Remote line test facility

A circuit for testing one subscriber line of a pair-gain telephone system, the subscriber line including an end-of-call indication relay to insert an end-of-call resistance connectable in the subscriber line, an opto-coupler for loop current and ring-trip detection and having a line voltage applied thereon, the circuit comprising a voltage source to provide from the line voltage an output voltage isolated from other subscriber lines, the circuit further comprising;

• a voltage comparator having a pair of inputs and an output;
• two resistances serially connected to provide a fraction of the line voltage for use as a reference voltage to one of the inputs to the comparator;
• and
• test means for testing leakage resistance on the line, comprising additional contact means on said end-of-call indication relay to connect the output of the comparator to the optical coupler in the test mode thereby enabling the said processor to process the comparator output signal, any leakage resistance forming a potential divider with the end-of-call resistance to which the other of the inputs to the comparator is connected, thereby enabling the comparator to compare the fraction of the normal line voltage generated by the leakage resistance in series with the end-of-call indication resistance with the reference voltage and to generate an output test signal indicative of the leakage resistance being above or below a predetermined value.