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Why DC is said to be more dangerous than AC?

  1. Dec 28, 2011 #1
    Why DC is said to be more dangerous than AC??

    For example, consider a 230V DC and 230V AC.., it is said DC voltage is highly dangerous than AC.., Why is it said so?? how much quantity of AC and DC can our body bear and remain harmless??:confused:
     
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  3. Dec 28, 2011 #2
    Re: Why DC is said to be more dangerous than AC??

    DC will be more effective at clamping your heart and hand muscles than AC. This makes it more lethal in practice.

    I don't have data to quantify the relative danger between the two.
     
  4. Dec 28, 2011 #3

    es1

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    Re: Why DC is said to be more dangerous than AC??

    http://en.wikipedia.org/wiki/War_of_Currents

    "Low-frequency (50–60 Hz) alternating currents can be more dangerous than similar levels of DC since the alternating fluctuations can cause the heart to lose coordination, inducing ventricular fibrillation, a deadly heart rhythm that must be corrected immediately.[14] However, any practical distribution system will use voltage levels quite sufficient for a dangerous amount of current to flow, whether it uses alternating or direct current. As precautions against electrocution are similar for both AC and DC, the technical and economic advantages of AC power transmission outweighed this theoretical risk, and it was eventually adopted as the standard worldwide."
     
  5. Dec 28, 2011 #4

    es1

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  6. Dec 28, 2011 #5
    Re: Why DC is said to be more dangerous than AC??

    That's good to know. There is a widespread misunderstanding that DC is worse.
     
  7. Dec 28, 2011 #6
    Re: Why DC is said to be more dangerous than AC??

    Its the amps that do you in typically.

    I could take a 9v transistor battery, hook up wires with a needle at each +/- terminal end, and kill you by stopping your heart by just puncturing your skin with the needles.

    Your blood would act as a conductive media, and you'd be a goner.

    A student died proving this would not work....because it did.
     
  8. Dec 28, 2011 #7

    dlgoff

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    Re: Why DC is said to be more dangerous than AC??

    I remember my ham radio mentor as a teenager saying, "ac will give you a buzz but dc will knock the snot out of you". Sorry I have no references, but he did make an impression on me.
     
  9. Dec 28, 2011 #8
    Re: Why DC is said to be more dangerous than AC??

    Hello, Srini

    I hope you realise that 230 volts DC and 230 volts AC develop the same power in a resistor, but they have different safety considerations.

    Don't forget that 230volts AC is 325 volts peak, 650 peak to peak. So an AC supply can actually apply greater voltage than 230 volts DC.

    The human body works on electricity.
    All the muscles are driven by electicity.
    When a source of sufficient voltage is applied the muscles work (contract).
    Every articulation in the body is controlled by (at least) two opposing muscles.
    For example clenching you hands and opening them.
    In general one set is much stronger than the other (try it with clenching/unclenching).
    When a supply such as the mains is applied it is strong enough to drive pretty well all the body muscles.
    So if you grab the mains by your hands both the clenching and unclenching muscles will be driven and the clenching ones will win.

    This is why it is said, "you cannot let go of DC, but you can (perhaps) let go of AC"

    This is the basis of the idea that DC is more dangerous than AC.

    go well
     
  10. Dec 28, 2011 #9
    Re: Why DC is said to be more dangerous than AC??

    It's Edison's war of currents backfiring.

    The way I heard of this is that muscles are controlled with pulses sent through nerves repeatedly (with AC), and the AC has similar frequency and is thus more effective at making you hold onto the wires. The nervous system generally sends a particular level of signal with particular frequency of firing of the neuron. I heard that you can easier let go of DC than AC for same current level. Also, the AC of same RMS has higher peak voltage and peak current.

    edit: I don't think there's much practical difference for 240v anyway. Both DC and AC would be quite hard to let go of, and the current would be very dependent to area of contact, skin conductivity, etc.
    edit: source
    http://www.allaboutcircuits.com/vol_1/chpt_3/2.html

     
    Last edited: Dec 28, 2011
  11. Dec 28, 2011 #10

    nsaspook

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    Re: Why DC is said to be more dangerous than AC??

