insulation resistance measurements.

    By the way here are some figures. These are for a 0.5% probability for an
    adult male (or a large hog) Let-go about 9ma, fibrillation -about 100ma
    most can take more. However fibrillation is time dependent so 20ma for 1
    minute may cause fibrillation.
     
  3. John Larkin

    John Larkin Guest

    Nope. Your math is right. A single contact with 240 (say, grabbing a
    hot wire) is harmless if you're well insulated from ground, as you
    will be wearing dry shoes, or standing on a non-conductive floor. Of
    course, your other hand may be grabbing a pipe or something, and that
    could be a lethal path. The idiots who get killed are standing
    waist-deep in water, boring a hole in the side of their boat with an
    old metal-case drill. I had a friend who was killed when he hit a
    power line putting up an antenna pole... that was about 8KV, I think.

    Actually, not a lot of people are accidentally electrocuted. AC is
    benign compared to, say, cigarettes or motorcycles.

    John
     
  4. Why not measure your resistance with a megger ?
    That will give you a true reading if you use the 250v scale.
     
  5. Albert L.

    Albert L. Guest

    The electrical resistance of the human body is dependent of the frequency of
    the current. At DC, what you measured with the multimeter, skin has very
    high resistance. But this resistance decreases quite a bit at 50/60Hz,
    therefore increasing the amount of current that can flow through the
    victim's body

    Albert
     
  6. John Fields

    John Fields Guest

     
  7. Baphomet

    Baphomet Guest

    The electrical resistance of the human body is dependent of the frequency
    of
    Actually, the body presents a very complex impedance. It is dependent on
    skin moisture and salt content, age (perhaps related to moisture content),
    but perhaps most of all...voltage. For example, at about 400 watt seconds,
    the body looks like about 500 ohms. At several volts, the body looks like
    10k to 1meg at the epidermis. The resistance of sub epidermal tissue varies
    widely.
     
  8. John Fields

    John Fields Guest

    ---
    When current flows through the body I don't believe an inductive or
    capacitive reactance comes into play at mains frequencies, so the
    impedance will be totally resistive. If that's the case, then 120VDC
    applied, say, from palm to palm will cause the same current to flow as
    120VRMS applied from palm to palm. Of course the _peak_ current will be
    higher in the AC case and there will be polarity reversals which could
    help to induce fibrillation, but that's not what I was disagreeing with
    the poster about.
     
  9. John G

    John G Guest

    John,
    I have to agree with you but the whole argument is a bit hypothetical as
    electrocution depends so much on the external conditions at the moment. i.e.
    Skin dampness, contact pressure, victims condition etc that there are no
    hard and fast rules except that the most dangerous voltages appear to be
    around the 100-300 volt range. Higher than that the contraction of muscles
    etc seems to save some people by throwing them off.
    Of course the whole argument is statistically very clouded because almost
    all the opportunity for electrocution is at 110 or 230 volts AC. Very few
    people are ever exposed to 250 volts DC for instance.
     
  10. Don Kelly

    Don Kelly Guest

     
  11. John Fields

    John Fields Guest

     
  12. John Fields

    John Fields Guest

    ---
    Don't preach to me you pompous little ****.

    The claim that was made was that merely by being at 60Hz a signal would
    cause a greater current to flow in the body and here's the poster's
    claim:

    "The electrical resistance of the human body is dependent of the
    frequency of the current. At DC, what you measured with the multimeter,
    skin has very high resistance. But this resistance decreases quite a bit
    at 50/60Hz, therefore increasing the amount of current that can flow
    through the victim's body."

    You may notice that no mention was made as to the physiological effects
    to be expected by the differences between the two signals.
    ---
    ---
    Tell that to the family of someone who touched something hot with the
    inside of a finger only to have it involuntarily wrap around the
    conductor in a death grip.
    ---
    Of course, my ass. Statistics re. electrocution has nothing to with it,
    and neither does the victim's condition nor any of the rest of it. The
    only thing that's importanr here is whether an AC signal of a particular
    voltage emanating from a particular source impedance will cause more
    current to flow than an equivalent DC signal feeding an identical load.

    ---
     
  13. The Captain

    The Captain Guest

    I well remember welding my elbow to a tuning coil! That was 8000
    volts at High Frequency, on a military RF transmitter. I have the
    scar to this day, forty years later. Of course there was no
    restricion on current to speak of and the thing that saved me was the
    muscle jolt that flung me away from the equipment.

    Unusual circumstances, I know, but it sure emphasised the lectures
    about electric shock I had received up to then.

    OK, getting back to 240 volt shocks as opposed to 110 volt shocks: I
    have heard, and I have no intention of putting this to the test, that
    the percentage of 110 volt shocks resulting in death is higher than
    that for 240 volts. This doesn't make any sense at first glance, but
    apparantly, 240 volts is more likely to cause just the kind of muscle
    spasm that saved my stupid young life, and fling you off the
    conductor. 110 volts, on the other hand, just lets you sit there and
    sizzle!

    Some general points; the condition of your skin, the path of the
    current, what you are wearing on your feet and several other factors
    will affect the path and strength of the current. Generally, don't
    sweat and mess with electricity. Sweat, blood lymph fluid and all the
    rest of the goop inside you is, from an electrical point of view, a
    solution of sodium chloride, and a pretty good conductor. If you
    sweat, this conductor is extended beyond the skin. This, by the
    way,is why the poor bastard in the electric chair had a sponge with a
    salt solution under the electric skull cap, clean conduction straight
    through the brain.

