Patent GB-881415-A (GB)
Title: Apparatus for testing an insulated electrical conductor
Inventor: CRUTCHLEY JOHN
IPC: G01R31/02
Filed: 1959-07-11
Abstract: 881,415. Testing cable insulations. ERICSSON TELEPHONES Ltd. July 8, 1960 [July 11, 1959], No. 23881/59. Class 37. In a cable insulation testing apparatus the cable is drawn through a test electrode to which is applied a test potential, and which is associated with a detector responsive to current caused by the test potential when the insulation under test breaks down; the detector comprising a transformer having a primary winding connected to the test electrode and a secondary winding connected to the control electrode of a switching device. The insulated cable 2, wound on a metal feed drum 1 is drawn through test electrodes 3, 4 by metal winding drum 5; the two test electrodes being formed from strips of metal chain mesh. The test electrodes together with an auxiliary test electrode 8, the latter comprising a metal rod 9 enclosed within a sleeve 10 of insulating material provided with slot 11, are connected to the circuit as shown in Fig. 2. The self-capacitance of an electrostatic voltmeter 13 is made equal to the self-capacitance of the auxiliary electrode 8 by a short length of conductor 14. A test voltage to the electrodes is obtained from a source of alternating current E applied to transformers T1, T2. The test electrodes 8, 3 and 4 are connected to the primaries L7 and L5 respectively of transformer T3, the secondary windings L8, L6 of which are connected to the control electrodes of two gas-filled tetrode valves V2, V1 and two neon lamps N2, N1, the latter protecting the valves V1, V2. When the portion of cable passing through the electrodes 3, 4 has no defect, the capacitive and inductive leakage currents which flow through the primary windings L5, L7 by way of earth are equal and opposite so that the potentials developed across the secondary windings L6, L8 and applied to the grids of V1, V2 are equal and the valves are biased so as not to conduct. If the winding drum 5 draws a portion of cable 2 with defective insulation within the electrode 3 then the resultant increase in current flows through resistor R3, winding L4, resistor R4 and winding L7, the latter causing a potential to be induced in both secondary windings L6, L8, connected in opposition, and passed to the grids of V1, V2, the valve which receives the positive pulse conducting and operating relay A. Thus, the relay A can be caused to operate regardless of whatever half-cycle the test voltage is in. The relay A causes the drum 5 to be stopped, the cable to be marked at the defective place, a warning to be given and the current source E to be disconnected. The position of the defect is then found more accurately using the test electrode 8, when the cable is passed through the slit 11 and by means of a spring-loaded switch (not shown) the detector circuit is disconnected and the fault located when a continuous spark is produced between the metal of the electrode and the cable. Should two defects appear close together and one be missed by test electrode 3 then on re-starting drum 5 this defect will be detected by test electrode 4.