T
IPS OF THE
T
RADE
C opperclad ground rods cost too much.
Changing to galvanized steel can take up to 58 percent out of ground rod cost.
Based on 1995 average prices, a 5 /
8
-inch 8foot-long copperclad ground rod cost
$6.25. Galvanized cost for the same size was $3.94 for a savings of $2.31 per rod.
Calculate the savings over a utility’s annual ground rod usage and the savings are substantial.
There is little difference in electrical resistance between galvanized and copperclad rods; and, in any event, the resistance of the ground rod is negligible when compared to the resistance of the earth itself. Also, use of copperclad rods requires special care to prevent galvanic action and rapid corrosion of anchor rods, anchors, piles, and associated hardware.
These are the major conclusions which may be drawn from a recent comparison study of galvanized-steel ground rods and copperclad-steel ground rods.
Resistance Study
Observation of a ground rod system shows that its resistance to the earth may be considered to have three components.
1. The resistance of the ground rod itself
Ground rods are adequate in cross-section so their resistance is a negligible part of the total resistance of the ground rod system. The resistance of a 5 /
8
-inch mild-steel ground rod 8-foot-long is .00196 ohms, as calculated from the resistivity of mild steel. 1
1

2
2. The interface resistance between ground rod and earth
Study by the National Bureau of Standards has shown that for a ground rod free of grease or paint, the contact resistance between the ground rod and earth is negligible.
2
3. Resistance of earth around ground rod
The resistance of the earth around a single ground rod is commonly calculated from the following equation developed by H. B.
Dwight.
3
R =
ρ
2
π
L
[ ln (
4 L a
) -1 ]
Where
ρ
= Average soil resistivity - (ohms-cm)
L = Ground Rod Length - (cm) a = Ground Rod Radius - (cm)
R = Resistance of Ground Rod to Earth - (ohms)
Figure 1 shows the earth resistance for a 5 /
8
inch-diameter ground rod (8-foot and 16-foot lengths) in soils of various resistivities as calculated by the earth resistance formula. Note that the resistances are high as compared to the resistance of the ground rod itself. In fact, the earth resistance is 600 to more than 10,000 times the resistance of the ground rod itself.
Corrosion of Ground Rods
The earth’s resistance is by far the major component of resistance in a grounding system. So, from an electrical standpoint, galvanized and copperclad rods are nearly identical. However, an installed ground rod should resist corrosion well enough to provide a sufficient metallic cross section throughout its anticipated life.
The National Association of Corrosion Engineers conducted a seven-year study of grounding materials at 20 test sites across the nation.
(4) The ground rods were examined at 1, 3, and 7 year intervals. The study was conducted both on individual ground rods and on various materials coupled to each other. The data on single copperclad rods, single galvanized rods, single
C
ONTINUED
,
NEXT PAGE
➥

Corrosion Weight Loss — (Grams Per Year)
A Copperclad Ground Rod
B Galvanized Steel Ground Rod
Average corrosion weight loss for 7-year period at 20 sites across the nation.
Data from N.A.C.E.
driven ground rod 1-
3-7 year test program
C Bare Steel Ground Rod
D
Galvanized Ground Rod Coupled to Copperclad Ground Rod
0 10 20 30 40 50 60
Figure 2 bare-steel rods, and galvanized rods bonded to copperclad rods are shown in Figure 2, which compares ground rod weight loss per year for the various materials.
Figure 2 shows, as expected, that the copperclad ground rod (A) has better corrosion resistance than the galvanized (B) or bare-steel ground rods (C). The graph also shows that when a galvanized rod is coupled to a copperclad rod (D), as would be the case with a copperclad ground rod and a galvanized anchor rod in a bonded system, a galvanized rod (D) corrodes at a rate of 10 times faster than would be the case if a galvanized ground rod (B) were used. This is one reason Rural Utility
Services (RUS formerly REA) suggests the use of galvanized steel for grounding. To quote RUS:
“Galvanized steel materials are suggested for grounding of all distribution lines, above ground and underground. This is a continuation of practices that have been recommended by RUS since the early
1960s for pole lines and, more recently, for underground construction.” (5)
The average corrosion rate of 5.6 grams per year of the galvanized ground rod shown in
Figure 2 can be shown to represent approximately 15 years of service before the galvanizing is gone. This assumes average coating of 2.26 oz/ft 2 of galvanizing.
Chance production SPC records on 5 /
8
inch ground rods show an average coating of 2.26 oz/ft 2 .
When the galvanizing is gone, a bare steel rod corrodes at an average rate of 15 grams per year as shown in Figure 2. Assuming this rate for the balance of nominal 30-year service life, or 15 years (30-15=15), the diameter of the ground rod will be reduced by approximately 0.019 inch.
C
ONTINUED
,
NEXT PAGE
➥
3

