When is silicone oil for transformers preferred over ester oils?

Silicone oil is preferred when maximum fire resistance and thermal stability are required, especially in indoor installations or high consequence environments where any ignition risk is unacceptable. It can be chosen where ester oil moisture behavior or material compatibility is a concern, or where very wide temperature range performance is needed. Ester oils provide good fire safety and biodegradability, so silicone is usually selected when the project prioritizes fire performance above environmental benefits and is willing to pay a higher fluid cost.