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

INTRODUCTION CHAPTER 4: EXTERNAL GROUNDING (EARTHING)
References to the specific industry codes and standards on which this chapter is based are provided
throughout. The requirements and guidelines in this chapter are provided to enhance personnel safety
and equipment reliability.
Safety of personnel and protection of sensitive electronic equipment from ground faults, lightning,
ground potential rise, electrical surges, and power quality anomalies is of utmost importance at any
communications site. Though unexpected electrical events like lightning strikes and power surges
cannot be prevented, this chapter provides design and installation information on communications site
grounding electrode systems that may help minimize damage caused by these events.
CAUTION
Grounding (earthing) and bonding alone are not enough to adequately protect a
communications site. Transient voltage surge suppression (TVSS) techniques, using
appropriate surge protection devices (SPD), shall be incorporated at a communications
site in order to provide an adequate level of protection. See Chapter 7, “Surge
Protective Devices,” for details and requirements.
A grounding electrode system shall have low electrical impedance, with conductors large enough to
withstand high fault currents. The lower the grounding electrode system impedance, the more
effectively the grounding electrode system can dissipate high-energy impulses into the earth.
WARNING
The AC power system ground shall be sized appropriately for the electrical service and
shall be approved by the local authority having jurisdiction.
All site development and equipment installation work shall comply with all applicable codes in use by
the authority having jurisdiction. Grounding systems shall be installed in a neat and workmanlike
manner (NFPA 70-2005, Article 110.12 and NFPA 780-2004, section 1.4). Where conflicting, the more
stringent standard should be followed. Government and local codes shall take precedence over the
requirements of this manual.
Unusual site conditions may require additional effort to achieve an effectively bonded and grounded
(earthed) site. See “Special Grounding (Earthing) Situations” on page 4-88 in these instances.
Consultation with Motorola Engineering or an engineering firm specializing in grounding electrode
system design is recommended.
Some of the benefits of a properly designed and installed low-impedance grounding electrode system
are described below (See ANSI T1.333-2001, section 4; ANSI T1.334-2002, section 5.1; BS
7430:1998; IEC 60364-1; IEEE STD 142-1991, section 1.3; IEEE STD 1100-1999, section 3.3.1; and
NFPA 70-2005, Article 250.4 for additional information):
• To help limit the voltage caused by accidental contact of the site AC supply conductors with
conductors of higher voltage.
• To help dissipate electrical surges and faults, to minimize the chances of injury from grounding
system potential differences.
• To help limit the voltages caused by lightning.
• To help maintain a low potential difference between exposed metallic objects.
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