Mac's Service Shop: Electric Shock
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This wiring scheme places the neutral on the outside edge of the light bulb so that contact with it while energized helps prevent electrical shock. Concern over electrical shock is a good attitude to adopt for just about any item that plugs into an outlet. Much has been done to mitigate the opportunity for personal injury since this Mac's Service Shop article entitled "Electric Shock" appeared in a 1965 issue of Electronics World magazine. At the time, there were still older radios and televisions that had an internal metal chassis which could be at the line voltage (usually 110-120 V) depending on which way the power cord was plugged into the wall. Polarizing wall receptacle slots (neutral on left is taller, hot on right is smaller) helped to assure that a lamp socket, for instance would have the outside area of the screw-in socket was at neutral and not hot, thus minimizing the chance of receiving a shock. Adding a safety ground to power cords that bonded to user accessible metal parts was a good first step, but at least in the early days many receptacles did not have the third hole to accept the ground prong. The only option for people determined to use the device was to cut off the ground prong or use an adapter. The better option was to assure no metal components were external to the product, and even adopting a "double insulated" practice where at least two layers of insulation stood between the user and the potentially (pun intended) harmful voltage. In 1977, OSHA enacted a "Double-insulated tools and ground-fault protection on construction sites" mandate. Mac's Service Shop: Electric Shock
Essential to the technician's safety is a thoroughgoing knowledge of the potential dangers of electric current. "Bet you can't guess where I was last night," Barney said to his employer. "I'd probably do better predicting the course of a Mexican jumping bean," Mac growled without looking up from the TV set he was aligning. "I was at a Red Cross class learning how to apply artificial respiration to a victim of drowning or shock." "Fine, but what brought this on?" "I may as well tell you. Last week when I was servicing that photoelectric counter at the spring factory, I darned near electrocuted myself. I'm not used to working on a wet cement floor, and I carelessly got hold of the hot 120-volt lead. For the first time in my life I couldn't let go, but fortunately I staggered backward and broke the connection. See what it did to my finger?" His extended forefinger revealed a narrow, deep burn edged with whitened, blistered skin. "Still smells like a butcher-shop incinerator," Barney said, sniffing the wound and wrinkling his nose in distaste; "but I decided that if I were going to spend the rest of my life working in a snake house, I'd better learn the poisonous and the harmless snakes and provide myself with some snake-bite serum. That's why I've really been boning up on just how electric shock injures or kills a human being. Also, when I found out that artificial respiration is the best method of reviving a victim of shock, I went to the Red Cross to learn the modern methods of restoring breathing." "You know you're busting to tell me what you've learned," Mac said, laying down his soldering gun and reaching for his pipe; "so why don't you sound off?" "Thought you'd never ask!" Barney exclaimed, heaving himself up on the workbench. "In the first place, it's the electric current that does the damage. Of course, we know current is a function of both voltage and resistance, but the resistance of the human body varies so widely it's impossible to tag one voltage as 'dangerous' and another as 'safe.' People have been killed by less than 50 volts and have survived contact with several thousand volts. "The resistance of the human body to electric current is divided between internal resistance, such as would be measured between two flayed areas of the body, and skin resistance. Internal resistance varies from about 100 ohms between the ears - and no cracks about the vacuum between my ears increasing this value - to about 500 ohms from a hand to a foot. Skin resistance varies from about 1000 ohms for wet skin to more than 500,000 ohms for dry skin. The skin area in contact with the voltage also affects this skin resistance. For example, a man sitting in a grounded tub of water with one hand on the hot side of the a.c. line may present no more than 500 ohms total resistance to the voltage present between the hot wire and ground. "Electricity damages the body in at least three ways: (1) it harms or interferes with proper functioning of the nervous system and heart; (2) it subjects the body to intense heat; and (3) it causes the muscles to contract. The first effect probably accounts for the most deaths. Normally, the heart contracts at a rate of about 65 beats per minute at the dictation of a built-in pacemaker. Electric current interferes with this pacemaking activity in two possible ways. The current may produce 'ventricular tachycardia' in which the heart beats very rapidly with greatly reduced efficiency that cannot sustain life for long. At currents between 100 and 200 ma., 'ventricular fibrillation' is induced in which the heart produces weak, random contractions that render it nearly useless for circulation of the blood." "What happens with still more current than 200 ma.?" "Oddly enough, the victim's chances may be better with the higher current because it causes clamping of the heart muscles and prevents the deadly fibrillation. One writer says that if the heart is exposed to this 100- to 200-ma. current, no power on earth can save him from the resulting fibrillation and death, but I think this needs some qualifying. I know that in certain types of heart surgery ventricular fibrillation has been deliberately induced for a certain length of time so that the quiet heart can be operated upon; then, another shock of a different sort has been used to restore the heart to normal operation. If this 'de-fibrillation' equipment could be used quickly enough on a shock victim, his life might be saved. "But let's go back to the effect of rising current. At 1 ma., the victim may feel no more than a tingling of the skin. Higher current can cause muscular contractions severe enough to break bones, and it produces a loss of voluntary control over the muscles that freezes a victim to the source of current. A man normally can free himself from a current of 9 ma. or less; a woman, from 6 ma. or less. "The electric current deadens the