September 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.
Common sense never
goes out of style, especially as it pertains to safety in the presence of electricity.
Most people who have worked in the electrical / electronics realm for a while are
aware that lethal electrocution
can occur with currents as low as 100 mA when it passes through the heart. Lower
values cause progressively less profound maladies, but in practice any level of
current great enough to be felt is not a good thing. I have written before about
having received a few pretty scary shocks when working on high voltage equipment
and many lesser jolts throughout my 50± years of exposure. Other than observing
my father's being leery of using of anything with an electric cord attached to
it, my first formal instruction about electrical safety was in my vocational classes
in high school. Instructor Russ Lorenzen taught us to keep one hand in our pockets
when working on live circuits, which of course was only to be done under the rare
circumstance when it is not possible to first turn power off. In practice that often
meant when doing so would be more inconvenient than the calculated risk of electrocution
;-). Seriously, though, a very often encountered qualifying scenario is when working
inside a live circuit breaker service entrance panel where pulling the electric
meter is the only way to remove power. Doing so requires a utility worker to break
the seal on the meter socket enclosure, and besides, often shutting off every circuit
in the panel, especially in a commercial or industrial environment, would be unreasonable.
How to Avoid Workbench Hazards
Don't Be Careless When Working with Electronics
Every year, thousands of electronics professionals and hobbyists suffer the painful
and sometimes lethal effects of electrical shock while at their workbenches. Most
are lucky enough to come away from the experience with a bruise, a broken bone or
a painful memory and a new respect for the power of electricity. Those who fail
to come away from it become statistics.
These accidents need never have occurred if the victims had adopted a sensible
work plan and geared themselves physically and mentally to avoid multiplying the
shock hazard. You can minimize the shock hazard on your workbench by using a few
simple expedients and exercising good common sense.
Fig. 1 - Recommended method of wiring cords and switches
on electronic gear.
In this article we will be discussing some of the practices you should adopt
whenever you work on line-powered and high-voltage circuits and equipment. We will
detail the conditions under which you should avoid working near potentially dangerous
voltages and describe what you can do to make your working environment a safer place
in which to work.
Safety Practices. Let us begin with the common denominator -
you. You can do everything possible to make your shop really safe, but if you are
a "walking disaster," accidents will follow you on the job.
First, never go to work on an electronic device-powered or not - while wearing
jewelry such as a wristwatch, ring, etc. The workbench is no place for jewelry or
other items like ties and dangling laces that can get hung up on the equipment in
an emergency or even be the cause of an emergency.
Be practical about what you wear on the job. You are at your best when comfortably
dressed. So, wear a long-sleeved shirt, buttoned at the wrists and open at the collar,
and rubber-soled shoes.
Whenever you are working on a circuit or chassis where high voltages are present,
keep your mind and eyes on what you are doing. Don't look away to observe a meter
reading or a scope waveform if you are touching a test prod to a point in a powered
circuit. Do your job the way a professional would: With the power to the equipment
under test turned off, connect the test leads. Turn on the power, take your reading,
and turn off the power. Only after the power has been turned off should you remove
the test leads from the equipment. If you do the job the unsafe way, your eyes have
to leave the work to take the reading, in which case the probe tip might slip. Chances
are that you will overreact and get yourself into more trouble.
It takes only about 10-20 μA of current coursing through the heart to cause
ventricular fibrillation, a usually fatal condition unless help and special equipment
are immediately available. Currents as low as 100 mA entering a hand and leaving
the body via the other hand or a foot can generate the fibrillatory current in the
heart. So, never reach into a high-voltage circuit with both hands, and never rest
one hand on the chassis while reaching into the circuit with the other hand. To
avoid temptation, keep your free hand in a pocket or behind your back.
If you plan to work on unpowered equipment in which high voltages are developed,
make certain that the line cord is unplugged and that you discharge all electrolytic
capacitors in the high-voltage circuits. Electrolytic capacitors can hold a potent
charge long after power is shut off; so, don't take chances. (Remember that charges
too small to be lethal can inflict secondary injuries like bruises, lacerations,
and broken bones as muscles violently and involuntarily contract upon contact. This
can be a life-saving move on the part of nature, by interrupting the through-the-body
circuit, but it doesn't help if you crack your skull against a shelf or tear your
flesh on a chassis.)
