Service manual

Relay Life
Electromagnetic relays are subject to normal wear-out. Relay life depends
on several factors. Two factors are loading and switching frequency effects.
Loading and
Switching
Frequency Effects
Relay Load. In general, higher power switching reduces relay life. In
addition, capacitive/inductive loads and high inrush currents (e.g., when
turning on a lamp or motor) reduce relay life. Exceeding the specified
maximum inputs can cause catastrophic failure.
Switching Frequency. Relay contacts heat up when switched. As the
switching frequency increases, the contacts have less time to dissipate heat.
The resulting increase in contact temperature reduces relay life.
End-of-Life
Detection
A preventive maintenance routine can prevent problems caused by
unexpected relay failure. The end-of-the life of a relay can be determined
using one or more of the following methods. The best method (or
combination of methods), as well as the failure criteria, depends on the
application in which the relay is used.
Check Contact Resistance. As a relay begins to wear out, its contact
resistance will increase. When the resistance exceeds a pre-determined
value, the relay should be replaced. Typically, a relay should be replaced
when the contact resistance exceeds 2.0Ω.
Check Stability of Contact Resistance. The stability of relay contact
resistance decreases with age. Using this method, the contact resistance is
measured several (5-10) times, and the variance of the measurements is
determined. An increase in the variance indicates deteriorating performance.
Replace Relays after Defined Number of Operations. Relays can be
replaced after a predetermined number of contact closures. However, this
method requires knowledge of the applied load and life specifications for
the applied load. For the multiplexers, maximum relay life is specified to be
10
8
operations at no load or 10
7
operations at rated load.
14 General Information Chapter 1