Ester oil is selected when fire safety, environmental sensitivity, or indoor and urban siting constraints are important. Natural ester is common for biodegradable, high fire point needs and can improve paper moisture management. Synthetic ester may be chosen for wider temperature range and better oxidation stability in some designs. Ester fluids can be beneficial in retrofills if compatibility is confirmed. Selection should consider dielectric properties, viscosity at low temperature, oxidation behavior, materials compatibility, and utility standards for the specific voltage class.
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