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Relay Life
Electromechanical relays are subject to normal wear-out. Relay life
depends on several factors. The effects of loading and switching frequency
are briefly discussed below:
Relay Load. In general, higher power switching reduces relay life. In
addition, capacitive/inductive loads and high inrush currents (e.g., turning
on a lamp or starting a motor) reduce relay life. Exceeding 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 also reduces relay life.
End of Life
Detection
A preventative maintenance routine can prevent problems caused by
unexpected relay failure. The end of the life of the relay can be determined
by using the three methods described below. The best method (or
combination of methods), as well as the failure criteria, depends on the
application in which the relay is used.
Contact Resistance. As the relay begins to wear out, its contact resistance
increases. When the resistance exceeds a predetermined value, the relay
should be replaced.
Stability of Contact Resistance. The stability of the 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.
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.
Replacement
Strategy
The replacement strategy depends on the application. If some relays are
used more often, or at a higher load, than the others, the relays can be
individually replaced as needed. If all the relays see similar loads and
switching frequencies, the entire circuit board should be replaced when the
end of life approaches. The sensitivity of the application should be weighed
against the cost of replacing relays with some useful life remaining.
Note Relays that wear out normally or fail due to misuse should not be considered
defective and are not covered by the product’s warranty.
64 Agilent E1364A Form C Switch Specifications Appendix A