October 1963 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
For some reason the subject of grounding has been very prominent
in my reading in the last few days. The chapter I just finished
reading in one of
David Herres' books on the
National Electric Code (NEC) covering grounding of
commercial and residential services, an article by H. Ward
titled, "Grounding and Bonding Systems," and now this article
by John T. Frye (of
Carl and Jerry fame) on grounding,
makes for a wealth of knowledge. Mr. Frye takes a unique approach
at teaching by exploiting his gift for story-telling. In this
article, electronics repair shop owner Mac gives technician
Barney a nice bit of tutelage on what constitutes a good Earth
ground and what does not. In some environments, treating the
soil with an electrically conductive substance is necessary
to establish a suitable ground without having to drive an unreasonable
number of ground rods.
Down-to-Earth Discussion - Resistance of a Ground
What factors affect the resistance of a ground? How is such
resistance measured and a low-resistance ground obtained?
Barney, a little late to work this bright October morning,
went bustling into the service department only to discover his
employer was not there. His relief was short-lived, though,
for Mac came backing through the rear door paying out a couple
of heavy insulated wires in front of him.
"There you are!" Mac exclaimed, glancing over his shoulder.
"Let's see now: we can't say the alarm didn't go off, the car
wouldn't start, or a train across the track held you up, can
we? We've already used those."
"Aw get off my back, will you?" Barney pleaded. "Can I help
it if our dog got sick in the night and I had to drop him off
at the vet's? Where do those wires go? What are you going to
do with them?"
"Allowing you to change the subject, they go to a couple
of rods driven into the earth out back, and I'm going to use
them to measure the resistance of our service bench and lightning
"Because our personal safety and the safety of our equipment
depends in a large measure on having low-resistance grounds."
"They have low resistance all right," Barney assured him.
"The wires going to them take care of that."
"I'm afraid not. While actually the resistance of a ground
is made up of the resistance of the lead, the resistance of
the rod, the resistance of the rod-to-earth contact, and the
resistance of the earth surrounding the rod, the resistance
of the first three is insignificant when compared to the fourth,
which is ordinarily so much higher."
"You mean the contact resistance between rod and earth is
"Right. Bureau of Standards tests show that if the rod is
free of paint or grease and the earth is packed close around
it, contact resistance is negligible. Now to understand earth
resistance, picture the ground rod as surrounded by successive
shells of uniform-resistance earth of equal thickness. The first
shell, the one nearest the rod, will have the smallest cross-section
of soil at right angles to the current flowing out from the
rod; so it will have the most resistance. The next shell with
a larger cross-section will have less resistance. As we keep
adding shells farther and farther from the rod, the cross-section
of each shell increases and its resistance goes down' until
we finally reach a point where the addition of more shells adds
next to nothing to the resistance of our ground.
"How far from the ground rod is that point?"
"Ninety percent of the total electrical resistance is generally
within a radius of six to ten feet from the rod."
"I suppose the kind of soil has a lot to do with the resistance."
"It does. The Bureau of Standards found the least resistance
in soil made up of fills containing more or less refuse such
as ashes, cinders, and brine waste. An average ground in this
material tested 14 ohms. Clay, shale, adobe, gumbo, loam, and
slightly sandy loam came next with an average ground resistance
of 24 ohms. Mixing this same soil with varying amounts of sand,
gravel, and stones shot the resistance up to 93 ohms. Finally,
when only sand, gravel, or stones with little or no clay, or
loam constituted the earth, the resistance rose to 554 ohms."
"Guess if we want a really good earth ground we should set
up in the middle of the city dump," Barney observed. "Does the
dampness of the earth affect the resistance?"
"Yes. When the moisture content of the soil falls below 20%,
the resistance goes up rapidly. For example, a given sample
of soil with 10% moisture has a resistance of about 350,000
ohms per cm.3 Increasing moisture to 20% brings this
down to 10,000 ohms per cm.3 and increasing it to
35% cuts this to 5000 ohms per cm.3" Moisture content
of the soil varies from about 10% in dry seasons to around 35%
in wet seasons, averaging out at around 16 to 18 percent. That's
why the resistance of a driven ground will often more than double
from a wet spring to a dry fall."
"How about temperature? Does it affect the resistance?" "I'll
say; especially when the ground freezes. The resistance of a
soil sample with a stable moisture content rose from 200 ohms
per cm.3 to 1500 ohms per cm.3 as the
temperature fell from 70° F. to 35° F.; then it really took
off. At 20° the resistance was up to 6000 ohms per cm.3
and at zero it was more than 40,000 ohms per cm.3
Where the ground freezes, it's especially important the ground
rod be long enough to reach below the frost line. In fact, the
ground rod should be long enough to reach down to the permanent
moisture level of the soil anyway. The top soil has the most
resistivity and is subject to wide variations in resistance
with changing seasons. The greatest reduction in resistance
is ordinarily encountered in going down the first six feet,
but the eight-foot rod is the most popular. In most - though
not all - cases, this length of rod will reach permanent moisture."
"Does the size of the rod have anything to do with the ground
"Not a whole lot. A comparison between 1/2-inch and 1-inch
rods driven into the earth reveals the latter, with twice the
diameter and four times the area, decreases the resistance only
about 10%. In general, the rod need only be large enough and
strong enough to withstand driving without bending."
"Where you getting all this dope on grounds? You got awful
smart all at once."
"I've been reading a booklet called 'Practical Grounding'
published by the Copperweld® Steel Company, Wire and Cable
Division, Glassport, Pa. They send this free for the asking.
Also I've been studying 'A Manual on Ground Resistance Testing'
published by the James C. Biddle Co. of Philadelphia and intended
for users of the Megger® ground testers manufactured by
that company. Thanks to these two authorities, I feel well-grounded
on the subject."
