Rockwell Automation 1772-L8_LW_LWP_LX_LXP,D17726.5.8 User Manual - Manual (Page 50)

Installing Your
Programmable Controller
Chapter 4
4-11
Ground loops may introduce objectionable ground currents causing faulty
operation of the programmable controller. If multiple grounding
connections cause faulty operation, refer to Article 250-21 of the National
Electrical Code for recommended methods of reducing the objectionable
ground current.
When ac power is supplied as a separately derived system through an
isolation/step-down transformer, you can connect it as a grounded ac
system or an ungrounded ac system. For a grounded ac system, connect
one side of the transformer secondary to the ground bus as in Figure 4.1.
For an ungrounded ac system, connect one side of each test switch for the
ground-fault-detector lights to the ground bus as in Figure 4.2.
Surge Suppression
EMI can be generated whenever inductive loads such as relays, solenoids,
motor starters, or motors are operated by “hard contacts” such as
pushbutton or selector switches. The wiring and grounding practices
described previously guard the processor system against the effects of
EMI. However, in some cases it may be necessary to use suppression
networks to suppress EMI at its source. Inductive loads controlled only by
solid-state output devices alone do not cause comparable EMI generation.
However, inductive loads on ac output modules that are in series or parallel
with hard contacts require suppression networks to protect the module
output circuits as well as to suppress EMI.
Connect suppression networks at the inductive loads. If you connect them
at the switching devices, the wires connecting the switching devices to the
inductive loads will act as antennas to radiate EMI. Figure 4.4 shows
typical suppression circuitry for different types of loads. Allen-Bradley
bulletin 700 relays and bulletin 509 and 709 motor starters have surge
suppressors available as an option. Table 4.B lists these Allen-Bradley
products and their suppressors.