Rockwell Automation 1770 Industrial Automation Wiring and Grounding Guidelines - Manual (Page 9)

Industrial Automation Wiring and Grounding Guidelines 9
Publication 1770-4.1 – February 1998
Some products have no visible groundable chassis and no ground lug
or ground terminal, but mount on a DIN rail. The FLEX I/O
products are in this category. The chassis of these products are
grounded only thru the DIN rail. For these products, connect an
equipment-grounding conductor directly from the mounting bolt on
the DIN rail to an individual bolt on the ground bus.
Grounding-Electrode Conductor — Connect the ground bus to
the grounding-electrode system through a grounding-electrode
conductor. The grounding-electrode system is at earth-ground
potential and is the central ground for all electrical equipment and ac
power within any facility. Use 8 AWG copper wire minimum for the
grounding-electrode conductor to help guard against emi. The
National Electrical Code specifies safety requirements for the
grounding-electrode conductor.
Shielded Cables — Certain I/O connections require shielded
cables to help reduce the effects of electrical noise coupling. Ground
each shield at one end only. A shield grounded at both ends forms a
ground loop which can cause a processor to fault.
Ground each shield at the end specified in the appropriate
publication for the product. Never connect a shield to the common
side of a logic circuit (this would introduce noise into the logic
circuit). Connect each shield directly to a chassis ground.
For some communication network cables, the shield connections are
unique to the particular cabling system. In some such cases, a dc
short to ground is not needed because a low-impedance ac path to
ground and a high-impedance dc path to ground are provided
internally at each node. Follow the specific instructions in the
publication provided for the specific communication network
cabling system.
Avoid breaking shields at junction boxes. Many types of connectors
for shielded conductors are available from various manufacturers. If
you do break a shield at a junction box, do the following:
•
Connect only category-2 conductors in the junction box.
•
Do not strip the shield back any further than necessary to make
a connection.
•
Connect the shields of the two cable segments to ensure
continuity along the entire length of the cable.