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Amount of Body Resistance

 
  
 
your  left  hand  to  either  foot.  The  second  path  is  the MOST  dangerous  path  since  the  current  will  flow through both your heart and your vital organs. Amount of Body Resistance Your  body  resistance  varies  greatly  in  different parts of your body. A value of 1500 ohms is commonly used as the resistance between major extremities of an average  human  body:  hand  to  hand,  or  hand  to  foot. Let’s use Ohm’s Law to figure how much current would flow through your body if you accidentally grabbed a wire carrying 120 volts alternating current (vac). Table  3-1.—Electric  Shock  Effects Ohm’s Law for figuring current is I = E/R. Let E = 120 VAC—The voltage you grabbed Let  R  =  1500  Ohms—Your  (average)  body resistance Now let’s compute it. I = 120/1500 I = .080 I = 80 milliamperes So if you grabbed a 120-vac wire, 80 milliamperes of  current  would  flow  through  your  body.  Now  use table 3–1 to determine the effect of 80 milliamperes of electric shock. You can see that you may not be around long enough to grab any more wires. You grabbed 80 milliamps  of  current!  That’s  15  milliamps  beyond  what could be fatal. It’s also 70 milliamps beyond the can’t- let-go  threshold,  and  62  milliamps  beyond  what  is needed to cause you to stop breathing. It’s important to remember that the 1500 ohms is just  an  average  value.  Body  resistance  varies  from person  to  person  and  may  often  be  LESS  than  1500 ohms. When your skin is moist, your body resistance could be as low as 300 ohms. Also, breaks in your skin at the point of contact reduce your skin resistance to nearly zero. Skin   resistance   is   only   important   when   you’re handling  voltages  of  less  than  240  volts.  If  you  get shocked by more than 240 volts, the voltage arc will burn  through  your  skin  and  leave  deep  third-degree bums where it enters your body. Time of Current Flow The longer you’re being shocked, the more chance there is for your heart to begin fibrillation. Fibrillation is the shocking of your heart into a useless flutter. Most people who die from electric shock die from fibrillation. Fibrillation in a normal adult is unlikely if the current in milliamperes  is  less  than  116/t,  where  t  is  the  shock duration  in  seconds.  The  longer  you  are  shocked,  the less current is needed to cause heart fibrillation. Here are   some   examples   of   shock   current   levels   and durations that would cause fibrillation: 21  milliamperes  for  30  seconds 44  milliamperes  for  7  seconds 3-2

   


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