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Which high voltage is more dangerous: AC, or DC, and why?

  1. Mar 19, 2012 #1
    I wish I had a plot of voltage on the X-axis and danger level on the Y-axis for AC and DC.

    I was talking to an engineer at work, he was telling me that DC is more dangerous at high voltage. I said I disagreed with him but did not do into detail as to why I thought so. I did say that I've always heard that DC had a tendency to be more dangerous. His response was that "DC only goes through you once, whereas AC continuously goes back and forth and rattles the heart". I think that with a limited current the body would be seen as a high impedance, and above some current they are going to be equally lethal. I did not make any reference to frequency, so he may have figured I meant only 60Hz as we have supplied here.


    My question is: which type of current is inherently more dangerous, and why?
     
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  3. Mar 19, 2012 #2
    Sorry I can't be of more help, but from what I understand if you have an AC supply that is, say, 120 volts, there might be certain times in it's cycle where the voltage may be higher than 120. That being said, AC seems more dangerous because you may be exposed to higher voltages than what you are told.

    However, I think the more important thing to think about is what the internal resistance of the source is. At very low internal resistance, even a 12 volt power supply like a car battery can turn deadly (a DC source, but I'm sure the same applies to AC). Thomas Edison believed it to be more dangerous than DC, and AC was used to execute criminals in the late 1800s and early 1900s, although that was used as a smear campaign against Tesla; no idea if he was right.

    On another note, very high frequency AC electricity will pass through the outermost layer of the skin (the so-called 'skin effect'), and not enter the vital parts of the body.
     
    Last edited: Mar 19, 2012
  4. Mar 19, 2012 #3

    Integral

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    Edison pushed AC over DC for the electric chair because he was producing DC and did not want his system associated with it. High voltage, with sufficient current capacity is dangerous, AC , DC don't much matter
     
  5. Mar 19, 2012 #4
    http://en.wikipedia.org/wiki/Electric_shock#cite_note-whitaker-1

    Also, if you were to hang at a very-high voltage AC transmission line, you could be dead, due to capacitive charging current through you.
    In DC there is no such current.
     
  6. Mar 20, 2012 #5

    NascentOxygen

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    Consider voltages in the hundreds of volts. I'm not sure where I heard this, but I have the notion that while AC delivers a shock every time it changes (that would be twice per cycle), contact with DC gives you only two shocks: one when you connect, and the other when you disconnect. While contact with AC may prevent you letting go, if the above is true then after getting a grip on DC I think muscle fatigue will set in and you'll be able to let go (but get a parting jolt as you do so).

    This is not to discount the heating effect of both (i.e., cooking effect), nor other associated dangers.

    https://www.physicsforums.com/images/icons/icon2.gif [Broken]As they say, there is no safe level, whether AC or DC.
     
    Last edited by a moderator: May 5, 2017
  7. Mar 20, 2012 #6

    Averagesupernova

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    I suppose it depends on what you define 'shocks' as, but I certainly wouldn't say it the way you have put it.
     
  8. Mar 20, 2012 #7
    I'm pretty sure you are constantly being injured when you hold onto a high voltage DC line, not just upon contact and removing contact.

    I don't know about the muscle grip thing though.
     
    Last edited by a moderator: May 5, 2017
  9. Mar 20, 2012 #8

    berkeman

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    Sorry, that is not correct.
     
  10. Mar 20, 2012 #9

    psparky

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    V=IR in either case.

    Should the "I" exceed your body's limit in either case....you are dead...or best case...serverly burnt.

    Also....what was said a month ago in a similiar thread....that notion that AC has certain cylces and this and that is rubbish. Our body clocks and the clock of an AC sin wave have nothing in common. Also, RMS value of AC is all that matters in my opinion.
     
  11. Mar 20, 2012 #10

    psparky

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    Let's look at this mathematically.

    Say we have a 120 volt battery with a body resitance of 2000 ohms.

    This produces a current of .06 amps and a power of 7.2 watts.

    Now let's take a wall receptacle with 120 volts RMS.

    This also produces a curent of .06 amps RMS and 7.2 watts

    7.2 watts = 7.2 watts.

    Watts are not available in RMS last time I checked. Watts....are watts.
     
    Last edited: Mar 20, 2012
  12. Mar 20, 2012 #11
    You're right about the RMS AC value determining how much damage you take, but doesn't muscle respond to electrical signals differently based on the signal characteristics?

    I would think that either AC or DC current would sort of saturate the muscles/nerves ion channels (just a guess), such that they are unresponsive, but I would guess that the right amplitude and frequency could cause the muscles to behave in a unique way.
     
