U.6.4
SEL-421/SEL-421-1 Relay User’s Guide Date Code 20020501
Testing and Troubleshooting
Testing Philosophy
The SEL-421 Relay is based on microprocessor technology; the relay internal
processing characteristics do not change over time. For example, if time-
overcurrent element operating times change, these changes occur because of
alterations to relay settings and/or differences in the signals applied to the
relay. You do not need to verify relay element operating characteristics as a
part of maintenance checks.
SEL recommends that you limit maintenance tests on SEL relays according to
the guidelines listed in Table 6.3 on page 6.3. You will spend less time
checking relay operations that function correctly. You can use the time you
save to analyze event data and thoroughly test systems needing more
attention.
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