ELEC 241 Lab

Laboratory 0

Safety

First the bad news:

Electricity can kill you.

How? The two most common ways are:

The good news is that in ELEC 241 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.

What strength voltages and currents are harmful?

Electricity is potentially the most dangerous commodity in general use by the public today. In spite of this, less than 1 percent of the 100,000 accidental deaths which occur annually in the United States are directly attributable to electrocution. Considering the ever-increasing use of electricity in the home and industry, we should be alert to protect and better this excellent safety record. An understanding of the effects of electric shock, high-frequency heating, and other electrically produced physiological phenomena on the human body should be part of basic knowledge. We present a brief survey of these topics in the hope that you will become aware of the very real dangers inherent in the application of electrical phenomena. We shall consider these dangers in each frequency band of the electromagnetic spectrum - from the power frequencies to cosmic rays.

Power Frequencies

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.

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The outer skin (epidermis) is roughly 0.1mm thick and has a resistance of the order of $10^5 \Omega /{\rm cm}^2$ 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.

Radio Frequencies

At frequencies above the power range, the primary bodily damage results from dielectric heating. Heating also occurs from both conduction current and radiation. The temperature of the human body is maintained remarkably constant by very complex temperature regulating mechanisms. These mechanisms, coordinated by the brain, control temperature by regulating the production and loss of heat. Circulation of bodily fluids contributes to the distribution of the heat. The change in diameter of the blood vessels, for example, regulates the volume and velocity of flow. However, not all areas are equally well regulated, and it is possible for local heating to occur. Such local heating is apt to be dangerous, particularly if vital organs are involved. A differential of $5^\circ$ C maintained for a sufficient time may be injurious or even lethal. Testes, for example, undergo degenerate changes with temperature rises as low as $1^\circ$ 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 ($f<1000$  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  ${\rm watt}/{\rm cm}^2$ for long-term exposures has been considered adequate. Surface burns resulting from R.F. currents are similar to those encountered at lower frequencies.

Microwave Frequencies

The effects of the lower microwave frequencies are similar to those of radio frequencies and a comparable tolerance figure for exposure should not be exceeded. As the frequency increases ($f>3000$  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.

Infrared, Visible Light and Ultraviolet Frequencies

The immediate effects of this band of frequencies are popularly known as sunburn. The eye, however, is particularly susceptible to excess exposure at these frequencies. Such exposure can easily occur when observing an electric arc without protection. This may result in hemorrhages of the choriod (the outer lining of the retina) and actual destruction of portions of the retina itself. In addition, actual immediate pain may be encountered because of violent contractions induced in the iris. Photophthalmia (temporary snow blindness) is due to ultraviolet radiation following undue exposure to sunlight, an electric arc, or a sun lamp.

X-Rays and Nuclear Radiation

The effects of this portion of the electromagnetic spectrum of the human body are still being studied. It has been found that the amount of energy absorbed by the tissue during exposure is the most important consideration. Damage to the reproductive organs has been found to be the most sensitive indication of excessive exposure. Controlled experiments with mice exposed to fast neutron radiation from an atomic reactor indicate the type of damage sustained by living animals. During the first 9 months of continuous exposure, no visible differences were noted. The mice then began to lose weight and gradually lose hair. Eye cataracts developed after a year and death followed. Limited exposure to nuclear radiation resulted in a shortened life span. We have yet to completely determine the consequences of the atomic bombs of World War II in terms of its biological effects. For this reason, plus the fact that the latent period between exposure and detection of damage may run into months and years, it behooves us to take all precautions against X-Ray and nuclear radiation.