The Fig. 3 scheme uses the inputs (CT, wiring, relay input
circuitry, and firmware) to issue the trip and to determine if
the breaker actually failed. This combined with multiple
operational copies of the BF function can potentially erode
security of the BF protection. Some multi-input relays reduce
this concern to a degree by allowing different CT, wiring, and
relay inputs for the BF function, as shown in Fig. 4.