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STANDARDS AND GUIDELINES FOR COMMUNICATION SITES GROUNDING (EARTHING) ELECTRODE SYSTEM COMPONENT AND INSTALLATION
REQUIREMENTS
4.4.1 GROUNDING (EARTHING) ELECTRODES
Grounding (earthing) electrodes are the conducting elements used to connect electrical systems and/or
equipment to the earth. The grounding electrodes are placed into the earth to maintain electrical
equipment at the potential of the earth. Grounding electrodes may be ground rods, metal plates, concrete
encased electrodes, ground rings, electrolytic ground rods, the metal frame of building or structure, and
metal underground water pipes (NFPA 70-2005, Article 250 (III)).
NOTE: Metallic underground gas piping shall not be used as a grounding electrode (NFPA 70-2005, Article
250.52), but shall be bonded upstream from the equipment shutoff valve to the grounding electrode
system as required by NFPA 70-2005, Article 250.104 and NFPA 780-2004, section 4.14.1.3.
4.4.1.1 GROUNDING (EARTHING) ELECTRODE RESISTANCE CHARACTERISTICS AND
SPHERE OF INFLUENCE
Around a grounding (earthing) electrode, such as a driven ground rod, the resistance of the soil is the
sum of the series resistances of virtual concentric shells of earth, located progressively outward from the
rod. The shell nearest the ground rod has the smallest circumferential area, or cross section, so it has the
highest resistance. Successive outward shells have progressively larger areas, therefore, progressively
lower resistances. (IEEE STD 142-1991, section 4.11 and MIL-HDBK-419A).
2X Rod Length
Rod Length
Concentric Shells
FIGURE 4-5 GROUNDING ELECTRODE SPHERE OF INFLUENCE
The effect of the concentric shells is that it takes a finite amount of earth for a ground rod to fully realize
its resistance value. This finite amount of earth is commonly known as the ground rod's sphere of
influence. The sphere of influence for a ground rod is commonly thought of to be a radius around the
ground rod equal to its length; the ground rod achieves approximately 94% of its resistance value at this
radius (100% is achieved at approximately 2.5 times the rod length) (IEEE STD 142-1991, section 4.1).
See Figure 4-5.
68P81089E50-B 9/1/05 4-9