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

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PHOTO: FOTOLIA/PERRUSH
Readers with experience and knowledge about electricity safety argue a DC shock can indeed be very hazardous.

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