May 1973 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Here is a timeless subject for anyone routinely subject to exposed high voltages. Most RF Cafe visitors already know that technically, it is the amount of electric current through the body that determines severity of electric shock, not the voltage. However, we also know that voltage does play a role because a certain voltage, per Ohm's law, is needed to induce a commensurate current. The body's resistance is determined primarily by perspiration (salt and water) and the path between contact points (e.g., across adjacent skin areas or hand-to-hand via the heart). MIL-STD-883 and JEDEC* have decided that the proper Human Body Model (HBM) for testing semiconductor survivability when subject to high voltages is 1.5 kΩ. I could not find out how that value was determined.
This is not the first time service shop owner cum wise teacher Mac McGregor has lectured his trusty technician Barney about the hazards of electric shock (see Mac's Service Shop: Electric Shock in the August 1969 issue of Electronics World).
* ANSI/ESDA- JEDEC JS -001- 2010 : Electrostatic Discharge Sensitivity Testing -- Human Body Model | MIL-STD-883, Method 3015.
Mac's Service Shop: Biological Effects of Electrical Shock
By John T. Frye, W9EGV, KHD4167
Matilda, office girl at Mac's Service Shop, paused in her typing to listen to the voices of her employer and his assistant coming through the open door of the service department. They were engaged in what Barney had solemnly assured her was going to be a "serious discussion."
"Okay, boss," Barney was saying, "I understand we're going to talk about what happens to the human body when it's exposed to various kinds and amounts of electricity, but where did this poop come from? Who dug it up?"
"The 'poop,' as you call it, represents the fruit of serious experiments by distinguished scientists in Russia, England, and this country, going back as far as 1775. On this card I've typed a list of some of the references I consulted while boning up on the subject, and I want you to read several of them. When through, you'll have a lot of respect for such names as Dalziel, Ferris, Lee, Kiselev, and Kouwenhoven. In our little talk, though, I do not intend to tie each finding to a certain experimenter. Instead I'll quote facts and figures that represent more or less a consensus, which will not be difficult because the separate findings lie reassuringly close together.
"Okay, how much voltage is safe?" "We're not going to talk about voltage.
Current is what affects the living organism, and current cannot be directly related to voltage applied to the human body because the resistance of the body varies so widely. For instance, dry skin has an average resistance of 100,000 to 500,000 ohms, but this falls to 1000 ohms when covered with perspiration and on down to 150 ohms when immersed in water. If areas are flayed so the electrodes are in contact with subcutaneous tissue, the resistance falls to only 100 ohms between the ears and to 500 ohms from flayed hand to flayed foot. Resistance also varies greatly with the area in contact with the electrode. Since current equals voltage divided by resistance, this means the same voltage can produce a great range of current. For example, 110 volts ac can produce 1 mA through dry skin, 110 mA through perspiring skin, and 0.75 ampere when applied to a body in a bathtub or shower. This explains, in part, why people have been electrocuted with less than 50 volts, while others have survived contact with thousands of volts."
Direct and General Shock. "Before we get started on the effects of various amounts of current on the body, let's make a distinction between the direct effect of electric shock and general shock to the nervous system. The latter is evidenced by news stories in which a person dies of excitement while watching a football game, of fright in a fender-dimpling accident, or even of joy at the receipt of good news. For these people, no current they can feel and be startled by, no matter how slight, is safe and may result in a fatality; although the death should be charged to nervous system shock rather than to the primary effect of electric current.
"Over the years literally hundreds of men and women have been tested for their response to electrical currents. One of the first things experimenters established was the threshold of perception current. The subject usually held a small current-carrying wire in each hand or substituted a brass plate tapped with a forefinger for one of the wires while the available current was gradually increased until it could be felt. The mean current at this point was found to be about 1.1 mA for men and 0.7 mA for women. To be on the safe side, the 0.5 percentile value of these measurements, 0.5 mA, is generally established as the current level below which there is little likelihood that an electric current will be felt when applied to unbroken skin of the hands. Some other areas of the body are more sensitive to electric current, the tongue being the most sensitive. Only 45 microamperes of current can be perceived by it, more in the way of a taste-bud stimulation than the usual sensation of shock."
"What happens when you increase the current?
