Insulating oil in service: is it fit for purpose?

Insulating oil is designed to dissipate heat, and act as a coolant and electrical insulator in power transformers. During operation it is subjected to heat and materials that are not compatible with the oil. Provided the insulating oil has been supplied to the agreed specification, it is the user´s responsibility to make sure that the oil is in good condition and fit for purpose. In order to evaluate the oil condition, oil samples should be taken from the equipment and tested. Details of the oil sampling technique are outlined in IEC 60475 (6). Frequency of oil sampling and testing is given in IEC 60422. Oil testing and classification of oil in service are briefly discussed