GB987876A - Improvements relating to the measurement of electrical impedance - Google Patents

Abstract

987,876. Impedence testing. EVERSHED & VIGNOLES Ltd. Jan. 3, 1963 [Oct. 3, 1961], No. 35626/61. Heading G1U. Apparatus for determining whether or not the line to earth loop impedance of an A.C. wiring system is above or below a predetermined safe value comprises line, neutral, and earth terminals 1, 2, 3 connectible to a system line and neutral 4, 5 supplied from the secondary of transformer 15 and to a system earth containing a loop resistance X returned to one side of the transformer primary. Neon indicators 16, 18 are connected between line and neutral and between line and earth, while a switch 10, limiter resistance 12, circuit breaker 11 and circuit breaker contacts 11a are series connected across the line and earth terminals of the apparatus; whose neutral and earth terminals are interconnected by variable resistance 20, circuit breaker 7 and circuit breaker contacts 7a. Initially the breakers are closed and on closure of switch 10 a predetermined current flows between terminals 1, 2 and sets up a voltage between terminals 2, 3. If the loop has impedance below a critical value, the current dependent on the value of resistance 12 operates breaker 11 and opens contacts 11a, while the voltage between terminals 2, 3 is insufficient to operate circuit breaker 7 and open contacts 7a. For high loop impedance the current dependent on the sum of loop impedance and resistance 12 may or may not operate breaker 18, while the voltage between neutral and earth 2 and 3 operates breaker 8 and opens contacts 7a. Operation or non operation of breaker 7 thus indicates an unsatisfactory or satisfactory value of loop impedance. The breakers may be of the hand restored toggle type.