GB1169243A - Dielectric Testing - Google Patents

Abstract

1,169,243. Insulation testing. IMPERIAL CHEMICAL INDUSTRIES Ltd. Jan. 10, 1968 [Jan.23, 1967], No.3363/67. Heading G1U. A dielectric specimen, which may be the insulation of a wire or a continuous insulating sheet, is tested while moving continuously by a method wherein the applied voltage is reduced to zero immediately a fault is detected. A linearly increasing A.C. voltage produced by modulating the output from a ramp generator is applied across the specimen as it passes through a test cell. When a discharge is detected in a resonant circuit coupled to the cell, a pulse is produced which triggers a multi-vibrator to consequently reset the ramp generator to zero, after which time the ramp generator output voltage starts to rise again. The maximum values reached by the ramp are recorded. The system is suitable for on line testing as coated wire is manufactured using an A.C. frequency of 1000 cps., and responds to discharges of 10<SP>-12</SP> coulomb, so avoiding a large discharge passing through the insulation when a fault is detected, so that no burning out occurs. A suitable test cell for the cable test incorporates a water filled cavity (1, Fig.3) through which the cable passes via rubber sealing diaphragms (3) held in place by nylon bolts (4) having central holes for the passage of the cable. The test potential is applied to the water filled cavity, while the cable core is connected to earth through an inductor, which together with the stray capacitance forms the required resonant circuit. A good specimen will show a record of discharge inception voltages (D.I.V.) which are uniformly high-faults will be shown by D.I.V.'s appearing in the record.