The features of large networks which are important in the design of selective protective systems are discussed.The characteristics of distance protective relays are considered, and the causes which lead to inaccuracy in their operation are enumerated. It is shown that nearly all these causes are due to the network and not to the relays. The conditions governing the minimum safe clearance times and minimum length of line that can be protected are discussed.An analysis is given of the magnitude and distribution, between earth and the conductors, of earth fault currents on systems with multiple earthing of the neutral point, and the sources of error resulting from this distribution and means of compensating for this error are given.Protective systems depending on direction of fault current are discussed and the “duplex” system is described in detail. It is shown in what way the conditions in a network during a fault may tend to occasion incorrect operation of relays of this type, and how such tendencies may be overcome in the design of the protective gear.The difficulties in the design of protective systems introduced by the distortion of the vector diagram during a fault are explained, and it is suggested that these difficulties may be overcome if the distortion itself is used for relay operation, as may be done by employing negative and zero phase-sequence components.