Prog

Discussion in 'Programmer's Corner' started by khalidrr, Jan 18, 2005.

  1. khalidrr

    Thread Starter New Member

    Jan 18, 2005
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    Why AC is more dengerous than DC?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    It's the other way around. AC shock will not freeze your muscles like DC, so you have a chance to pull away from contact with a hot terminal. In practice, AC voltages are often much higher - like power lines - and can do fatal damage almost instantly. High DC voltages will be more lethal than the same AC level because you can't break contact.
     
  3. n9xv

    Senior Member

    Jan 18, 2005
    329
    1
    Hi,

    Actually it is the "other" way around, AC does tend to "freeze" your muscels whereas DC tends to make your muscels contract and "throw" you from the shock!
    I have been bitten by as much as 1200-VDC from transmitter supplies and nearly thrown my shoulder out of place from the momentum of my arm flying back at me at 80-MPH!

    I've turned many a screw driver and pliers into dangerous projectiles via this method. I know many TV repairmen that have displaced themselves through a cheap plaster wall after making contact with 15-30 kVDC from that big red wire that connects to the top of the CRT picture tube!

    In any event, thats not to imply that DC wont kill you either. Remember, whether its DC or AC, its not the voltage that kills you - its the current. Voltage sets up the condition for current to flow, but its the current that does the damage. Most text say that as little as 100-mA can cause significant bodily damage and that 300-mA is by all means enough to kill you! - - - The current is the culprit!

    Be carefull,

    n9xv
     
  4. apontier

    New Member

    Jan 21, 2005
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    I've heard many times that it's not the voltage that kills it's the amperage. Ohm's law says that when voltage goes up amps go down. So that would lead me to believe the higher the volts safer you are. I know this is not true. I don't know the amperage at 69kv, but I know it would kill you. I wonder if at some point the voltage does become the danger, more than amps. Could the force be as deadly as the amperage?
     
  5. n9xv

    Senior Member

    Jan 18, 2005
    329
    1
    Hey pontier, better check that Ohm's law formula again. As the voltage increases, the current increases proportionally (for a given resistance). Only an increase in resistance will decrease the current (for a given voltage). The static discharge from your feet rubbing the carpet can build up to several thousand volts! But when you touch someone and discharge that voltage, you dont kill them because the current is infinitely small. However, if that several thousand volts were beeing delivered by a big industrial generator capable of sustaining several hundered amps for instance, then it would vaporize you like a gnat in a bug catcher on a hot summer night! - - - Voltage never hurt anyone but current is a murderer!

    Incidentally, when linemen work on those ghastly power lines, they actually charge themselves up to the same potential as the line they are working on so as to not create a condition for current flow. That is, no current can flow between two equal voltages of the same polarity.


    By the way, I am not trying to be a smarty pants :D or anything I just really enjoy communicating with other like minded scientist/engineers/technicians etc.

    n9xv
     
  6. Brandon

    Senior Member

    Dec 14, 2004
    306
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    Its current, current current.

    I've been hit with kilovolts, up to the megavolts range. As long as its not driven, it won't do a thing to you. In the end, voltage is just a difference in potential. If there is very little potential to start with and you put most of it on one side, you have a huge voltage with almost no potential to do anything. Put a drop of water in a cup, and take a similar cup and fill it. HUGE difference (voltage) but very little water (current). Do the same thing and put 10 gals on one side and 10,000 on the other and you have a swimming pool crashing down on you.

    Moral of this thread.. Wear gloves and keep one hand in your pocket.

    As with ohms law, its ideal and for a DC system for the most part. If you talk about caps and flyback voltage on inductors, ohms law will apply, but for only a milli or micro second, then you have your transient response of the circuit. You have a cap charged to 1MV but the cap is only 1 pF, and put a 1 ohm resistor across it you will get 1 Megaamp for about 1 femtosecond or something crazy like that, then it will be gone.
     
  7. vineethbs

    Well-Known Member

    Nov 14, 2004
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    i remember a pic i saw in one of my old physics texts (halliday and resnick ), it showed a powerline repair guy on a helicopter repairing a live line , can anybody tell me how its really done ,( if i remember r8 , the lines were pretty thick )
     
  8. Firestorm

    Senior Member

    Jan 24, 2005
    353
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    this is done through very insulated gloves and tons of safety precautions...i dont kno the details but would b glad to find them if no1 else knos...also itd be pretty hard to keep a heli steady 4 a long time while fixin a line lol...l8er

    -fire
     
  9. vineethbs

    Well-Known Member

    Nov 14, 2004
    56
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    thx firestorm :) , i just chked out the text again , the heli had some kind of attachment on which the repair guy sat , i can't make out anything else
     
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