cable.
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A long cable connects the grounds of two electrical devices, and the mounting structure or
grounding rod also directly connects the grounds of each device to the local earth ground.
The two paths, in this case, are the connecting cable and earth itself.
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When electrical devices are connected to a common metal chassis such as an instrument
tower, the structure can create a ground path in parallel to the ground wires in sensor
cables running over the structure.
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Conductors connected to ground are found in most cables that connect to a data logger.
These include sensors cables, communications cables, and power cables. Any time one of
these cables connects to the same two endpoints as another cable, a ground loop is
formed.
22.10.2 Detrimental effects
The harm from a ground loop can be seen in different ways. One consideration is the
electromagnetically induced effect. This will manifest as AC noise or an AC pulse. As seen in
Figure 22-1 (p. 185) the parallel conductive paths form an electrical loop that acts as an antenna
to pick up electromagnetic energy.
Figure 22-1. Stray AC magnetic fields picked up in loop antenna
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Relatively small electromagnetic energy: This could come from AC current on a nearby
power cable, or RF energy transmitting through the air, and can cause electrical noise that
either corrupts an analog signal or disrupts digital communications.
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Larger electromagnetic energy: The antenna loop scenario can have a more damaging
22. Tips and troubleshooting185
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