August 1959 Popular Electronics
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular
Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged. See all articles from
of electrical shock have been around as long as experiments in electricity
and electrical appliances have been around. For that matter, even
ancient men unfortunate enough to have come into contact with an
electric eel or a lightning bolt, or even those who rubbed against
sheep's wool in an arid environment and then reached for a metal
implement, know the pain of an electrical shock... or worse. This
article in the August 1959 edition of Popular Electronics warns
readers of the dangers lurking at the end of every electrical cord.
One of the cartoons shows a guy being zapped while using an
electric drill. About a year after graduating from high school,
a friend of mine was using a power saw in a garage that had a damp,
dirt floor. Even as late as the mid 1970s there were still a lot
of power tools that had metal bodies, and usually had no ground
wire. Electrocutions were not uncommon. My friend died from his
contact with 120 VAC.
Today's power tools are always listed
as "double insulated" by the Underwriters Laboratory. They provide
at least two layers of protection that shield the user from contacting
live components. The National Electric Code stipulates that only
tools with the official double insulation mark (a squared inside
another square) may be used without a safety ground connection.
Even a double insulated tool cannot guarantee your safety when being
operated on a damp dirt floor if there is low enough resistance
to the inside of the tool via mud or water; only a ground fault
interrupter circuit (GFIC) can do that.
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Shocking But True
Ignorance of electrical safety rules could cost you your
By Forrest H. Frantz, Sr.
of us have been "bit" at one time or another. If the shock was a
mild one, we said "ouch" or "d--n" and that was that. We gave it
no further thought. However, we should think about it. An appliance
that in one situation may produce a slight tickle, in another may
jolt us right into a hospital bed or worse. Each year electrical
shock takes approximately 800 lives in this country, and these 800
executions are usually accomplished with less power than it takes
to press a shirt!
Shocks are caused by a combination of
two factors: voltage and current. The relationship between voltage
and current can be compared to that of a gun with a bullet in its
firing chamber, with voltage being the gun itself, and with current
- the actual death-dealer-being the bullet. The current, or bullet,
is harmless until it is "fired" by the voltage. Thus neither voltage
nor current is dangerous until they exist in the proper relationship.
This explains why a 50-volt jolt occasionally will cause death,
while an automobile's ignition system - with its several thousand
volts of shocking power - is rarely a killer.
The amount of current
Watch That Current!
in a "shock" circuit
is determined by the applied voltage in relation to the sum
of the internal resistance of the power supply, the resistance
of the body, and contact resistance at the points where the
circuit is completed.
your radio while taking a bath.
This is an easy way to start a one-way trip to Paradise.
the third wire from a 3-wire
portable electric tool. If the motor shorts out to the frame,
the grounded wire will protect you from shock.
electrical equipment while
you are standing on a damp surface. If you do, you have a good
chance of getting 117 volts through your body.
The amount of current that
flows depends on the amplitude and source of the voltage, the physical
size of the individual who is shocked, the portion of his body through
which the current flows, and the condition of the skin at the points
The amount of shock current can be calculated by (I = E/R). Assuming
a voltage of 80 volts and an internal body resistance of 400 ohms,
a fatal current of 200 milliamperes would flow. Fortunately, in
addition to the internal body resistance, there is contact resistance
between the voltage source and the skin. Dry skin contact resistance
is between 100,000 and 600,000 ohms, but this figure decreases rapidly
as the contact area increases and the skin becomes damper. A small
amount of perspiration can lower skin contact resistance to 50,000
ohms or less, while complete wetting of the skin and increased contact
area can reduce the contact resistance to between 500 and 1000 ohms.
Wet-body contact might involve a total resistance of about
1000 ohms from the right to the left side of the body. With a voltage
source of only 50 volts, the current would be 50 ma., a deadly level.
After initial contact has been made, contact resistance
decreases. If the decrease in contact resistance lowers the total
resistance to 500 ohms, the current will rise to 100 ma. - enough
to cause ventricular fibrillation, a heart condition that results
in death. Current-Limiting Factors
what about the cases where several hundred volts from a Geiger counter
battery or thousands of volts from an automobile spark coil or a
laboratory high-voltage machine do not cause death? In calculating
the effect of a 50-volt shock, it was assumed that no internal resistance
existed within the voltage source and that current was limited only
by body resistance. This is not always the case. Any electric battery
or electric generator has an internal resistance. If this internal
resistance is low in comparison to body resistance, total body resistance
will determine the amount of current flow.
