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Relay Life
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) reduces 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 preventive maintenance routine can prevent problems caused by unexpected relay
failure. The end of the life of the relay can be determined by using one or more of
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 pre-determined 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
o
variance indicates deteriorating perf
rmance.
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 can be replaced when the end of relay 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.
Appendix A
Relay Life 53
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