"The next thing the experimenters wanted to know was the maximum let-go current. As 60-Hz ac current is increased from the threshold value, the muscles begin to contract until finally voluntary control of them is lost. Beyond a certain value, called the let-go current, it is impossible for the subject to release a current-carrying electrode. Average values for this let-go current were found to be 16 mA for men and 10.5 mA for women. Again applying the 0.5 percentile values, these became 9 m A for men and 6 mA for women. Incidentally, this 3/2 ratio of current tolerance for men and women prevailed throughout most of the tests - if Women's Lib members will pardon the observation! It was also found that 60-Hz ac is four to five times more dangerous than dc because ac causes more severe muscular contractions and produces sweating that lowers the skin resistance. Let-go current increases with frequency. At 5000 Hz, the let-go current is more than three times the value at 60 Hz.
"Currents greater than about 18 mA contract the chest muscles and stop breathing during the shock, but breathing resumes when the current ceases. However if the current continues, collapse, unconsciousness, and death result in a few minutes from lack of oxygen to vital organs. Currents much above 20 mA are considered too dangerous to apply deliberately to human beings; so experiments must be conducted on animals and the data extrapolated to man. While this leaves much to be desired, it's not easy to recruit volunteers for destructive testing."
"I'd think not!" Barney muttered.
"Most tests on animals were directed toward determining the maximum non-fibrillating current in adults. Ventricular fibrillation is a medical term describing a usually fatal interference with the heart's electrical activity. Just as a car's ignition system keeps all the cylinders firing in a smooth and powerful sequence, so does the heart generate and conduct timed pulses of electrical current that contract ventricular muscle fibers in a coordinated and rhythmic fashion to produce maximum blood-pumping efficiency. An abnormal stimulation of the heart can make it start acting as a car would if someone instantaneously scrambled the spark plug wires. Muscle fibers contract independently in an uncoordinated, asynchronous fashion that sets the heart to quivering and destroys entirely any effective pumping action. For all practical purposes, the heart stops and asphyxial death occurs in a few minutes. Once ventricular fibrillation starts in man, it rarely stops naturally before death."
Experimental Results. "Experimenters have electrocuted hundreds of animals including horses, calves, sheep, pigs, dogs, and smaller animals with gradually increasing 60-Hz current while monitoring the heart's action with an ECG to detect the onset of fibrillation. Were Matilda listening - which, of course; she isn't - I'd assure her the animals were anesthetized before the experiments." He paused, and Matilda's fingers began guiltily pecking away at the typewriter keys.
He went on, "Here are some conclusions reached: current through the heart - for example, from hand to hand or hand to foot - is needed to produce fibrillation. That's why, if you must work on a hot circuit, you should keep one hand in your pocket and make sure you're standing on insulating material. Whether or not fibrillation occurs depends on the weight of the subject, the strength and duration of the current, and when a short shock occurs with regard to the heart's cycle. Ventricular fibrillation is unlikely to occur in a normal adult if the shock intensity is less than 116/t1/2, mA, where t is in seconds. This means the maximum non-fibrillating current for a 1-second shock is 116 mA; but for a 4-second shock it is only half that value. As the weight goes down, so does the maximum non-fibrillating current. A value of current an adult can endure safely may electrocute a child."
"What does the timing of short shocks have to do with it?"
"There is a recovery period just after the ventricle has contracted when it is particularly susceptible to being triggered into fibrillation by stimulation. This corresponds with the 'T-wave' of the ECG display. If a random self-generated pulse of the heart, called a 'premature ventricular contraction,' falls on a T-wave, it can trigger fibrillation so that the heart, especially a diseased heart, can, in effect, electrocute itself. A pulse of external current penetrating the heart at this sensitive moment can have the same effect.
"Currents of 100 to 200 mA are the values most likely to produce fibrillation when applied to the exterior of the body. Above 200 mA the heart muscles are clamped so tightly by the current that there is no movement, not even the quivering of fibrillation. A heavy current of short duration may well be less dangerous than one in the range of 100 to 200 mA. This also explains why fibrillation is stopped in a cardiac intensive care unit by a heavy-current counter shock that stops the random, ineffective activity of the heart completely and allows it 'to start all over' in the proper manner."
