Because of the desired bias for security, duplicated
(redundant) BF protection is not common. However, it might
be considered beneficial if the zone protection and BF
functions are independent with respect to their input signals
(CTs and wiring), hosting relays (hardware and firmware), and
tripping outputs (relay output contacts). This strict approach
does not prevent integration of the BF functionality with the
ZPRs, as explained later in this paper.