Installation guide

Network Infrastructure for EtherNet/IP™
Designing the Infrastructure
4-41
Grounding of Shielded Cables
Shields play an important role in providing noise immunity for systems. However, an improperly
installed shielded cable can cause problems due to voltage offsets in a grounding system. To
minimize the effects of ground offsets, the shield should be isolated at one end of the cable. In this
case, the shield should be isolated at the device. The ground can be applied at the switch or other
infrastructure component. It should be noted that some European wiring practices recommend
grounding the cable shields at both ends, requiring that particular care be taken to avoid ground
loops. The quality of a plant’s grounding system must be thoroughly evaluated prior to using this
approach.
Eliminating ground loops is extremely important in reducing noise interference caused by
ground offsets or local ground transients. Ground offsets occur when there is a potential difference
between two earth-ground points in a system as shown in Figure 4-4. This potential difference can consist
of continuous dc or ac voltages as well as transients.
Figure 4-4 Ground Noise Loop in Shielded Cable.
A cable that connects the two points together provides a secondary travel path for current. The current
through the communications cable’s shield will couple noise into the communications system. This noise
will have a direct impact on the signal-to-noise performance of the system.