Feedback on C.D. Prewitt: the Electricity Safety Debate Continues

C.D. Prewitt's past assertions about the superiority of DC in terms of electricity safety draw fire from two readers, who argue AC and DC are both dangerous.
By Grover Mull and David A. King
September/October 1973
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Readers with experience and knowledge about electricity safety argue a DC shock can indeed be very hazardous.
PHOTO: FOTOLIA/PERRUSH


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I just had to answer C.D. Prewitt's remarks on electricity safety in MOTHER EARTH NEWS. My major gripe is that some people may assume that because he's done some foolish things unharmed, they may do the same and survive.

The fact is that each person's bodily resistance to electricity is different, and even that varies from time to time. As a rather crude test, we duplicated Mr. Prewitt's experiment on thirty people working in our plant's assembly area. Our instrument was an ohmmeter with an internal 1.5V battery, and the resistance was measured by having a person hold one of the device's probes in each hand and squeeze the tips between thumb and forefinger. Here are the results, expressed as a chart of bodily reactions to various current levels.

  Resistance       Current@ 115V

320,000 ohms       .37 milliamperes
25OK               .46
120K               .96
ll5K              1.00
ll0K              1.05
8OK               1.45
75K               1.53 Threshold of sensation
70K               1.65
55K               2.10
50K               2.30
48K               2.40
46K               2.50
44K               2.62
42K               2.75
40K               2.88
38K               3.03
30K               3.84
27K               4.26
25K               4.60
20K               5.75 Mild sensation
10K              11.50 Pain
  6K              19.20 Muscular paralysis
 
The bodies of 30 individuals averaged 67K ohms of resistance to electricity

As you can see, the hand-to-hand resistance varied over a surprisingly wide range among our thirty subjects. We made the check because some of the assemblers were getting mild shocks from our heat welders (15V). We had thought it was impossible, but the test shows that—on a hot, humid day—five or six of these people might indeed feel discomfort.

This is a good reminder, I think, that low voltages may not be as safe as we often believe, and that 115 volts becomes very dangerous for some people even under the best conditions.

I believe that any shock is potentially hazardous and that all electrical circuits should be treated with great care. What one gets away with in practice is quite misleading. An entirely safe situation can change in a second to an extremely hazardous circumstance ... for example, if one merely brushes against a good ground such as a water pipe.

I hope Mr. Prewitt's thoughts haven't killed anyone.

Grover Mull
Beaverton, Oregon

I'd like to offer some comments on C.D. Prewitt's article in MOTHER EARTH NEWS ... but first of all, to get the matter of practical experience vs. theory out of the way, I should mention that I'm retired after having spent most of my life in electricity and electronics. That's something like 46 years working with power. If you add the school years spent merely in experimenting with it, make that about 50 years. In addition, I have had many years of college-level training in this field. Thus I think I can offer a pretty good combination of practical experience and theory.

Now to Mr. Prewitt's remarks: First and most important are his comments on electrical shock, if only because such ideas pose a major threat to anyone who takes them seriously. Electricity is dangerous, and it is not imagination or fright which causes the hazard! Shock, as Volta discovered something like 200 years ago, will cause a violent muscular contraction in a living body (or even in one newly dead, as with Volta's frog legs). If this powerful spasm happens in the heart muscle, it can throw the organ into ventricular fibrillation ... precisely the condition brought about by a coronary thrombosis. If someone who knows how to deal with this emergency is not at hand, the accident is quickly fatal.

Passing a current from hand to hand is an excellent way to send a goodly portion of that power through the heart, since major blood vessels run from the arms to that organ. Thus a hand-to-hand contact is the most commonly lethal of all electrical shocks. Only the head-to-foot (as used in execution in the electric chair) is more dangerous, because in that case the current passes through the brain. A person working with electricity is much more likely to get a hand-to-hand shock than any other variety. (A current passing from finger to finger is not particularly dangerous for the simple reason that the heart or brain is not included in its circuit.)

