Abstract: A protection arrangement for coupling a two-wire line to a SLIC includes surge protecting triacs between each wire and ground, and normally closed contacts coupling the wires to the SLIC. The triacs are triggered via varistors coupled to the wires on the SLIC side of the contacts, which are opened in response to over-voltages on the line, as detected via a diode bridge and threshold circuit. The triacs are thereby not triggered during over-voltage conditions so that their power dissipation is reduced during such faults, whereas they respond rapidly to surge conditions which accordingly need not be handled by the contacts.

Abstract: A loop closure detector for a two-wire telephone line comprises a four-arm resistive bridge, in which the four arms are constituted respectively by a first battery feed resistor, the two-wire line in series with a second battery feed resistor, and two resistors forming a potential divider across the battery voltage which is coupled across one diagonal of the bridge. A voltage sensing circuit, constituted by a comparator or a light emitting diode, is connected across the other diagonal of the bridge to provide an output representing a loop closure state of the line. A ring-ground detector is also described using a similar bridge and voltage sensing arrangements.