FOERFARANDE FOER INMATNING AV EN PROEVNINGS- OCH HAOLLSTROEM FRAON EN BELAEGGNINGSKOPPLING TILL DEN 'ANKOMMANDE' C-LEDNINGEN VID EN TELEANLAEGGNING

摘要

In telephone exchanges the so called 'c' line carries the holding current for at least one holding relay responsible for maintaining the coupling condition between two subscriber lines or one subscriber and an intermediate exchange office. It occurs that spurious, induced voltages reduce the holding current in the 'c' line below 22mA (the allowable minimum) causing the relay and the entire connection to breakdown. To provide a safeguard against such occurrences an electronic current sensor is provided which monitors the holding line current and which, when the current drops below a predetermined value, causes an additional current source, operating from a substantially higher voltage gradient than that which the holding current normally derives from, to be fed into the 'c' line thus very rapidly restoring the current level. As and when thereafter the current level rises above a predetermined current value the current sensor causes the additional current source to be switched off again. This method eliminates spurious breakdowns of telephone connections during the coupling-in or the conversation stages.