Patent GB-520748-A (GB)
Title: Improvements in or relating to the testing of electrical insulation
IPC: G01R27/26
Filed: 1938-10-31
Abstract: 520,748. Capacitance and power-factor measurements. REYROLLE & CO., Ltd., HARLE, J. A., KIRKWOOD, 1. W. A., and LIVINGSTON, E. A. Oct. 31, 1938, No. 31404. [Class 37] In testing the insulation of metal-clad switchgear &c. for watt-loss the connections are such that the capacity of the insulation under test is in series with the capacity to earth of the external circuits or apparatus to which the insulated conductor is connected so that the whole or nearly the whole of the voltage of the testing transformer is applied across the insulation under test and little or none of such voltage appears across ,the earth capacity of the external circuit. The application to the testing of an orifice insulator A of the condenser bushing type employed in metal clad switch gear is shown. The insulator is, surrounded by a normally earthed metal casing B and encloses a conductor C connected to a feeder cable and having its end within the insulator connected to a metal flange C<1> which is connected to the innermost metal layer A<1> of the insulator A. The insulator has an outer metal sheath A<2> within and insulated from the casing B. A conductor D from the sheath A passes through an insulator B<1> and is normally connected to the earthed casing by a removable link D<1> protected by a cover B<3>. The testing circuit comprises a Schering bridge E, the testing voltage being supplied from the secondary winding F of a transformer to opposite corners E<1>, E<3> and a vibration galvanometer G being connected between the other corners E<2>, E The bridge arm E<1>, E<2> contains a standard condenser E the arm E<2>, E 3 contains a resistance E 5 shunted by a variable condenser E<7>, and a variable resistance E<8> is connected in the arm E<3>, E<4>. The corner E<1> is connected to the sheath A<2> through the conductor D, the conductor C and the corner E<4> being earthed at H<2>, H'. The insulation between the sheath A<2> and the casing B, and also the insulator B<1> must be capable of standing the full test voltage. The earth capacity C<2> of the feeder cable &c. is short-circuited by the earth connection H<2>. The normally earthed casing B may be connected to the corner E<3> of the bridge. Alternatively, the casing B may be earthed, a second set of tests being made to separate the losses in the insulator A from those in the secondary insulation between the sheath A<2> and the casing B. For this second set of tests the conductor C is connected to'the corner E<3> so that the dielectric losses in the secondary insulation. alone are measured. Alternatively the connections H<1> H<2>, H<3> are removed and the conductor C is connected to the corner E<4> but not to earth, as shown at H<4>. The corner E<3> is connected to the casing B as shown at H<5>. Thus, the series circuit including the capacity of the insulator A and the earth capacity C<2> is joined between the corners E<1>, E<3> of the bridge, the variable resistance E<8> shunting the earth capacity C<2>. In a modification, the corner E<1> is connected both to the casing B and the sheath A<2>, the casing B being insulated from earth. In another arrangement, the corner E<1> is connected to one of the internal condenser layers, the sheath A<2> being connected to the corner E<3>. With this arrangement, the earth connection to the conductor C may be omitted, in which case the earth capacity C<2> is not short-circuited but is in series with the capacity between the conductor C and the intermediate condenser layer in the test arm E\ E <4>. The test may be carried out whilst the conductor C is alive and in service. Specification 482,444 is referred to.