Relay contacts wear according to the amount of relay switching, amount of spark created at
the time of open or closure, and presence of airborne contaminants.
However, there are some steps you can take to help prolong the life of relay contacts:
• Switch the relay on or off only when the application requires it.
• If you have the option, switch the load on or off at a time when it will draw the least current.
• Take measures to suppress inductive voltage spikes from inductive DC loads such as contactors and
solenoids (circuit given below).
Adding external contact protection may extend relay life beyond the number of contact
cycles listed in the specification tables for relay modules. High current inductive loads such as
clutches, brakes, motors, direct-acting solenoid valves, and motor starters will benefit the
most from external contact protection.
The RC network must be located close to the relay module output connector. To find the
values for the RC snubber network, first determine the voltage across the contacts when open,
and the current through them when closed. If the load supply is AC, then convert the current
and voltage values to peak values:
Now you are ready to calculate values for R and C, according to the formulas:
R ( ) =
C (µF) =
10
I
2
V
10 x I
x
, where x
50
V
1 +
C minimum = 0.001 µ F, the volta
g
e ratin
g
of C must
b
e
V, non-polarized
R minimum = 0.5
, 1/2 W, tolerance is ± 5%
=
Inductive Field Device
+
–
PLC Relay Output
Output
Common
Input
Common
Supply
R
C
Chapter 3: I/O Wiring and Specifications
3–11
DL205 Installation and I/O Manual, 2nd Edition
3
