D
OCUMENT
301900,
R
EVISION
D
E
PI
S
ENSOR
U
SER
G
UIDE
9
clamps are available from electrical supply houses to make these
connections.
The requirements and techniques for producing the low impedance ground
will depend on the installation and its location. In a building or structure
where close lightning strikes are unlikely, a metallic water pipe, exposed
section of rebar, or an AC safety ground may be adequate.
For a remote installation subject to significant lightning activity such as a
mountain peak, much greater effort is required to provide an adequate
ground.
In lightning-prone conditions, a copper clad grounding rod inserted into the
ground is an acceptable solution if the climate is relatively humid or if the
rod is embedded in a marsh, clay, or wet sand ground where soil resistivity
is less than 50-100 ohms.
A 1-cm (0.5") diameter rod 4 meters long (12 feet) will assure an acceptable
grounding resistance of about 10 ohms. Grounding rods are generally
available in a variety of lengths and can be connected together to get overall
depths exceeding 12 meters (40 feet).
In dryer regions, arrays of copper rods driven into the earth help to reduce
the grounding resistance. However, the total ground resistance is not
reduced in direct proportion to the number of rods in the array, and the rods
should be spaced apart by double their length to avoid "saturation."
In a high lightning threat area, a star configuration of radial metal straps
buried just below the surface with a 2 meter grounding rod at the end of
each radial is a good approach. A 6-to-8-leg star of 50 m (150 feet) length
with a grounding rod at each end provides a ground resistance of less than
10 ohms even in a soil with resistively of several thousand ohms.
In desert areas, chemical doping and drip irrigation techniques may be
required to provide adequate grounding.
The Grounds for Lightning and EMP Protection
by Roger Block, second
edition, published by Polyphaser Corporation provides an excellent
practical treatment of grounding techniques.
In sites exposed to high levels of EMI/RFI, such as, hill top “Antenna
Farms” additional EMI/RFI protection may be required. The Kinemetrics
EpiSensor Protection Enclosure (KMI 301931) can be purchased for such
sites.
Powering the EpiSensor
When the cable has been made and tested, you can apply power to the
sensor. If you are providing power, please refer to Chapter 6 for detailed
requirements.
