If the supply to the protected portion of the system is constant, and if the magnitude of the fault current is

determined primarily by the location of the fault on the line, the selection of a more inverse time

characteristic is more desirable to provide selective coordination with adjacent line protection. However, if

the capacity of the supply varies significantly over a period (such as a day), a less inverse time or even

the definite time characteristic, may be preferred to provide smoother coordination.

Limited Region of Operation

A limited region-of-operation option is available to provide additional protection against false tripping on

mutually coupled lines. Faults on adjacent lines that share the same poles, towers, or right-of-way may

induce currents on the protected line which appear as fault currents in the tripping direction. The limited

region of operation mode provides discrimination between faults on the protected line and faults on the

adjacent line. To order this option, specify option 3-5 or 3-6.

One consideration in applying a phase directional overcurrent relay is the definition of trip direction. For

most applications, the setting of the relay directional element is based upon the impedance characteristics

of a given circuit. This angle is then used as the maximum torque angle and any current flowing in the

half-plane defined by this angle is considered to be in the trip direction.

However, there are at least two situations where the half-plane trip region is not adequate. They are when

load current approaches the fault current and when leading current flows in the non-trip direction above

the relay pickup setting.