Physiological effects of electricity

Discussion in 'Feedback and Suggestions' started by Hartlus, Oct 16, 2013.

  1. Hartlus

    Thread Starter New Member

    Oct 16, 2013
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    I wrote this in notepad thinking I would just be sending an email but now I see that I have to post to a message board. Oh well. Let me preface by saying that I am not an authority on anything. I learned everything from somewhere else.

    ---

    One very important error and one not so important:

    You described using a sensitive ohm meter to determine the body's electrical resistivity. This method is incorrect or at least not very good. The method that I have heard described is to create a circuit through a small power supply to the body with the ohm meter in between. ie. power supply positive > ohm meter > left hand > right hand > power supply ground. Sounds scary but the ohm meter has a high impedance which prevents any appreciable amount of current to flow through the body. The high impedance is probably also the reason that just a probe in each hand is inaccurate. Perhaps your meter is sensitive enough to obtain an accurate reading but your visitor's meter may not. This could cause a person to take an inaccurate reading and lull them into a false sense of security regarding the electrical components that they are concerned with.

    You stated that you measured 1M ohm with your meter but NIOSH states (according to wikipedia anyway) that under dry conditions the resistance offered by the human body may be as high as 100,000 ohms. If that is correct, you are going around with the idea that you can handle ten times more voltage than you really can. Not safe.

    Another error that isn't really critical is that you state there is a necessary amount of voltage to induce Tetanus. Perhaps you thought that that was some clever word play but it doesn't come off that way. Tetanus is caused by some biological crap. Perhaps I should check my thesaurus but that should be reason enough to not use the word tetanus in this context.

    As an aside; the reason women are less resistant to shock is because electricity is sexist. Just kidding. The idea that women have less resistance is based on empirical data rather than, well, something more sciency. There are physiological differences between the average man and woman that affected the empirical data but it's these differences that affect resistivity rather than sex. Just think of the body as a human resistor (or capacitor) comprised of protein, liquid, salt, fat etc. A short sweaty fat man with short fat arms makes a better conductor than a tall lean woman. And then there are kids; their short little arms and pudgy little bodies don't stand a chance.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I believe I did also. :p
    I would have loved to have inherited knowledge.
    Max.
     
  3. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    I know I've read over 260k between the arms squeezing a the tips of the meter between my thumb and forefinger ... with a simpson 260; also if I moistened my fingers before that resistance test, I could lower the resistance.. I also know that the resistance is much, much lower, according to what I've read in the NEETS books ... as low as 30 ohms under humid conditions (think out to sea). I'd bet you'd read low ohms on your tongue ... and I've read where open wounds have a much lower resistance (using your blood stream as a conductor).

    So, what does all of this mean ... be safe. I've not measured my resistance with a digital multimeter. I can tell you from empirical data, that a 900 volt, 300 mA power source will hold a 200 lb person for more than two seconds. I can tell you I've known kids that tested a 9V battery using their tongue ... Not sure how accurate it was, but they described getting a tingle ... or was that tetanus.

    from https://www.osha.gov/dte/grant_materials/fy07/sh-16610-07/01_pg-module_1.pdf
     
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  4. Georacer

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    Nov 25, 2009
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    @Hartlus,

    Can you please give a link and a quote to the part where it says that the resistance from arm to arm is 1MΩ? I can't seem to find it.
     
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  5. Hartlus

    Thread Starter New Member

    Oct 16, 2013
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    It appears that I gave the wrong page title. This is the page that I was reading: http://www.allaboutcircuits.com/vol_1/chpt_3/4.html

    6th or 7th paragraph down, first sentence: "Measuring electrical resistance with a sensitive meter, I measure approximately 1 million ohms of resistance (1 MΩ) between my two hands, holding on to the meter's metal probes between my fingers."

    I don't know if that's right or wrong, it just doesn't agree with what I've read.
     
    Last edited: Oct 17, 2013
  6. Hartlus

    Thread Starter New Member

    Oct 16, 2013
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    I was concerned that I would sound like someone who knew what they were talking about when in fact I had just spent a lot of time on the internet. I was trying to figure out if a 3 VDC @ 1.1a power supply could kill or seriously harm someone. 3v just seems so small to even bother worrying about.

    I want to use a regulator to provide the current for an electroplating project (electroplating being the project). I know someone else wanting to do the same thing so I was going to put one together for her. The setup is basically this: Tub full of saturated cupric sulfate and maybe some acid, a copper anode sitting in the solution, an iron cathode or item to be plated, and a power supply that can pass current at low voltage (only around .5v actually).

    The concerning factors or risks are:

    Cupric sulfate solution is very conductive.

    Skin provides most of the resistance through the human body and may be compromised by the cupper sulphate.

    It would seem perfectly natural to pick the elctrodes up at the same time in different hands. Even more natural if something was spilling or falling.


    The OSHA pdf was very helpful. Thank you for that. Do I understand correctly that if the guy made up of resistors was wet then the resistance is 1,000 ohms per arm?

    I originally planned for 0-3V so a wet woman would receive at least 15ma and experience loss of muscular control (sounds familiar). If I tone it down to 0-1V she'll only receive .5ma and not feel a thing. Heh. Yet, if she were to receive 50 ma she would get tetanus. Go figure.

    But now you say that resistance can be as low as 30 ohms out at sea and the cupric sulfate is at least as conductive as any other salt water. So I guess I'm still concerned.

