Rockwell Automation 1747-L40 SLC 500 Fixed Hardware Style - Manual (Page 35)

Chapter 1
Selecting Your Hardware Components
1–16
Selecting Contact Protection
Inductive load devices such as motor starters and solenoids may require the
use of some type of surge suppression to protect the controller output
contacts. Switching inductive loads without surge suppression can
significantly reduce lifetime of relay contacts. The figure below shows the
use of surge suppression devices.
Output Device
Varistor
Output Device Output Device Output Device
RC Network
Surge
Suppressor
Output Device Output Device
Diode (A surge suppressor can also be used.)
Ċ +
Surge Suppression for Inductive DC Load Devices
Surge Suppression for Inductive AC Load Devices
Contact Protection Methods for Inductive AC and DC Output Devices
These surge suppression circuits connect directly across the load device.
This reduces arcing of the output contacts. Suitable surge suppression
methods for inductive AC load devices include a varistor, an RC network, or
an Allen–Bradley surge suppressor. These components must be
appropriately rated to suppress the switching transient characteristic of the
particular inductive device.
For inductive DC load devices, a diode is suitable. A 1N4004 diode is
acceptable for most applications. A surge suppressor can also be used. See
table on page 1–15.
We recommend that you locate the suppression device as close as possible to
the load device.