I wrote the following article for
Monitoring Times magazine of Brasstown, North Carolina.
I've decided that more folks might want to read the article as published in the December 2013 final issue. Please note that the article is copyrighted and may not be reproduced without permission of
Monitoring Times or the author.
Ground Fault Protection and the AC-DC radio
The Ground Fault Interrupter, also known as a "Residual Current Device", is one of the greatest life-saving inventions of the past fifty years although seldom recognized as such. Invented to improve electrical safety in the gold mines of South Africa, improved versions of the device are now required where inadvertent grounding is possible through contact with plumbing, concrete floors, frequently wet areas, or other hazardous locations.
The GFI works by comparing the electrical current flowing between the line and neutral conductors of an AC circuit. Any difference in current between those two conductors is likely caused by a "ground fault" that routes electricity from the line to a different path than the neutral. That path might include a human being contacting the line through a faulty power tool and getting current flowing through the body with possible deadly consequences.
The GFI comes commercially in several varieties. The most familiar one is the GFCI outlet (Ground fault Circuit Interrupter) now required in bathrooms, kitchens, garages, outdoor power outlets, and a variety of other electrically hazardous locations. The GFCI has the usual three contacts found in regular outlets; line, neutral and safety ground. A GFCI compares both the difference between line and neutral and difference between neutral and safety ground. The common household GFCI is designed to shut off the power when as little as 4 to 6 milliamps is detected as a ground fault. That tiny current is below the threshold of shock level that might cause death.
A second version is the ALCI (Appliance Leakage Current Interrupter). This device is permanently attached to the power cord, replacing the ordinary plug. Like a regular power plug, it only has two poles, line and neutral. It is polarized with one wider blade that only fits the neutral side of an outlet. Inside of what looks like a fat plug with two buttons is the same type of circuit for comparing and shutting off power in the event of a ground fault. The ALCI is designed to trip just like a GFCI when a fault of 4 to 6 milliamps is detected. It is most commonly found as the large plug on a hand-held hair appliance such as a hair dryer or curler. A true ALCI has two buttons, one for "test" and one for "reset". Instructions often suggest testing every month or so by pressing the test button. That simulates a ground fault. Pressing the test button should cause the ALCI to instantly shut off power to the appliance. The reset button does exactly what it says. It resets the ALCI so that power and ground fault protection are resumed.
The AC-DC radio.
In the 1930s, in order to reduce the manufacturing cost of radios, the AC-DC set was introduced. An AC-DC set could operate directly off the power line without needing a power transformer. The tube filaments, some with ballasts, are all in series adding up to power line voltage. These are also called "hot-chassis" sets because one side of the power line is connected to or switched directly to the chassis. Depending upon the orientation of the power plug, the chassis would be connected either to the line or to the neutral. For the typical set which switched the power line to the chassis by means of the on-off switch on the volume control, the chassis might be connected to the line or to the neutral. If the chassis happened to be switched to neutral when the set was turned on, it would be "hot" when turned off because the chassis would then be connected to the line by way of the cold tube filaments.
Millions of such sets were produced. The hazard from electrical shocks from such sets was well known in the 1930s and later. Since most of the sets were wood or plastic as were the knobs, the hazard was somewhat minimized unless it was easy to touch the chassis. Consumer Reports magazine of that era named sets as "unacceptable" if the chassis or chassis bolts protruding under the cabinet could be touched or come into contact with any metal surface. Later AC-DC sets used "floating ground" in which the direct power line connection to chassis was changed so that a resistor and capacitor were connected between that line and the chassis. That reduced the possible shock level.
These old AC-DC radios are now collectible. To those of us whose hobby is restoring old radios, use of an isolation transformer when servicing an AC-DC set is a requirement. But what about those who use the radios after we service them, including our children or grandchildren? They are not familiar with the hazards. Can we make those radios a bit safer? We could mount an isolation transformer inside the cabinet if there is room, but that is not possible for most sets. One answer to making the sets a bit safer is replacing the radio set power cord with an ALCI plug and cord taken from a hair appliance.
The most hazardous AC-DC radios are the early 1930s sets with an open back. Even after World War II, radios with the power line switched to the chassis were still being produced. As an example, the popular Setchell-Carlson model 427 shown here, often nicknamed the "Frog Eye" because of its design, is of 1947 vintage. It has a fully exposed chassis accessible from the bottom that has one side of the power line switched directly to that chassis. It and similar radios with a chassis that can be directly touched are crying for a safety improvement.
A second category of most dangerous is a hot-chassis AC-DC set with metal cabinet. Larger AC-DC sets that were originally intended for shipboard-use, such as the Scott SLRM, MacKay 128, the National NC-44 usually have enough room inside for a permanent isolation transformer. Smaller metal AC-DC sets usually don't have room for a transformer. Some examples of these sets in my collection include the Hallicrafters S-38 and S-41, the Echophone EC-1, and the Minerva Tropic Master. All of these switch the power line to chassis. Most have insulators between the chassis and the metal cabinet but that insulation may have deteriorated or broken down over time. In some cases, the flimsy cardboard back is broken or missing. Again, safety improvements are warranted.
Some AC-DC kit radios with the power line switched to chassis, such as some from Meissner, did not come with a cabinet, just a chassis. Again, rewiring and an ALCI plug and cord will reduce the hazard. Metal-panel AC-DC kit radios such as the Knight Ocean Hopper, Space Spanner, Lafayette KT-135 and similar sets, most of which have the safer floating grounds, are good candidates as well. Wireless broadcasters from Knight, Lafayette and others are also transformerless AC-DC designs and should be improved for safety. Three-way portables are also transformerless sets and can also benefit from an ALCI for improved safety.
Rewiring AC-DC sets
For those AC-DC radios that switch the power line directly to the chassis, I often rewire the sets to take advantage of the fact that ALCI plugs are polarized. I want the wide neutral blade to connect directly to the chassis or, in the case of sets using floating B- ,usually to the negative side of the electrolytic filter capacitors. That means the power switch is connected only to the line side (the narrow blade on the ALCI plug). The power switch is then rewired to feed the rectifier and the series filaments. If you are hesitant to rewire the power switch in the radio, don't let that stop you from replacing the power cord with one with an ALCI plug.
An ALCI is not a fuse.
An ALCI is a people protector, not a set protector. An ALCI only protects from fault currents, not from an overload on the usual line to neutral current. Many AC-DC sets are wired so that a part of the rectifier tube filament burns out in case of an overload. Adding an appropriate fuse in a well-insulated holder inside the radio on the line-side before the power switch will provide additional short-circuit protection.
Buying hair appliances with ALCI plugs
Some new hair appliances no longer come with ALCI plugs. I buy used hair dryers and curlers at thrift stores and resale shops, usually for less than $3 each. Be aware that a true ALCI must have both the test and reset buttons. A cheaper protector, called an "immersion detection circuit interrupter" (IDCI) has no buttons or only a reset button. It uses a sense wire in the appliance itself and only shuts off if it detects the appliance coming into direct contact with water. Avoid those as they are not true ALCI devices and will not provide proper ground fault protection for a radio.
How about tube-type record players?
Most of the little tube-type amplifiers in older record players are also transformerless designs. The safety of these will also be improved with an ALCI plug and cord upgrade.
Protecting unsuspecting future users of our nice but rather hazardous AC-DC radios can be done by simply modifying the sets with a bit of modern technology. If you have room for an onboard isolation transformer, then install one. If not, an ALCI plug and cord replacement will greatly improve the safety of an AC-DC "hot chassis" device.
Date:3-5-14
The Crystal calibrators from Bud and Heathkit were the previous items on the bench.