Relay Life; End-Of-Life Detection - Agilent Technologies 75000 C Series Service Manual

Relay Life

End-of-Life
Detection
10 General Information
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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., 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.
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 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
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 2.0 Ohm.
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. Alternatively, 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 Agilent E1463A, maximum relay life is specified
7
at 5 X 10 operations with no load and 3.5 X 10
worst-case rated load.
4
operations at the
Chapter 1