Patent GB-531591-A (GB)
Title: Improved arrangements of insulated metal-clad electrical apparatus affording facilities for testing the insulation
IPC: G01R27/26
Filed: 1939-07-27
Abstract: 531,591. Power-factor and resistance measurements ; couplings. ENGLISH ELECTRIC CO., Ltd., and BLANDFORD, A. R. July 27, 1939, Nos. 21850/39 and 10111/40. [Classes 37 and 38 (i)] A high tension conducting member within or passing through a bushing by which it is insulated from the surrounding protecting metal is in two parts normally electrically connected, but of which the second can be readilv electrically disconnected from the first, and so from the rest of the high-tension system, for the purpose of testing the insulation between the second part and earth or the surrounding metal, for example, for measuring power factor and insulation loss by a Schering or other bridge method. As shown in Fig. 1, a lead from H.T. apparatus is led out through an earthed metal spout 1 lined with a condenser type insulating brushing 2, the cable entering a terminal socket within the bushing. The terminal comprises an annular support 3, secured within the bushing 2, and a terminal socket 4 which is normally electrically connected to the support 3 but can be disconnected so that test connections can be made between the support 3 and earth. The support 3 is a cup which is a tight fit within or partially recessed into, the interior of the bushing 2 and is connected permanently to the inner layer of foil 2b by filling a hole in the bushing with soft metal 2c. Connection is made to the apparatus by a tail-piece 4a on the socket passing through a clearing hole in the base of the cup 3. The socket is provided with fixing lugs or a flange and is separated from the cup by insulating material 5. The parts 3, 4 are secured together without electrical connection by screws 8, which may be of insulating material or insulated by bushes and washers. A further screw 6 normally makes electrical connection between the parts 3, 4 but can be unscrewed to break the connection for test purposes. For carrying out the test a plug is inserted in the bushing from the open end of the spout and carries prongs which make contact respectively with the cup 3 and the socket 4. In a modification, Fig. 4, the terminal socket 4 is integral with the support, and a separate metal insert 9 is partially recessed into the bushing and is normally connected to the terminal by springs 10. When making a test, a plug 11 having an insulating member 11a, carried by one of the contact members 11b is inserted so as to break contact between the springs 10 and ring 9 and make contact between the ring 9 and conducting part 11b. A plug 11c makes contact with the socket 4. When applied to a complete three-phase single bus-bar pillar of draw-out switch-gear, three spouts 1 are provided on the bus-bar chamber so that the bushings in each may be tested separately. Fig. 5 shows the application to a bushing 2 with a conductor 4 passing completely through and fitting within the bushing. A ring 9 at one end of and outside the bushing 2 forms part of and is normally connected to the conductor 4. The end of the bushing is stepped down to expose a short length of the foil 2b, to which the ring 9 is permanently connected. Secured to the conductor 4 is a spring contact clip 10 normally in contact with the ring 9. When making a test, a plug 11 having two conducting prongs insulated from one another is inserted between the spring 10 and ring 9. Alternatively, the spring clip may bear against the annular end face of the ring and may be part of and project from a washer held on the conductor 4 and against the end of the bushing 3 by a nut on a screw-threaded part of the conductor.