TABLE OF CONTENTS Sensitivity to phase-to-phase fault detection can be increased by the use of negative-sequence relays (type 50/51 Q) because a balanced load has no negative-sequence ( I 2) current component. This is also the situation for phase-to-earth faults if type 50/51 relays are used since a balanced load has no zero-sequence ( I 0) component.
Instantaneous overcurrent and time-delay overcurrent negative-sequence units are common features of the new multifunction relays. It is important to ensure that the settings of these units are checked for co-ordination with phase-only sensing devices such as downstream fuses and reclosers, and/or earth-fault relays.
To determine the pick-up setting of the negative-sequence element, it is necessary to keep in mind that the magnitude of the current for a phase-to-phase fault is 
(87 percent) of the current for a three-phase fault at the same location, as indicated in eqn. 5.11. On the other hand, the magnitude of the negative-sequence component for a phase-to-phase fault can be obtained from the following expression taken from Section 2.2: I a2= 1/3( I a + a 2 I b + aI c). For a phase-to-phase fault, I a = 0 and I b = - I c. Therefore the magnitude of the negative-sequence component is 
(58 percent) of the magnitude of the phase-fault current. When the two factors ( 
and 
) are combined, the 
factors cancel, leaving a 1/2 factor.
From the above it...
