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Electric shock in welding

  1. Nov 19, 2012 #1
    in welding operation , you never hit by electric shock,why????
     
  2. jcsd
  3. Nov 19, 2012 #2

    davenn

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    only if you are using it correctly :) use it incorrectly and you will likely to get a shock
    the welding rod handle is an isolator, and no part of your body is completing a circuit between the welding rod and the material being welded


    Dave
     
  4. Nov 19, 2012 #3
    if you hold electrod in your hand and hold handle in another hand , you dont get shock. why???
     
  5. Nov 19, 2012 #4

    Borek

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    It is not exactly true that you won't get an electric shock. Check the voltage of the typical welding power supply to understand what is going on.
     
  6. Nov 19, 2012 #5
    what is voltage need to get electric shock?
     
  7. Nov 19, 2012 #6

    Borek

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    Ask uncle Google.
     
  8. Nov 19, 2012 #7
    Typically for safety evaluations - 50V is considered the starting point. For a welder, particulaly DC), with long cable or large system it may be possible to get an inductive kick - from the current turning on or off ( usually this only occurs when you turn it off) - Think of a DC welder.
     
  9. Nov 19, 2012 #8
    Depends on Amps. Its the amps that will most likely harm you.
    Here's an analogy
    Lets say voltage is a baseball, and Amperage is how fast the baseball is moving. If you have a baseball (lets say 120v) but low amperage (lets say the ball is just tossed in the air), its not going to do much damage to you if it hits you(may do a little). BUT if that 120v has HIGH amperage, its like that same baseball coming at you at 100mph, and that's going to hurt! That's why they say its the amps that get you, not the voltage. (its kinda like kinetic energy is to the baseball, as amps are to voltage in a loose sense)
     
  10. Nov 19, 2012 #9

    davenn

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    not entirely correct

    even a small voltage and current can cause a good tingle, think of a 9 V battery on the tongue or other damp area.
    on skin the combination of voltage and current only need to over come your skin resistance to give you a shock. It only takes 30mA through the chest to put the heart into fibrilation.

    the old saying is ... Volts Jolts, Current Kills

    cheers
    Dave
     
  11. Nov 23, 2012 #10
    This is not true at all. Voltage alone decides the deadlines since current does not play a real role in dielectric breakdown. The Ohm's law cannot be used: if I touch the +12V rail of my PC's power supply while grounded nothing will happen although it is rated for 15A. Even 600A at several volts can't possibly do any damage. Now if I touch a CFL rated at 40mA while connected to mains a lot will happen and I could die.

    Low voltage high current is inherently safer than high voltage low current. That is why safety equipment for living things is rated for voltage and not amps.
     
  12. Nov 23, 2012 #11

    Averagesupernova

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    Please don't post garbage like this. Now it is unlikely that several volts from one hand to the other will be able to push 600 amps through, but it is in fact current that kills. How many volts do you think it would take to push the amount of required current to kill through the heart from an open chest wound? The volts that it takes to kill depends on conditions of the body and points of contact. The current through the heart it takes to kill is pretty much constant.
     
  13. Nov 23, 2012 #12
    A typical shock that you get when touch someone or something is between 4000V and 10 000V. When the amps are not very high, you need a lot of voltage to feel something.
     
  14. Nov 23, 2012 #13

    jim hardy

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    To original question:
    Dry skin and dry welding gloves are both good insulators. They prevent current getting to any nerves that could feel it.

    Welders are typically ~70 volts open circuit to strike the arc, dropping to ~25 volts to maintain it.
    Dry skin will protect you against 70 volts, but wet skin will allow even a 12 volt car battery to give your hand a good "zap". Hence the old saying "dont stand in a puddle when changing a lightbulb".

    From my own experiments with a Simpson 260 meter and DC source,
    i can definitely feel 0.001 ampere if it gets past my skin. At 1/10 that value there's the beginning of sensation, but that value probably varies between individuals.

    Literature gives thresholds of 0.02 to 0.05 amps through one's chest as capable of stopping a heart , i presume that's why GFCI breakers are about that sensitive.

