INSTALLATION OF MONITORING STATIONS - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 77

contact area if they are all electrically connected. At least one 12.5-cm ( 2 -in)-diameter,
2.4-m (8-ft)-long grounding rod is needed to provide an adequate soil contact area.
A deoxidation agent should be applied to all mechanical grounding connections to
ensure low resistance to ground. The grounding rods should be free of nonconducting
coatings, such as paint or enamel, which can interfere with a good soil contact. All
grounding rods must be driven below surface. Where rock is encountered, the rod can
be driven in at a 45 ◦ angle or buried in a trench at least 0.6 m (2 ft) deep (the deeper
the better). Lastly, all grounding rods must be wired together to provide electrical
continuity. The above-soil ends of the rods and their electrical conductor attachments
should be protected from damage.
It is helpful to know the resistivity of the soil to select the proper grounding
system. This is the electrical resistance to current flow within a unit volume of soil,
usually located near the earth's surface. It can be approximated by measuring with
a multimeter the resistance between two conductive rods driven into the soil to a
specified depth and distance apart. The resistance between the grounding system and
the earth should be less than 100 ohm. In general, the lower the resistivity of the earth,
the better the earth ground it will provide. Soils with low resistivity (e.g., moist dirt)
quickly dissipate any voltage potential that develops between two points and provide a
better earth ground. High resistivity soil (e.g., dry sand) can build up a large potential
voltage or current that may be destructive. If the resistivity is high, several grounding
rods may be required. Where the soil can freeze, grounding rods should be driven
below the frost line.
Soil resistivity often changes seasonally. The value in early spring, following a
winter thaw, may not reflect the soil conditions during the midsummer lightning
season. In addition, towers in arid climates may be prone to electrostatic discharge if
system grounding is poorly done. When in doubt, take the conservative approach and
provide added protection. It is, in the long run, the least costly route.
On existing towers, the tower's grounding system should be evaluated. If it is
deemed adequate, the data logger's ground may be connected to it. If not, a separate
earth grounding system should be installed and then physically tied into the existing
ground system.
Data Logger and Sensor Grounding. Lightning protection devices, such as
spark gaps, transorbs, and MOVs, should be incorporated into the data logging system
electronics to supplement grounding. Anemometers and wind vanes are available with
MOVs as part of their circuitry, or can usually be outfitted with them. Their primary
purpose is to limit the peak surge voltage allowed to reach the protected equipment
while diverting most of the destructive surge current. The protection offered for each
data logger should be verified with the manufacturer. Additional protection equipment
may be needed in lightning-prone areas.
Tower Grounding. Lightning protection equipment must be installed on the
tower and connected to the common ground. An example of a lightning protection kit
consists of an air terminal installed above the tower top, sometimes referred to as a
lightning rod , along with a long length of heavy-gauge (10 gauge or less), noninsulated