"Oh brother! Let's get on with the testing," Barney suggested,
making a wry face at the pun. "How come you need two more grounds
to test the one here in the shop?
Why don't you just measure the resistance between our ground
and a water pipe?"
"Because a water pipe ground has resistance, too; and when
you measure the resistance between two grounds you simply get
the series resistance of both grounds and don't know how much
of the total resistance belongs to the ground you're trying
"So how are you going to get around this?"
"I'll show you. Write down on the blackboard measurements
taken between pairs of grounds as I make them with the v.o.m.
Call our bench ground 'A' and those two outside grounds 'B'
and 'C.' Here we go:
"Notice I took two readings of each resistance, reversing
the probes and averaging the readings to nullify the effect
of the stray d.c. voltage. We see the resistance of A + the
resistance of B = 80 ohms. A + C = 85 ohms. Adding these two
equations together, we get: 2A + B + C = 80 + 85 or 165 ohms.
From that let's subtract the equation: B + C = 95 ohms. That
leaves: 2A = 70 ohms, or A = 35 ohms. We've 'used' the other
two grounds to get at the resistance of A and then made them
cancel themselves out! For good accuracy, the resistance of
the auxiliary grounds should approximate that of the one being
measured and they should be at least 20 feet from that ground
and from each other in order to prevent overlapping of their
'effective resistance areas.' "
"Hey, that's neat! I see, though, the presence of that stray
d.c. voltage kind of messes things up."
"You're right, and it and the stray a.c. voltage are almost
always found in some degree between two rods driven into the
earth. We can get away from the d.c. by using a.c. and computing
the resistance. We simply use an a.c. ammeter to measure the
amount of current a given amount of a.c. voltage sends through
a pair of rods. The resistance is equal to E/I. Or we can use
a Wheatstone bridge operating on an alternating current of say
1000 cycles and balance the bridge with a pair of headphones.
This last method would get away from any errors introduced by
stray 60-cycle a.c. between our rods . In either case, we would
do the computation exactly as we did when we measured resistance
with the v.o.m."
"You spoke of a 'Megger' instrument designed to measure ground
resistance. Does it use one of the methods we've just been talking
"No, it uses still another 'fall-of-potential' method in
which an auxiliary ground rod is driven some distance away from
the ground to be measured and another rod is driven about half
way between the two grounds. An a.c. current is fed through
an ammeter to the ground being measured and the farthest test
ground. Voltage appearing between the ground being measured
and the mid-point ground is read with a high resistance a.c.
voltmeter. The resistance wanted will equal the measured voltage
divided by the measured current.
"The 'Megger' uses this basic method to give a direct reading
of the ground resistance. It consists essentially of a hand-cranked
d.c. generator whose output flows through the current coil of
an ohmmeter and then goes to a current reverser that changes
it into a.c, to be applied to the farthest-apart grounds. The
a.c. voltage appearing between the center ground and the ground
being measured is fed back through a potential commutator that
restores it to d.c. for application to the potential coil of
"Hold it!" Barney interrupted. That makes two ohmmeter coils."
"There are two coils. This ohmmeter is like none you ever
saw. A low-resistance current coil and a high-resistance potential
coil are mounted on the same shaft that moves the pointer and
they work in opposition in the field of a permanent magnet.
No hair-springs keep the pointer at one place. It assumes a
position dictated by the ratio of the current through the current
coil and the voltage applied to the potential coil. The ohms
scale is much more nearly linear than that of our v.o.m. The
current reverser and the potential commutator are mounted on
the same shaft as the generator armature and so are synchronized
for all hand-cranked speeds. Changing the voltage and frequency
of the output of the instrument by turning the crank at different
speeds has no effect at all on the resistance reading.
"To use the instrument, you only have to run leads from three
binding posts to the proper grounds. One test ground should
be about 50' from the ground being tested, and the other should
be at 100'. These auxiliary ground rods need only be driven
2' or 3' deep. You turn the crank and read the resistance of
the ground directly on the meter. If stray a.c. makes the reading
erratic, you simply turn the crank faster or slower to shift
the test frequency away from the 60-cycle stray current."
"If a fellow was going to do a lot of ground testing or needed
high accuracy, that would be the ticket," Barney observed; "but
these other computational methods will work fine for us. How
low a resistance do you need, and how do you go about lowering
the resistance of a ground that is too high?"
"Electrical codes require the resistance of a driven electrode
shall not exceed 25 ohms, but the lower the better. Ours, as
you can see, is not low enough after the prolonged drought.
I think I'll first try going deeper with 'Copperweld' Sectional
Rods that are threaded on both ends so one can be driven full
length into the earth, another screwed on the top with a special
coupler, that driven full length, an so on. Low-resistance soil
is often encountered 20' to 40' below the surface. In a typical
test, a ground that measured 270 ohms at 8' measured only 10
ohms at 40'.
"Another possibility would be to drive several other 8' rods
and connect them to our present ground. If these new grounds
are kept at least 5' from our present ground and from each other,
three more rods should cut our ground resistance to about one-third
"Or we could chemically treat the ground around our present
rod to lower its resistance. This should be done by digging
a foot-wide-foot-deep circular trench out about a foot and a
half from the rod and filling it with magnesium sulphate, copper
sulphate, or ordinary rock salt. This works best where ground
resistance is quite high. The improvement fades away with time
unless the treatment is renewed every few years."
"A ground always seemed such a simple thing to me," Barney
said with a sigh. "You just drove a rod into the earth and that
was it. Now it seems terribly complicated."
"There is no such thing as a simple subject," Mac parodied;
"there are just uninformed people!"
Posted March 11, 2015