Protective relaying circuit

Claim 1

PROTECTIVE RELAYING APPARATUS COMPRISING, (A) AN A.C. AMPLIFIER RESPONSIVE TO FIRST AND SECOND SIGNALS, WHICH SIGNALS HAVE PREDETERMINED DIFFERENT FREQUENCIES, (B) A DISCRIMINATOR ENERGIZED BY THE OUTPUT VOLTAGES OF THE AMPLIFIER, SAID DISCRIMINATOR PROUDCING A D.C. OUTPUT VOLTAAGE HAVING A SIGN DEPENDING UPON THE SIGNAL RECEIVED BY THE AMPLIFIER, (C) A PAIR OF D.C. AMPLIFIERS, ONE AMPLIFIER PROVIDING AN OUTPUT VOLTAGE WHEN THE DISCRIMINATOR OUTPUT VOLTAGE IS OF ONE SIGN AND THE OTHER AMPLIFIER PROVIDING AN OUTPUT VOLTAGE WHEN THE DISCRIMINATOR OUTPUT VOLTAGE IS OF REVERSE SIGN, (D) A FIRST RELAY HAVING AN OPERATING COIL ENERGIZED BY THE OUTPUT VOLTAGE OF SAID ONE D.C. AMPLIFIER AND A SET OF NORMALLY-OPEN CONTACTS, (E) A SECOND RELAY HAVING AN OPERATING COIL ENERGIZED BY THE OUTPUT VOLTAGE OF SAID OTHER D.C. AMPLIFIER AND HAVING A SET OF NORMALLY-CLOSED CONTACTS, (F) A CONTROL MEMBER HAVING AN OPERATING COIL, (G) A SOURCE OF VOLTAGE, AND (H) CIRCUIT ELEMENTS CONNECTING THE CONTROL MEMBER TO THE SOURCE OF VOLTAGE THROUGH BOTH SAID SETS OF RELAY CONTACTS.