Abstract: A switching arrangement that can be connected in a communications channel comprising a pair of lines, between sets of terminal equipment, which comprises:(i) a pair of series switching circuits, each of which can be series connected in one of the lines and will open when subjected to an overcurrent in its associated line; and/or(ii) one or more shunt switching circuits connected between the lines, or between one or both the lines and earth, and will close when subjected to an overvoltage in its associated line;wherein the or each switching circuit can be remotely actuated by means of a test signal sent along the channel.

Abstract: A circuit protection arrangement that is intended to be series connected in a line of an electrical circuit comprises a transistor switch 1 that controls the circuit current and has a control input, and a control arrangement that controls the voltage of the control input of the switch. The control arrangement comprises a comparator circuit 3 that compares a fraction of the voltage across the switch 1 with a reference voltage, for example supplied by a Zener diode or bandgap diode, and opens the switch if the fraction is greater than the reference voltage. The comparator circuit is powered by the voltage drop in the line across the switch so that the arrangement can be formed as a two-terminal device that requires no external power source.The arrangement has the advantage that it exhibits a relatively small variation in performance with respect to temperature variations.

Abstract: A number of arrangements for protecting an electrical circuit from an electrical overcurrent are disclosed. In the arrangements the base or gate of a switching transistor (4, 101, 161, 222, 401) is held in a voltage divider, one arm of which comprises a control transistor (6, 102, 162, 222, 404) whose base or gate is itself held in a voltage divider. When an overcurrent is experienced the control transistor switches on causing the switching transistor to turn off. The arrangement may include a shunt switching circuit to shunt the overcurrent across the load. It may include a resetting circuit that will attempt to reset the switching circuit after the overcurrent has been experienced. In one embodiment the arrangement includes means such as a battery, dc--dc voltage converter or Seebeck device in order to reduce or eliminate any initial voltage drop across the switching circuit.