GB568765A - Improvements in or relating to the measurement of electric current and more particularly current of a short or transient duration - Google Patents

Abstract

568,765. Electromagnetic ammeters; locating faults. DAMANT, E. L. Aug. 31, 1943, No. 14205. Convention date, Nov. 3, 1942. [Class 37] In an instrument for measuring a transient or variable current relatively movable magnetic elements of high retentivity are mounted together in the field of a single current coil so as to operate an indicating element when permanently magnetized by the current in the coil, the current coil being also adapted to carry a, demagnetizing current for zeroizing the instrument. Fig. 1 shows the apparatus arranged for measuring a phase-to-earth fault current on a three-phase system. A current transformer B is arranged in the earth lead from the neutral point N to earth E and has its secondary connected to a rectifier R. The rectified current passes through the instrument AF. A fault at F causes current to flow through the current transformer B, the secondary current being rectified and energizing the instrument AF. The instrument AF is similar to a moving iron repulsion or attraction type of meter except that the magnetic material has high retentivity. The fixed magnetic member Fi, Fig. 2, may be a bar or flat plate or a combination of bars or plates, or may be of triangular or other shape formed to the circumference of a circle. The magnets may be laminated or built up of wires. The movable magnetic member M may or may not be similarly shaped to the fixed member Fi. Passage of current through an enclosing coil C magnetizes the members M, Fi permanently, so that they repel one another, causing a pointer P to be displaced. The instrument may be connected across an ammeter shunt in a D.C. system. After the reading is taken, the magnets are demagnetized by means of a gradually diminishing alternating current. The Specification as open to inspection under Sect. 91 comprises also the following subjectmatter. The current may magnetize a bar of magnetic material of high retentivity, the permanent magnetism remaining therein serving as a measure of the maximum value of the current. The location of the fault is determined by reference to a curve showing the calculated fault current which would flow when a fault occurs at different points in the system. Alternatively, the system may be calibrated by placing faults at various points and measuring the fault current for each fault point. The instrument may be calibrated in A.C. or D.C. amperes or in distance from the point of supply when used for fault location. Maximum or minimum values of temperature, fluid level, &c. may be measured by measuring a current controlled thereby. This subject-matter does not appear in the Specification as accepted.