Electrical Safety

Discussion in 'General Electronics Chat' started by rustynail, Feb 13, 2006.

  1. rustynail

    rustynail Thread Starter New Member

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    I am looking for electrical safety material as it applies directly to direct current circuits. Does anyone have the name of a reference book or publication that addresses this subject. I have plenty of material on AC circuits, but not DC. Thank you.
  2. chesart1

    chesart1 Senior Member

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  3. n9xv

    n9xv Senior Member

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    chesart1 provides some great links. I would add that you can sum it up like this;

    Its the current that kills! As little as 100-mA to 300-mA of current actually flowing through the body can be leathal.

    As far as the current is concerned, current is current. That is, 100-mA to 300-mA whether its AC or DC.

    The only difference is, DC bucks you like a mad bull! If you come in contact with high voltage DC, it causes the muscels to retract in such a way that you'll be sent flying in the opposite direction.

    AC has the effect of "glueing" you to the point of contact. With AC you cant let go. I'm speaking of 60-Hz AC here. The muscels contract & retract at such a rate as to not allow you to break free. Some people argue the above because they have been shocked by AC and experienced the "bucking" phenomena from AC. Thats true to a point. Beyond 120-VAC @ 60-Hz and with enough current, you'll likely not be letting go.

    AC at RF frequencies (> 10-KHz) the effect is to burn the point of contact.
  4. JoeJester

    JoeJester AAC Fanatic!

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    I know of emperical evidence that a 900V, 300 mA supply will hold a body on the supply for a second or two. The saving grace was that person's body resistance at the time was pretty high ... 200k or so ... so only 4 or so mA was flowing. It took a good mf'r scream for that person to remove themselves from that supply.

    A person's body can be as low as 300 ohms, so potentials as low as 30 volts could be hazardous to your health.

    Module 1 of the NEETs book describes safety. Visit Davidson College for a copy.
  5. alva

    alva Member

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    Not quite correct about the old adage that current kills.
    Current WITH high voltage kills. High voltage being around 40+ volts.
    The higher voltage forces the current through skin much easier.
    If you hook up a 6 volt battery and a 6 ohm resistor creating 1000mA of current,
    I don't think many people will feel a thing if they touch the circuit unless they're in a bathtub.


  6. JoeJester

    JoeJester AAC Fanatic!

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    All physicalogical conditions concerning electrical shock use current in their descriptions.

    In most circuits, and your completing the circuit when you touch it and ground, the R [human body], determines the I flowing.

    Ever know of someone who stuck a nine volt battery on their tongue? I'm not advocating that you do such things, it's just an illustration. The tongue [being wet with saliva] is considerably lower in resistance than say, between the hands, will allow more current to flow from that 9 volt battery ... so they perceive it.

    1 mA is perceptible.
    10 mA is sufficient to prevent voluntary control of the muscles.
    100 mA can be fatal if it's over 1 second.

    Tests have shown that body resistance under unfavorable conditions may be as low as 300 ohms, and possibly as low as 100 ohms from temple to temple if the skin is broken.
  7. aac

    aac Active Member

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    Your body has resistance too. It has to be added to the 6 Ohms. So with your circuit you won't feel anything because you won't get 1000mA. If your dry skin is dry, you might have 150kOhms. You won't even get 1mA. It is the current that kills you, no doubt. It sure doesn't take 40V to get enough current so you can feel it. Try putting your tung on a 9V battery really quickly.
  8. alva

    alva Member

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    yeah and swallowing the 9v battery can kill you a lot easier.
    For the current to have fatal or near fatal consequences, it has to enter into the body and that takes a certain amount of voltage. A slight sensation to the skin is not harmful, static electricity has more effect than that.

    That 100mA they're talking about is current into the heart and that takes some doing with low voltage devices. I imagine there are scenarios where it can happen like being in salt water carrying a 12V car battery and holding the terminals in your hands and wearing metal jewelry, etc. but under ordinary circumstances - dry surface, dry shoes, etc. it isn't going to happen.

    Most lethal shocks occur in a damp environment (e.g. bathroom or basement) where the body becomes less resistant as noted above so the High voltage AC current can do its damage. I've been shocked before, 120V AC appliance, and know the pain of my right arm, feeling like it weighs 200 pounds and a throbbing headache that occurred with it. It's one reason I don't like to work with AC and I stay with mainly low voltage DC where the amps are higher and the shocks are less!

    P.S. Don't put batteries in your mouth or any other part of your body! Just in case.

    P.S.S. Don't take chances with either AC or DC low or high voltage either. Just in case.
  9. n9xv

    n9xv Senior Member

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    "Please dont eat the batteries"

    was'nt that a TV show in the late 70s?

    Anyway, it absolutely is the current that kills. When they say X number of milliamps can cause a certain amount of damage etc, its meant that, that level of current must actually be flowing in the body. Not if you touch something with X amount of current thus providing an alternate branch for current. You can charge up a .01-uF capacitor to a few 1000-volts. Touch it and no ill effects because there is no useful current. We all walk past 120-VAC outlets everyday, voltage is present but no damage is done until you make contact with the outlet providing a suitable path for current to flow. The voltage must be sufficient to push the current through you in order to do the damage. The specified levels of current must be flowing through you, not through what your touching.
  10. JoeJester

    JoeJester AAC Fanatic!

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    Folks ... 15, 500 volt and 21,000 volt circuits don't wait for you to touch them. They will reach out and touch you when the air gap between you and it is close enough. It is very important you read the safety precautions in the technical documentation. The a$$ you save may be your own.

    That 900VDC 300 mA supply I talked about was the supply's rating. Luckily, the only visibile scar was the entrance point on the thumb. Like I said, the body resistance was high enough to prevent death, but low enough that the person hung on the circuit for about three seconds. Loss of involuntary control of muscles is described as approximately 10 mA.

    Touching your tongue to that battery will result in an involuntary muscle reaction. That indicates a current greater than 1 mA [perception] but less than 10 mA. I doubt these are exact figures for every human, but they are close enough for discussion purposes.

    I'm sure when shock therapy was the fad, the doctors had made the observations to arrive at the physiological effect figures.
  11. thingmaker3

    thingmaker3 Retired Moderator

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  12. rustynail

    rustynail Thread Starter New Member

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    Thank you all for the great discussion and leads. In the end, it does come down to current passing through the body. The amount of voltage necessary to drive the fatal current, depends on the body resistance which is constantly changing and influenced by many factors. Thank you.
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