Development and certification of a procedure for determining the leakage current of electrical-engineering periclase powders

Abstract

Electrical-engineering periclase powders (EPP) are used as high-temperature electrical insulating fillers in tubular electrical heating elements (TEH) and other electrical heating devices and equipment. When carrying out acceptance tests, the quality of EPP is evaluated according to GOST 13236-83 ['Electrical Engineering Periclase Powders. Specifications'].Until recently, in the Soviet practice, the electrical insulation properties of EPP have been evaluated on the basis of bulk resistivity and dielectric strength according to the specifications and the quality control of the electrical heating devices has been carried out on the basis of the experimentally determined leakage current of the product included in the electrical circuit.Our long-term experience in the production of the EPP-based TEH indicated the existence of a complex functional relationship between the magnitude of the leakage current measured in an operating electrical device and numerous factors including those not covered by GOST 13236-83. At the present time, the leakage currents measured using the specimens simulating an operating TEH is being treated as the most reliable characteristic of the powders that reflectsthe totality of their specific features and, also, the production technology of TEH and their service conditions. The procedure described in this paper is intended for obtaining quantitative data concerning the leakage current that make it possible to classify the experimental EPP with respect to their quality on the basis of a comparative study and to determine the fields of their application. The method involves measurement of the current passing through an electrical insulating filler incorporated between the high-resistance heating element and the casing of the TEl{ in air at room temperature and normal pressure maintaining a given specific load. The measurement is carried out using an unsealed TEl{ after subjecting it to prior heat treatment in order to reduce the effect of the unstable climatic conditions under which the specimens are produced. The procedure developed by the 'Dinamit Nobel' company (Germany) was taken as the primary standard.It reflects most of the specific features of the procedures followed for determining the leakage current at the foreign enterprises. Figures 1 and 2 show the schematic of the specimen and the measuring circuit. The process of specimen preparation includes all the stages of production of the casing, the current leads, and the coil, assembly of the resistance heating element, and preparation of the structural elements, the powders, and the TEH itself including the operations of filling up, reduction, and treating of the end regions. The recommended materials and the equipment are given in the No. 2 revision of GOST 13236-83. A distinguishing feature of the process of specimen preparation is that the intermediate operations are controlled strictly in order to minimize the effect of the technological factors involved in the production of the specimen on the magnitude of the leakage current.Befo...