Most transformers use mineral insulating oil because it balances good dielectric strength, cooling capability, availability, and cost. Where fire safety or environmental constraints are stronger, natural ester fluids such as FR3 or synthetic esters may be selected because they have higher fire points and better biodegradability. Silicone oils can be used where very high fire resistance is needed. The selection is based on dielectric requirements, thermal loading, site fire codes, spill risk, maintenance strategy, and compatibility with existing materials, seals, and paper insulation. Manufacturer approval and standards compliance are required.
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