Service manual

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., 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
The contact resistance should be checked periodically to 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 2.7 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 E1465A/E1466A/E1467A maximum relay
life is specified at 10
7
operations with no load and 10
5
operations at the
maximum rated load.
Replacement
Strategy
The replacement strategy also depends on the application. If some
relays are used more often, or at higher load, than the others, the relays
can be individually replaced as needed. If all of the relays see similar
loads and switching frequencies, the entire circuit board can 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.
10 General Information Agilent E1465A/66A/67A Service Manual