A liquid having the property of insulating the coils of transformers, both electrically and thermally. There are two broad classes, (1) natural and (2) synthetic. The natural type includes refined mineral oils (petroleum fractions), which have low viscosities and high chemical and oxidative stability. The synthetic types are (1) chlorinated aromatics (chlorinated biphenyls and trichlorobenzene) known collectively as askarels; (2) silicone oils; and (3) ester liquids such as dibutyl sebacate. All these types are nonflammable but combustible. Flash points are 250–300F (121–148C).
