Dielectric Dissipation Factor and Resistivity @ 90 ºC (IEC 60247)

These tests measure the leakage current through oil. This is a measure of the degree of deterioration or
contamination of that oil by soluble polar products and/or ageing products/colloids (resulting from oxidation of the
oil it self or from the solution in the oil of external contaminants or materials used in the construction of the
equipment).These tests are very sensitive to the presence of contaminants. Hence any changes in the concentration
of contaminants may be monitored – even when the degree of contamination is so slight as to be undetectable by
chemical methods.These tests are not limited to transformer insulating oil – they are also applicable to the fluids in
cables, oil circuit breakers and other electrical equipment.The recommended limit for these tests depends on the
type of electrical equipment and its application. In general, high values of the dissipation factor may influence the
power factor and/or the insulation resistence of the transformer.

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