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Travelling waves on e.h.v. transmission lines often cause considerable distortion of relaying measurands, and, in this paper, modern methods of digital simulation are used to examine the performance of crosspolarised mho relays under more realistic conditions than have hitherto been possible using conventional conjunctive test methods. It is shown that travelling-wave effects can cause a significant delay in the fault-detection process and that, in this respect, the bandwidth of the voltage transformers (v.t.s) is an important influencing factor. The extensively used block-average comparator arrangement is examined in detail, and it is shown that a recently developed processing technique can provide faster fault-clearance times when coupled with a small degree of overreach. The paper concludes with a brief consideration of the performance of the block-average comparator mho relay in a long-line application.