Severing A Ground Loop; Soil Moisture Example - Campbell CR300 Series Product Manual

22.10.3 Severing a ground loop

To avoid or eliminate ground loops, when they are detected, requires severing the loop.
Suggestions for severing ground loops include:
Connect the shield wire of a signal cable to ground only at one end of the cable. Leave the
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other end floating (not connected to ground).
Never intentionally use the shield (or drain wire) of a cable as a signal ground or power
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ground.
Use the mechanical support structure only as a connection for the safety ground (usually
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the ground lug). Do not intentionally return power ground through the structure.
Do not use shielded Cat5e cables for Ethernet communications.
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For long distance communications protocols such as RS-485, RS-422, and CAN, use a
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Resistive Ground (RG) terminal for the ground connection. The RG terminal has a 100-ohm
resistor in series with ground to limit the amount of DC current that can flow between the
two endpoints while keeping the common-mode voltage in range of the transceivers. The
transceivers themselves have enhanced voltage range inputs allowing for ground voltage
differences of up to 7 V between endpoints.
For exceptional cases, use optical or galvanic isolation devices to provide a signal
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connection without any accompanying ground connection. These should be used only
when ground loops are causing system problems and the other methods of breaking a
ground loop don't apply. These devices add expense and tend to consume large amounts
of power.

22.10.4 Soil moisture example

When measuring soil moisture with a resistance block, or water conductivity with a resistance cell,
the potential exists for a ground loop error. In the case of an ionic soil matric potential (soil
moisture) sensor, a ground loop arises because soil and water provide an alternate path for the
excitation to return to data logger ground. This example is modeled in the following image:
22. Tips and troubleshooting 187