June 1953 QST
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights hereby acknowledged.
My career involving controlled movement of electrons began with working around high voltages and currents. In electrical vocational classes, the instructor lectured on the potential (pun intended) harm that could be done to life and property because of ignorance or inattention to the job at hand. Safety practices were reviewed prior to allowing us plebes to handle not just 3-Ø, 480 V supplies for motors, but also for household AC circuits and low voltage sources with high current outputs. We all had the opportunity to get 'lit up' with 115 Vac and even 230 Vac if we dared (a practice that would never be allowed in today's environment). We watched films (no VCRs back then) showing poor souls who had been the victims of electrocution. It was all enough to convince me that the experts weren't kidding. Nevertheless, even with a healthy respect for being hurt, there were times when I would accidently come in contact with a live wire and feel that unmistakable 60 Hz tingle and internally induced buzzing sound. I still have my linemans' pliers from nearly 40 years ago that have scars from melting caused by inadvertently providing a path to ground while grasping a hot wire with the jaws. One time I burned a large screwdriver in half while working inside a circuit breaker panel. One of the major rules, covered in this article, is to when possible keep one hand in your pocket when working with live circuits. That minimizes the chance of a current passing through your heart, which is one of the most certain scenarios for death by electrocution.
How to Live Longer
Safety Technique for Ham Equipment
By Donald H. Mix * W1TS
Over a period of years a goodly number of hams have met death at the hands of their equipment. The really surprising thing is that there have not been more of them, considering the high voltages often used in transmitters. Especially in these days when thousands of new hams are coming into the game with limited background in things electrical, it is important to make everyone of them realize the very real danger of death or serious injury that may lie in wait behind an innocent-looking panel.
This article deals with the things that every operator should observe in building and operating his equipment with a minimum of danger not only to himself but to the rest of his household. Read it carefully and make a firm resolution that you won't be the next one!
Several years ago, shocked by the sudden death by accidental electrocution of one of its foremost members, the Headquarters staff held a series of conferences. The object was to study current ham practices, particularly in regard to transmitter construction and adjustment, and to formulate a set of rules that should minimize chances of accidental injury or death. This set of recommendations was published in the March, 1939, issue of QST. Although we continue to lose good, but not always careful, hams at the rate of about three a year through high-voltage accidents, we hope that the publication of the ARRL Safety Code has served to save the lives of many others. For this reason, we feel that it is worth while to remind all hams, once again, that any piece of electrical gear is capable of dealing out sudden death.
TVI, still" just around the corner" when the Safety Code was first published, has not been without its beneficial influence on the design and construction of ham gear. Not only has it forced us into the production of cleaner signals, but the requirement of shielding enclosures has made ham rigs vastly safer to operate and work around. Today, the greatest hazard probably is the one that lurks around the test bench with its upended chassis and temporary power leads. This should be borne in mind when reading the recommendations that follow.
There are two ways by which the danger may be minimized. The first is that the operator train himself to follow a procedure of operation and adjustment that will minimize his chances of coming in contact accidentally with any exposed high-voltage point. The second consists of methods of construction that will provide a minimum of exposed high-voltage points with which contact can be made. The second should actually be considered supplementary to the first to reduce the hazard, should the operator forget. It is seldom possible to construct a piece of equipment that can be handled indiscriminately with any degree of safety while the power is turned on. Of course, no ham knowingly will touch a danger point with the power turned on. But by following certain rules, the chances of a thoughtless act having a tragic result can be minimized.
Rules for the Operator
Fig. 1 - Reliable arrangements for cutting off all power to the transmitter. S is an enclosed double-pole knife-type switch, J a standard a.c. outlet, P a shorted plug to fit the outlet and I a red lamp.
A is for a two-wire 115-volt line, B for a three-wire 230-volt system, and C a simplified arrangement for low-power stations. All are discussed in detail in the text.
A) Kill all power circuits completely before touching anything behind the panel or inside the chassis or enclosure.
The easiest way to make sure that this rule is followed is to make the operation of turning off all power a simple one. A single main switch should be provided that cuts off all power from the equipment. The use of one plainly-labeled switch for this purpose, instead of several individual switches. for various circuits, obviously not only makes it easy for you to become automatic in eliminating all danger, but it also makes it readily possible for someone else not familiar with control-circuit details to cut the power off quickly should you forget and get "hung up" across the high voltage.
Toggle switches and other similar spring-operated switches, or relays of the sort most frequently used by hams for power control, are not sufficiently reliable for this important purpose. While automatic interlock circuits are fine as secondary devices to help protect you in case of forgetfulness, don't let your life depend on them. They have been known to fail, particularly when they control relays that may stick closed at any time.
There are at least two devices for cutting off all power to the transmitter that are about as foolproof as anything could be. The arrangement is shown m F1g. 1. A and B are similar circuits for two-wire (115-volt) and three-wire (230-volt) systems. S is an enclosed double-throw knife switch of the sort usually used as the entrance switch in house installations. J is a standard a.c. outlet and P a shorted plug to fit the outlet. The switch should be located prominently in plain sight and members of the household should be instructed in its location and use. I is a red lamp located alongside the switch. Its purpose is not so much to serve as a warning that the power is on as it is to help in identifying and quickly locating the switch should it become necessary for someone else to rescue you.
