Protective relays are designed and manufactured to high
standards of reliability. Mean time between failures (MTBF)
reaches 300 to 400 years for best-in-class relays [8] [9]. Still,
there is always a non-zero probability of an internal
component failure. Built-in self-monitoring is designed to
maximize security and avoid unintended operation by
detecting internal problems under practical component failure
scenarios. Therefore, the MTBF viewed from the security
perspective (meaning only considering failures that may lead
to unintended operations) is considerably better than 400 years
for best-in-class relays.