Overlapping Protective Zones:

A separate zone of protection is given around every system element that was show in the figure. Any failure which might occur along within a given zone will cause the tripping or opening of all circuit breakers inside that zone. For failures which occur within a region where two protective zones overlap, more breakers will be tripped than are necessary to disconnect the faulty component; however, if there were no overlap of protective zones, a fault in a region among the two zones would result in no protective action at all. Thus, it is desirable for protective zone overlap to ensure the maximum system protection.

Figure: Protective Relaying Zones