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Clearly a method of discriminating between a fault and normal or overload conditions is a necessity, and such a method is available if use is made by suitable circuitry of the fact that the currents in a three phase system can be resolved into their positive, negative and zero sequence components. (a) Zero-phase sequence systems: Faults involving earth give rise to zero-sequence currents which can be used as a basis for discrimination. The value of discriminative methods which isolate the zero-sequence component usually resides in their ability to ignore load currents and phase-tophase short circuits, thus permitting the use of earth-fault relay settings lower than load current values.

To understand this function of back-up protection it is necessary to explain that with every protective installation there is associated a 'protected zone' which is def'med, for a unit system, as the zone lying between the two or several sets of current transformers which together with the relays constitute the protective system; and, for a non-unit system, the zone lying between the current transformers and the point or points on the protected circuit beyond which the system is unable to detect the presence of a fault.

This perhaps rather fanciful approach illustrates very well the essentials of a unit protection system, which can be stated thus: (i) there must be fault-sensing means at each end of the protected circuit; (ii) there must be means of communication between the ends so that the fault-sensing means at each end is simultaneously acquainted with conditions at the other end (or ends), so that its tripping function can be exercised or not as appropriate. 28 Protection principles and components (h) Phase-comparison method: In situations where the means of communication between circuit ends is unable to provide a linear transmission of current amplitude information, a comparison between relative phase angles is often possible.