The Skin Effect And The Human Body

Discussion in 'General Electronics Chat' started by K-Young, Dec 7, 2009.

  1. K-Young

    Thread Starter Member

    Aug 22, 2009
    27
    1
    I've been doing some research on electrical safety and for the most part everything seems to make sense to me, but the Skin Effect on the human body is confusing me. I know how the Skin Effect works in electrical conductors, but how does it work on the human body. This video illustrates my point:

    http://www.youtube.com/watch?v=qf6AvUdccTk

    At very high frequencies the human body can tolerate voltages that would be normally lethal. Granted that the human body can be an electrical conductor, how does this work?

    Also, I've read some places that the skin effect can trick you also. For example in that video, the guy might not feel much but there is some damage being done to him internally. This would be more true if the voltage was much higher(I believe people who make Tesla coils can tell you more about this).
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    It should be clear that in conductors, the term "skin effect" refers to the tendency of the current to stay confined to the surface layer, rather than flow evenly throughout the conductor. This is why conductors designed to handle microwave frequencies tend to resemble plumbing. http://en.wikipedia.org/wiki/Skin_effect

    This has no relationship to human skin, which is not a conductor.
     
  3. K-Young

    Thread Starter Member

    Aug 22, 2009
    27
    1
    If the human skin isn't a conductor, than how does one get an electric shock?
     
  4. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    I think beenthere was simply stating that skin isn't a very good conductor. Our skin has much too high of a resistance for it to be known as a "conductor". It would more directly be known as an insulator; only under surfeit electrical conditions will the skin "conduct" well enough. I have to admit though, your question is very interesting.

    Austin
     
  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287
    Another "don't try this at home" trick. I used to demonstrate the human light bulb...well, human wire anyway. I'd set up a 1,000 watt ham radio transmitter to operate on around 14 MHZ at a couple hundred watts. I'd have a wire coming from the antenna terninal I'd grab the threaded part of a heat lamp, and touch the button part of the base to the transmitter wire. The bulb would light up fully...my hand being half the circuit. Never felt a thing, unless I didn't grab the base TIGHT enough, in which case I'd get a NASTY r.f. burn!

    I should be glowing in the dark with all the experiments like this I used to do. :)

    eric
     
  6. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    This is definitely an interesting question. I'm going to throw in my opinion/hypothesis and see if it holds up to scrutiny.

    I think a key aspect to this discussion is that human skin is a good insulator at DC and it becomes a better conductor as frequency increases. This relates to the famous Edison vs. Tesla (i.e. DC vs. AC) battle. Edison was trying to show that DC is safer by electrocuting animals with AC current, but AC wound up more practical in the end.

    The ironic thing is, if we go to higher frequencies, well beyond 60 Hz, the good conductivity of skin, saves us due to the skin-effect (pardon the unavoidable pun). Basically, the mostly-dead skin tissue carries the current, and no disturbance is done to living tissue, since high frequency AC can't penetrate much beyond the skin depths (both the electrical "skin-depth" and the biological skin depth). This idea should not be carried too far, because eventually higher and higher currents will cause enough heating to cause biological damage.

    Among of the most harmful and noticable aspects of currents that enter the body are disturbance of electrical control signals and contraction of muscles. Of course, the heart is the most sensitive to this. However, if the electricity does not even reach surface muscles and nerves, we are much less likely to feel the effects.
     
    Last edited: Dec 8, 2009
  7. Mike33

    AAC Fanatic!

    Feb 4, 2005
    349
    25
    To expand on steveb's answer above...I have read that in Tesla's opinion, frequencies higher than about 2kHz have no effect on humans, even at extremely high voltages and currents!! The inability of such small wavelengths to penetrate the layer of dead skin at the surface of the body may well be the cause.
    Again, please don't try this at home! LOL. That would be dumb....
     