"But too much current actually sears tissue the way those electric hot dog cookers sear weiners," Barney pointed out.
"Right. It's a grisly subject, but we know a little about what heavy currents do to the human body from examination of the bodies of electrocuted criminals. In a typical electrocution, 2000 volts was fed to moistened, sponge-lined electrodes applied to the shaven skull and a leg. The voltage was immediately dropped to 500 volts and then raised and lowered between these two values of voltage every 30 seconds for two minutes. The current varied from 4 to 8 amperes. A temperature of 128°F was measured at the site of a leg electrode 15 minutes after the electrocution, and the blood was found to be profoundly altered electrochemically."
"Let's get off that subject," Barney said with a shiver. "A few years back, I remember reading stories in both Time and the prestigious Wall Street Journal quoting some authority as saying that 1200 patients were accidentally electrocuted in hospitals every year. Know anything about that?"
"That was in 1969, when Dr. Carl Waldemar Walter of Boston's Peter Bent Brigham Hospital and professor of surgery at the Harvard Medical School was alleged to have said that. His statement hit hard because Dr. Walter was chairman of the National Fire Protection Association that drafts safety codes for electrical devices. The figure was hotly disputed by many members of the AMA as being exaggerated and, to my knowledge, was never really proved true or false; but the story did serve to focus attention on this type of electrocution that can and does happen.
"Dr. Arbeit, writing in the June 19, 1972, issue of the JAMA, cites fifteen cases of accidental electric shocks delivered to hospital patients, at least five of which were fatal; and an ECG machine or monitor was involved in fourteen of these cases. Such devices have normally grounded the patient to eliminate serious 60-Hz interference with the tracings. From our talk about leakage currents last month, we know a grounded body is a sitting duck for any current leaking out of any other line-current-operated device he touches, such as a bedside lamp, radio, TV, electric bed, or another diagnostic or therapeutic piece of equipment.
"Many hospital deaths, however, do not result from macroshock currents, applied to the exterior of the body. Instead they are caused by microshock currents that bypass the high-resistance skin layer and are delivered directly to the interior via wires or fluid-filled tubes inserted into one of the body orifices or through cuts made in the skin. Where such conductors enter the thorax and approach the heart, extremely small ac currents can cause death. Some medical authorities say currents as low as 20 microamperes can trigger fibrillation when applied directly to the heart by means of a pacemaker lead or through a diagnostic catheter inserted into the heart chambers. At this level, even static electricity becomes a threat.
"This accumulated knowledge about what happens to the body when it is exposed to electrical shock is beginning to bear fruit. In July of last year, the Underwriters' Laboratories finally brought out a realistic safety standard for medical and dental equipment, UL 544. We've already talked about established standards for leakage current in household appliances. Equipment manufacturers, finally provided by the medical fraternity with hard information regarding how much current is safe in a variety of situations, are designing and marketing equipment that should be harmless when properly used, as they always said they would."
"The Effects of Electric Shock on Man," C. F. Dalziel, reprinted from IRE Transactions on Medical Electronics, May 1956. For sale by Supt. of Documents, U. S. Govt. Printing Office, Washington, DC. Price, 20¢.
"Lethal Electric Current," C. F. Dalziel and W. R. Lee, IEEE Spectrum, February 1969.
"Standards for Safety Medical and Dental Equipment," Underwriters' Laboratories, 1285 Walt Whitman Rd., Melville, NY 11746.
"Controlling the Electrocution Hazard in the Hospital," Sidney R. Arbeit, et al, Journal of American Medical Association, Vol. 220, No. 12, June 19, 1972.
"Intensive Heart Care," Bernard Lown, Scientific American, Vol. 219, No.1, July 1968.
"American National Standard for Leakage Current for Appliances," #C101.1, American National Standards Institute, Inc., 1430 Broadway, New York, NY 10018.
Posted April 3, 2018
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive 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 Radio & Television News
magazine (which itself started as simply Radio News), and then changed
its name to Mac's Service Shop after the magazine became
World. 'Mac' is electronics repair shop owner Mac McGregor, and Barney
is his eager, if not somewhat naive, technician assistant. 'Lessons' are taught
in story format with dialogs between Mac and Barney.