The outlet J should be placed in some corner out of sight where it will not be a temptation for children or others to play with. The shorting plug can be removed to open the power circuit if there are others around who might inadvertently throw the switch while you're working on the rig. If you take the plug with you, it will prevent someone from turning on the power in your absence and either injuring themselves or the equipment or perhaps starting a fire.
Those who are operating low power and fecl that the expense or complication of the switch isn't warranted can use the shorted-plug idea as the main power switch. In this case, the outlet should be located prominently and identified by a signal light, as shown in Fig. 1C.
The test bench ought to be fed through the main power switch, or a similar arrangement at the bench, if the latter is remote from the rig.
B) Never permit anyone else to switch the power on and off for you while you are working on equipment.
Always do the power switching yourself. Having someone else do it is too risky, even though you are giving the orders. There's too much chance of a misunderstanding and, after all, you don't want to make someone else bear the responsibility for your execution.
C) Never put your hands into any gear without first using a grounded probe at all exposed points.
Bleeder resistors, even though conservatively rated, are not infallible, and a fully-charged filter condenser can be just about as lethal as the supply with the power turned on. The probe will also serve as a secondary line of defense in case you should forget to turn the power off. Short-circuit the power supply with the probe instead of your arms. Always jab a bare terminal with the probe before changing plug-in coils. This is a good habit to form, even though you may be using parallel feed in your present rig. It may save your life some day when you're working on another piece of gear of your own or belonging to someone else. Touch the probe to all exposed points just in case a burn-out makes one point cold while others are still hot.
The sketch of a suitable homemade probe is shown in Fig. 2. The handle of the probe ought to be long enough so that you don't have to put your hand close to the equipment to use it. A total length of about 18 inches should be safe. The insulating handle and wire are to save you in case the ground lead opens up. It is obvious, of course, that you must be sure that there is a solid circuit between the grounding point of the probe wire and the negative terminal of the plate supply (and positive terminal of the bias supply).
Fig. 2 - A homemade insulated grounding probe. Such a device should always be used to ground exposed high-voltage points before touching anything behind the panel. It serves to protect the operator against charged filter condensers or if he forgets to turn the power off.
If you are working on gear on the test bench, make sure that power supplies and probe are connected to the same ground and that you use the probe at each power-input terminal before touching anything underneath the chassis.
D) When shooting trouble, make sure that you are well clear of the gear before turning on the power.
Don't place yourself in an awkward or unbalanced position while you push the key with one hand and try to stretch around a corner to see what's happening behind the panel. If you should slip or lose your balance, you might fall into the high voltage. Use a push button, or other momentary-contact switch on the end of a cord for power control, so that you can keep yourself well in the clear. The push-button-type switch opens automatically and this might at least keep you from frying if an accident happens.
E) Stay clear of grounded metal while you're working on the rig.
Don't lean against the cabinet, chassis, or other grounded metalwork. And, above all, don't wear headphones while you're working on equipment or changing coils. While the voltages in a receiver, exciter or speech amplifier may not be above 2.50, your chances will still be pretty slim if your head is at one end of the circuit. A good rule to follow is to keep one hand in your pocket. There will be less danger of a shock passing through a vital part of your body. If the rig or test bench is in the basement, you ought to provide a rubber mat around it.
F) Never adjust variable links by hand. Adjustable links can be highly dangerous with a series-feed tank circuit. Not only does manual adjustment bring your hand too close to the tank coil, but the clearance between the link and the tank coil is never very great, making contact between the tank coil and link all too easy. If the coil design doesn't provide a means of fitting the link with a panel control, use a long stick for adjustment. Shielded links or links grounded at one side or a center tap will prevent high voltage from appearing unexpectedly on the link cable.
G) Don't work on equipment (or try to change plug-in coils) when you're tired or otherwise not up to snuff mentally.
Mental or physical fatigue is invariably accompanied by a certain amount of absentmindedness. Wait until you are fully alert.
H) Use special care when checking tank circuits with an absorption wavemeter.
The use of absorption wavemeters in checking tank-circuit resonances and harmonics has become everyday practice, and yet what could be more dangerous than a bare coil stuck in a hot tank circuit while the operator watches the indicator rather than where he is putting the coil? Use a well-insulated link line between the tank and wavemeter coils and ground the case of the wavemeter (which should always be of metal) with a clip lead.
I) Never pull test arcs from the transmitter tank circuit.
This warning really shouldn't be necessary, of course. It is singled out from other foolhardy conduct around a transmitter only because a great many of us have a natural temptation to do it. The r.f. may not cause anything more than a nasty burn, but an r.f. arc isn't an insulator and can easily conduct d.c. through a pencil or screwdriver. Play safe and resist the temptation.
J) Teach members of your household how to apply artificial respiration.
