Effect of Soil Microörganisms on Rubber Insulation

Abstract

Abstract Evidence is presented that some of the rare failures of nonleaded rubber-insulated underground cables were caused by soil microörganisms. Laboratory conditions deliberately chosen to favor microbial attack made possible greatly accelerated tests in which failures like those found after ten years or more in service have been produced in a few weeks or months. These failures are characterized by discrete, minute low resistance spots. Drastic decrease of insulation resistance was used as the criterion of failure. The exact distribution of low-resistance spots has been shown by an electrolytic test. Parallel tests in sterile and in active soil proved that the failures in soil were caused by living microorganisms. Soil exposure tests have been made on natural and synthetic rubbers and several other plastic insulations. Natural rubber compounds were found to be the most susceptible. In some soils GR-S compounds proved stable up to two years, but failed in a more active soil. The exact mechanism of attack is not known. In some cases there is drastic electrical failure without visible surface effect. In actual operation of underground cables the exact combination of conditions necessary for producing failure by microbial attack can occur only rarely. The purpose of the severe laboratory tests is to make possible the development of insulations stable under all soil conditions.