The earth resistance of an electrode is dependent on several factors, including soil resistance, contact resistance of the electrode to the earth, and resistance of the rod(s), couplers, and connections.

An effective, deep-driven grounding installation includes the following considerations:

Installing ground rods beyond 10 feet deep presents several problems. Sectional rods must be used (typically 10-12 feet long) and coupled together to achieve the desired depth. The coupler is a larger diameter than the rod and therefore forms a hole bigger than the rod itself (Photo 2). This creates a coupler void limiting soil contact to the rod surface of the additional sections. Only the first section will maintain full rod to soil contact.

Manual driving of the rods with sledge hammers, pipe drivers arid other means cannot provide adequate force to penetrate hard soils. Mechanical or powered drivers are necessary for deep driven rods. The rod material and coupler design must be able to withstand the force necessary to drive through hard subsoil.

The first rods installed in 1988 were done by climbing a ladder and holding an electric hammer on top of the rod. This procedure was both awkward and dangerous to the installer. A driving machine was then constructed to better facilitate this part of the process. Photo 3 shows a picture of the machine. This machine consists of a support frame with leveling jacks and wheels. A vertical assembly holds an electric impact hammer and can be manually cranked up and down by the operator. The electric hammer is equipped with a special driving tool that prevents “mushrooming” of the rod and actually reforms the rod end.

Due to the extreme forces required to penetrate hard soils, it was found that screw type couplers were mechanically failing. The threads were being stripped causing poor rod-to-rod contact. A new type of tapered spline coupler was found to be the most reliable coupler used. A test rod was driven and then pulled to check the mechanical durability of the coupler. Photo 4 shows a cut-open section of the test rod. This drive-on coupler design simplified the process by making it possible to use smooth rods of any length. This allowed deep-driven systems to be installed inside buildings with minimal ceiling heights (as in Case Study 3).

To maintain full rod-to-soil contact, a slurry mix of sodium bentonite (a naturally occurring clay) is injected into the coupler void as the rods are installed. This provides a conductive material between the rod surface and soil over the depth of the rod. A typical 60-foot ground rod requires 2 to 5 gallons of bentonite. A test was done to determine the resistance effect of the bentonite in the coupler void. Figure 5 shows a comparison graph of three ground rod installations without the bentonite. Note how the dry rods showed a fluctuating resistance as compared to the graph in Figure 1.

CONCLUSIONS

As shown by the data presented, the average 8 to 10-foot ground rod will not meet minimum NEC code requirements for earth resistance. The resistance of a shallow (10 feet or less) electrode will vary greatly as seasonal conditions change. Due to high earth resistance, the typical shallow electrode is unable to maintain an electrical system at earth potential during transient voltage conditions and lightning surges. Where stable resistance values of less than 5 Ohms are required, electrode depths of 30-60 feet are necessary.

REFERENCES

1. The IAEI Soares Book on Grounding, 4th Edition, page 128.

2. ANSI/NFPA 70-1991, National Electric Code, Article 250.

3. ANSI/IEEE Green Book, Std 142-1982.

4. NEC Article 250-81, (b), (FPN).

5. NFPA 78, Appendix 1.

6. ANSI/IEEE Std 142-1982, Green Book, Section 4.1 Table 5.

7. NEC Article 250-84.

	DEEP EARTH GROUNDING VERSUS SHALLOW EARTH GROUNDING Page 4 of 4
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	INSTALLATION METHOD The earth resistance of an electrode is dependent on several factors, including soil resistance, contact resistance of the electrode to the earth, and resistance of the rod(s), couplers, and connections. An effective, deep-driven grounding installation includes the following considerations: Selection of rod material Selection of coupler type Diameter and length of rod(s) Type of driving equipment Installation procedures Testing procedures Wire termination Installing ground rods beyond 10 feet deep presents several problems. Sectional rods must be used (typically 10-12 feet long) and coupled together to achieve the desired depth. The coupler is a larger diameter than the rod and therefore forms a hole bigger than the rod itself (Photo 2). This creates a coupler void limiting soil contact to the rod surface of the additional sections. Only the first section will maintain full rod to soil contact. Photo 2. Ground rod and coupler. Manual driving of the rods with sledge hammers, pipe drivers arid other means cannot provide adequate force to penetrate hard soils. Mechanical or powered drivers are necessary for deep driven rods. The rod material and coupler design must be able to withstand the force necessary to drive through hard subsoil. The first rods installed in 1988 were done by climbing a ladder and holding an electric hammer on top of the rod. This procedure was both awkward and dangerous to the installer. A driving machine was then constructed to better facilitate this part of the process. Photo 3 shows a picture of the machine. This machine consists of a support frame with leveling jacks and wheels. A vertical assembly holds an electric impact hammer and can be manually cranked up and down by the operator. The electric hammer is equipped with a special driving tool that prevents “mushrooming” of the rod and actually reforms the rod end. Photo 3. Ground rod driving machine. Due to the extreme forces required to penetrate hard soils, it was found that screw type couplers were mechanically failing. The threads were being stripped causing poor rod-to-rod contact. A new type of tapered spline coupler was found to be the most reliable coupler used. A test rod was driven and then pulled to check the mechanical durability of the coupler. Photo 4 shows a cut-open section of the test rod. This drive-on coupler design simplified the process by making it possible to use smooth rods of any length. This allowed deep-driven systems to be installed inside buildings with minimal ceiling heights (as in Case Study 3). Photo 4. Drive-on spline rod coupler. To maintain full rod-to-soil contact, a slurry mix of sodium bentonite (a naturally occurring clay) is injected into the coupler void as the rods are installed. This provides a conductive material between the rod surface and soil over the depth of the rod. A typical 60-foot ground rod requires 2 to 5 gallons of bentonite. A test was done to determine the resistance effect of the bentonite in the coupler void. Figure 5 shows a comparison graph of three ground rod installations without the bentonite. Note how the dry rods showed a fluctuating resistance as compared to the graph in Figure 1. Figure 5. Dry rod resistance graph. CONCLUSIONS As shown by the data presented, the average 8 to 10-foot ground rod will not meet minimum NEC code requirements for earth resistance. The resistance of a shallow (10 feet or less) electrode will vary greatly as seasonal conditions change. Due to high earth resistance, the typical shallow electrode is unable to maintain an electrical system at earth potential during transient voltage conditions and lightning surges. Where stable resistance values of less than 5 Ohms are required, electrode depths of 30-60 feet are necessary. The case studies have shown that installing deep driven electrodes is effective and practical for both new and existing facilities. The new method of installing deep-driven ground rods provides a universal means of effective earth grounding. REFERENCES 1. The IAEI Soares Book on Grounding, 4th Edition, page 128. 2. ANSI/NFPA 70-1991, National Electric Code, Article 250. 3. ANSI/IEEE Green Book, Std 142-1982. 4. NEC Article 250-81, (b), (FPN). 5. NFPA 78, Appendix 1. 6. ANSI/IEEE Std 142-1982, Green Book, Section 4.1 Table 5. 7. NEC Article 250-84.
	Previous	Contents

INSTALLATION METHOD

CONCLUSIONS

REFERENCES