Abstract: A method of locating faults in a telephone system including the steps of connecting antiparallel rectifying switches (ARS's ) that inhibit alternating current (A.C.) flow in one direction but not the other between subscriber equipment and respective telephone company lines, applying an A.C. test signal to a telephone company line and measuring a resulting signal, and determining whether a fault is on the telephone company line or customer equipment based upon the resulting signal. Also disclosed is an ARS that includes a first input and output, a second input and output, and paths between respective inputs and outputs providing low turn-on thresholds in one direction between respective inputs and outputs and high turn-on thresholds in the other direction, the directions of low and high being the same for both sets of inputs and outputs.