Help with 256V DC (50mA) safety concerns

Discussion in 'General Electronics Chat' started by kvderic, May 28, 2008.

  1. kvderic

    Thread Starter New Member

    May 28, 2008
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    Hello all,

    I'm currently utilizing an M10 series DC/DC converter from American Power Designs pushing 256V at 50mA in a circuit design (input is 24VDC). What sort of precautions should I be taking when doing measurements around this circuit. Obviously, I don't want to shock myself or kill myself on accident. Thanks ahead of time.

    -Eric
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    1) Remove jewelry (rings, watches) or insulate them heavily with tape.
    2) Wear rubber-soled shoes.
    3) Sit on a wooden chair/stool/box.
    4) Wooden floor or thick rubber mat.
    5) Keep one hand in your back pocket.
    6) Remove power (unplug), wait 1 minute, and use a jumper to discharge circuit to ground.
    7) Always measure for voltage before handling said circuit.
    8) Do not use a wrist grounding (ESD) strap when working on high voltage circuits.
    9) ALWAYS WEAR EYE PROTECTION.
     
    Last edited: Jun 4, 2008
  3. kvderic

    Thread Starter New Member

    May 28, 2008
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    Wookie, can you elaborate on number 8. I know the wrist strap says "not recommended above 250V" and I believe it; I just wanted to know if you could give me a more detailed answer. Is the resistance to GND not high enough for this amount of voltage?

    -Eric
     
  4. paulktreg

    Distinguished Member

    Jun 2, 2008
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    "A low-voltage (110 to 220 V), 60-Hz AC current traveling through the chest for a fraction of a second may induce ventricular fibrillation at currents as low as 60mA. With DC, 300 to 500 mA is required. If the current has a direct pathway to the heart (eg, via a cardiac catheter or other electrodes), a much lower current of less than 1 mA, (AC or DC) can cause fibrillation. Fibrillations are usually lethal because all the heart muscle cells move independently. Above 200mA, muscle contractions are so strong that the heart muscles cannot move at all."

    You won't kill yourself as long as its current limited at 50mA but it might make you jump!
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Ventricular fibrillation can be triggered at currents lower than 60mA.

    I am NOT a medical doctor. I cannot tell you at what current levels you may experience ventricular fibrillation. I strongly suggest that you do not tempt fate, because fate may take you up on it.

    If you really want to know, ask your doctor. Have him test you.

    What I suggested are merely prudent guidelines from years of experience. They are not all-encompassing, but should keep you comparatively safe if you chose to follow them.
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
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    I also like to wear safety glasses when working with anything capable of making a big spark or blowing the top off a chip or capacitor. Particles in the eye would be unpleasant, or worse.
     
  7. silencer

    New Member

    Jun 3, 2008
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    Wrist straps are really just to prevent ESD damage to sensitive components. When dealing with high voltage, you want to insulate yourself from any path to ground, not create one. Resistance of the human body can vary greatly depending on many factors, being as high as a few megs or more, or as low as 1k or less. You can do the math and see with 250V, you can draw a lethal current. This would be especially true if you wear the strap on one hand and make contact with the other--your heart would be directly in the path of the current.
     
  8. silencer

    New Member

    Jun 3, 2008
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    One other thing that just came to mind as I read your post that hadn't been mentioned yet... just because the power supply is rated at 50mA doesn't mean it couldn't put out much more. Those current ratings usually specify the maximum continuous current it can source. A power supply can operate well above its rated output for short periods of time, without damaging the components or tripping breakers, fuses, relays, etc. Generally speaking, the shorter the duration of the transient, the more current you could draw. I'm just guessing, but for a short enough fault, you could probably draw ten times or more your rated continuous output.

    So regardless of what maximum current your body could withstand, suffice it to say that you could get hit with more. I would echo the previous suggestions and play it safe.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Thank you for mentioning that. It was a glaring omission from my post. I've added it to my original post.

    I've been wearing glasses since the 3rd grade, so I always have them on. As more advanced protective lenses have become available over the years, I've opted for the better ones. As I am generally only without safety glasses on when I am asleep in bed, I seldom think about it; but the first thing I do when I wake up is to put them on.

    Eyesight is a precious and very fragile commodity. Protect it. You only get one set of eyes per lifetime.
     
  10. kvderic

    Thread Starter New Member

    May 28, 2008
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    Thanks for the input guys. Greatly appreciated.
     
  11. paulktreg

    Distinguished Member

    Jun 2, 2008
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    Agreed but AC only.
     
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