The Zone Protection is that part of a power system guarded by a certain protection and usually contains one or at the most two elements of the power system. The zones are arranged to overlap so that no part of the system remains unprotected. Figure (1.1) shows a typical arrangement of overlapping zones of protection.
When it becomes desirable for economic or space saving reasons to overlap on one side of a breaker blind spots occur as shown in Fig. (1.2).
It can be seen that for a fault at X the circuit breakers of zone B ,including breaker C will be tripped; however, this does not interrupt the flow of fault current from zone A, the relaying equipment–of zone B must also trip certain breakers in zone A. This is all right for fault at X, but for faults in zone B to the right of breaker C the operation of breakers in zone A is not useful. How far this unnecessary operation can be tolerated will depend on the particular application.