Rockwell Automation 1770 Industrial Automation Wiring and Grounding Guidelines - Manual (Page 15)

Industrial Automation Wiring and Grounding Guidelines 15
Publication 1770-4.1 – February 1998
Transient 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
guidelines are based on the assumption that you guard your system
against the effects of transient emi by using surge-suppressors to
suppress transient emi at its source. Inductive loads switched by
solid-state output devices alone do not require surge-suppression.
However, inductive loads of ac output modules that are in series or
parallel with hard contacts require surge-suppression to protect the
module output circuits as well as to suppress transient emi.
Figure 10 shows 3 examples of where to use suppressors. In
example 1, although the motor-starter coil is an inductive load, it
does not need a suppressor because it is switched by a solid-state
device alone. In example 2, the relay coil needs a suppressor
because a hard-contact switch is in series with the solid-state switch.
However, in both examples 1 and 2, we show a suppressor on the
motor and solenoid because it is an inductive load switched by the
hard contacts of the motor starter or relay. Even if they have no
interaction with the control system, regularly cycled loads of this
type need suppression if conductors connecting to these loads
are: 1) connected to the same separately derived system as that
of the control system; 2) or routed in proximity with conductors
of the control system as per the routing guidelines.
In example 3, the pilot light has a built-in step-down transformer
that needs a suppressor because it is an inductive load being
switched by the hard contacts of a contact output module; without
suppression, the transient emi would be generated inside the I/O
chassis. Lights with built-in step-down transformers that are
switched by hard contacts external to any I/O chassis may not need
to be suppressed because the noise spike they can generate may be
only approximately one tenth that of a relay or motor starter.
In all cases, the ac power coming into the I/O modules must be
switched by the CRM contacts. Therefore, a suppressor is needed
across the line at the load side of the CRM contacts as shown in
Figures 7 and 8. The application (voltage, net load of I/O circuits)
dictates the specific suppressor needed across the line at the load
side of the CRM contacts.
Surge-Suppression