GB531762A - Improvements in or relating to electrical testing arrangements - Google Patents

Abstract

531,762. Detecting faults. GENERAL ELECTRIC CO., Ltd., CROFT, E., H., and RANDALL, H. July 24, 1939, No. 21478. [Class 37] In a trolley bus in which electrical apparatus is insulated from the chassis by double insulation, the neutral conductors between the primary and secondary sets of insulation are connected to a testing point so that both primary and secondary sets of the insulation may be tested. The leakage testing equipment for a trolley bus supplied with D.C. from trolley wires 1, 2, the negative wire 2 being earthed, comprises a socket element 3 on the bus and a plug element 4 at the testing station. Some of the electrical apparatus of the bus is provided with double insulation, the secondary sets B of which comprise porcelain insulators for insulating the frames C, of the apparatus from the chassis 31, the apparatus being insulated from the frames by the usual primary insulation A. The socket element 3 comprises eight fingers 20 ... 27, six 21 ... 26 of which are connected to apparatus frames, the remaining fingers 20, 27 being connected to opposite sides of the positive live switch 7 with the interposition of a high resistance 28 between the fingers 20 and the power circuit. A contact 29, adapted to engage a contact 30 on the plug element 4, is connected to the chassis 31. The plug element 4 has six contacts 32 ... 37 connected together and arranged to engage the fingers 21 ... 26, and two contacts 38, 39 connected together and arranged to engage the fingers 20, 27. When the plug is inserted, all the frames C are connected together, the switch 7 is bridged by the high resistance 28, and the contact 30 is connected to the chassis. A milliammeter 47 is connected between the contact 30 and earth and indicates any leakage from the power circuits to the chassis. By depressing a pushbutton 48 the secondary sets of insulation B are short-circuited and a leakage test is obtained across the primary sets of insulation A. By depressing a second push-button 49 the primary sets A are short-circuited and a test is obtained across the secondary sets B. In a modification the testing connections are made by a drum switch on the bus.