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Thread: When does DC voltage become dangerous?

  1. #91
    HKJ's Avatar
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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by BobBarker View Post
    Also, just for general information... If you attach a full-wave bridge rectifier to your ac line (or transformer)... you WILL NOT get 120v RMS... You WILL get a rectified 169v (you multiply the input by 1.41 to get your output)
    That is not correct, you will still have the exactly same RMS voltage minus the drop in the rectifier.
    But add a capacitor and you get the a DC voltage with a RMS value close to the peak voltage of the original AC voltage (i.e. 1.41 times more voltage).
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  2. #92

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    I've definitely gotten a charge out of this thread, which has sparked several observations.

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  3. #93
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    Default Re: When does DC voltage become dangerous?

    Just wondering, if I'd touch + and - of, let's say, 1000V DC with two of fingers on same hand, would the damage be limited to hand, or current will in some way go through whole body?
    Sorry if this has been asked here, just too much posts to check all of them.

  4. #94
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    Default Re: When does DC voltage become dangerous?

    99% of the time, you will only lose those fingers and the meat in between... but that 1% is still there looking to kill you. so basically, don't go around touching 1000v DC or AC systems unless you really don't value your life.
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  5. #95
    Flashaholic InHisName's Avatar
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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by flashflood View Post
    I've definitely gotten a charge out of this thread, which has sparked several observations.

    Resistance is futile.
    Protect the family Joules.
    Stay current to maximize your potential.
    Watts your problem? More power to you!

    Oy -- Thank God it's Faraday.
    (I know, I know -- revolting!)
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  6. #96
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    Default Re: When does DC voltage become dangerous?

    The most damage would be done where the current leaves your body. You need to isolate yourself from any ground. Use isolated tools. People in my line of work die from -48V unfused power. One guy was sweaty and bald bent down to get his wrench was touching a bus and his head hit the frame(which is bonded to ground). That room still smells funky and I get the chills when I work in there.

    Amperage is what does the damage.

    Voltage is just the pressure behind the current. Voltage needs to be high enough and your body's resistance needs to be low enough for the current to flow through you. Without any amperage though it is just like a static shock from the carpet. High voltage but harmless. Just fun to tease the cat.
    Last edited by ToyTank; 04-20-2012 at 10:02 PM.

  7. #97
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    Default Re: When does DC voltage become dangerous?

    To add a theoretical example to Mr Happy's post. Lets say you had an old 350 Cubic Inch V-8 Chevy engine. A few decades ago, the starter motors for these engines pulled about 220 Amps at 12 Volts - OR 2,640 Watts of power were used to accomplish the work of turning over the engine if the starter was in good shape. Fairly thick cables are needed to move 220 Amps of current. IIRC something like #1's or aut or 2/0. In theory, (lots of other complications would occur) if you could find a 24 volt starter for that same engine, the starter would pull only 110 Amps at 24 Volts but you still are using the same 2,640 Watts of power (24 x's 110 = 2,640 Watts) to accomplish the same work. So higher voltage allows you to move less current while maintaining the same power level. Saves a lot of copper!

    I wouldn't say that "Amperage is what does the damage". How many times have we touched a leaking 40KV spark plug wire only to quickly pull your arm away and have your elbow strike something and hurt like heck, if not draw blood.
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  8. #98
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    Default Re: When does DC voltage become dangerous?

    Back at school they taught me that from 40V, DC voltage can become deadly.
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  9. #99

    Default Re: When does DC voltage become dangerous?

    http://www.darwinawards.com/darwin/darwin1999-50.html

    anything 9v and over is potentially lethal in the right circumstances

  10. #100

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    Quote Originally Posted by InHisName View Post
    I appoint you to be in charge of the daily humor.
    Balancing your d(humor)/dt will be a daily delicate balancing act.
    I know you can do it, as long as you don't get overcharged.
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  11. #101
    Flashaholic ToyTank's Avatar
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    Default Re: When does DC voltage become dangerous?

    Quote Originally Posted by BVH View Post
    I wouldn't say that "Amperage is what does the damage". How many times have we touched a leaking 40KV spark plug wire only to quickly pull your arm away and have your elbow strike something and hurt like heck, if not draw blood.
    The current itself did no damage, it was your nerves reacting causing the reflex. If that had been a 200 amp you'd lost your arm.

    I stand by my statement that amperage is what does the damage, but yes you need enough voltage to overcome your resistance and complete the circuit.

    Current = Voltage / resistance. I can't find what the average resistance of a human is though. I'm sure there would be variation among people and individuals sweaty and not etc. But if your resistance is higher than the voltage than current= Zero and there is no circuit. Wikipedia... electric shock

    A person can feel at least 1 mA (rms) of AC at 60 Hz, while at least 5 mA for DC. The current may, if it is high enough, cause tissue damage or fibrillation which leads to cardiac arrest. 60 mA of AC (rms, 60 Hz) or 300–500 mA of DC can cause fibrillation.[2][3]

    Body resistance


    The voltage necessary for electrocution depends on the current through the body and the duration of the current. Ohm's law states that the current drawn depends on the resistance of the body. The resistance of human skin varies from person to person and fluctuates between different times of day. The NIOSH states "Under dry conditions, the resistance offered by the human body may be as high as 100,000 Ohms. Wet or broken skin may drop the body's resistance to 1,000 Ohms," adding that "high-voltage electrical energy quickly breaks down human skin, reducing the human body's resistance to 500 Ohms."[9]
    The International Electrotechnical Commission gives the following values for the total body impedance of a hand to hand circuit for dry skin, large contact areas, 50 Hz AC currents (the columns contain the distribution of the impedance in the population percentile; for example at 100 V 50% of the population had an impedance of 1875Ω or less):[1
    0]


    End Quote

    conductors/fuses are rated for amperage not by voltage or wattage.

    Voltage is rated by the insulating properties of the sheath.

    I always like the water analogy. For lower resistance and more current you need bigger ID pipes or wires. If you want to push the water/electricity further you need higher PSI/voltage and thicker walls on your pipe/wire. Over simplified but I still like it.
    You need enough voltage to get the amperage to pass through your body for the amps to be dangerous
    Last edited by ToyTank; 04-21-2012 at 12:23 PM.

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