Patent GB-456766-A (GB)
Title: Improvements in or relating to earthing arrangements for electric apparatus
IPC: H02H11/00
Filed: 1935-05-15
Abstract: 456,766. Resistance measurements. REYROLLE & CO., Ltd., A., Hebburn-on- Tyne, LEYBURN, H., 212, Wingrove Road, Newcastle-on-Tyne, LACKEY, C. H. W., Runswick, Benson Street, Chester-le-Street, Co. Durham, and CLOTHIER, G. D., 87, Jesmond Park West, Newcastle-on-Tyne. May 15, 1935, No. 14239. Addition to 429,503. [Class 37] In arrangements for testing earthing circuits of the kind described in the parent Specification, in which the indicating device comprises an ammeter or other measuring instrument, means are provided whereby a measurement is obtained which is substantially independent of any potential difference between the neutral conductor and earth, so that the instrument not only tests the continuity of the earth circuit but also measures its resistance. The testing equipment shown comprises a transformer with a primary winding B which can be connected between one of the conductors E<1> of the three-phase supply E<1> .. E3 and the neutral conductor E by means of a flexible conductor D and plug D<1>, an A.C. ammeter H with a main winding H<1> connected to one end of the transformer secondary winding B<1> through an adjustable resistance G and an auxiliary winding H<2> connected through an adjustable resistance G<1> to the other end of the secondary winding and, through the plug D<1>, to the neutral E, and a three position two-armed switch J<1>. In the normal position of the switch, shown in Fig. 1, the arm J<3> engages a contact L<1> connected to a neon indicating lamp P. The arm J<3> s connected through a flexible lead M to a wander plug M<1>, and if the latter is placed against the frame R of the apparatus under test, the lamp P lights up if the plug D<1> has been correctly inserted in its socket D<2>. The switch is then depressed to its lower or calibrating position, in which the arms J<3>, J4 engage contacts L<2>, L<3> and complete the circuits through the windings H<1>, H<2> of the ammeter. The current flowing through the winding H<1> depends on the secondary voltage of the transformer, i.e. on the voltage of the supply system, while the current flowing through the winding H<2>, if any, will be due to the voltage between the neutral conductor E and earth. The resistances G, G<1> are adjusted to bring the pointer of the ammeter to the full scale position by turning a knob G<5>, and the switch J<1> is then raised to the testing position, thereby opening the circuit of the winding H<2> and connecting the winding H<1> in series with the transformer winding B<1> and the earthing connections. The ammeter reading, multiplied by a factor depending on the setting of the knob G<5> which is indicated on a scale G<6>, gives the ohmic resistance of the earth circuit. In the case of an arrangement using a D.C. supply as the source of testing potential, a moving coil ammeter which is unaffected by alternating currents is used, or alternatively, filtering or choking coil arrangements are provided to prevent the ammeter from being influenced by alternating current due to the neutral potential. In one such arrangement, Fig. 3 (not shown), the testing equipment comprises a handoperated generator with means for connecting it to the neutral conductor and, through a moving-coil ammeter, to a wander plug which can be applied to the casing of the apparatus under test.