Normally, two zones of protection overlap at a breaker.
With dual-redundant protection systems, up to four zone
protection relays (ZPRs) that have access to the breaker
current signal may integrate the BF protection for the breaker.
How many integrated BF elements should be enabled for a
given breaker? Is it safe to make trip and BF decisions based
on the same current transformer (CT) output, wiring, and input
circuitry of a relay? How do we keep the integrated BF simple
enough to maintain and avoid unintended operation caused by
human errors? How do we balance the rewards of avoiding
external breaker failure initiation (BFI) signals prone to noise
and test errors with the danger of using the same relay input
data for primary zone tripping and BF tripping for a much
bigger zone?