    So it's CURRENT that your nerves sense, and it's VOLTAGE that pushes that current through your skin to where the nerves are.

    Your welder was designed to be fairly safe so long as you use good sense.

    If you ever played with a "Plasma Globe" you have seen the high voltage break down the gas into streamers that'll follow your finger around the plasma globe. That's an example of high voltage but very low current - the nerves in your finger can't even feel the tiny current where it touches the globe's surface. It's milllionths of an amp, well below threshold of feeling.

    you should look up meanings of ampere, volt and ohm.
     
  15. Nov 23, 2012 #14
    what?? this is completely false. If this was true then the harmless "plasma orbs" (jim hardy's comment) that sit on peoples desk tops would severely shock you.
     
  16. Nov 23, 2012 #15
    Exactly. So it's voltage alone that decides whether that fixed puny amount of current flows. Lower voltage: it won't flow. Higher voltage: it will flow. The source itself could supply 90mA or 543624A - if the voltage is not high enough, nothing will happen. 20 V across the hands can't push those deadly 90mA. 0.2 V across the heart can't push those deadly 90mA.

    Considering that even a 1.5V button cell can deliver such current, you have to wire around 50 of them in series to raise the voltage to make them really dangerous. On the other hand even if you wire millions of such cells in parallel, thus making a system capable of delivering huge currents, you still wouldn't be able to electrocute a mouse.


    They will shock you pretty severely if you touch the output of the transformer that's inside. You should not assume that all things suitable for sitting on desks are safe.
     
  17. Nov 24, 2012 #16

    davenn

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    Thanks for the backup Averagesupernova :)

    I havent had a chance to respond to cyclix's rubbishing of me
    I suggest he does a bit of study of human physiology and how its affected by electric currents before spouting off with bad replies

    Dave
     
  18. Nov 24, 2012 #17

    Averagesupernova

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    Cyclix, I don't understand how you don't understand. Just as I posted and you somewhat agree the amount of current through the heart that it takes to kill does not change much. The amount of voltage it takes to do this varies widely on conditions of the body. We were told in school that even what is considered safe voltages (50 volts and less) can cause death when placed from hand to hand. If the hands have been wet for a while so there is very good conduction to deep tissue significant current can flow. We were also told that if the heart rate is low to begin with it is even easier to cause heart failure. I hope I don't need to say that I have never experimented with it. The flip side of this is with very dry hands it takes alot more voltage to do the same thing. So it is NOT voltage alone that determines what it takes to kill you. Conditions of the body vary widely and this is another variable. I also hope that I don't need to mention that a high voltage supply that is only capable of supplying a couple of mA is not likely to stop the heart. However, it is not wise to get into the habit of thinking that way.
     
  19. Nov 25, 2012 #18

    jim hardy

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    that's it exactly.

    I assure you, from personal experience, that
    when you are out in the ocean soaked with salt spray
    and your boat's battery terminal comes loose
    you cannot hold onto the positive wingnut to tighten it
    because your salt soaked skin allows the battery's measly 12 volts
    to push curent into your fingers
    and hand
    and arm
    and you even feel it in your feet where the current exits into the bilgewater.

    Your hand and arm muscles contract and pull away from the battery.

    And you "get the feel" of current stimulating your nerves.
    Because it's painful, the lesson sticks.

    old jim
     
  20. Nov 25, 2012 #19

    Averagesupernova

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    We had a prof. in school that would refer to it as 'reach out and touch ya'.
     
  21. Nov 25, 2012 #20
    There is a fallacy in this argumentation. You are involving variable resistance in the equation but not adjusting the other important parameter (voltage) at the same rate. If we lower the voltage with the same factor as the resistance we will see again that if the input voltage is not enough to push our given dangerous current, nothing happens. The power supply itself could be rated for 1000s of A but if its voltage is below a certain threshold for a given resistance then nothing will happen. The only way for some living thing to get harmed in this setup is by increasing the voltage.

    Next time you hear "Amps kill you, Volts don't" ask that person what would he rather touch, 15A at 12V or 40mA at mains voltage. The latter supplies hundreds of times less "amps". Just don't let them try to disprove me by experiment.
     
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