Safety device for electrical transformers, comprising: - a casing provided with an internal chamber suitable to be connected in hydraulic communication with an oil-containing vessel and an oil-containing conservator of an associated electrical transformer, for oil flowing through said internal chamber from/to said oil vessel to/from said conservator; characterized in that said safety device also comprises: - a first oil outflow detecting vane which is movably connected to said casing and is arranged along the path of the oil flow, said first oil outflow detecting vane being suitable to move from a rest position when oil outflows from said vessel with an oil speed greater than an alarm oil speed threshold; - a second oil outflow detecting vane which is movably connected to said casing and is arranged along the path of the oil flow, said second oil outflow detecting vane being suitable to move from a rest position when oil outflows from said vessel with an oil speed greater than a safety oil speed threshold, said safety oil speed threshold being greater than said alarm oil speed threshold; - first detection means for detecting a movement of said first oil outflow detecting vane from its rest position, said first detection means being suitable to be connected to alarm means for signalling when the detected movement of said first oil outflow detecting vane from its rest position, exceeds said alarm oil speed threshold; - second detection means for detecting a movement of said second oil outflow detecting vane from its rest position, said second detection means being suitable to cause the intervention of a circuit breaker device suitable to disconnect said electrical transformer from an associated power grid when the detected movement of said second oil outflow detecting vane from its rest position exceeds said safety oil speed threshold. - a casing provided with an internal chamber suitable to be connected in hydraulic communication with an oil-containing vessel and an oil-containing conservator of an associated electrical transformer, for oil flowing through said internal chamber from/to said oil vessel to/from said conservator; - a first oil outflow detecting vane which is movably connected to said casing and is arranged along the path of the oil flow, said first oil outflow detecting vane being suitable to move from a rest position when oil outflows from said vessel with an oil speed greater than an alarm oil speed threshold; - a second oil outflow detecting vane which is movably connected to said casing and is arranged along the path of the oil flow, said second oil outflow detecting vane being suitable to move from a rest position when oil outflows from said vessel with an oil speed greater than a safety oil speed threshold, said safety oil speed threshold being greater than said alarm oil speed threshold; - first detection means for detecting a movement of said first oil outflow detecting vane from its rest position, said first detection means being suitable to be connected to alarm means for signalling when the detected movement of said first oil outflow detecting vane from its rest position, exceeds said alarm oil speed threshold; - second detection means for detecting a movement of said second oil outflow detecting vane from its rest position, said second detection means being suitable to cause the intervention of a circuit breaker device suitable to disconnect said electrical transformer from an associated power grid when the detected movement of said second oil outflow detecting vane from its rest position exceeds said safety oil speed threshold.