The functions of ground preference in carrier-current relay schemes

Almost all of the more common present-day carrier relay systems employ a feature known as ground preference which enables the ground relays, during faults involving ground current, to take preference over the phase relays in controlling the starting and stopping of the carrier signal. The specific system conditions which require this feature are not well understood, nor is it generally understood that under certain conditions ground preference can cause trouble, or that even in some cases phase preference is “preferable.” It is the purpose of this paper to discuss, in general, the reasons for and conditions pertaining to the preference feature.