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An experimental general-purpose process-control digital computer system provides high-speed phase-and ground-distance fault protection of one 230 kV transmission line. The stored program performs all of the relaying functions using the output of an analog-to- digital converter which reads the instantaneous value of the power- system currents and voltages. Model line tests described in a companion paper (II -Test Results) form the basis for preliminary performance evaluation. An automatic oscillograph and diagnostic programs will allow a study of field performance.