The effects of fungi on the direct current surface conductance of electrical insulating materials.

The extent of visible fungus growth on pressuresensitive electrical insulating tapes, as well as the effects of fungi on the mechanical strength of the tapes, was reported previously (Berk and Teitell, 1951). Fungi seriously decreased the tensile strength of tapes with cellulosic backings but had no more than slight effects on the tensile strength of tapes with elastomeric backings. This report deals with the second phase of the investigation, the effects of fungi on the electrical properties of the pressure-sensitive insulating tapes and, particularly, the effects on surface conductance. The literature is somewhat controversial concerning the relative importance of moisture and fungus growth on the failure of electrical equipment in service (Leutritz and Hermann, 1946; Leutritz, 1948). The Summary Technical Report of the National Defense Research Council (1946) on "Tropical Deterioration of Electric and Electronic Equipment" states, "Fungi are also important agents of deterioration in electric and electronic equipment. Hyphal strands of surfacegrowing fungi can introduce leakage paths which reduce insulation resistance and establish couples which promote electrolytic corrosion." Titus (1945) considered fungi to be secondary causes of operational failures but stated that fungus filaments, when bridging terminals, might cause flash-overs. Proskauer and Smith (1945) stated that microorganisms contribute to the retention of moisture on surfaces that would normally dry out quickly, and the presence of these microorganisms might produce paths of low electrical resistance. Leutritz and Hermann (1946) searched the literature from 1820, the year the galvanometer was invented, to 1945 and found no experiments that showed the effects of fungi on insulation resistance. They then studied the effects of humidity and fungus on unfilled and variously filled plastics and concluded that the decrease in insulation resistance was due to water absorption or adsorption and not due to fungi. There have been several additional investigations of the problem since 1945. Witt, Chapman, and Raskin (1952) tested various plastics exposed to fungus and humidity; they used nitrogen atmospheres to prevent fungus growth where only humidity was desired; and they reported that the presence of moisture overshadowed any effects that fungi might have produced. Luce and Mathes (1951) and Gauvey (1951) worked with plastic hook-up wire insulation and, using a technique of fumigation to prevent fungus growth, were able to separate the effects of humidity and fungi. They found that for many good plastic insulating materials there was a definite decrease in the direct current surface resistance caused by fungi and separate from any lowering caused by moisture. This decrease in resistance was detected electrically before any visual growth of fungi took place. Leonard and Patouillet (1951) reported on a laboratory method that compared creepage resistance results from sterilized and viable inocula, and they were able to show that viable fungi lowered the resistance. Dubois and Herou (1952) investigated the effects of mold on various plastic materials. They used as criteria the time required for travel of high voltage sparks and the number of drops of water or ammonium chloride solution required for a short circuit. They found that with some materials there is a degrading effect from the molds, which they attributed to the secretion of an electrolytic solution capable of penetrating the material. In studying a slightly different aspect of the problem, Blake, Kitchin, and Pratt (1950) and Blake and Kitchin (1949) reported that soil microorganisms can cause failure of rubber insulation. The decreases in volume resistance were so great that sensitive measuring methods were not necessary.