Testing relays at the ends of a transmission line must include the presence of two synchronized time pulses, one at each end of the line, to generate a verifiable test result. GPS based precision clocks can provide the master time source required.
Many protection schemes require the proper coordination of relay settings at two ends of a transmission line. These settings are determined by engineers using models of the power system. Verification of proper operation is performed using two relay test sets, each connected to a relay at the two ends of the line. These test sets must be synchronized, so that they can generate fault waveforms with the proper time relationship at the two ends of the line.
These signals are presented to the relays, and proper operation of the relays and communications network can be verified. The higher the accuracy of the synchronization, the more precise the test results. Various relay test sets take different time signals (Arbiter clocks can support them all); see manufacturers requirements for performing a relay test.