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