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Electrical Safety

Discussion in 'Electronic Basics' started by phaeton, Aug 14, 2007.

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  1. phaeton

    phaeton Guest

    Hello again from the dungeon...

    As I start to get into building line-driven power supplies and such,
    where should I start REALLY being careful? 40V? 60V? 110V? I realize
    that current is what does the damage, but you don't have it without
    voltage. (I also realize that other factors will factor).

    What is the realistic neighborhood of where I shouldn't be grabbing
    wires and/or switching out caps or resistors in a live circuit?

    The physics and shop teacher in HS talked about 110 AC Line current
    like it would knock you dead instantly. An electrician that was
    installing a 208v-->480v step up transformer said "110 won't hurt you
    at all. It's just enough to scare you a little bit".

    I'm not about to strip a power cord and bite down on it, obviously,
    but he's not the first to say 'I've been bit by 110 a bunch of times.
    It's no big deal'..

  2. There should be no need to be doing that at all.
    But if you want a ballpark - anything mains powered (except plugpacks
    which are ok).
    If you don't know the answers to these questions then you shouldn't be
    playing with mains powered stuff at all. The mains can kill you.

    Generally speaking, anything over 50V or so is potentially dangerous
    and should be treated as such.

  3. DJ Delorie

    DJ Delorie Guest

    IIRC, the cutoff for "safe" is somewhere around 60v, which determined
    the rail voltage for telephone switches or something.

    I've gotten bitten by 120v before. It hurt, but caused no permanent
    damage, not that I'd want to do it again. "Safe" is different than "I
    got away with it" too - a little current in the wrong place is fatal,
    especially if it has to cross your chest to get to ground.

    As for lower voltages, I've destroyed ICs by inserting bypass caps
    without discharging them first. Safe for me, not safe for the chips.
  4. John Larkin

    John Larkin Guest

    48 volts.

  5. I think do some bit of research into body count or annual body count as
    a function of voltage.

    110-120 volts AC is "up-there", maybe 2nd place to industrial 460-480
    volts AC or so. Body count of 110-120 volts AC exceeds that from 440-480
    volt AC range on US Navy ships among those where both of these voltage
    ranges are present.

    I suspect that 110-120 volts AC at 50-60 Hz has a high body count due to
    high carelessness due to survival rate of shocks from 110-120 volts AC
    50-60 Hz being high, though short of 100%.

    - Don Klipstein ()
  6. Bob Masta

    Bob Masta Guest

    Sorta like the drunk: "I've driven home from the bar while totally
    sloshed a bunch of times. It's no big deal."

    Or the guy who never wears his seat belt.

    Or the cyclist or biker who never wears a helmet.

    It only has to be a "big deal" one time.

    But even at low voltages, there is a good reason
    to switch things off: If you work on live equipment,
    sooner or later you will slip and short something and
    let the magic smoke out. (Ask me how I know this!) <g>

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!
  7. Chuck

    Chuck Guest

    I was surprised by some of the answers

    First off, disregard the notion that 120
    volts can't kill you. The effect it has
    on you depends on many factors, such as
    whether the current passes through your
    chest (i.e., heart), and how much
    current passes through your body. The
    latter depends, for example, on whether
    you are an older tradesperson with
    calloused, dry fingers.

    But if you have soft, moist fingers and
    contact high enough voltages (definitely
    less than 120 volts), you may easily
    pass enough current to kill. The point
    here is not that you need to measure
    your skin moisture, but rather that you
    need to have a good understanding of
    Ohm's law and why someone might escape
    direct "contact" with lethal voltages.

    Statistics show that even among
    experienced industrial electricians, the
    greater cause of injury and death is
    the consequence of massive muscle
    contraction causing falls from ladders,
    or being thown violently into some
    deadly object.

