Patent GB-576451-A (GB)
Title: Improvements in spark testing apparatus for insulated electric cables and wires
IPC: G01R31/02
Filed: 1944-05-10
Abstract: 576,451. Locating faults in insulation. HENLEY'S TELEGRAPH WORKS CO., Ltd., W. T., and SAVAGE, J. H. May 10, 1944, No. 8890. [Class 37] In spark-testing apparatus for electric cables or wires, wherein the cable is passed continuously through a test electrode, e.g., as in Specification 451,768, the electrode consists of a number of groups of chains adapted to form a tube surrounding the cable, each chain forming a segmental part of the tube wall and being supported at two spaced supports one of which is a guide through or over which the chain may move against the action of force tending to hold a part of the chain between the supports in contact with the cable surface. The force may be provided by the weight of a part of the chain belowthe guide or by an additional weight or spring. Two chains per group may be used, but a larger number is preferable. Fig. 1 shows diagrammatically one group of chains, comprising four chains C1 .. C4, supported from points S1 .. S4, respectively, located above the electrode axis O and passing through guide supports G1 . . G4. The chains are held in tension by weights W suspended from their lower ends, and in the absence of a cable, hang to form a quadrilateral embracing the electrode axis. Fig. 1a shows how the chains are deflected when a cable is present in the electrode. Figs. 1b to 1f (not shown) show how the chains contact the cable when eccentric to the electrode axis and for varying sizes of cable. The complete electrode consists of a number of these groups of chains one behind the other in the direction of travel of the cable. To enable the cable to be inserted in the electrode without being threaded through the various chain groups, the support points S1, S3 and S2, S4 respectively, are mounted on arms A1, A2, Fig. 2a, carried by brackets B1, B2, pivoted at P so that the arms may be turned down to the position shown to permit insertion of the cable after which the arms are turned up to the position indicated in Fig. 2c (not shown), where the chains embrace the cable as in Fig. 1a. The lower support points G1 .. G4 are guide apertures in a trough-shaped plate F. In practical form the chain group support points S1, S3 and S2, S4 are fingers 8, 9, Figs. 3, 5 disposed in pairs on longitudinally extending bridge members 7 secured at each end by insulating supports 6 to sector shaped end plates 5 rockable about pivots 12 in fixed upstanding arms 4 of a bracket 3 secured to a metallic base 2. The fingers on the one bridge 7 are staggered with respect to those on the other. Insulating supports 10 carry a trough-shaped plate 11, four longitudinal rows of holes in which form the lower guide supports for the chains Cl .. C4 of the various groups. The guide holes G1, G2 may be slotted transversely so as to reduce the length of chains C1, C2 and the vertical movement of the weighted ends thereof. These guide are also displaced axially relative to the correspondnig support points 8, 9. Providing that the cable is run through in the direction of arrow IV the friction of the chains in their guides is thus reduced. The trough 11 may instead of being carried on insulators 10 be carried on chains or rods secured to the insulators 6, or each end may be supported by a pair of insulators extending horizontally one from each end of a bar centrally supported at pin 12. These arrangements avoid insulating supports below the cable run where they are prone to contamination. To prevent damage to the electrode by projections on the cable the chains C1 .. C4 may be connected to their respective weights W or springs by a frictional grip which is released when the pull reaches a predetermined amount. In the arrangement shown the weights W are held on the chains by split pins 13 which grip frictionally in holes in the weights and release the weights should the chains be momentarily caught up by a projection on the cable. Alternatively, the length of each chain beneath the lower support may be increased and the weight W dispensed with. The electrode is screened by fixed end walls 14, side walls 15, and by cover plates 16 secured to the arcuate edges of sector plates 5. The screening walls and cover plates may be of perforated sheet metal or gauze but the side walls 15 and covers 16 are preferably made of a transparent plastic for visual observation of the cable under test. Any suitable operating gear may be used to swing the sector plates 5 about their pivots 12. As shown handles 21 are connected at each end of the electrode to links 19 pivoted at 20 and connected by a pin and slot mechanism 23, 24 to a vertically movable link 22 pivotally secured at each end to links 25 pivotally fixed at their other ends to the sector plates 5, and biased by a tension spring 26. This spring holds the sector plates in either the open or closed position, movement of the plates being effected by substantially vertical movement of the operating handles 21. The high tension supply to the electrode may be controlled by a press-button switch 27 inserted on the primary side of a stepup transformer supplying the electrode, the switch being closed by a spring contact 28 on one of the links 19 only when the electrode is in its operative position. As an additional safeguard the connection from the H.T. supply terminal 29 to the bridge 7 is by a chain 30 which contacts the earthed base 2 when the electrode is opened. The bridge members 7 may move lineally instead of rotatably as shown. According to the Provisional Specification, the plate housing the guide supports for the chains may carry at each end two pivotally mounted brackets from each of which extends a longitudinal bar connected with the corresponding bracket at the other end. These bars may carry the arms on which the support points are provided.