Chapter 3 ______________________________________________________ Installation of the Mast
VAISALA _______________________________________________________________________ 79
- The equipment grounding system and lightning protection
grounding system should be located as far as possible from each
other. Minimum recommended distance is 5 m (16 ft.). Due to this
the lightning rod grounding cable is routed down from the top of
the mast via the guy wire which is insulated from the mast
construction, refer to number 2 in Figure 66 on page 78. The
equipment grounding should be done near the concrete pad for the
mast as illustrated in Figure 66 on page 78.
- The recommended grounding resistance is 10 Ω or less for both the
grounding of lighting protection rod and for the equipment
grounding. Soil conditions (sand, rocks, etc.) and ground resistance
measurement determines the design and construction of the
grounding system. Buried ground rods and / or buried wire can be
used for the ground network. The type and combination used
depend on soil conditions (ease of installation) and on the value of
measured ground resistance (the most effective way of reducing
resistance to the required value of 10 Ω or less). Measurements of
the earth resistance shall be made at least 48 hours after rainfall.
- The connection between ground rods and equipment frames and
copper cables should be made with appropriate compression lugs,
bolts, nuts, and lock washers.
- Optionally, the top of the earth electrode can be housed inside
covered pit as illustrated in Figure 67 on page 80.
Table 3 Examples of Soil Resistivities, Ohm-Meters
Soil Type Median Variation
Topsoil, loam 26 1 ... 50
Inorganic clays of high plasticity 33 10 ... 55
Silty or clayey fine sands with slight plasticity 55 30 ... 80
Fine sandy or silty clays, silty clays, clean clays 190 80 ... 300
Silty sands, poorly graded sand-silt mixtures 300 100 ... 500
Well graded gravel, gravel sand mixtures 800 600 ... 1 000
Due to the above reasons and variations, the grounding accessories
presented in this manual (rods, connectors, etc.) are only examples of
the proper ones, thus recommended to be supplied locally.
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