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December 1965 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
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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
 By John T. Frye
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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