When Not to Work. Many electronics men go to work on circuits
or equipment when they should be doing something else - like resting. There are
definitely times when you should avoid going near electronic gear if you plan to
Hot, muggy environments cause a worker to perspire profusely and sap energy.
A body covered with high-salinity perspiration becomes a fairly good conductor of
electricity. Not only is the resistance over the surface of the skin reduced by
perspiration, it provides a more direct current path between the skin and the interior
of the body.
Cold environments can be equally hazardous. Cold has a numbing effect on the
body, particularly in the extremities - like the fingers that hold test probes.
Fingers that lose their normally acute sense of touch can easily make mistakes and
do so all too often. Either heat the area or stay away.
Never approach a job if you are tired, angered, or emotionally upset. And don't
try to work off excess energy at your workbench. (Go lift weights or do some jogging;
it's safer.) Under these conditions, your concentration is apt to wander - which
is as bad as your eyes wandering.
Fig. 2 - How to make a workbench with a metal top safe by
addition of insulation.
The best time to go to work is when you are relaxed and alert. Stop working when
you become fatigued or bored, and take frequent rest breaks.
Your Equipment and Workshop. Many electronics men who practice
proper safety measures give little thought to their test equipment and workshops.
This is particularly true of the hobbyist who works in a basement or attic where
environmental conditions are hardly conducive to safety.
Line-powered test gear is a particularly sore point. Under no circumstances can
a line-powered instrument be considered safe if it is equipped with a two-conductor
line cord. It is even less safe if only a single-pole, single-throw power switch
is used. All two-conductor line cords should be replaced with three-conductor cords,
and all instruments should be equipped with double-pole single-throw switches. The
recommended method for wiring the cords and switches into your gear is shown in
Fig. 1. While you are at it, carefully inspect all power cords and plugs, replacing
any that are frayed, loose, or worn.
Plug three-prong plugs into appropriate sockets or into adapters to mate them
to two-conductor house wiring systems. If you use adapters, slip the spade lugs
on the grounding wire under the outlets' wall-plate mounting screw and tighten down.
When you have several instruments that have to be used simultaneously, your best
bet is to use a circuit-breaker or fuse-protected heavy-duty powerline outlet box.
In this event, you need only one adapter in a two conductor house wiring system.
If you want to be really safe at your workbench, consider installing a ground-fault
interrupter (GFI) in the bench's power system. The GFI is a fast-response device
that disconnects power from the load whenever leakage current exceeds a specific
amount (typically 5 mA). Don't install the GFI into the room's entire electrical
system, or it might extinguish the lighting when it trips - a safety hazard in itself
as you grope around in the dark and trip over things .
Finally, make your work area safe and livable. In a damp basement where the floor
is of raw concrete or in an attic where the floor is of unfinished lumber, lay vinyl
flooring. Both areas will benefit enormously from a few sheets of hardboard nailed
over exposed studs and rafters. Before installing the hardboard, however, make sure
that there is adequate weather insulation between the exposed studs and rafters.
A casement vent in the basement or a through-the-wall vent, each equipped with an
exhaust fan, to allow free circulation of air will keep either area relatively dry
and odor-free. While you are about fixing up your work area, install adequate lighting.
Any good book on home improvements will tell you how to do these things.
Wood is the best material for an electronics workbench, but if you must use a
table with a metal top, it will have to be made safe. You will need two sheets of
3/4-inch plywood cut to 1/8 inch longer and wider than the dimensions of the table
top. Cement the plywood sheets together and clamp overnight. Then top them with
a ribbed synthetic rubber runner, held in place with contact cement, to provide
a durable non-skid work surface. Finally, glue and nail a hardwood frame around
this assembly as shown in Fig. 2. When finished, the worktable surface should
slip over the metal table top. Do not fasten the work surface to the top of the
If you do everything we have outlined above, your chances of being injured or
worse in your workshop will be very remote. But, again, we must caution you. Don't
relax your guard or take shortcuts. To do so, you are only inviting trouble.
Posted March 21, 2023
(updated from original post on 9/5/2017)