Campbell Scientific CR200/CR200X-series Dataloggers - Manual (Page 67)

Section 7. Grounding
Grounding the CR200(X) and its peripheral devices and sensors is critical in all
applications. Proper grounding will ensure the maximum ESD (electrostatic
discharge) protection and higher measurement accuracy.
7.1 ESD Protection
ESD (electrostatic discharge) can originate from several sources, the most
common, and most destructive, being primary and secondary lightning strikes.
Primary lightning strikes hit the datalogger or sensors directly. Secondary
strikes induce a voltage in power lines or sensor wires.
The primary devices for protection against ESD are gas-discharge tubes (GDT).
All critical inputs and outputs on the CR200(X) are protected with GDTs or
transient voltage suppression diodes. GDTs fire at 150 V to allow current to be
diverted to the earth ground lug. To be effective, the earth ground lug must be
properly connected to earth (chassis) ground. The power ground and signal
grounds have independent paths to the ground lug.
The 9-pin serial port is another path for transients. Communications paths such a
telephone or short-haul modem lines should be provided spark gap protection at
installation. Spark gap protection is often an option with these products, so it
should always be requested when ordering. Spark gaps for these devices must be
connected to either the earth ground lug, the enclosure ground, or to the earth
(chassis) ground.
A good earth (chassis) ground will minimize damage to the datalogger and
sensors by providing a low resistance path around the system to a point of low
potential. Campbell Scientific recommends that all dataloggers be earth
(chassis) grounded. All components of the system (dataloggers, sensors,
external power supplies, mounts, housings, etc.) should be referenced to one
common earth (chassis) ground.
In the field, at a minimum, a proper earth ground will consist of a 6 to 8 foot
copper sheathed grounding rod driven into the earth and connected to the
CR200(X) Ground Lug with a 12 AWG wire. In low conductive substrates, such
as sand, very dry soil, ice, or rock, a single ground rod will probably not provide
an adequate earth ground. For these situations, consult the literature on lightning
protection or contact a qualified lightning protection consultant.
In vehicle applications, the earth ground lug should be firmly attached to the
vehicle chassis with 12 AWG wire or larger.
In laboratory applications, locating a stable earth ground is challenging, but still
necessary. In older buildings, new AC receptacles on older AC wiring may
indicate that a safety ground exists when in fact the socket is not grounded. If a
safety ground does exist, it is good practice to verify that it carries no current. If
the integrity of the AC power ground is in doubt, also ground the system
through the buildings, plumbing or another connection to earth ground.
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