  13. Mar 20, 2012 #12
    Perhaps skin moisture and other body properties have different capacitances such that their impedance is not 2000 ohms across all frequencies.

    Also, does dielectric breakdown depend on if the field is AC or DC? Because I know that the skin layer breaks down once a voltage threshold is reached, and then the impedance drastically reduces and causes much more harm.
     
  14. Mar 20, 2012 #13
    I was a;ways taught that DC was more dangerous because if you caught it on the palm of your hand you would clamp on, but with AC your muscles would spasm and you'd let go of the cable.
     
  15. Mar 20, 2012 #14

    psparky

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    It's amazing to hear all the theories....as no one seems to have concrete proof. I think I could give a contradiction to every one of the points above...including my own!!!!

    Perhaps all the details given above only matter if you are at the threshold of electricution......around the .1 amp or whatever it is. This seems to be typcially in the 120 volt range. But at 480 volts AC or DC....if you are in the wrong place at the wrong time....you dead.....Fred.

    Perhaps someone should do a simulation to put this to rest once and for all......(and when I say this.....I am joking....DO NOT TRY THIS AT HOME.....you will likely die!)

    But....lets say someone really had to know the difference. If they had a death wish....they could plug two wires into a 120 volt receptacle and hold on for 3 seconds and let go....record how they felt.

    Then for DC....they could hook up 10 twelve volt car batteries in series......hold on to the wires for 3 seconds as well........and then record how they felt. Assuming they could let go.....maybe they couldn't!

    If someone were insane enough to do this.......a spotter near by would be recommended to disconnect the wire in the case you couldn't let go after 3 seconds.

    Have both set ups in place as well during experiment....so your body would have exact same moisture in it and on your hands.

    Again...do not try this because you will likely die. But it's a cool thought I think.
     
  16. Mar 20, 2012 #15
    I think we need a bioengineer or neuroscientist who understands both biological effects and electrical effects to give the full story.

    Other than that, I think its clear that high voltage at any frequency is dangerous when it can put enough current through your body. There are a lot of variables that determine how dangerous electrical shock is to a person more than just frequency. Just one example, if the potential is between your left hand and your right foot, you will have a much higher chance of dying than if the potential is between your right thumb and right index finger.

    And having seen books from the 1950s and 1960s on experiments done with radiation and other health hazards, I'm pretty sure there are already studies that give data on what conditions are dangerous and lethal.
     
    Last edited: Mar 20, 2012
  17. Mar 21, 2012 #16

    NascentOxygen

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    I speak from experience on the AC description. I haven't experienced a shock from DC (touch wood) so cannot vouch for that.
     
  18. Mar 21, 2012 #17

    NascentOxygen

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    It depends on what one understands by "dangerous". The OP never said full body conduction, nor to limiting interest to the extent of heating to the point of cooking.

    Most deaths from contact with electricity are due to disturbance of the heart rhythm, not organ overheating. Contact can be fleeting and injury only superficial, but unless the heart is returned to an effective beat, death is inevitable.

    Some initial trials using electrocution as capital punishment were particular gruesome, some tests reported insufficient voltage, some clearly way too much current.
     
  19. Mar 21, 2012 #18

    jim hardy

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    I've felt both 130VDC and 120 VAC with my fingertips many times.

    They both "tingle". I cannot tell one from the other. Do not try it.

    Regarding "unable to let go", either will cause that effect.
    Old timey electricians routinely brush a circuit with the back of their hand before touching it. That's to verify it is truly de-energized. They use backof hand so that contracting muscles will naturally pull their fingers away not lock them around it.
    They will use a fingertip to test a circuit for voltage, but old electricians usually have the strong, callused hands of a workingman so their skin is tougher than average folks.

    Anybody know of a study on the neurological effects? I was unable to find one.
     
  20. Mar 21, 2012 #19

    NascentOxygen

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    But was that smooth DC, or just rectified AC?
    I don't intend to!
    The same way as those on the land test an electric stock fence before climbing through, and not just once, either, especially in the face of reassurance that it really truly honestly is "OFF". :biggrin:
     
  21. Mar 21, 2012 #20

    Integral

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    Not sure what kind of experiance you are talking about. All you learn from getting shocked is, not to do that again.

    I would expect damage/pain from an AC source would be very similar to its RMS equivelent in DC.

    BTW, just for the record, I once took 500V DC through about a cm of my little finger. Quite painfull and left a burn. I have also taken a few 120VAC shocks, with nothing other then pain as a reminder.
     
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