I'd like to shed some light on Mr. Prewitt's experiences with batteries which led him to the conclusion that electricity is less dangerous than is usually thought. You can get a "shock" of several thousand volts from an induction coil such as that used on an automobile, and that jolt will not kill you ... for the very simple reason that there is no appreciable amount of current available from the source. It is current that kills. Connect a human body to a high-tension line carrying as much voltage as an induction coil yields, and that body will be burned to a crisp in short order. My father saw such a happening when the line was carrying only 500 volts DC. DC, mark you, not AC ... either kind of current can kill if it passes through the right part of the body.

Mr. Prewitt's chain of dry cells would not yield a current to compare to that from a powerline for the simple reason that the internal resistance of the batteries is much higher than the impedance of the transformer feeding a household load. To put it differently, the voltage of a chain of dry cells drops appreciably when a load is connected. That of a powerline— adequately wired —does not. You can draw up to several hundred amperes out of a powerline for a few seconds. Mr. Prewitt's dry cells, on the other hand, would yield perhaps 35 amperes or so on short-circuit. Had he been using a chain of lead-acid storage cells, he would have had quite a different experience ... since those devices yield a current of several hundred amperes on short-circuit.

About the relative safety of AC and DC: The reason a person feels more of a shock from alternating than from direct current is very simple, and is twofold. First, you get two shocks whenever you connect to a DC source: one when you connect, a second when you disconnect or when the power is discontinued. Alternating current (AC) is, effectively, connected and disconnected twice for each alternation ... four times for each cycle. Thus a 60 Hz current such as that from most power mains in the USA will deliver 240 shocks for each second you are in contact with the line.

Another factor in electrical shock is the peak voltage and current involved. Although the values commonly given for alternating current are the RMS (root mean square) or effective values, what you feel is the peak value: A so-called 110-volt AC line will deliver 155.551 volts rather than the 110 you might expect. This figure is the RMS value times the square root of 2 (about 1.4141). Multiply that factor by the RMS voltage or current being delivered by an AC line, and you have what you'll feel as a shock if you connect to that line.

Now to Mr. Prewitt's "practical" advice on the use of DC in preference to AC. Like most such suggestions, his counsel is "iffy." If you want to use standard appliances such as washing machines and dryers as well as thermostatically controlled heating appliances such as waffle irons, smoothing irons, percolators, etc., your best choice is AC for the simple reason that motors are more economically made to operate on alternating current. Thermostatically controlled heating appliances don't work well on DC because every time the circuit is broken direct current pulls a wicked arc, which quickly burns out the rather delicate points used in a thermostatic switch.

If, on the other hand, you are going to use only a light load such as incandescent lamps—and if the power is to come from a private plant—then DC is the logical choice if only because a bank of storage batteries can be charged from a generator of some kind and used as a reservoir to carry the load most of the time. For example, it's practical to charge storage cells from wind-driven generators and thus have some electricity for the cost of upkeep only and essentially without contamination of any kind. A more practical source for most locations, however, is a gasoline-driven generator which can be started up whenever the batteries need a boost.

If you produce your own power and want to use a full line of household appliances which will need a substantial part of your generator's capacity, an alternating current powerplant is the better choice because it eliminates the added expense of buying DC motors for your appliances. Incidentally, some household items use "universal" brush-type motors which will operate on either AC or DC ... provided that the DC voltage is equal to the RMS or effective value of the AC voltage the mechanism is designed for. Often a universal motor will run a little faster on DC than it will on AC. If this matters, perhaps you will want to provide some means of controlling the voltage to the device.

Thus Mr. Prewitt's advice to get a DC power source if at all possible is not necessarily sound. As I said before, this is an "iffy" question. Each person must apply common sense to his situation and then make the decision that best fills his needs. I urge my readers to seek expert advice before selecting a powerplant for home use. Which is the better type of current depends on the possible sources and the uses to which it will be put. No one answer will fit all circumstances.

Let me again remind Mr. Prewitt that I have had upward of fifty years' experience with electricity ... experience backed with the theoretical training of an electronic engineer. I have spent most of my life working with electricity and I can say with all earnestness that I am not afraid of it. Still, like fire, electricity is an excellent servant but a mighty poor master. I respect both fire and electricity very deeply and would advise others to do likewise.

I would like to suggest to Mr. Prewitt that he buy himself some textbooks and back his practical experience with the theory required to make him an authority on the subject.

David A. King
Layton, Utah


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