Insulating oil is oil that stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high voltage capacitors, and some types of high voltage switches and circuit breakers. Its functions are to insulate, suppress corona and arcing, and to serve as a coolant. Today, most transformers use a fluid that achieves a much higher performance level than standard naphthenic mineral oil with far less risk. Mineral oils invariably have an issue with corrosive sulphur that can render them problematic in service, and attempts to balance this out with copper passivators are insufficient compared to readily-available, safer alternatives