center in the brain that controls breathing. At 30 ma., breathing becomes labored and it finally ceases completely at values approaching 75 ma. At Or about 100 ma., ventricular fibrillation begins. Beyond 200 ma. the heart muscles are clamped." "What about really heavy currents measured in amperes?" "We know about the effect of these from autopsies performed on criminals executed in the electric chair. In a typical execution, 2000 volts single-phase a.c. is applied to moistened sponge-lined electrodes fastened to the shaved head and one leg. Immediately the voltage is dropped to 500 volts and then raised and lowered at 30-second intervals for a total application of two minutes, during which period the current varies from 4 to 8 amperes. There is little doubt circulation and respiration cease at the first contact, and it is believed consciousness is blotted out instantly. The temperature of the body rises abruptly. A temperature of 128°F has been measured at the site of the leg electrode 15 minutes after the execution. The blood is profoundly altered biochemically." "Let's change the subject," Mac said with a little shiver. "I imagine the path through the body has lots to do with the shock danger." "And you're right. A current passing from finger to elbow through the arm may produce only a painful shock, but that same current passing from hand to hand or hand to foot may well be fatal. That's why the practice of keeping one hand in your pocket while working on high-voltage circuits and standing on an insulating material is a good one. "A.c. is said to be four to five times more dangerous than d.c. For one thing, a.c. causes more severe muscular contractions. For another, it stimulates sweating that lowers the skin resistance. Along that line, it is important to note that resistance goes down rapidly with continued contact. The sweating and the burning away of the skin oils and even the skin itself account for this. That's why it's extremely important to free the victim from contact with the current as quickly as possible before the climbing current reaches the fibrillation-inducing level. "The frequency of the a.c. has lots to do with the effect on the human body. Unfortunately, 60 cycles is in the most harmful range. At the house-current frequency, as little as 25 volts can kill. On the other hand, people have withstood 40,000 volts at a frequency of a million cycles or so without fatal effects." "Well, now that we have the victim thoroughly shocked, what can we do to revive him?" "Apply artificial respiration at the earliest possible minute and keep applying it until a doctor pronounces the victim dead. In one study, about three out of four who received artificial respiration within three minutes of the shock lived; but of those who got it four minutes after the shock, only 14% survived. In another study involving 700 victims, 479 had stopped breathing, 323 of those were saved by artificial respiration. Most recovered in 20 minutes, but some took as long as four hours to start breathing on their own. It may even take as long as eight hours to revive a victim, and during this period no pulse may be discernible and a limb-stiffening condition similar to rigor mortis may be present. These are manifestations of shock and are not to be taken as evidence the victim has died." "Well, this has been a most illuminating conversation," Mac said, knocking the ashes from his pipe against the heel of his hand. "Let's see if I can recapitulate your major points: "1. A very little current can produce a lethal electric shock. Any current over 10 ma. will result in a painful and serious shock. "2. Voltage is not a reliable indication of danger because the body's resistance varies so widely it's impossible to predict how much current will be made to flow through the body by a given voltage. "3. The current range of 100- to 200- ma. is particularly dangerous because it is almost certain to result in lethal ventricular fibrillation. Victims of high-voltage shock usually respond better to artificial respiration than do victims of low-voltage shock, probably because the higher voltage and current clamps the heart and hence prevents fibrillation. "4. A.c. is more dangerous than d.c., and 60-cycle current is more dangerous than high-frequency current. "5. Skin resistance decreases when the skin is wet or when the skin area in contact with a voltage source increases. It also decreases rapidly with continued exposure to electric current. "6. Prevention is the best medicine for electric shock. That means having a healthy respect for all voltage, always following safety procedures when working on electrical equipment, and constantly keeping in mind that you don't need to take hold of both 120-volt wires to kill yourself. Touching the hot wire while in contact with a good ground will fry you just as quickly. "7. In case a person does suffer a severe shock, it is important to free him from the current as quickly as can be done safely and to apply artificial respiration immediately. The difference of a few seconds in starting this may spell life or death to the victim. And keep up the , artificial respiration until a physician pronounces the victim dead." "Hey! That's excellent," Barney applauded. "I didn't know you were such a good listener. I thought you were just a talker. I might conclude by saying that about 750 persons died from electric shock in industry last year, as did 150 who were electrocuted in the home. Considering that we who work with electricity are supposed to be well informed of its danger, that's not very encouraging." "No, but I think it's the old story of familiarity breeding contempt," Mac said. "Working with electricity day after day, we tend to get careless until an experience such as you had in the spring factory wakes us up. It could well be that shock will save your life."
Mac's Radio Service Shop Episodes on RF Cafe This series of instructive technodrama™ stories was the brainchild of none other than John T. Frye, creator of the Carl and Jerry series that ran in Popular Electronics for many years. "Mac's Radio Service Shop" began life in April 1948 in Radio News magazine (which later became Radio & Television News, then Electronics World), and changed its name to simply "Mac's Service Shop" until the final episode was published in a 1977 Popular Electronics magazine. "Mac" is electronics repair shop owner Mac McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant. "Lessons" are taught in story format with dialogs between Mac and Barney.
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