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Down-to-Earth Discussion - Resistance of a Ground
October 1963 Electronics World

October 1963 Electronics World

October 1963 Electronics World Cover - RF Cafe  Table of Contents 

People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Electronics World was published from May 1959 through December 1971. See all Electronics World articles.

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 Silver in QST 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?

John T. Frye (Electronics WOrld image) - RF Cafe

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 arrester grounds."

"Why?"

"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 low?"

"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 resistance?"

"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 to measure."

"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 about?"

"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 the ohmmeter."

"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 its

present value.

"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