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Human Body Capacitance and Resistance

Discussion in 'Electronic Design' started by Winfield Hill, May 30, 2010.

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  1. A repost of a comment I made elsewhere, for discussion here.

    The subject is ESD Human Body model values. I'm enamored by
    a 1989 symposium paper by Richard Fisher, of Sandia Nat'l Labs,
    where he created a "Severe Human ESD Body Model." His model
    had worst-case numbers meant for use in electrostatic-discharge
    circuit-protection analysis, etc.

    Fisher's Severe Body Model consists of two parts, the body and an
    arm with hand reaching out to zap something. The body part has
    400pF of capacitance in series with 250 ohms and 0.5uH. Then the
    arm and hand part bridges the body terminals with 10pF, and finally
    we have another 110 ohms and 0.1uH in series to complete the model
    and connect it to the poor real-world victim. The body capacitance
    is higher than you may see elsewhere first because the body is
    sitting down, and second because it's a worst-case body. We won't
    go further into what that means. :)

    You charge the 400pF capacitor to a voltage of your choosing.
    20kV is a nice high number. During discharge we get a fast spike
    of current from the 10pF, with sub-ns risetime to dangerous levels,
    with up to 5A peak current, and lasting up to 5ns into the "load."
    This is followed by a slower discharge of the 400pF capacitance,
    lasting up to 200ns.

    This would be followed by, ahem, a postmortem.

    As for the effect of high moisture and humidity, as said, these can
    affect things, e.g., lowering resistances to the low levels we see
    in Fisher's Severe Model, but it also means the maximum electrostatic
    voltage developed on the 400pF capacitor is likely to be much lower.
    I suspect Fisher would prefer to take the dry-air high voltage with
    the moist-skin low resistances for his Severe case.
     
  2. Those resistances seem to be quite low for the resistance of the skin.
    If the resistance would be that low, you would get severe burns each
    time you touched the 230 V mains.

    Those resistances in the order of a few hundred ohms would make sense,
    if we assume that the spark penetrates the skin and the current
    propagates in fluids under the skin.

    RF burns from touching an antenna connected to a 100 W transmitter can
    be quite painful, put it can be hard to detect, where the RF current
    penetrated the skin.

    With the component values given, it would form a lossy resonant
    circuit with a resonant frequency in the upper HF range, thus
    expecting a few cycles at that frequency, until the oscillation dies
    down.
     
  3. The skin is the big unknown.

    Once opened by a wound, etc., the resistance is VERY low.

    Open heart defib is only 2mA on the paddles.

    Arm-to-arm with open wounds is only 10mA as opposed to 40mA on dry skin
    to cause a fibrillation.

    Area of contact is also a factor in lowering skin resistance (lowering
    of the interface resistance into the salty blood conductor of our inner
    body).

    As for an electrostatic event causing a fibrillation, it is not very
    likely until one gets up into the lightning bolt voltages. The time of
    stroke becomes a factor.
     
  4. legg

    legg Guest

    So0me references, if you don't already have them:

    http://www.aecouncil.com/Papers/aec1.pdf
    http://www.globalsmtindia.in/documents/ESD_DAMAGE_MODELS_AND_CHEMICAL_KINETICS-PART_I.pdf
    http://www.barefoothealth.com/science/body_voltage_study.pdf

    Combining dry ait high voltage with moist skin low resistance sounds
    like a typical solutiion, when a committee avoids the use of it's
    individual brains.

    RL
     
  5. Shaun

    Shaun Guest



    Once again Dimbulb is wrong, "always wrong".

    Don't you understand Ohms Law, if resistance goes up (dry skin), current
    goes down, YOU IDIOT!.
    Also it take the same amount of current through the body (dry skin, moist
    skin or open wounds) to cause their heart to fibrillate. The closer you are
    to the heart the less current it takes because more of that current will
    flow directly though the heart.

    Resistance goes down for open wounds(assuming the current goes into the open
    wound) because there is a direct connection to the internal fluids of the
    body, hence less voltage is needed to cause fibrillation. With dry skin
    (high resistance) it takes more voltage to cause dangerous amounts of
    current to flow.


    The current levels that you specified are also wrong!

    20 uA of current is considered hazardous to the Heart if it passes directly
    through the heart.

    500 uA of current is the maximum amount of leakage current allowed in
    medical equipment through ground and if the ground connection was broken,
    you touch the case of the device and ground, that current will flow though
    you.

    These specs are for patients at high risk, poor health condition.
     

  6. You are the idiot. Every statement I made was about OPEN skin. You
    know, intimate access to the blood pathway.

    Learn to read, dipshit.

    Get a clue while you are at it. Then... grow the **** up.
     
