Installing Your
Programmable Controller
Chapter 4
4-17
Before grounding your processor system, consult the following sources
of information:
National Electrical Code, published by the National Fire Protection
Association of Boston, Massachusetts
local codes and ordinances
Mounting Processor Components
After planning your layout, you can begin mounting the chassis. In
mounting each chassis:
Make sure each chassis lies flat. If the I/O chassis does not lie flat, shim
it with washers so that the chassis is not warped, when the nuts are
tightened. Warping an I/O chassis could damage the backplane and
cause poor connections.
Make good electrical connections between each I/O chassis, the
backpanel, and the enclosure
Remove paint or other nonconductive finish from studs and the
backpanel so that good electrical contact is made at each bolt or stud.
After you have established all layouts, you can begin mounting and
properly grounding each chassis.
Grounding is important for safety in electrical installations. With
solid-state controls, proper grounding (including elimination of ground
loops) has an added value of helping reduce the effects of electrostatic and
electromagnetic interference. Providing a low-impedance path to
earth-ground potential will reduce the chances of EMI causing your
processor system to malfunction.
An authoritative source for safety grounding requirements is the National
Electrical Code. Article 250 of the code provides such data as the size and
types of conductors and methods of safely grounding electrical
components. As defined in the code, a grounding path must be permanent
and continuous, and must be able to safely conduct ground-fault currents
that may occur in the system to ground with minimum impedance. Also,
the connections to a grounding conductor must be a permanent nature.
Local codes and ordinances dictate which grounding method
is permissible.
Figure 4.8 showed mounting assembly details for stud-mounting of a
chassis or ground bus to a backpanel, bolt-mounting of a chassis or ground
bus to a backpanel, and stud-mounting of a backpanel to the back wall of
the enclosure. You can mount the chassis with either bolts or welded studs.
