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The effect of particles and moisture on the 60 Hertz breakdown strength of tranformer oil was investigated. Different sizes and concentrations of iron, copper and cellulose particles were used. The breakdown voltage was determined according to ASTM D1816 using VDE electrodes with an 80 mil (2 mm) gap. The breakdown strength of oil decreased approximately 25 percent when the moisture content was increased from 5 ppm to 20 ppm. A further reduction in the breakdown strength of oil occurred when iron, copper or cellulose particles were added to the oil. The transformer oil showed very low values of 60 Hertz breakdown strength as the result of the combined effects of moisture and particles' type, size and concentration.