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:
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Connect the shield wire of a signal cable to ground only at one end of the cable. Leave the
other end floating (not connected to ground).
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Never intentionally use the shield (or drain wire) of a cable as a signal ground or power
ground.
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Use the mechanical support structure only as a connection for the safety ground (usually
the ground lug). Do not intentionally return power ground through the structure.
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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
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.
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For exceptional cases, use optical or galvanic isolation devices to provide a signal
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
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