Vaisala AWS330 - Manual (Page 40)

User's Guide ______________________________________________________________________
38 __________________________________________________________________ M211296EN-B
- 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, and so on) and the ground resistance
measurement determine 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
must be made at least 48 hours after rainfall.
- The connection between ground rods and the equipment frames and
copper cables should be made with appropriate compression lugs,
bolts, nuts, and lock washers.
Table 17 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 soil variations, the grounding accessories
must be supplied locally.