    I once told my 9 year old to lick a 9V and tell me what the voltage was. This is because I am a bad father. He did without flinching and reported that it was about 8.7V. So of course I had to check and I'll be damned if it wasn't exactly 8.7v. Well, this went so well that I decided to give him a new 9v. Again, no flinching. Reports 9.4v and it's dead on again. I suppose some fathers would be proud but I had really wanted to see him jump.

    I need to go to bed. This is why I have to stay off forums.

    Thanks for the help. I'm sure I'll be back asking you why my circuit won't work.
     
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    While OSHA may have standards in their training manual, everyone is different.

    30 volts @ 100 mA is 300 ohms. For your 3 volts to be dangerous, you would need a low body resistance. 30 ohms for 100 mA, 300 ohms for 10 mA, etc.

    Safety first ...

    Here's some information on Electrical Safety from the Neets Series Module 1
     
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  8. WBahn

    Moderator

    Mar 31, 2012
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    If you get through the skin resistance (say poke a needle into each of a person's wrists) then you have a quite low-resistance path through the body that goes across the chest cavity. Say it is 100Ω. Now connect a 9V battery. You've got 90mA, which is very possibly a lethal current.

    The chances of a situation like this occuring accidentally are all but zero, but it highlights the need to exercise care around even relatively low voltage supplies if there is a chance that we can get tangled with them when our skin resistance is compromised (and wet, sweaty skin is highly compromised). Personally, I doubt I would ever be much concerned with 12V and below. But anything above about 24V and I'm going to exercise a degree of caution -- and that caution is going to go up in exponential proportion to the voltage.
     
    Last edited: Oct 17, 2013
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  9. Wendy

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    Mar 24, 2008
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    I have to agree with WBahn on this one. I have had 12VDC from a car battery cause a tingle, but I did not feel any threat, even though a car battery has a lot of potential electrical and mechanical violence in it. Short them out and they have been known to explode, though I have never seen it happen.

    Current path is also critical, I used to check 9V batteries on my tongue. Not smart, but not too dangerous, since the current is only traveling ¼" or so. I'm a lot more worried about the chemicals I am accidentally ingesting.
     
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  10. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Knowledge is important when working around circuits.

    I like to say one final thought ... "the ass you save may be your own."
     
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  11. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I've been shocked by 24VDC before when I was sweaty. I played with 24VDC with my bare hands for a long time before that. I don't any more. It wasn't as bad as 120VAC but it was enough to make me jolt and throw my tool.

    I don't think 3V is enough to hurt you even if drenched in electrolyte, but I won't bet your life on it.
     
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  12. Hartlus

    Thread Starter New Member

    Oct 16, 2013
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    I would not have bothered worrying about such a small voltage but if I make a circuit for someone else it's not my life I'm betting on. I'll just etch a shock warning on the board and recommend gloves. Gloves are good anyway when dealing with chemicals.

    Thanks everyone! Next time I'll be asking for help in one of the help forums.
     
  13. WBahn

    Moderator

    Mar 31, 2012
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    I still check 9V batteries on my tongue.

    I just turned signatures on and so can't help but commenting on yours.

    "Good enough is the enemy of best."

    True, but "Best is the enemy of steady paycheck."

    I tell students that if the solution is "good enough", then it's good enough! Stop! Bill the customer and move on. The customer is NOT going to be any happier to get a "better" solution a week late and $1000 more expensive when the solution you had a week ago met their needs.

    Actually, the two aren't that incompatible, but it does require a more holistic view of the metric of "best" than most engineers take.

    We like making circuits that perform better. Which is understandable and admirable. But we are being paid by someone that has a broader metric of performance that includes cost and schedule. Just as a circuit with higher bandwidth may not be "better" even if high bandwidth is the goal if the circuit sacrifices too much in other areas such as power consumption or stability, so too may a circuit not be "better" even if it meets all of the electrical performance specs better if the circuit sacrifices too much in other areas such as cost to design it or time to deliver it.
     
  14. Wendy

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    Mar 24, 2008
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    I think I said that. :D It was a saying I picked up from a friend who was a self taught engineer. He was doing well last I heard.
     
  15. WBahn

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    Mar 31, 2012
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    Said what? Said it where? I'm sure you have at some point some place (and probably many times).
     
  16. Wendy

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    Mar 24, 2008
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    I believe we were talking about my signature?
     
  17. WBahn

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    Mar 31, 2012
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    Okay. I thought it was obvious that we were talking about your signature since I was responding to a from a post from you that I had quoted. But maybe it wasn't obvious that that was the signature I was referring to and I should have been more explicit. Anyway, I thought that what you said in your response to what I said was that you had said what I was saying in response to what you had said in your signature, I just didn't see where you had said that.

    Now, wrap your mind around THAT line of reasoning! :eek:

    I think it is even accurate. :D

    But all is well.
     
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  18. killivolt

    Active Member

    Jan 10, 2010
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    Try saying that out loud, it's even more difficult than to read it.

    BTW, "Meterman 38XR while pinching with wet fingers I'm 154k or so.

    Whats yours?
     
    Last edited: Oct 19, 2013
  19. WBahn

    Moderator

    Mar 31, 2012
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    Hmmm. I didn't have any problem saying it out loud. But then I could also feel my mind drawing pretty heavily upon the knowledge of the point that was being conveyed to keep my mouth on track. So perhaps I was only able to say it smoothly because I was sufficiently familiar with it. Interesting.

    I don't know if I want to pinch a "Meaterman":eek:; just has this masochistic "feel" to it.:D
     
  20. killivolt

    Active Member

    Jan 10, 2010
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    I does sound bad when you say it that way, doesn't it.:p

    Maybe I should have the bought the MeterMan brand rather than what my poor spelling suggested.
     
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