There is a general agreement that in service conditions the quality of insulating fluids gradually deteriorates under the impact of electrical, thermal and chemical stress. Two ASTM methods are used to monitor the deterioration of liquid insulation step by step. A comparison is made between the performances of a commercial based mineral oil and ester fluids. The results obtained using a laboratory grade spectrophotometer and a ratio-turbidimeter, indicate that the absorbance increases by a significant and easily observable margin with ageing rate. Under the same aging condition, synthetic ester ages considerably slower than mineral oil; natural ester showing an intermediate behaviour.
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