The performance of a distance relay near its zone
boundaries is not very predictable because of various types of errors. Consequently, it becomes necessary to use multiple zones of protection to cover the entire line dependably and securely. Zone 1 relay operates instantaneously (no intentional delay – i.e. in about one to two cycles) while a fault in Zone 2 causes the relay to operate with an added delay (generally of the order of 20 to 30 cycles). In this fashion, the entire line is protected even where the zone boundary is not very precisely determined. In addition to these two zones, often a third zone (with an additional time delay about one second) is provided at each end in order to provide remote backup for the protection of the adjacent circuits. It should be noted that often, due to system load, it is not possible to obtain a secure Zone 3 setting on high voltage networks.