Abstract: An overcurrent protection arrangement comprises a switching circuit that is intended to be series connected in a line of the circuit to be protected and which will allow normal circuit currents to pass but will open when subjected to an overcurrent. The arrangement includes a pulse generator which will generate pulses when the switching circuit has opened that reset, or attempt to reset, the switching circuit to its conducting state. Only a predetermined finite number of pulses or pulses for a predetermined finite time are generated, so that the circuit will quickly function again after having been subjected to a current transient but will not continually attempt to reset in the case of equipment faults.

Abstract: A two terminal circuit protection arrangement that: (1) is intended to be series connected in a line of the circuit; (2) comprises (a) a series switching transistor (1) that controls the line current, (b) a control transistor (4) that controls the base or gate voltage of the switching transistor and is responsive to an overcurrent through the switching transistor, and (c) a voltage source, for example a battery (3), a dc-dc converter (58/59) or a Seebeck device (43), applied to the base or gate of the switching transistor which biases the switching transistor into or toward conduction in normal operation; and (3) is capable of being remotely reset into a conducting state by stopping current in the line. The arrangement enables the initial voltage drop that is required to turn the switching transistor (1) on to be reduced or eliminated while requiring relatively little current from the voltage source. The arrangement can be reset remotely by briefly removing the voltage source or load from the circuit.

Abstract: A switching arrangement that can be connected in a communications channel that comprises a pair of lines between sets of terminal equipment, which comprises: (i) a series switch (1,2) for connection in each of the lines; (ii) a shunt switch for connection between the lines; and (iii) a control circuit (5) that can actuate the series switches (1,2) and can actuate the shunt switch on receipt of a signal sent along the channel: wherein the control circuit (5) can actuate the shunt switch and the series switches (1,2) on receipt of one or more signals but the shunt switch will remain closed over a different time period than that during which the series switches remain open, in order to allow different tests to be performed on the channel, and wherein one or more of the switches comprises a solid state switch.

Abstract: A maintenance termination unit (MTU) for use in telephone circuits. The MTU comprises a switching arrangement which can be connected between sets of terminal equipment in a communications channel comprising a pair of lines. The switching arrangement comprises: a DC voltage window detector circuit which, in use, is connected between the lines and is responsive to a voltage between the lines; and series switching circuits, which, in use, are connected in the lines. The arrangement may also include a shunt switching circuit, which, in use, is connected between the lines. The arrangement includes low pass filters associated with the switching circuits. The DC voltage window detector circuit will actuate the switching circuits when the DC voltage between the lines is within respective predetermined bands, but will not actuate the switching circuits when the DC voltage between the lines is not within the respective predetermined bands.