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Adding Relay and
Circuit Protection
Chapter 1
The Form C switch has space for adding relay and circuit protection. Relay
protection can be added by placing a protective device across the specified
pads. This is done by adding metal oxide varistors (MOVs) between the
common (C) and normally open (NO) or normally closed (NC) terminals.
As the voltage goes up, the varistor draws current to protect the relay.
Circuit protection can be added by placing a protective device in series with
the common lead. This is done by adding a resistor between the common
(C) terminal and your circuit. When installing circuit protection, a jumper
must be removed first.
To install these protective devices it is necessary to remove the sheet metal
covers from the module. The locations for installing the devices are labeled
as shown in Table 1-2, where xx = the channel number. Do not install a
capacitor in any of these locations. Figure 1-13 shows locations where these
protective devices can be added.
Table 1-2. Protective Devices Board Locations
Relay Protection
VxxO
Varistor location across common (C) and normally
open (NO).
VxxC
Varistor location across common (C) and normally
closed (NC).
Circuit Protection
JMxx
Resistor or fuse location in series with common (C).
xx = Channel Number
Figure 1-13. Adding Relay and Circuit Protection
Circuit Protection
C
NO
NC
VxxC
VxxO
Relay
Getting Started 25
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