On the other
hand, if the internal resistance of a battery or generator is almost
as great or greater than body resistance, current flow is partially
limited. If the internal resistance of the voltage source is many
times the body and contact resistance, current flow is limited to
an almost constant value. Thus, in a 1000-volt generator with an
internal resistance of 100,000 ohms, the current could never exceed
Electrostatic generators such as Wimshurst
machines or small van de Graaff generators can develop hundreds
of thousands of volts. But, although these machines produce high
voltages, their power output (volts x amperes) is small. When contact
is made with a "hot" van de Graaff generator, the current flow is
limited to a low value, as it is with automobile ignition systems.
However, live experiments should be avoided because there are exceptions!
Effects of A.C. Shocks
. Special physiological
effects of electrical shocks are determined by the frequency of
the voltage. While both d.c. and a.c. can cause burns, low-frequency
alternating current - and this includes the standard 60-cycle house
current - affects the nervous system.
At current values
between 8 and 15 ma., a.c. shocks are painful, but most individuals
retain enough control of their muscles to withdraw from contact.
Currents between 15 and 20 ma. cause pain and loss of muscular control.
The victim cannot voluntarily withdraw from contact. Unless the
current is interrupted, the victim becomes exhausted and lapses
into unconsciousness. When a.c. shock currents reach values between
20 and 50 ma., pain is very intense and paralysis of the breathing
muscles will cause suffocation.
When a 60-cps alternating
current of between 100 and 200 ma. is applied to the body, the frequency
superimposed over the heart's normal beat can disrupt its timing.
Since the heart is being told to pump at a rate of 72 times a minute
by the nervous system, and, at the same time, it receives external
stimuli from the house power supply at the rate of 60 per second,
it becomes confused and begins to flutter aimlessly. This is ventricular
Currents greater than 200 ma. stop the
heart's movements completely, rather than causing ventricular fibrillation.
If exposure to the shock is not prolonged more than three or four
minutes, however, the heart will sometimes resume its action.
. The knowledge that it takes two contacts to
cause a shock tempts some people to take foolish chances. Don't
work on your house wiring until the power switch is turned off.
Although you may have one of the wires completely taped up, if you
happen to be working with the "hot" wire and then you back into
a cold water pipe, zowie! If you're standing on damp ground or on
a concrete floor, you can get a shock right through your shoes.
A number of people have been killed each year because they
touched light fixtures, switches, or radios while standing in a
bathtub. If you're taking a bath and you want to change the radio
station, don't - it's such an undignified way to leave this world.
Switches are normally insulated from the a.c. line; but a defect
in wiring, or an insulation breakdown, can cause a fatal accident.
If these things seem unlikely, keep in mind the 800 Americans who
die each year from "unlikely" shocks.
tools are a "sneak-path" threat. To minimize the chances of sneak-path
electrocution, most portable tool manufacturers provide a grounding
wire connected to the frame of the tool. If the insulation breaks
down or a short from the motor to the frame occurs, current will
return to ground through the grounding wire (if it's grounded, of
course) instead of through the user. The short will usually necessitate
the replacement of a blown fuse. This is small trouble, however,
compared to what might happen otherwise.
The a.c./d.c. radio
and other a.c./d.c.-operated electrical devices are additional sources
of danger. In the earlier a.c./d.c. devices, one side of the line
was connected directly to the chassis. If the line cord for one
of these units is inserted so that the chassis is connected to the
hot side of the line, body contact from chassis to ground (even
though the equipment is not turned on) can result in electrocution.
At present, most a.c./d.c. equipment is manufactured with
the chassis connected to one side of the line through a capacitor.
But even this measure does not completely eliminate the shock hazard.
At 60 cycles, a 0.5-μf. capacitor has a reactance of 5300 ohms,
and a 0.1-μf. capacitor has a reactance of about 26,500 ohms. If
the body is placed in series with a 0.5-μf. capacitor across the
line, electrocution can occur. Although electrocution is unlikely
from body contact to the a.c. line through a 0.1-μf. capacitor,
a shock will occur. The experimenter should proceed with caution
when working with a.c./d.c. equipment. If possible, isolate the
equipment from the line with an isolation transformer.
. In any shock emergency, the
circuit should be broken as quickly as possible, preferably with
a switch. If you can't get to a switch, remove the victim from the
circuit. But take precautions to avoid becoming a second victim.
Remember, the victim is "hot," and if you simultaneously touch him
and a good ground, you won't be in a position to help anyone.
If the victim is not able to breathe after he is removed
from the circuit, don't waste time calling a doctor. Apply artificial
respiration immediately and keep it up until someone else brings
With the observation of basic safety rules, electronics
is not a dangerous hobby. But a complete knowledge of the "enemy"
and his "tactics" is your best insurance against painful and possibly
serious accidents. Don't take chances with house wiring, don't touch
any electrical fixture when your hands or feet are wet, and don't
work with a.c./d.c. equipment unless you are aware of its shock
hazard. Posted 1/25/2012