4
Of course, this diameter decrease will cause the ground rod resistance to increase; but as seen below, the increase is negligible.
Resistance of 5 /
8
-inch ground rod reduced .019" in dia.
= 0.002088 ohms.
Original resistance 5 /
8
" ground rod
Resistance increase
= 0.001962 ohms.
= 0.000126 ohms.
This corresponds to a resistance increase of 6.4 percent. The resistance increase is obviously quite small when compared to the resistance of the earth as shown in Figure
1 (previous page) (0.01 percent to 0.0005 percent of the resistance of the earth).
How to Install Ground Rods
Ground rods should be installed near the ground wire and the base of the pole. The rod must be driven to its full length into the ground and, after driven in place must have a resistance less than 25 ohms (National Electric Safety Code Rule 96.8). If the resistance is greater than 25 ohms, an additional ground rod must be used in parallel.
The ground wire must be wrapped around the mounting hardware bolt and the mounting hardware tightened firmly to assure a continuing bond.
By using Chance galvanized ground rods, you can save money at no sacrifice of performance. Chance has a complete family of rods and the necessary clamps. Talk to your representative and cut your costs. ■
References:
(1) Resistivity of mild steel from page 15-6 of Marks Handbook 8th Edition.
(2) “Grounding Connections for Electrical Systems”, O.S. Peters, U.S. Dept. of Commerce, National Bureau of Standards, Technological Paper 108, June 1918.
(3) “Calculation of Resistance to Ground”, H.B. Dwight, Electrical Engineering, Vol. 55, December 1936.
(4) “Driven Ground Rod 1-3-7 Year Test Program”, Final Report, Draft No. 2 September 1971, National
Association of Corrosion Engineers.
(5) Manual on “Underground Corrosion Control in Rural Electric Systems”, REA Bulletin 161-23, October
1977, Part IV, page 18.

®
UNITED STATES
HUBBELL POWER SYSTEMS, INC.
210 N. Allen
Centralia, Mo 65240
Phone: 573-682-5521
Fax: 573-682-8714 e-mail: vjengle@hps.hubbell.com
ASIA
HUBBELL S.E. ASIA PTE. LTD.
159 Sin Ming Road
#05-08 Amtech Building
Singapore 575625
Phone: 65-4544772
Fax: 65-4544775 e-mail: sajeev@hubbell.com.sg
CANADA
HUBBELL CANADA, INC.
870 Brock Road South
Pickering, Ontario L1W 1Z8
Phone: 905-839-1138
Fax: 905-831-6353 e-mail: jpearl@hubbell-canada.com
EUROPE
HUBBELL POWER SYSTEMS
Ronald Close
Woburn Road Industrial Estate
Kempston, Bedford
MK42 7SH, England
Phone: 44-1-234-843632
Fax: 44-1-234-841435 e-mail: jhopwood1@aol.com
MEXICO
HUBBELL DE MEXICO, S.A. DE. CV
Av. Coyoacan No. 1051
Col. Del Valle
03100 Mexico, D.F.
Phone: 525-575-2022
Fax: 525-559-8626 e-mail: vtasdf@hubbell.com.mx
TAIWAN
HUBBELL TAIAN CO., LTD.
12th Floor 66 Sec. 2
Chien Kwo North Road
Taipei, Taiwan
Phone 02-515-0855
Fax: 02-515-0898
Fax: 02-515-0899 e-mail: skao@hubbell-taian.com.tw
® ® ® ®
®
573-682-5521 Fax 573-682-8714 http://www.hubbellpowersystems.com
View from Vol 3 No 2
April 1997
NOTE: Because Hubbell has a policy of continuous product improvement, we reserve the right to change design and specifications without notice.
© Copyright 2000 Hubbell • 210 North Allen Street • Centralia, MO 65240 05-2002WB
5