Abstract: A protection arrangement for a two-wire line, such as a telephone subscriber line, includes a transformer having two primary windings and a secondary winding. The primary windings are connected in series with the wires of the line with phases such that common mode (longitudinal) currents on the line induce a current in the secondary winding, whereas loop currents do not. The induced current in the secondary winding is amplified and used to trigger a protection device, such as a triac or SCR and diode bridge arrangement, to short circuit the line for the duration of any excessive current thereon.

Abstract: A circuit for protecting sensitive telephone equipment from voltage or current overloads on the TIP or RING line includes a resistor serially positioned in the line and a triac triggered by a voltage differential across the resistor. A pair of fast diodes is positioned on the line adjacent to the telephone equipment for momentarily clamping the voltage on the line at that point between ground and the voltage supply of the equipment. The mementary clamping of the voltage creates the voltage differential across the resistor which triggers the triac. The fast diodes are capable of passing current for a time sufficient to allow the triac to begin shunting current to ground. An enhanced form of the protection circuit includes a second pair of diodes extending in parallel with the fast pair of diodes and positioned on the line between the pair of fast diodes and the triac. Inclusion of the second pair of diodes allows the circuit to utilize a triac with a slower turn-on time and a higher on-state voltage.

Abstract: The contacts of an isolation relay are positioned between a line interface circuit and the associated feed resistors, the relay being controlled to open the contacts in response to an excessive common mode alternating current through the feed resistors. The common mode a.c. is detected by a sensing amplifier, and is integrated and compared with a threshold level in a dectector unit. If the common mode a.c. is excessive, a zero crossing predictor enables the detector unit to control the isolation relay and open its contacts at a zero crossing of the a.c. A phase shifter in the zero crossing predictor compensates for the response delay of the isolation relay. As the excessive current is now interrupted, the sensing amplifier responds to the line voltage. When this falls below a predetermined peak level, a delayed resetting of the protection arrangement is automatically effected.