Quantcast Chapter 9 Personnel Protection

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MIL-HDBK-419A
CHAPTER 9
PERSONNEL PROTECTION
9.1 ELECTRIC SHOCK.
Electric shock occurs when the human body becomes a part of an electric circuit. It most commonly occurs
when personnel come in contact with energized devices or circuits while touching a grounded object or while
standing on a damp floor. The major hazard of electric shock is death. Fatalities from shock total about 1,000
annually. In addition, numerous injuries occur each year due to involuntary movements caused by reaction
The effects of an electric current on the body are principally determined by the magnitude of the current and
the duration of the shock. The current is given by Ohm's Law, which, stated mathematically, is I=V/R where V
is the open circuit voltage of the source and R is the resistance of the total path including the internal source
resistance, and not just the body alone. In power circuits, the internal source resistance is usually negligible in
comparison with that of the body. In such cases, the voltage level, V, is the important factor in determining if
a shock hazard exists.
At the commercial frequencies of 50-60 Hz and at voltages of 120-240 volts, the contact resistance of the body
primarily determines the current through the body. This resistance may decrease by as much as a factor of 100
between a completely dry condition and a wet condition. Thus, perspiration on the skin has a great effect on its
contact resistance.*  At voltages higher then 240 volts, the contact resistance of the skin becomes less
important. At the higher voltages, the skin is frequently punctured, often leaving a deep localized burn. In this
case, the internal resistance of the body primarily determines the current flow.
9.1.1 Levels of Electric Shock (9-1) (9-2).
The perception current is that current which can just be detected by an individual. At power frequencies, the
perception current usually lies between 0 and 1 milliamps for men and women, the exact value depending on the
individual. Above 300 Hz, the perception current increases, reaching approximately 100 milliamps at 70 kHz.
Above 100-200 kHz, the sensation of shock changes from tingling to heat. It is believed that heat or burns are
the only effects of shock above these frequencies.
The reaction current is the smallest current that might cause an unexpected involuntary reaction and produce
an accident as a secondary effect. The reaction current is 1-4 milliamps. The American National Standards
Institute (9-3) limits the maximum allowable leakage current to 0.2 milliamps for portable two-wire devices and
0.75 milliamps for heavy movable cord-connected equipment in order to prevent involuntary shock reactions.
*For calculation purposes, the resistance of the skin is usually taken to be somewhere between 500 and 1500
ohms.
9-1





 


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