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 converter for connection between an electrographic sensor and an electrographic controller, and comprising: (a) a first set of contacts for connection to the sensor; (b) a second set of contacts for connection to the controller; and (c) an emulator for emulating at the second set of contacts: (i) a four-wire sensor when the first set of contacts is connected to a five-wire sensor and a second set of contacts is connected to a four-wire controller; and/or (ii) a five-wire sensor when the first set of contacts is connected to a four-wire sensor and a second set of contacts is connected to a five-wire controller.

Abstract: A switching device for a telecommunications channel. The device comprises a voltage detector, a switch, and a storage device. The voltage detector produces an output voltage if the voltage between the lines of the channel exceeds a predetermined level, for example 110 volts. The switch can increase an impedance in one of the lines (preferably causing open circuit) in response to an input voltage. The first storage device, for example a capacitor, produces the input voltage when it reaches a charge V.sub.2 ; is initially at a voltage V.sub.1, for example zero, that is less than V.sub.2 ; after charging by the output voltage from the voltage detector, is at a voltage V.sub.3 greater than V.sub.2 ; and discharges from V.sub.3 to V.sub.1 in the absence of the output voltage. The voltage detector comprising a low-pass filter such that an AC signal on the channel produces an output voltage that charges the first storage device to a charge below V.sub.2.

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.

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.