When double-circuit transmission lines are operated with one of the circuits isolated and earthed at both ends, relaying conditions can arise that influence the choice of the forward-reach settings of distance protection. Earth-fault conditions during single-circuit working in this way give rise to apparent-impedance values presented to the protection that are generally less, and to errors in impedance measurements that are correspondingly greater, than for the same fault conditions during normal operation, when both circuits are in service together. The paper reports computer studies which seek to compare the discriminative performance of distance protection under earth-fault conditions during double-circuit and single-circuit operation and which, in particular, provide an account of the relaying conditions likely to be encountered in single-circuit operation. Apparent impedance values and associated relaying errors are summarised in tabular form for fault and prefault conditions relevant to a typical 400 kV interconnection, on the basis of which an assessment of the discriminative performance of distance protection in single-circuit working is made.