The present invention relates to a dielectric-insulation or arc-extinction fluid for an apparatus for the generation, the transmission, the distribution and/or the usage of electrical energy, the fluid being a mixture comprising a fluoroolefin and oxygen. According the fluoroolefin is a monohydrofluoroolefin containing from 4 to 5 carbon atoms, the hydrogen atom being bound to a carbon atom of the double bond or directly adjacent to the double bond.