  7. Shaun

    Shaun Guest

    No it wasn't all about open skin. You said dry skin take 40mA to cause
    fibrillation, that is WRONG, like you always are. You know nothing you
    fucking idiot. Why do you even post on the usenet, you should know by now
    that you get shot down every time. Just give up! and do us all a favour.

    You were WRONG on the mA ratings that you posted and your theory about
    current required to cause fibrillation was wrong. You are always wrong.

    Just give up.
     


  8. Wrong. The key is the pathway, and how much of the total current
    actually flows through that part of the pathway that is also comprised of
    the heart. The heart only needs a couple milliamps through it to
    fibrillate or defibrillate. As one gets further and further from the
    heart as far as the current source and exit is concerned, the amount of
    current needed to get the heart at 2mA increases, because the pathway is
    millions of parallel resistors of which only a few thousand relate to
    current flowing through the heart. The current in is one value, and the
    current out is that same one value, but the current in each of the
    millions of parallel resistors differs for each and not all contribute to
    any flow that would relate to fibrillation or pass through the heart. So
    entry point and egress point are very important. There are pathways that
    would not cross the heart at all. Mode of entry is also important. Dry
    skin has a high resistance. Blood does not. If punctures by the voltage
    or by other means is involved, the current to cause fibrillation lowers
    because the skin is not longer in the model.

    You lose. Again.
     

  9. That is what I said, dumbfuck.
     

  10. NOT voltage, idiot. CURRENT causes fibrillation.

    You lose, again.
     

  11. I made HV medical device supplies. I know about what the limits are
    and what the design constraints are.

    Operating room defib paddles run at around 2mA during a cycle.

    Grow the **** up.
     
  12. Those numbers are for AC powered devices.

    Give us the handheld numbers, obsolete boy.
     
  13. And that is through the side of the chest and the chestplate bone.

    It takes far less in an open heart surgical procedure.
     
  14. Guest

    300J from chest to back, the best path possible. BTDT, burned a bit.
     
  15. Robert Baer wrote...
    Let me point out that I was thinking of a component postmortem.
    JEDEC standard JESD22-A114D spells out the familiar HBM ESD
    test model: 100pF in series with 1500 ohms.

    Fisher's Severe HBM was created from hundreds of measurements
    reported in the literature. His interest was in the worst-case
    observations. His 360-ohm value of body resistance is lower
    than you observe with simple ohm-meter measurements, etc., but
    keep in mind it's a high-voltage measurement. Is it reasonable
    to hope that our outer-skin-layer insulation can withstand a
    say 20kV discharge and maintain high-resistance?
     
  16. Well, since people regularly charge themselves up to at least 30kV (a 1
    cm spark) I would say not.
     
  17. Dirk Bruere at NeoPax wrote...
    Right you are. As I clarified earlier, I was thinking of a
    component postmortem.
     
  18. Mycelium

    Mycelium Guest


    Except that he was referring to the chip. D'oh!
     
  19. 30 KV per centimeter is the breakdown gradient of air. But that number
    only correlates to spark gap length when the eectric field is even just
    before breakdown.

    In the usual case of people charging themselves up by shuffling their
    shoes on carpet, they make sparks in gaps with uneven electric field. So,
    less than 30 KV can make a 1 cm spark. If one end of the spark gap is a
    sharp point or the tip of a wire maybe AWG 22 (approx. .63 mm) or smaller,
    then a 1 cm spark can occur from about 11 KV, easily from 12KV.

    Also, these sparks often appear bigger than they are. I find 1 cm to be
    uncommon, but I find 8 mm fairly easy to achieve with favorable shoes and
    a fairly favorable carpet and favorable humidity. So, I think 10 KV is
    common but much more is not.

    However, I remember experiencing one apartment with one exceptionally
    favorable carpet and I somewhat remember making 15 mm, possibly 18 mm
    sparks (corresponding to probably about 17 to possibly 20 KV).
     
  20. Shaun

    Shaun Guest

    If you had read my hole post you would have seen that I mentioned that right
    after quote above. See below

    Quote: "Don't you understand Ohms Law, if resistance goes up (dry skin),
    current
    goes down, YOU IDIOT!.
    Also it take the same amount of current through the body (dry skin, moist
    skin or open wounds) to cause their heart to fibrillate. The closer you are
    to the heart the less current it takes because more of that current will
    flow directly though the heart.

    Resistance goes down for open wounds(assuming the current goes into the open
    wound) because there is a direct connection to the internal fluids of the
    body, hence less voltage is needed to cause fibrillation. With dry skin
    (high resistance) it takes more voltage to cause dangerous amounts of
    current to flow.

    end quote

    I never lost in the first place, but you always lose!

    You must have taken that paragraph right out of a book, bravo! I know you
    don't have the brains to come up with that your self or even put together a
    paragraph like that.
     
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