Many lives have been saved after electrical shock by the prompt and proper application of artificial respiration by a bystander. The usual methods used by physicians in detecting life in victims of accident often do not apply in the case of electric shock. Cases are on record where victims, apparently dead, have been brought around after as much as four hours of seemingly hopeless pumping. Regardless of other indications, hope should not be given up until there is definite indication of rigor mortis. Prompt action is highly important. Don't wait for a pulmotor to arrive. Instruction sheets can be obtained from your local Red Cross office and often at fire or police stations. If possible, always have someone else in the room while you're working on equipment.
However, a most-important point is that everyone should be warned of the danger of contact with a person who has been injured and who may still be in contact with the high voltage. Make sure that members of your household understand that the power must be turned off first. There have been several instances where women and children have been killed or injured in attempting to remove an unconscious person still in contact.
Equipment Design and Construction
So much for the rules of operator conduct in the presence of electrical equipment. If they are followed religiously, there is small chance of an accident. However, forgetfulness is a part of human nature and the unfortunate part of it is that electricity has no sympathy for a first-time offender. The first moment of forgetfulness may be your last! Therefore, it is the sensible ham who will construct his equipment so that it won't be easy to get hurt even if he forgets.
1) Panel Controls and Metering
It shouldn't be necessary to point out that every control shaft extending through the front of the panel should be at ground potential. To make certain of this, ground every control shaft to the panel either directly or by the use of panel-bearing units wherever an insulating shaft coupling is used. The frames of key or metering jacks must be fastened to the grounded panel. Never mount them with insulating washers. This, of course, essentially dictates metering and keying in the cathode or center tap.
If you want metering in the positive leads, use meter switching instead of jacks. Meters, unless connected in the cathode or centertap lead, should be recessed so that there will be no danger of contact with the adjusting screw.
2) Power Supply
Enclose all power supplies, or construct them so that contact with any part of the circuit is impossible. You don't want to stick your knee into the power supply while you're concentrating on keeping clear of the r.f. units. Power supplies seldom need adjustment, and there is no reason why they can't be enclosed. However, most modern power-supply components are designed so that they may be mounted with their terminals protruding under the chassis where there is normally little chance of accidental contact. Rectifier plate caps should be of the insulated type. Similarly there should be no exposed terminals. Use insulated plug-type connectors designed for the proper voltages at the test bench as well as at the rig. In using these connectors, make sure that all live connectors are of the female type. For example, always use a female connector on the power-supply chassis, and another at the transmitter end of the cable. Use a male connector on the transmitter chassis.
All negative plate-supply and positive bias-supply terminals should be connected to the chassis and the chassis connected to a water pipe or other good ground connection. Then there will be no chance of your serving as a ground wire should some component break down, making the chassis hot. For the same reason, all transformer and choke cores and other metalwork not normally a part of the electrical circuit should be grounded to the chassis. If the power supply is to be mounted above other units, its chassis should be provided with a bottom cover plate.
Every power supply should be equipped with a conservatively-rated bleeder resistor. If the bleeder resistor is placed on top of the chassis for ventilation, cover it with screening or perforated metal.
In these days of TVI, it is good practice to use shielded wire for external power cabling. If the shielding is grounded, it will protect you in case of an insulation breakdown.
3) R. F. Units
The requirement of grounded shielding enclosures for TVI makes the use of coil switching highly desirable for the sake of convenience. It also provides a great factor of safety. Since it is almost impossible to change a plug-in coil in a shielded rig without making contact with grounded metal as well as the coil, it ought to be made a general rule to use parallel feed in all circuits where plug-in coils are used. If you must use series feed, use coil switching. However, it doesn't do much good to use parallel feed if there are other exposed high-voltage points in the vicinity of the coil. Use insulated plate connectors for tubes with cap terminals, insulated plate leads, and place r.f. chokes under the chassis or cover up choke terminals and those of blocking condensers.
Pi-section tank circuits do not provide a d.c. path to ground in case the blocking condenser should blow. A receiving-type r.f, choke connected across the low-impedance output will provide a protective path to ground and keep the plate voltage off the coax line should the blocking condenser fail.
4) Audio Equipment
The rules of construction set forth for power supplies will serve equally well for most speech amplifiers and modulators. An important additional precaution that should be taken is that the microphone stand and enclosure should always be connected to the microphone-cable shield which, in turn, should be grounded to the chassis. At least one ham has met death by failing to do this.
5) Auxiliary Equipment
Particular attention should be paid to the use of pick-up links for monitors and 'scopes. Too often such links are made in haywire fashion from any piece of wire that happens to be handy. Use well-insulated wire and ground one side of the link.
In conclusion it should be reiterated that no voltage, including those in the lower hundreds, can be considered free from danger. With a good contact, tests have proved that the maximum that a person can take and still have the power to release his grip is 40 volts a.c.! Handle every circuit with caution and remember that the lowly 115 volts has more electrocutions to its credit than any other. None of the measures recommended above involves any considerable amount of complication or expense. It's just a matter of taking the time to make sure that they are complied with.
* Assistant Technical Editor, QST.
Posted July 27, 2016