Engineering Expert Witness Blog

“Contact with live electrical wiring, equipment, and light fixtures was the main cause of electrical deaths and injuries among electrical workers, followed by contact with overhead power lines. Among non-electrical workers, contact with overhead power lines was the major cause of death. Other causes included contact with energized metal objects, machinery, power tools, and portable lights.”

     From an engineering viewpoint, the body’s electrical resistance is an important variable.   Electrical resistance of an object is a measure of how freely electrical current can flow across the object when a voltage is applied across it.  Resistance is measured in units called “Ohms.”    Resistance of a person’s body can depend on skin dryness, perspiration level, thickness of the skin, the distance that the electrical current travels through the skin, and other factors. The typical human body has a hand-to-hand electrical resistance somewhere between 1,000 and 2,000 Ohms, but resistance across other parts of the body can be much higher.  There is a wide variation in body resistance between individuals, so the same voltage level may result in different effects.

     Most electrical injuries occur from alternating current (AC) at levels above 50 Volts at the low frequencies typically maintained by electric utilities.  The North American utilities normally generate power at a frequency of 60 cycles per second.  The “cycles per second” unit of frequency is also referred to as “Hertz,” usually abbreviated as “Hz.”  At 60Hz frequency, the threshold for perception occurs with electrical currents as low as 0.0001 Amps.  The “can’t let go” electrical current for adults is approximately 0.010 to 0.015 Amps. This is the current that causes involuntary muscle contractions severe enough to prevent the person from letting go of the source of the electrical shock. 

     Electric currents as low as 0.050 Amps at 120V, 60Hz, have been known to cause death.  Just to give you an idea of how small that current is… a table lamp with one 40 Watt incandescent bulb draws 0.333 Amps from a 120 Volt, 60 Hz household electrical outlet!  In the interest of electrical safety, the National Electrical Code (NEC) considers 0.005 Amps at 120V, 60Hz to be the safe upper limit for children and adults. 

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