GB350669A - Improvements relating to electrical measuring instruments - Google Patents

Abstract

350,669. Resistance measurements. EVERSHED & VIGNOLES, Ltd., ROLFE, G. B., and PERRY, C. E., Acton Lane Works, Chiswick, London. May 1, 1930, No. 13460. [Class 37.] An instrument of the type having a self-contained generator and a deflecting instrument is provided with a switch having three positions, in which the connections are arranged for insulation tests, Wheatstone bridge tests, and Varley loop tests. The generator has two armatures or armature windings A<1>, A<2> which are connected in parallel, as shown, for bridge and Varley loop tests and in series for insulation resistance tests. The switch S shown is of the rotary barrel type with fixed contacts 1 .. 16. In the position shown, connections are made for the Wheatstone bridge test. The potential coil P, in series with a resistance R, is connected in parallel with the armature windings A<1>, A2. The negative terminal of the generator is connected through contacts 10 .. 13 to the junction D of the ratio arms U, V, the positive terminal being connected through a resistance L and contacts 8, 2 to the junction of the variable arm Wand the resistance to be measured, which is connected between the earth and line terminals E, K. The current coil C is in series with a resistance T, which is short-circuited at contacts 6, 7. One end of the coil C is connected to the frame of the instrument, indicated by dotted lines, and thence by contacts 4, 3 to the junction of the arms U, W, the other end of the coil being connected through contacts 7, 6, 5 to one end of the ratio arm V and the link terminal K. For the Varley loop test the connections are the same, except that the connection between contacts 2, 8 is broken and the contacts 1, 9 are connected, thus connecting the positive generator terminal to a terminal M instead of to the terminal E. The terminal M is connected to a good earth, the line terminal K to the looped line, and the terminal E to the faulty line. For insulation testing contacts 2, 8 remain connected, contacts 11,12 and 15,16 also being connected. This puts the armature windings Al, A2 in series with each other and with the current coil C, resistances T, L and the insulation resistance to be measured, the potential coil P, in series with the resistance R, being connected across the generator terminals. The adjustable resistance W and ratio arms U, V are not effective in this test. The switch S may be of other types, or may consist of a two-way switch with positions for bridge and insulation tests and an auxiliary switch for changing from bridge to Varley loop connection.