The scheme does not use external BFI signaling and
therefore reduces the risk of human errors and noise-induced
spurious BFI signals.
Wiring of the BFT signals is more extensive as compared
with the standalone scheme because multiple relays issue the
trip and lockout signals for any given breaker.
On the other hand, DTT for remote breakers can be easier
to engineer if the zone devices have the DTT capability in the
first place. However, we should notice that when issuing a
BFT, the Zone 1 IED needs to reach all remote breakers,
including remote breakers in Zone 2, and vice versa. As a
result, the built-in DTT signaling needs to be cross-wired
between the two ZPRs.
Maintenance of the Fig. 3 scheme is relatively simple due
to clarity of the zone protection location and BF functions and
the design symmetry (all of the zone IEDs provide the
internally initiated BF function; no external BFI signals or
devices are used).