Grounds - Campbell CR300 Series Product Manual

2.3 Grounds

Proper grounding lends stability and protection to a data acquisition system. Grounding the data
logger with its peripheral devices and sensors is critical in all applications. Proper grounding will
ensure maximum ESD protection and measurement accuracy. It is the easiest and least expensive
insurance against data loss, and often the most neglected. The following terminals are provided
for connection of sensor and data logger grounds:
Signal Ground ( ) - reference for single-ended analog inputs, excitation returns, and a
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ground for sensor shield wires.
5 common terminals
o
Power Ground (G) - return for 3.3 V, 5 V, 12 V, current loops, and digital sensors. Use of G
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grounds for these outputs minimizes potentially large current flow through the analog-
voltage-measurement section of the wiring panel, which can cause single-ended voltage
measurement errors.
6 common terminals
o
Earth Ground Lug ( ) - connection point for heavy-gage earth-ground wire. A good earth
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connection is necessary to secure the ground potential of the data logger and shunt
transients away from electronics. Campbell Scientific recommends 14 AWG wire, minimum.
NOTE:
Several ground wires can be connected to the same ground terminal.
A good earth (chassis) ground will minimize damage to the data logger and sensors by providing
a low-resistance path around the system to a point of low potential. Campbell Scientific
recommends that all data loggers be earth grounded. All components of the system (data
loggers, sensors, external power supplies, mounts, housings) should be referenced to one
common earth ground.
In the field, at a minimum, a proper earth ground will consist of a 5-foot copper-sheathed
grounding rod driven into the earth and connected to the large brass ground lug on the wiring
panel with a 14 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,
search for published literature on lightning protection or contact a qualified lightning-protection
consultant.
In laboratory applications, locating a stable earth ground is challenging, but still necessary. In
older buildings, new VAC receptacles on older VAC wiring may indicate that a safety ground
exists when, in fact, the socket is not grounded. If a safety ground does exist, good practice
dictates to verify that it carries no current. If the integrity of the VAC power ground is in doubt,
2. Wiring panel and terminal functions 10