    DC arcs are usually more destructive than AC on switch gear. DC rated switches of the same voltage and current also cost more.
    http://www.eaa.org/sportaviationmag/2005/0502_switches.pdf [Broken]
     
    Last edited by a moderator: May 5, 2017
  12. Dec 30, 2011 #11
    Re: Why DC is said to be more dangerous than AC??

    THANX for ur useful piece of info.., :rolleyes:
     
  13. Dec 30, 2011 #12
    Re: Why DC is said to be more dangerous than AC??

    Hope it was helpful - it was more or less exactly as explained to me by the Public Safety Inspectorate lecturer (a Physics Phd) when I did my last electrical safety course.

    go well
     
  14. Dec 30, 2011 #13

    psparky

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    Re: Why DC is said to be more dangerous than AC??

    My usual two cents:

    I have always heard that DC has a much higher "grab" to it. Once it grabs hold of you, you can't let go. AC on the other hand....tends to "throw you" off the circuit.

    At any rate, do not allow your body to be a load and you should avoid either case.

    In regards to the "it's the amps that kill you".....well, maybe so, but amps are a result of Volts and resistance. V=I*R. So the "amps" are onlyl 1/3 of the equation. There simply aren't devices that deliver pure amps. What they do deliver is voltage. And depending on the resistance they receive between the terminals.....Voltage divided by resistance equals the "amps" you speak of.

    So if someone does get killed by electricity, it was the combination of the volts, amps and resitance.

    Here's a fine example. Your 12 volt car battery. Take both of your index fingers and put one on each terminal. What do you feel....nothing. Taking a random guess at human body resistance of 2,000 ohms.........12/2,000= .006 amps. You simply can't notice it. Even if you lick your fingers (resistance goes down with water) you still will not likely feel anything....now your tongue....don't go there.....that may kill you.

    Great, now lets say you were to hypothetically take a wrench and put it across the battery terminals.....all hell is going to break loose....big sparks...perhaps a wrench glowing bright red.

    Let's guess the wrench's resistance to be something like .05 ohm. 12/.05= 240 amps!!!!
    240 amps is enough annialate a human....yet when a human touches the terminals.....only .006 amps flow.

    One more thing.....when you do start your car.......your car starter motor is almost a dead short....about 600 amps flow for a second or two. That's why the cable on your positive terminal is humongous!!!! Also that is why they tell you to crank your car for only a few seconds at a time......you are litterally burning the motor and wire otherwise!!!!

    Incidentally, they say it takes .1 amps steady to kill a human....it's right at the threshold.
    But to get to this .1 amp.....you need a voltage source and the appropriate amount of ohms.

    Here's another test when you are re-wiring outlets or switches or whatever with 120 volts.
    If your hands have a lot of drywall dust on them, and you do grab the line to neutral with your fingers.....you barely feel the shock. On the other hand...if your hands were soaking in water or whatever and you grab the line to neutral.....holy hell will you feel that.

    Again, amps are derived from voltage and resistance....not the other way around.

    Incidentally, if you ever asked the question why cars are DC rather than AC.....good question....simple answer: Invent an AC battery and cars will be AC! Until then...we are stuck with DC. The alternator in the Car is AC btw....it just gets converted to DC by a rectifier!!!

    I would like to gurantee that all the above statements are 100% true....except for the statements that are 100% false.
     
    Last edited: Dec 30, 2011
  15. Dec 30, 2011 #14

    jim hardy

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    Re: Why DC is said to be more dangerous than AC??

    tell me if i'm cluttering board here.

    they told me 0.02 amp through your heart can stop it.

    i worked around old timey electricians.
    there's some common sense habits they employ.
    you'd never notice because they seem so casual about it.

    1. Before you touch anything potentially 'hot', place one hand behind your back. That way you won't make a circuit from one hand to the other, which goes throough your chest.


