Development of an Instrument Used for Hot-line Insulation Diagnosis of House Wiring

Abstract

Recently many electric installations of consumers have been constantly energized. Therefore, the insulation of house wiring is diagnosed by measuring the earth leakage current with a clamp-type current transformer (CT) at the power-source earth line.In this situation there is a technical problem that the earth leakage current measured may include, in addition to active current due to resistances of installations and house wiring to the earth, reactive current due to the capacitance of house wiring to the earth.
The authors developed a clamp-type CT equipped with a simple gadget that could remove reactive current(within 4.5mA) component, this provides accurate measurement of the minute active current (beyond 0.01mA). The feature of this instrument is that its measuring technique is based on an algorithm named as hyperbolic measurement method that is capable of arithmetic operation using only scalar quantity when the bias current is made to flow on the CT.
As a bias current, a constant current (Ib) in phase with the source voltage and an antiphase constant current (-Ib) are superimposed on CT, respectively, in the first and second computation stages. The scalar values of the resultant currents are used in the computations.
This paper describes how to remove electromagnetic noises from the outside as well as the basic theory of the technology developed, composition of an experimentally manufactured instrument, and its performance test results.