  8. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    I'm forced to be the first one to debunk my own theory. I found data to support the idea of lower skin resistance at higher frequency. However, the effect does not appear strong enough. There is maybe a 1 order of magnitude reduction in skin resistance, but this seems too small to suport the "electrical-skin-effect" in the high kHz range. I didn't find data at RF frequency, so maybe the idea can be salvaged, but I'm not hopeful.

    I conclude from this that the electrical-skin-effect must be taking place just under the human skin where conductivity is much higher. High conductivity and high frequency are both needed to support a small electrical-skin-depth.
     
  9. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    :rolleyes: Mike and Steve you are so wrong
     
  10. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    Please explain.

    Am I wrong, or am I wrong about being wrong? Or, both?
     
  11. K-Young

    Thread Starter Member

    Aug 22, 2009
    27
    1
  12. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    High frequency electrical safety is quite clear - avoid making contact.

    As far as instantaneous damage is concerned, making contact so the current from a damper diode in an old vacuum tube TV set traverses your hand/arm produces a long blackened burn under the skin. It is very painful. The frequency is 15,750 Hz.

    The lesson is obvious - never come in contact with a conductor unless you can insure it carries no voltage. Stay far away from high voltages and frequencies.
     
  13. K-Young

    Thread Starter Member

    Aug 22, 2009
    27
    1
    Wait what....?

    Where are you getting these stroies from? I hope it's not from first-hand experience?:eek:
     
  14. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    First-hand experience. Ask me about changing over 900 volts worth of B batteries in the box supplying a Beckman DU spectrophotometer. DC hurts, too.

    I never touch a conductor unless I know it's deenergized.
     
  15. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    Very interesting, and it seems correct. To verify, I consulted the book, "The Handbook of Biological Effects of Electromagnetic Fields" 2nd edition, edited by Polk and Postow. This verifies that the skin effect does not become significant for humans and other large vertebrates until frequency is greater than 10MHz.

    One important point mentioned is that the standard formula for skin depth assumes a good conductor with  p={{\sigma}\over{\omega \epsilon}} much greater than 1. However, most biological materials have 0.1<p<10 over a wide frequency range. The book quotes a more general expression for skin depth valid for this range.

    The bottom line is that this skin depth explanation appears to be unfounded, unless we start talking about > 10 MHz. Even internally, we are not good enough conductors to be protected by the skin effect. At least, not in the kHz and low MHz range. Maybe KL7AJ's example at 14 MHz is high enough frequency for that explanation, or maybe he was protected by another mechanism? - Like being a real man who doesn't need a voltmeter.:p
     
  16. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    True enough. Always use your third hand.
     
  17. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    A corollary of this is: "never use your third leg". :eek:
     
  18. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    I haven't had time to do much figuring on this but two questions immediately spring to mind.

    Firstly the electromagnetic waves of frequency greater than 200mHz would be smaller than a human (1.5m wavelength).

    Secondly propagation of waves (=AC) depends upon the ratio of conduction current to displacement current in the medium concerned.

    when ωε > σ propagation is capacitive ie the medium behaves as a dielectric.

    I don't have figures for skin but seawater which should be similar

    ε = 80 and σ = 4
     
  19. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    The electrical properties for human skin and body tissue are very complex. Unfortunately you will also find many myths that are pure nonsense. So you should only thrust scientific papers. And certainly not folklore. But it is an interesting field. I have found some free sources
    http://www.scribd.com/doc/23422819/Electrical-Conductivity-of-Tissue-at-Frequencies-Below-1-MHz
    http://books.google.com/books?id=YUl9bz-om1kC&pg=RA2-PA598&lpg=RA2-PA598#v=onepage&q=&f=false
    http://books.google.com/books?id=vttnCrG2GawC&pg=PA301#v=onepage&q=&f=false
    http://www.fys.uio.no/elg/bioimp/pdf/depth.pdf
    http://www.bem.fi/book/index.htm
     
  20. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    SteveB,

    That is immortalized by Snappy Sammy Smoot (Skip Williamson's cartoon character) - "Never wee-wee on yer TeeVee".
     
Loading...