standards and guidelines for communication sites - Radio And ...

MINIMUM SITE GROUNDING (EARTHING) REQUIREMENTS CHAPTER 4: EXTERNAL GROUNDING (EARTHING)
4.7.4 GROUNDING (EARTHING) ELECTRODE SYSTEM RESISTANCE
REQUIREMENTS
In order to disperse lightning energy into the earth without causing dangerous over-voltage, the shape
and dimensions of the grounding (earthing) electrode system are more important than a specific
resistance value of the grounding electrode system. However, a low resistance grounding electrode
system is generally recommended (IEC 61024-1-2). Attempts should be made to reduce the grounding
electrode system resistance to the lowest practical value (MIL-HDBK-419A, section 2.2.3).
NOTE: Although grounding electrode system resistance is important and should be met whenever possible, it
alone does not determine the suitability of the grounding electrode system to properly dissipate and
control lightning energy. The resistance of the grounding electrode system is only a general measure of
merit. Proper design and installation of the grounding electrode system, installation of ground rings,
ground rods, radial grounding conductors, and the bonding of systems and equipment, is as important as
the resistance to earth.
Effective grounding electrode system shape and dimensions are achieved through the proper installation
of the required and recommended grounding electrode system components listed throughout this
chapter. The required and recommended grounding electrode system components are, but not limited to,
the following:
• Building ground ring. See “External Building and Tower Ground Ring” on page 4-22.
• Tower ground ring. See “External Building and Tower Ground Ring” on page 4-22.
• Ground rods properly installed and spaced around the building and tower ground rings. See
“Ground Rods” on page 4-11.
• Radial grounding conductors for high lightning prone geographical areas, sites that are normally
occupied (such as 911 dispatch centers), sites with high soil resistivity, or when bedrock prohibits
the driving of ground rods. See “Radial (Counterpoise) Grounding Conductors” on page 4-24.
• Proper bonding of all grounding electrode system components. See “Bonding to the External
Grounding (Earthing) Electrode System” on page 4-40.
• Proper bonding of all ancillary equipment. See “Bonding to the External Grounding (Earthing)
Electrode System” on page 4-40.
The grounding electrode system resistance requirement is determined based on the classification of the
site. Communications sites are classified by this standard into two categories:
• Type A: See “Type “A” Sites - Light Duty” on page 4-46.
• Type B: See “Type “B” Sites - Standard Duty” on page 4-47.
4.7.4.1 TYPE “A” SITES - LIGHT DUTY
Type “A” sites shall have a grounding (earthing) electrode system resistance design goal of 25 ohms or
less for a single grounding electrode (NFPA 70-2005, Article 250.56; MIL-HDBK-419A, section
2.2.2.1). If the design goal of 25 ohms cannot be achieved throughout the year with a single grounding
electrode, then the grounding electrode shall be augmented by at least one additional grounding
electrode (NFPA 70-2005, Article 250.56). See “Soil Resistivity Variability and Factors Affecting Soil
Resistivity” on page B-1 for information regarding the seasonal variations in grounding electrode
resistance.
4-46 68P81089E50-B 9/1/05