Patent GB-482444-A (GB)
Title: Improvements in or relating to electric switchgear
IPC: F15B13/01
Filed: 1936-09-29
Abstract: 482,444. Switchgear structures. REYROLLE & CO., Ltd., A., and HARLE, J. A. Sept. 29, 1936, No. 26418. [Class 38 (v)] [Also in Group XXXVI] In switchgear of the kind having currentcarrying members surrounded with insulation supported by metal casings or other metal parts which are normally earthed, in order that the insulation may be subjected to routine watts-loss tests to determine if deterioration thereof is occurring, each metal casing or other normally-earthed part is provided with an independent, detachable earth-connection and is insulated from adjacent earthed metal parts, whereby it can be isolated to permit testing of the associated insulation. Open-type gear having insulating bushings supported by earthed rings or flanges can be tested in this way, as well as metal-clad gear. Figs. 1, 2 show a metal-clad, compound-filled, three-phase, horizontal draw-out structure having duplicate bus-bars arranged in separate chambers D, D<1> which communicate through metal trunks D<2> with the chambers of adjacent panels. A transformer chamber E contains current transformers E<2> on bushing insulators E<3> surrounding conductors E<1> which lead from fixed sockets F<5> in orifice insulators, through further bushing insulators G<2> into a cable-dividing box G. A further set of orifice insulators E<4> is provided for a withdrawable potential transformer H, and orifice insulators F with socket contacts F3 are associated with each bus-bar-set. The movable gear comprises an oil-immersed breaker A having fixed plug contacts B<4> and removable selector plug contacts B<5>. Chambers D, D', E are mutually insulated by being supported on insulating feet D', E<6>, and separately connected by readily-detachable leads to a common earth-bar J, Fig. 4. The band joints D<5> between the trunks D<2> and chambers D, D<1> are each provided with an insulating ring D<6>, and an insulating layer G<3> isolates the cablesheath from the box G. Each bushing and orifice insulator in the fixed part of the gear, (Fig. 7) has an embedded metal layer K which, instead of being connected to the earthed metal casing K<1>, is connected to the common earth-bar J by a lead J<1> passing through insulation K<2>. In the insulators of the movable part, however, the leads are detachably connected to the metal casing, Fig. 8 (not shown). In both cases an insulating screen K<3> separates the insulator itself from the adjacent metal casing. Similar metal screens may be provided in or on any insulation which is to be tested separately. Fig. 10 shows how the Schering capacity-bridge is applied to the testing of bus-bar-chamber insulation. The two ends of the testing-arm of the bridge are connected respectively to the normally-live conductors within the insulation to be tested and to the earthing-lead J<1> which has been detached from the common earth-bar J. The other arms comprise the standard condenser L<2> and resistances L<3>, L<4> with variable shunting-condenser. The supply transformer is shown at L and a vibration galvanometer at L<1>. In order to ensure that the capacity between adjacent casings due to the introduced insulation does not affect the measurement, the adjacent casings are brought to the potential of the tested one by means of a capacity M and resistance M<1> adjusted so that the potential of the junction-point equals that of the equipotential points of the bridge. Alternatively, one side of the secondary of L may be earthed and the junction-point connected to the casing under test, Fig. 11 (not shown), or the junction-point may be connected to metal screens in the insulation between two adjacent casings, Figs. 12, 13 (not shown). A method of making the test is described in the Specification.