The study of apparent negative value in measuring dielectric loss

The measurement of the dielectric loss of insulating material or electrical apparatus with the Schering bridge at power frequency is one of the most fundamental dielectric tests for evaluating insulation behavior. It has been pointed out that a negative error or even an apparent negative reading of tanδ may result if there is a tee network of interference present in the measuring circuit or within the test sample itself, but this problem has not yet been investigated fully. The purpose of our investigation is to find the origin of such phenomena. In this paper we discuss such phenomena occurring in three-terminal measurements of dielectric materials, and draw a conclusion that the negative error of tanδ reading at power frequency is not due to the presence of resistance in the guard electrode circuit as conventionally supposed. We suggest a possible model of the tee network of interference present inside the sample itself and also point out the fact that another form of the tee network interference, the induction effect through electric field, may also cause a negative increment of the tanδ reading.