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 45± years of exposure. Other than observing my father's being scared sh**less 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 stay healthy.
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 table.
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 September 5, 2017