Surge Suppressors

This chapter provides information on the use of surge suppressors to protect the relay output contacts of the relay output module when switching inductive loads. It also describes recommended surge suppressors for AC and DC load applications.

Because of the potentially high current surges that occur when switching inductive load devices, such as motor starters and solenoids, the use of some type of surge suppression to protect and extend the operating life of the I/O module's output contacts is required. Switching inductive loads without surge suppression can significantly reduce the life expectancy of relay contacts. By adding a suppression device directly across the coil of an inductive device, you prolong the life of the output or relay contacts. You also reduce the effects of voltage transients and electrical noise from radiating into adjacent systems.

The following figure shows an output with a suppression device for the 5034-OW4I relay output module. We recommend that you locate the suppression device as close as possible to the inductive load device.

For DC outputs driving inductive loads, use an appropriate diode for surge suppression. These surge suppression circuits can be connected directly across the load device.

Suitable surge suppression methods for inductive AC load devices include a varistor, an RC network, or an Allen-Bradley® surge suppressor, as shown in the following figure. These components must be appropriately rated to suppress the switching transient characteristic of the particular inductive device. See Table 1 for recommended devices.