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Safe voltages and transformer secondary

Discussion in 'General Electronics Chat' started by Elerion, Jul 12, 2017.

  1. Elerion

    Elerion Member

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    I read many times, form many sources, that 60 VDC or 30 VAC are potentionally dangerous. I know that it is current what matters, but higher voltages lead to greater currents (assuming same resistance, of course).
    I also know that primary and secondary windings of a transformer are dangerous.
    But all references I read talk about quite big transformers.

    Is using a 24 VAC secondary, or audio transfomer (whose secondary winding is usually low voltage) safe ? (avoiding primary windings, of course).
    Is using a low VA transformer safe? (assume, a 20mA secondary winding).

    Thanks.
     
  2. dr pepper

    dr pepper Well-Known Member

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    The majority of trannies even 3va ones can produce dangerous current if the volts is high enough.
    In the Uk voltages higher than 50v are considered dangerouse, but then it depends on the circumstances.
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Safe for what? Safe for you not getting a shock or safe so that if the transformer is shorted then it won't cause a fire?
    In Canada, PA systems in schools must not produce voltages above 25V RMS so that little kids who are crawling around in the ceiling won't get shocked.

    If your skin resistance is reduced by bathing for an hour in salt water and a voltage is applied between both arms then your heart might stop if the voltage produces a current higher than about 30mA RMS of AC.
     
  4. dave

    Dave New Member

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  5. AnalogKid

    AnalogKid Well-Known Member

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    Yes, for casual contact with fingers. If you've ever touched the tip of your tongue to a fresh 9 V battery, you know what can happen when a normally safe voltage contacts unprotected body parts.

    ak
     
    • Informative Informative x 1
  6. Elerion

    Elerion Member

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    I meant safe for not getting a shock.

    Obviously, care should be taken. But my question was about, would it be safe if accidentally touched?
    99.9% of time I work with low voltages, and not worried at all about the effect on the body.
    Just to know when to be absolutely conscious and be alert, as I'm planning a medium voltage project.
     
  7. chemelec

    chemelec Well-Known Member

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    In Canada, I believe it is also considered relatively safe if it is Under 50 volt.
     
  8. OBW0549

    OBW0549 Member

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    I've never been worried about shock hazard when working with 24 VAC: I've never been shocked, although a few times I've felt a very slight "tickle" from the AC when I've gone right across the source. But shocked? No.
     
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  9. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Same in working on 24Vdc airplanes. I can "feel" which bus bars and circuits are live and which ones are not.
     
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  10. Colin

    Colin Member

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    A voltage as low as 3v can kill you. Look at the baby who swallowed a coin cell and died.
     
  11. JimB

    JimB Super Moderator Most Helpful Member

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    Safe Voltage is not a binary thing.

    You cannot simply say

    Volts <= X is safe
    Volts > X is dangerous

    There are many variables which affect the result, and there isn't a simple equation which ties them all together.

    JimB
     
  12. chemelec

    chemelec Well-Known Member

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    Yes but that was Not Electrocution.
     
  13. AnalogKid

    AnalogKid Well-Known Member

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    Your post is completely misleading. The child died because the cell ruptured, and the battery chemicals ate through her intestine causing unstoppable bleeding. It had nothing to do with voltage, I-squared-R electric current heating, or any other electrical phenomenon.

    ak
     
  14. Elerion

    Elerion Member

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    Sure. But there are voltages which are safe Let's say, 5V.
    Well, with this post I just wanted to know where to set a reasonable limit.

    By the way, not the same, but related.
    I found a couple of multimeters, which manual states:
    "To avoid personal injury, please do not attempt to
    input voltage higher than 30V rms."

    When doing frequency measurements.
    The multimeter is CAT-IV 600V, CAT-III 1000V

    Why is this?
     
  15. Colin

    Colin Member

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    "Yes but that was Not Electrocution."

    "Your post is completely misleading. The child died because the cell ruptured, "

    YOU ARE WRONG !!

    Here is the report from the hospital:

    And when a button battery lodges, it sends an electrical current through the tissue. "[This current] is causing more damage because it is splitting the water surrounding the button battery and forming hydroxide, which is an ingredient in lye," This passes into whatever tissue is nearby, including the aorta or the trachea.
     
  16. MaxHeadRoom78

    MaxHeadRoom78 Active Member

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    By far the highest number of injuries from electrical source is arc flash over, approximately 1,000 deaths per year due to electrical injuries are reported in the United States alone, as someone who has worked extensively in the electrical industry I can vouch for this.
    The injury is far more extensive than a thermal burn. Due to subdermal effects.
    This generally involves high voltage, high energy sources.
    Max.
     
  17. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    take a look at this



    The title is ever so slightly misleading, but if you watch the vid you realize why the title is what it is
     
  18. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    So are you, claiming that the child was 'electrocuted' - which is the theme of this thread.
     
  19. Elerion

    Elerion Member

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    Sorry if I get back to the point :)

    Why could a 600V CAT-IV multimeter be dangerous for (just) frequency meter function:
    "To avoid personal injury, please do not attempt to input voltage higher than 30V rms."

    I've seen this warning on Tenmas' and some other brands.
    I'm sure most of us have a multimeter or many. Is this a common thing?
     
  20. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Possibly it's just the manufacturer 'covering themselves' - rather like toys for babies having labels saying 'unsuitable for children under 5'.

    There's no specific 'safe' voltage, the UK mains at 240V requires considerable effort to kill yourself with, yet a number of people seem to manage it by accident each year?. The American 110V system is obviously lower, yet people die on that each year. I've always heard that some farms in America use 55V mains (for safety reasons), yet some even die on that.

    As far as meters go, safe practices and understanding what you're doing (and why) will keep you far safer than any random voltage rating.
     
  21. Superdat

    Superdat Member

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    You can't just say it's Amps that kill. It's the combination of amps,volts and resistance. Or to put it another way oms law.
    A car draws tens of amps at 12 volts when the starter is engaged but if you get hold of the 12v lead while touching the car's earth nothing happens (to you).
    Although it varies depending what you read, a current of 100-200mA can kill. I'm sure I've read of about 20mA somewhere.
    To make the math easy I'll use 100mA. At 24V amd 100mA the resistance (of a human body) would need to be 24/100mA=0.24k or 240 ohm.
    I just measured myself hand to hand (after a shower about 30mins ago) and I'm 3.5Mohms.
    So ohms law again (using 3Mohm for ease), 24/3M = 8uA. Massively below even the lower 20mA level.
    Soft tissue as in the case of the child has a much lower resistance. So unless you're in the habit of laying in a bath full of super salty water with one electrode in your mouth and the other up a different orifice, 24vac or dc won't kill you. It would make you exit the bath rather quickly though.
    I haven't tried measuring myself this way, if for the sake of science, you wish to, feel free ;-)
    Measuring across my tougue gives about 1.5M so 16uA not even close to 20mA.
    220vac hurts, but 220/3Mohm = 73.3uA or approx 0.1mA, still along way below the kill level.
    So even at 220vac you have to be very unlucky or stupid or both! The bath method is definitely not recommended.

    Long time ago when I was in the Navy (RN) we were in West Virginia and it was very hot and humid.
    I was working on a 24vdc system and got a "I've had a shock" sensation. Not exactly a 220vac shock but I could tell it was electricity.
    When checking later, holding leads in each hand, I could say yes or no to a 24v being turned on or off and I wasn't looking either ;-)
     
    Last edited: Jul 15, 2017

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