GB549500A - Improvements in electric protective arrangements of the earth leakage type - Google Patents

Abstract

549,500. Protective cut-out arrangements. BRITISH THOMSON-HOUSTON CO., Ltd., TRENCHAM, H., and MORTLOCK, J. R. Aug. 13, 1941, No. 10282. [Class 38 (v)] [Also in Group XXXVI] An arrangement for giving warning of or protecting against the deterioration of the insulation of a section of metal-enclosed conductor, such as a busbar, comprises insulating the metal sheath of the conductor from earth and utilizing the current flowing to earth through the resultant capacitance to modify, after rectification the impedance of an A.C. reactor, the alternating current flowing through which is used for operating a detector. The insulation between the three-phase conductors A .. C and sheaths is denoted by condensers A1 .. C1 the lower plates of which are connected together and to earth through rectifier R arranged to supply a winding D of a reactor E, the winding F of which is in an A.C. circuit containing a relay G. Deterioration in the insulation of Al or B1 or Cl will cause a larger current to flow in coil D and so lower the impedance of the reactor sufficiently to permit operation of the relay. Greater sensitivity can be obtained by replacing the relay G of Fig. 1 by a similar arrangement of rectifier R1 and reactor E1, the relay G1 being similarly arranged in the cascaded circuit, Fig. 2. Alternatively or additionally, the circuit of G can include a bridge-rectifier the output of which supplies a further winding on core E so as to amplify the change in impedance of coil F, Fig. 3 (not shown). In Fig. 4 (not shown) a condenser is in the relay circuit so as to resonate at certain conditions of the current flow in winding D. In Fig. 5, two sets of conductors have the outputs of their rectifiers R, Rl, connected to balancing windings D, D1 on the reactor El, normal out-of-balance being compensated by rectified current fed to a coil K. A condenser L in the circuit of coil F is shunted by a rectifier R1, the output of which supplies an amplifying winding H, a small change in the reactance of F increasing the volts across L and hence the current through H which further diminishes the reactance of F and ensures operation of the relay G. The arrangement shown in Figs. 3 and 4 of Specification 549,470 may be applied to the present arrangement for determining the faulty insulation Specification 541,419 also is referred to.