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

STANDARDS AND GUIDELINES FOR COMMUNICATION SITES GROUNDING (EARTHING) ELECTRODE SYSTEM COMPONENT AND INSTALLATION
REQUIREMENTS
• The conductors shall be bonded directly to the tower and tower ground ring (ANSI T1.334-2002,
section 5.4). If it is not practical to bond all conductors to the tower, the tower shall have additional
grounding conductors bonding it to the tower ground ring; 70 mm2 (#2/0 AWG) or coarser
conductor is recommended in this case.
• Conductor bonding shall comply with “Bonding to the External Grounding (Earthing) Electrode
System” on page 4-40
• The conductors shall be constructed of 35 mm2 csa (#2 AWG) or coarser, bare, solid, tinned or untinned,
copper. See “Grounding (Earthing) Conductors” on page 4-28 for conductor specifications
and installation requirements. (ANSI T1.334-2002, section 5.4)
• The conductors shall be buried at least 457 mm (18 in.) below ground (ANSI T1.334-2002,
section 5.4). When topsoil conditions allow, it is recommended to bury the conductors to a depth of
at least 762 mm (30 in.) (ANSI-J-STD-607-A-2002, section C.9.2); this is especially important in
areas where the frost line may reach 457 mm (18 in.).
• The minimum length of each conductor shall be 7.6 m (25 ft.). If the desired resistance to earth is
not achieved at 7.6 m (25 ft.), the radial conductor may be extended to help obtain the desired
resistance (ANSI T1.334-2002, section 5.4). The maximum effective length for a single radial
conductor is generally considered to be approximately 24.4 m (80 ft.). Adding additional
conductors is generally more effective than extending the length of a single conductor.
NOTE: When multiple radial conductors are used, the conductors should be of different lengths to help prevent
resonant “ringing” of the tower from a lightning strike.
NOTE: Low resistance in radial (counterpoise) grounding configurations is desirable, but not critical. Low
resistance in the dissipating path of strike currents into the earth is of secondary importance when
compared to the primary objective of controlling voltage gradients and voltage differences between
structures and equipment close to the tower (ANSI T1.334-2002, section 5.4).
• When soil conditions allow, the effectiveness of the radial grounding conductor may be increased
by including a ground rod every 4.9 m (16 ft.) (or twice the length of the ground rods) installed as
described in “Ground Rods” on page 4-11. See Figure 4-19 on page 4-24 for an example of ground
radials.
Figure 4-20 on page 4-26 shows the resistance characteristics of a radial grounding conductor. The
resistance to earth of a straight horizontal electrode (radial grounding conductor) may be calculated as
follows:
R =
ρ
-----πL
ln
2L
( 2aD)
12 /
----------------------
– 1
Where:
D