Feedback on C.D. Prewitt’s Observations About DC Electricity and Electrical Safety

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DC electricity can be hazardous too.

I got very disturbed by C.D. Prewitt’s article on electricity in MOTHER EARTH NEWS. Electricity can kill . . . even a 1 1/2 volt D cell if it’s applied just right. Shocks can cause death by fibrillation (a condition where the heartbeat gets out of step). Currents can kill either by causing lung seizures or by creating such violent muscle contractions that the person is thrown against something.

One of the original reasons behind the development of alternating current was its safety. If someone comes in contact with direct current under certain conditions, his muscles (which are controlled by electricity from the nerve fibers) will become locked into the circuit. On the other hand, medium-voltage AC gives the muscles a slightly better chance to jerk away. (One man made a mousetrap by running 1,000 volts AC through two concentric rings. A mouse trying to get the bait in the middle would be shocked, causing the animal to jump involuntarily across the room and die from fibrillation.) With DC, this “jerking” reaction becomes extreme. I was once thrown ten feet across a room and almost knocked out as I hit the wall by 100 volts of direct current. Luckily, the current was such that I didn’t freeze into the circuit.

Prewitt’s article says, “Putting a hand on 100 volts DC causes little sensation . . . .” He doesn’t mention that this is possible only when the hand is dry and even covered with powder to raise contact resistance. It could prove fatal, however, if one placed two wet or sweaty hands on the same amount of current . . . causing it to pass directly through the heart. All the power that’s needed is 200 milliamps which could be provided by 500 ohms or mildly sweaty palms. (Hints: Always keep one hand behind your back, wear rubber shoes, and avoid leaning against a metal surface when working with voltages over 20. When working with currents of under 20 volts, watch out for situations where you might puncture your skin . . . thus allowing the electrical flow to have a metal-to-bloodstream contact. Electrical shocks are warnings: heed them.)

When trying to decide what current to use, consider also that DC causes just as much radio interference as AC, and, depending on how it’s generated, AC can be quieter. Fuel cells are fairly quiet, but brush contact sound on DC generators is fierce . . . note the capacitors across automobile generators to keep the noise-level down. Low-quality AC inverters are apt to be noisy because of the square wave they generate. Alternators with slip rings can be very quiet, however. You can get TV’s and radios that work on DC, mostly in the nine to fourteen-volt range. But how do you get that voltage from 100 volts DC without designing a complex regulator? What’s more, there aren’t any decent record players for DC. The good ones use 60 cycles to regulate their speed. Unless you’re an electronics expert stick to 110 volts AC . . . except in the case where you want only electric lights or have access to an AC-DC motor (most sewing machines have them). Think safety . . . electricity can kill.

Peter Grant
Kingston, N.Y.

I want to point out a terrible piece of advice in MOTHER EARTH NEWS. The last statement comparing the danger of AC versus DC is not accurate.

If you make good contact, as with one wire in each hand, or with any moisture in the contact, 100 volts of DC will grab and cook you . . . like it or not! The steady flow of direct current contracts muscles — including the heart — and holds them that way. The effect of not being able to let go is deadly.

With 100 volts of AC the same is true, but AC is constantly swinging from 0 to 140 volts and back, then to minus-140 and back. The average effective power is still 100, but the 140-volt peaks and the buzzing result of the voltage change 60 times per second allow you to feel it sooner. More important, the “grabbing” effect on the muscles is greatly reduced by the pulsing rather than the steady flow of electricity.

I feel obligated to point out the greater danger of DC since I recommended using it in my article (“A Practical Self-Contained Power System For Bus Or Cabin”,  MOTHER EARTH NEWS) . . . and since it almost got me a few times.

Stephen F. Willey
Cotati, California