[Table of Contents]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 (if any) are hereby acknowledged.
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
See all articles from
Shocking But True
Ignorance of electrical safety rules could cost you your life
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 of contact.
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
But 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 10 ma.
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.
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 fibrillation.
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.
Portable electric 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. Emergency
. 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 a doctor.
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.