With double-breaker arrangements, the current in the
protected zone on at least one side of the breaker will be the
summation of the current flowing through two breakers. In the
past, this summation was always done external to the ZPR.
For this configuration, the ZPR cannot provide BF detection.
With some modern relays, the currents from each of the two
breakers are brought into the relay, and the current entering
the zone of protection is summed internally to the relay. This
type of relay can provide BF detection for dual-breaker
arrangements. In this respect, we could have three scenarios