ELECTRIC CABLE WITH PROTECTIVE DEVICE

Abstract: An electric supply cable (2) feeds a source of electric power from a sending end of the cable (2) to a receiving end of the cable (2) and includes, at or adjacent the receiving end of the cable (2), electrical impedance means (6) interconnecting the live and neutral lines of the cable (2) whereby, on conduction by the cable (2), the predetermined load of the impedance means (6) is imposed upon the cable (2), and, at or adjacent the sending end of the cable (2), electrical sensing means (8) constantly monitoring the load on the cable (2), the sensing means (8) being interconnected with switch means (10) such that, in the absence of the predetermined load of the impedance means (6), the switch means (10) are activated to disconnect the cable (2) from the source.

Description: ELECTRIC CABLE WITH PROTECTIVE DEVICE TECHNICAL FIELD This invention relates to electric cables incorporating protective devices, and has particular, though not exclusive, application to temporary supply cables for installation to enable repair to permanent supply cables . BACKGROUND ART It is well established practice in the electricity supply industry to incorporate in electrical power supply systems means to reduce the risk of electrical injury arising from a variety of abnormal operating conditions. For example, over current protection devices, such as fuses, commonly guard against the risk of overheating resulting from damage or interference to the insulation system of a cable, while earth fault detection devices guard against the risk of initially small currents flowing to earth as a result of damage or interference to the cable . In some power supply systems, additional circuitry is provided within the cable, typically flow and return lines additional to the conventional lines, to monitor constantly the integrity of the cable, and indirectly to indicate any breach in continuity. However, none of the above devices are capable of detecting an open circuit such as could occur if the individual wires of a conducting cable were inadvertently broken or disconnected and did not subsequently contact one another to effect a short circuit. Regional electricity companies are from time to time required to install temporary supply cables to customers ' properties in the event of faults arising in the permanently connected supply cables to enable repair to the permanent cables whilst minimising loss of supply to the customer. Such an expediency is however only practical if, in the particular circumstances, the temporary cables can be installed more quickly than the time that would be taken to repair the permanent cable . There is therefore a need for speed of installation of the temporary cables, and such speed must be achieved without unduly increasing the potential danger associated with the supply. Bearing in mind that the requirement for the temporary supply is indicative of a situation having a potentially higher risk of electrical danger, it would be desirable to be able to ensure that the installation of the temporary supply cable, as well as being quick, was such as to ensure adequately safe operating conditions. DISCLOSURE OF THE INVENTION According to the present invention there is provided an electric supply cable in which a source of electric power is fed from a sending end of the cable to a receiving end of the cable, characterised by, at or adjacent the receiving end of the cable, electrical impedance means interconnecting the live and neutral lines of the cable whereby, on conduction by the cable, the predetermined load of the impedance means is imposed upon the cable, and, at or adjacent the sending end of the cable, electrical sensing means constantly monitoring the load on the cable, the sensing means being interconnected with switch means such that, in the absence of the predetermined load of the impedance means, the switch means are activated to disconnect the cable from the source. Thus it will be appreciated that, in the event of, for example, an inadvertent open circuit in the cable, the sensing means senses the disconnection of the impedance means, and the switch means are activated to disconnect the cable from the source of power. The impedance means at or adjacent the receiving end of the cable may be reactive, inductive, capacitive or resistive, and may comprise, for example, an electrical resistor, a capacitor, or a low rated light bulb. Conveniently the sensing means at or adjacent the sending end of the cable comprises an inductive coil which feeds a signal to an electric device powered by the source preferably with the means to sense, compare and amplify signals to operate the switching means. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawing shows an electric cable according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, the cable is indicated generally at 2 and may be a two phase cable with live and neutral lines only, or a three phase cable with live lines only, or may additionally include an earth connection. The cable 2 has a sending end connected to a power source, and a receiving end for connection thereto of system to be powered. The sending end of the cable is connected to the power source through a protective device indicated generally at 4, while the live and neutral lines of the cable 2 at the receiving end thereof are interconnected by an impedance 6, typically a low rated light bulb, whereby, on conduction through the cable 2, a predetermined electrical load is connected across the impedance 6. The protective device 4 includes sensing means 8 incorporating an inductive coil which feeds a signal to an electronic device powered by the source, and switching means 10 operation of which interrupts the electrical supply from the source to the cable 2. Under normal operating conditions, and once the cable 2 is installed as required, current flows through the impedance 6 whether or not a load is connected to the receiving end of the cable 2. The protective device 4 is so arranged that, as long as the sensing means 8 detect the existence of the predetermined load of the impedance 6, the switching means 10 remain closed to maintain the circuit complete and intact . In the event that the conducting circuit of the supply cable becomes discontinuous, for example by inadvertent breakage or disconnection of one or more of the conducting lines without said lines touching one another, the predetermined load on the impedance 6 will be removed. This change in electrical condition is immediately detected by the sensing means 8, the switching means 10 are opened, and the cable 2 is disconnected from the power source. Thus any potential danger consequential of the open circuit is immediately eliminated. A preferred application of the cable of the invention is as a temporary supply cable installed, for example to a domestic residence, to enable repair to a damaged permanent supply cable. In such circumstances, the sending end of the temporary cable is connected to a suitable power source, for example the adjacent house or a convenient public service supply such as a lamp standard, and the receiving end of the cable is connected to the electric meter of the residence in question. It will be appreciated that the protective device incorporated in the temporary supply cable enables very high standards of safety to be provided with a minimum of installation requirements, such cables being capable of being installed in a wide range of situations by virtue of their inherent increased flexibility and compact size compared with conventional cables in which protection is achieved either mechanically by armoured sheaths or electrically by the incorporation of auxiliary circuits. It is anticipated that the cable of the invention will have numerous applications in the industrial, commercial and domestic fields where continuous monitoring of circuit integrity will offer benefits hitherto not practically available .

Claims: CLAIMS 1. An electric supply cable (2) in which a source of electric power is fed from a sending end of the cable (2) to a receiving end of the cable (2) , characterised by, at or adjacent the receiving end of the cable (2), electrical impedance means (6) interconnecting the live and neutral lines of the cable (2) whereby, on conduction by the cable (2) , the predetermined load of the impedance means (6) is imposed upon the cable (2) , and, at or adjacent the sending end of the cable (2), electrical sensing means (8) constantly monitoring the load on the cable (2), the sensing means (8) being interconnected with switch means (10) such that, in the absence of the predetermined load of the impedance means (6) , the switch means (10) are activated to disconnect the cable (2) from the source. 2. A supply cable as claimed in claim 1 in which the impedance mean (6) comprise an electrical resistor, a capacitor or a low rated light bulb. 3. A supply cable as claimed in any one of claims 1 to 3 in which the sensing means (8) at or adjacent the sending end of the cable (2) comprises an inductive coil which feeds a signal to an electric device powered by the source and which communicates with the switching means (10) .

Inventors: MAY HARRY STANLEY, MAY, HARRY, STANLEY
Applicants: MAY HARRY STANLEY, NORTHERN ELECTRIC DISTRIBUTION, MAY, HARRY, STANLEY, NORTHERN ELECTRIC DISTRIBUTION LTD
Application year: 1999
Main classification: H02H3
IPC codes: H02H3