Current detection circuit arrangement for an automatic telephone exchange

A circuit arrangement for an automatic telephone exchange for detecting currents in a system of two conductors having earth as the common return conductor, comprising:

• a pair of terminals for said conductors, a supply battery and resistor, each one of said terminals being connected to different poles of said supply battery by way of said resistor and further comprising a first pair of resistors arranged in series between the one pole of said supply battery and the other terminal and a second pair of resistors arranged in series between the other pole of said supply battery and the one terminal, the junction of the resistors of said first pair and the junction of the resistors of said second pair constituting a pair of connecting points for the connection of a detector for detecting the differential-mode component of the currents in the two conductors, means for the detection of the common-mode component of the currents in the two conductors by forming the sum of the voltages at the said junctions of said first and second pair of resistors, respectively, and means for comparing said sum of the voltages with the potential at a point of reference potential, an indication of the common-mode component of the currents in the two conductors being derived from said last-mentioned means.