![\includegraphics[scale=0.650000]{../241/bodyequiv.ps}](img213.png)
The good news is that in ELEC 243 laboratory, you will not be exposed to dangerous voltage levels unless you mishandle the equipment. Before we get close enough to the equipment to mishandle it, let's take a closer look at just how you can get hurt if you do.
Electric Shock. The ratio of fatalities to injuries for electric shock accidents is very high in comparison to the corresponding figure for all other accidents. Death due to electric shock is fast and permanent.
Shock Sensitivity. Because of a person's highly developed nervous system, they are sensitive to very small currents. For example, the tongue will give a sensation of taste at currents as low as 45 microamperes. The threshold of feeling on the hand is found to be on the order of 5 mA DC and 1 mA at 60 Hz. The shocks due to currents at this threshold are usually considered annoying rather than dangerous. However, they are startling when not anticipated, and may cause involuntary movement that sometimes results in serious injury. As the value of current increases above the threshold, one becomes aware of sensations of heat and contraction of the muscles. Sensations of pain develop and voluntary control of the muscles in the path of the current becomes increasingly difficult. Finally a value of current is reached where the victim "freezes" to the circuit. The value of current at which a victim can just release the electrode is referred to as "let-go" current. The average let-go current for healthy people is about 16 mA rms (60 Hz). It is important to note that let-go currents of as low as 5 mA have been measured. Experience has shown that an individual can withstand, with no ill aftereffect except for possible sore muscles, repeated exposure to their let-go current for at least the time required for them to release the conductor.
Effect of Frequency and Waveform. Gradually increasing direct current produces sensations of internal heating. Sudden changes of current, however, produce powerful muscular contractions, and interruption of the current causes very severe shocks. Experiments at 10 kHz indicate that the let-go current is approximately 3 times the 60 Hz value. For non-sinusoidal waveforms, the peak value seems to be the critical factor in muscular stimulation.
Nature of Bodily Damage. As far as gross electrical effects are concerned, the body can be represented by the equivalent circuit shown in the figure below.
when dry. The inner skin
(dermis) and the internal organs are of relatively low resistance
because of their high salinity.
It is generally believed that the
effects of electric shock are due to the current flowing
through the body.
Electrical burns are a result of heating the skin. The
immediate formation of blisters at the point of contact causes the
skin to lose its protective resistance and more serious damage
may thus occur.
Burns of this nature penetrate quite deeply and, while
they seldom become infected, heal very slowly.
Electrical current is most dangerous when vital organs are in its path
through the body. Current may cause stoppage of breathing due to
excessive contraction of the chest muscles. Temporary paralysis of
respiration may also occur if the current produces a block in the
nervous system that prevents signals from reaching the lungs. In
either case, it is imperative that an approved form of artificial
respiration be applied immediately and continued without interruption
until competent medical examination has been made.
Ventricular Fibrillation. Passage of electrical current through the heart is considered particularly dangerous. The heart exerts its pumping action as a result of complex rhythmic motion controlled by periodic electrical impulses to the muscle tissue. The regularity of this motion assures proper blood circulation. Electric current passing through the heart completely upsets this rhythmic motion and results in random muscular contractions. The heart no longer effectively pumps, but quivers like so much "Jell-O". This condition is known as ventricular fibrillation and is nearly always fatal. Energy of the impulse of current is responsible for this hazard and an estimate of the danger threshold for fibrillation is 13.5 watt-seconds.
C maintained for a sufficient time may be injurious or
even lethal.
Testes, for example, undergo degenerate changes with
temperature rises as low as 
C.
The amount of bodily temperature rise depends on the specific area
exposed and its efficiency of heat elimination. Other factors
contributing to temperature rise are intensity and the duration of
exposure. At frequencies below the microwave region (
MHz)
about 40 percent of the incident energy is absorbed. These
frequencies cause deep (internal) heating and are very dangerous:
such heating is not well indicated by sensory elements of the skin. A
tolerance figure of 0.001
for long-term exposures has been
considered adequate.
Surface burns resulting from R.F. currents are similar to those
encountered at lower frequencies.
MHz), the incident energy is
absorbed by the skin with an efficiency of 40 to 50 percent.
The effects of such radiation are much the same as is encountered with
infrared and sunlight.
The possibility of the formation of eye cataracts must be considered
at microwave frequencies. These cataracts result in impairment of
vision or even blindness as a result of the formation of a white cloud
in the normally transparent cornea of the eye. Cataracts are thought
to occur in a manner analogous to the formation of the "white" of an
egg upon heating. Experiments with rabbits have indicated that
cataracts have formed in 3 to 9 days following a single 15 minute
exposure to a 100 watt source of 12 cm waves at a distance of 5 cm.
While the exposure indicated here is probably extreme, care should be
taken when working with high-powered radar sets.