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Electrostatic dissipation

  1. Feb 2, 2014 #1

    EMC

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    Hi Guys--and Gals?

    Here's a wierdie; One is always collecting potential on our bodies, frequently called electrostatic voltage.
    I've found sound research that it is beneficial to "earth" or ground ourselves as often as possible to drain this off. Those last words are the question: does this potential discharge require more than microseconds or perhaps does it require some time for the excess electrons that have moved into orbits in one's bodies molecules to detach?
    There are opinions that posit that it does take time so it is beneficial to sleep on a grounded conducting sheet with many health benefits anyone have any good information about this?

    cheers
    Michael
    Electronic Measurement Consultants
     
  2. jcsd
  3. Feb 2, 2014 #2

    davenn

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    Hi Michael
    welcome to PF

    have you ever walked across a carpet and then touched a metal doorknob etc ?
    you feel that zap and hear the crack of the spark ?

    the discharge is pretty much instantaneous :smile:

    Dave
     
  4. Feb 2, 2014 #3

    sophiecentaur

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    The rate of discharge would be related (order of magnitude) to the resonant frequency of the object in question. A 1.5m person would, perhaps (depending on the details of the connection) behave like a resonant monopole. A 1/4 wave monopole of 1.5m would resonate at about 100MHz. This would mean that the discharge might be expected to take a few 10ns periods (as the oscillations of the spark die down).

    An alternative approach could be based on the speed of light being 0.3m/ns - which would give a timescale of, perhaps 5ns for a charge pulse to travel across the body.

    There could be other valid ways of looking at it but I think it would all be over after, perhaps 100ns.
     
  5. Feb 2, 2014 #4
    Hello Michael,

    Electrostatic charging is caused by the difference in affinity for electrons between two materials which come in contact and are pulled apart. Rubbing seems to enhance the exchange.

    Most early "Hard" discharge models were based upon an RC model of the human body that gave a simple exponential decay. Later models including inductive effects. The following paper sums it up.

    http://www.aecouncil.com/Papers/aec1.pdf

    For ESD protection in the lab, it's desirable to discharge the body slowly lest a sudden change in voltage is capacitively coupled to delicate equipment. Thus wrist straps have large value resistors between the person and ground.

    I think physiological effects are minor for small discharges, and going overboard for grounding is likely silly. However, it is nice to avoid the occasional zap. That said, it's hard to beat cotton and a 30-45% relative humidity to stay comfortable.
     
  6. Feb 3, 2014 #5

    meBigGuy

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    Human body model for static discharge has a typical discharge rise time of 840ps and a typical decay time constant (not fall time) of 150ns. Min and max range from 11ps to 1.8ns rise time and 50ns to 1.5us decay time constant. The electrostatic model for human body discharge is complex, to say the least.

    Using a 1K, 150pf simplified model for a 10KV discharge you get a 10A pulse with a 1ns rise time falling to about half in 100ns. (typical for static discharge testing)

    I'm dubious (putting it lightly) about "sound" research that implies health benefits for sleeping on "grounded sheets" or staying discharged. (other than avoiding getting hit after touching your spouse on the ear) I'll bet it's hard to find a "paper" by someone who isn't selling the solution. But I do know that staying discharged will favorably affect the health of any CMOS circuits you may be handling.
     
  7. Feb 3, 2014 #6

    Bobbywhy

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