One of the most widespread liquid electrical insulating materials is transformer oil. It is used to fill the pores  in the fiber insulation and the space between the windings, thereby increasing the dielectric strength of the  insulation and improving the heat removal from the windings and the core of a transformer. The oil is also  used in high-voltage current breakers. The most important quality parameters of insulating oils are their  dielectric properties, fast heat transfer and moisture resistance.