    2. Before you take hold of something, brush it with the BACK of your fingers to see if it's 'hot'. Even if you checked it with a meter just a few seconds ago.
    That's because electric current makes your muscles contract. If you grab something with your fingers and it's hot, your hand will clamp down on it and you can't let go.
    If you instead touch it with the BACK of your fingers, the shock will make them pull away. I call that one "the digital voltmeter safety check".

    3. before you grab something with your hand, place your elbow against something.
    That way if it is REALLY hot and blows your arm off it'll only take it to the elbow.

    4. of course always wear good work shoes, and no gold chains. put wedding ring in your watch pocket. wood or fiberglass ladder not aluminum.

    5. avoid water. As Sparky said wet skin conducts right into your bloodsteram. This is REAL evident if you're in a boat in the ocean and wetted with salt spray... you can't even hold onto the 12 volt boat battery

    just a practical thought,,

    old jim
     
  16. Jan 1, 2012 #15
  17. Jan 1, 2012 #16
    Re: Why DC is said to be more dangerous than AC??

    i experimented with electricity in my childhood., was curious to know how it would feel like., inserted a metal wire in plug-point and switchd on power.,:redface: i gt shock of more intensity and smehow repelled away.., if it was DC that would had been my last day..,!! :):cool:
     
  18. Jan 1, 2012 #17
    Re: Why DC is said to be more dangerous than AC??

    That's obviously the most stupidest thing to do.
     
  19. Jan 2, 2012 #18

    psparky

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    Re: Why DC is said to be more dangerous than AC??

    To add to what Jim said....If your messin with residential 120 volt and you are using the back of your fingers....again, add some good old drywall dust to your fingers.....U will just barely feel the heat if it is hot.

    Keep in mind too....If you are only grabbing one hot wire.....you are likely not going to get shocked in a standard dry condition. If you've checked the "dead" circuit with your voltmeter and tick tracer....you need to hand test it two points on one hand.....u must complete the circuit!!
     
  20. Jan 15, 2012 #19
    Re: Why DC is said to be more dangerous than AC??

    I was asked this question at my job interview (more accurately, the "which is more dangerous" variant).

    I have heard both sides of it, and the only honest answer I could give is "IDK". Explanations like studiot's (This is why it is said, "you cannot let go of DC, but you can (perhaps) let go of AC") are always kind of...I call them "witchcraft" explanations, simply because there isn't really anything hard and substantial about them and seem superficially reasonable but have little things that don't really seem as reasonable as they first appear.

    For example, the "might let go of AC thing" is usually explained because of the zero crossing point. However, in the US that zero crossing happens 120 times per second, and only for the briefest of moments. There is simply no way for a slow human to take advantage of something happening that fast. It's on a completely different timescale than we are.

    Also note from Kholdestare's article:
    "Even when the current is stopped, the victim may not regain voluntary control over their muscles for a while, as the neurotransmitter chemistry has been thrown into disarray."

    "Alternating current (AC), because it alternately reverses direction of motion, provides brief moments of opportunity for an afflicted muscle to relax between alternations."

    Two different paragraphs containing information that seems to contradict itself.

    Another reason why the AC/DC wouldn't matter.

    Fact: Mains level electricity will give you a major kick in the *** regardless of whether it is AC or DC. Other factors (voltage, grounding points, skin connection) seem to be overwhelmingly more important than frequency.

    About the only "reasonable" sounding thing I've heard about AC vs DC is that AC more easily disturbs the heart's rhythm.
     
  21. Jan 16, 2012 #20

    psparky

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    Re: Why DC is said to be more dangerous than AC??

    I think I have to agree with Jiggy-Ninja.

    The time clock switches 60 times per second.....yes, very different clock than ours....

    And the fact that frequency is the last of our concerns.....voltage, grounding, wet or dry skin seems to be the big factors!!!

    Here's another question that I just thought of....In the movie, the Green mile....they hook the hot line to the top of the head with a wet sponge.....where was the other line hooked up at? I would assume you would need the other line somewhere else also hooked on with a sponge. This was not clear in the movie....which is often the case in Hollywood.
     
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