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Differential feeder protection, in general, uses a single relaying quantity derived from all phases of the system being protected. The paper contains a comprehensive analysis of the nature of this relaying quantity for the whole range of single shunt faults and all likely combinations of phase-sequence quantities extracted from the faulted system. The analysis includes the well-established case of the summation transformer, this device having an output which contains positive, negative and zero phase-sequence components of current. The analysis is presented in the form of curves giving the relaying quantity in terms of the important components of the various phase-sequence impedances of the power system. From these curves it is possible compare the different ways of deriving the single-phase relaying quantity and thence to specify the general rules for selection phase-sequence networks. In order to provide a comprehensive method for determining a differential relaying quantity in magnitude and phase, a general chart has been developed; from this, the magnitude and phase of the relaying quantity, derived from any specified phase-sequence network, may be obtained graphically in terms of the positive and zero phase-sequence reactances and the zero-sequence resistance of the power system. Finally, the effect of load current on the magnitude of a derived relaying quantity has been considered. The corresponding effect on phase has not been treated since it has already appeared elsewhere, although the results have been included for purposes of comparison and completeness.