Even though the basic functions of the oil used in transformers are (a) heat conduction and (b) electrical insulation, there are many other properties which make a particular oil eminently suitable. Organic oils of vegetative or animal origin are good insulators but tend to decompose giving rise to acidic by-products which attack the paper or cloth insulation around the conductors. Mineral oils are suitable from the point of electrical properties but tend to form sludge. The properties that are required to be looked into before selecting an oil for transformer application are as follows: INSULTING PROPERTY : This is a very important property. However most of the oils naturally fulfill this. Therefore deterioration in insulating property due to moisture or contamination may be more relevant. VISCOSITY : It is important as it determines the rate of flow of the fluid. Highly viscous fluids need much bigger clearances for adequate heat removal. PURITY : The oil mu...