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DISTANCE-TYPE relays have been used successfully for many years to protect against phase faults on transmission lines. However, distance relays to protect against ground faults in the same satisfactory manner as for phase faults have not been generally available. The chief reason for the delay in producing these is that a ground fault is much more involved than a phase-to-phase fault so that the simple use of line-to-ground voltage and line current does not produce the right answer. The error is caused by the fact that the zero-sequence-line impedance is not equal to the positive sequence impedance, therefore distance response is varied by changes in the distribution of the positive, negative, and zero sequence currents caused by changes in system connections outside the protected line section.