    Any amount of current can kill you if it is applied to the heart at
    the point where the nerves are firing and it messes up the rythm
    completely. There was an interesting article in, I believe,
    Scientific american, a few years ago about just this. Apparantly,
    during most of the heart's cycle, the effects of current were minimal,
    but durting a few periods during the cycle the heart is particularly
    prone to damage and failure and a shock then will drop you like a
    slaughtered ox.

    This explains why some people walk away from a shock which should have
    been lethal, while aother die from a relatively minor shock.

    Now, about that bear fence; presumably you have a problem with bears,
    which means you are living in North America. Have you condsidered the
    amount of deep doo doo you could drop into if your fence electrocuted
    a child, for example. The litigation would probably bankrupt you!
    Also, if it's designed to hurt a bear, it will be lethal to other
    animals, deer for example who may touch it with their noses, not to
    mention smaller animals.

    Now unless you really hate all mammals, that bear fence sounds like a
    rather poor idea. Aren't there any alternatives?

    John
     
  14. The Captain

    The Captain Guest

    Well, trying to ignore the pointless invective and un-needed insults,
    one significant difference between a DC voltage and an AC voltage
    which measure the same on a voltmeter, is that The DC voltage will be
    measured as the actual voltage while the AC voltage will be the RMS
    voltage, which is actually .707 of the peak voltage. Therefore, the
    peak voltage and current of an AC source will be 1.414 times the
    equivalent DC voltage.

    And, sorry to disabuse you of your obviously dearly held opinion, the
    condition of the victim obviously does have a great deal to do with
    the outcome of any shock. Someone with heart desease will be far more
    likely to die from an electric shock than someone who is fit and well.
    Not all the time and sometines, for reasons I have described
    elsewhere in this thread, a fit person will die and an unfit person
    live, but that's just chance, and therefore only measured
    statistically.

    Also, check my other post, there seems to be, for un-obvious reasons,
    a greater chance of surviving a 240/230 volt shock than a 110 volt
    shock.

    I realise that you are probably going to scream insults at me for
    disagreeing with your opinions, but please try to restrain yourself
    and answer, if you wish to, in reaonable terms. To do otherwise is
    extremely unprofessional.

    John
     
  15. John Fields

    John Fields Guest

    ---
    Yes, I addressed that issue in an earlier post which, apparently, you
    missed.
    ---
    ---
    Other than just being pig-headed or perhaps not paying attention to what
    you've been reading, I really don't understand why you have such a
    problem dealing with the fact that regardless of the condition of the
    person being "shocked", the current passing through their body as a
    result of being connected across the OHMS range of a multimeter will be
    the same as the RMS current passing through their body if it were to be
    connected to an AC voltage source with the same RMS output voltage as
    long as the output impedances of the AC and DC sources were identical.
    That was my contention in the beginning, it is now, and unless you can
    prove me wrong it will be my contention in the future
     
  16. Possibly to the likelihood of fibrillation, due to that frequency
    being close to a resting pulse rate? Remember Galloping Gertie, the
    Tacoma Narrows (?) bridge? Resonant frequencies and all that fun
    physics.

    Tom
     
  17. John Fields

    John Fields Guest

     
  18. Don Kelly

    Don Kelly Guest

    ------------
    Sorry- 50-60 Hz current perception and shock levels are lower than DC
    perception and shock levels. I can't give you a reference but it came
    originally from tests done many years ago and reported in AIEE transactions
    (prior to IEEE) There may be a reference to this in the EPRI high voltage
    book. As to why - I have no idea.
     
  19. Charles Jean

    Charles Jean Guest

    Maybe it's a matter of relativity. I know I must watch out about
    charged capacitors in power supplies, flybacks in TV, etc even when
    the damn things are unplugged, so I'm expecially careful in those
    situations. Digital logic, 15V analog stuff, NAH! Get between 24V
    and 120, I get a little bit spooky. Leave me alone, I want to think
    everything over one more time before I go in on this one! Changing my
    electric water heater out(the only thing I would work on at 220) takes
    real discipline, and a long time. I go over my checklist at least 10
    times. No metal of any kind-rings,bracelets, necklaces, etc. Breaker
    off? yes, checked it 5 times already and verfied it with meter
    movement. Tennis shoes on? Standing on folded towel? Then take a
    deep breath, one hand in the pocket, make a fist with the other one
    and touch one of the conductors with the back of my fist. Then unhook
    both conductors cover them with some wire nuts while doing the rest of
    it.

    My point is that my precautions seem to go up with the voltage I'm
    working with. Maybe they shouldn't, but that's the case. I know some
    people that simply won't work on 240, but will do the occasional lamp
    or fixture job. Could this be the reason for the statistics?

    A higher percentage of those working on 240:
    a) are professionals or know what they are doing?
    b) ergo, are safer at the job
    c) get most of the HV work because of us weenies?
    d) there's just more home 120 than 240, luring the moths to the flames

    A friend of mine replaced the recirculating water pump on his
    evaporative cooler unit and it keep tripping the breaker. He called
    me and offered beers if I would help him(it was 115 degrees in AZ at
    the time!) I was analyzing the problem as I drove over there. How
    hard could this be? Two pump wires, each connected to a pump switch
    wire? Yup, when I got there, I found all four wires glommed together
    in a single wire nut!

    Eschew Obfuscation!
     
  20. Vivek Gani

    Vivek Gani Guest

    An architecture student told me once that being shocked by 120 volts AC
    is actually worse than 240 volts AC, saying that with 120 volts you
    won't easily notice that you're being shocked and thus just remain
    holding the wire, whereas with 240 volts, you'll instantly back off.
    Also, this summer I accidentally was shocked by 240 volts by touching an
    open connection in a breaker box (this was 240 volts in the US). I
    just had a weird feeling in my finger for a second then jumped back when
    I realized that I was being shocked.
    Eh, just thought I'd throw in my two cents...
     
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