    I would try to learn a great deal more
    about the dangers of working with high
    voltages before attempting to do so. Do
    a Google search on some of these issues
    and consider soliciting opinions from
    the deceased to balance those available
    from the fortunate! ;-)

  8. Nobody

    Nobody Guest

    Bear in mind that it isn't just a case of being directly harmed by the
    electricity itself. Even 50V can give you enough of a "surprise" that you
    reflexively pull your arm away and ram a soldering iron into your
    face or send a tank of FeCl3/NaOH/etc flying.
    I've been bit by 240 a bunch of times (in the UK, where 240V is 240V
    above ground, not +/-120V). At least one of those was the "bad" case:
    right hand on the live, left hand resting on the grounded chassis. Mind
    you, that was when I was 16-17, young and healthy; the consequences might
    be a bit more serious these days.
  9. Anonymous.

    Anonymous. Guest

    Here in Britland we have an old saying,

    "It's volts that jolts
    But mills that kills"
  10. I've been bit by 240 a bunch of times (in the UK, where 240V is 240V
    Ah, we do tend to forget that fact (it's not 240 a.g. here in the USA). But
    touching 480 3-phase (what's that a.g.?) "got my attention".
  11. Tim Wescott

    Tim Wescott Guest

    And if it has high current capability it can have nasty interactions
    with jewelry. By brother bears a scar on his wrist from a metal watch
    band that got shorted (thanks to a wrench) between a truck battery and
    ground. That was just 12 volts, but it stripped the skin off his wrist
    underneath the segmented metal watch band.


    Tim Wescott
    Wescott Design Services

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at
  12. Doug Miller

    Doug Miller Guest

    Ours are still 240V line-to-line, though -- grabbing one hot lead of a US 240V
    circuit in each hand is every bit as dangerous as grabbing the hot lead of a
    UK 240V circuit in one hand and the neutral in the other.

  13. In the US, it is:

    There are some old electricians,
    There are some bold electricians,
    But there are no old, bold electricians.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  14. Sjouke Burry

    Sjouke Burry Guest

    Just today i was bitten by 240v twice.
    As long as you dont actually grab hold of a wire,
    its fells and acts like a warning to to stay away
    from the hot wires.
    Any time I have to work on a hot circuit, I have
    learned not to grab things, and use proper insulated tools.
    And any time you let your attention slip a little bit,
    you get a stinging lesson to re-train your reflexes. :)
    It also sometimes causes nice sparks.
    Oh, and make sure your footware is isolating you from ground.
  15. Jamie

    Jamie Guest

    Here in the states, 50 Volts or more, you're in a whole new world.
  16. Rich Grise

    Rich Grise Guest


    Good Luck!
  17. Rich Grise

    Rich Grise Guest

    Well, if he can drive safely, it's not.
    That's trivial: Look in front of the car. If there's something there,
    don't go there.
    Oh, feh. This is just too much. A helmet on a freaking BICYCLE?

    You're supposed to learn how to ride the bike so that you don't fall
    on your head.

  18. Since I do a lot of bike riding for delivering delivered restaurant
    food, I have to ride bikes while being a restaurant worker. That is my
    "day job" and I am an engineer as my side job since I have some need to
    make my living from my brain only about 15 hours per week, so I have to
    make my living from my feet quite a bit.
    Restaurant workers and those juggling 2 lines of work due to brain load
    issues should be expected to have "senior moments" at any age.

    I expect that most who do major work or major commuting with a bicycle
    are only a step or 2 better off - as in still having *some* vulnerability
    to a crash where a helmet makes a difference in brain injury outcome.

    For this matter, about a year and a half ago I did suffer a crash that
    had me landing on my head. First time after maybe 1/4 million miles of
    cycling. Prior to that I had a crash resulting from a minor brief brain
    lapse where I failed to avoid a smallish known obstacle and went flying
    and my head missed a tree trunk by only about 10 inches (about 30 cm).

    - Don Klipstein ()
  19. Bob Masta

    Bob Masta Guest

    Then again, I guess there's no sense protecting anything that
    isn't getting used anyway... <g>

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!
  20. Al in Dallas

    Al in Dallas Guest

    I work around 480V, but I've heard 12V can kill if there's enough
    current available. I had to spend a whole day watching videos of
    arc-flash accidents.

    When I was a kid, I got my pinkie stuck on 120V. Since it was just a
    trickle along one finger, it didn't hurt me, but DJ's advice is
    important: "Safe" is different than "I got away with it" too - a
    little current in the wrong place is fatal, especially if it has to
    cross your chest to get to ground.
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