Electrical-fault detector for detecting fault currents having an appreciable harmonic content

Claim 1

AN ELECTRICAL-FAULT DETECTOR COMPRISING A POWER SUPPLY LINE, SENSING MEANS COUPLED TO SAID POWER SUPPLY LINE FOR DEVELOPING A SIGNAL DEPENDENT ON THE LINE CURRENT, SAID SENSING MEANS COMPRISING A TRANSACTOR HAVING A NON-LINEAR FREQUENCY RESPONSE WHEREBY TO ACCENTUATE THE MAGNITUDE OF THE HIGH FREQUENCY COMPONENTS RELATIVE TO THE LOW FREQUENCY COMPONENTS OF SAID SIGNAL, FILTER MEANS FOR SEPARATING THE FUNDAMENTAL AND LOW HARMONIC FREQUENCY COMPONENTS FROM SAID SIGNAL, CONTROL MEANS RESPONSIVE TO THE REMAINING PART OF SAID SIGNAL AND OPERABLE TO DERIVE A CONTROL SIGNAL THEREFROM, AND DETECTOR MEANS OPERABLE IN RESPONSE TO SAID CONTROL SIGNAL UPON THE MAGNITUDE THEREOF EXCEEDING A PREDETERMINED VALUE WHEREBY TO DETECT ONLY SUCH FAULTS AS PRODUCE A DISTORTED LINE CURRENT HAVING AN APPRECIABLE CONTENT OF HARMONICS OF HIGHER ORDER THAN SAID LOW FREQUENCY COMPONENT.