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.

| September/October 1973

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.

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