A method for deriving at least one operating parameter P of a fluid-insulated electrical apparatus, which is a gas-insulated medium or high voltage switchgear or a transformer and which comprises an insulation fluid with at least three components X, Y, and Z with component concentrations c X, c Y, and c Z, and which derived operating parameter P is dependent on a dielectric breakdown strength E bd of the insulation fluid of the electrical apparatus and defines an operational operating state or a failure state of the electrical apparatus, the method comprising the method elements of - measuring by means of at least one sensor a plurality of measurement variables (ρ, p, T, λ, η, c S), wherein the measurement variables (ρ, p, T, λ, η, c S) are indicative of at least a pressure (p) and a temperature of the insulation fluid, - assigning the at least three components X, Y, and Z to at least a first component group A with a group concentration c A and to at least a second component group B with a group concentration c B, wherein at least the first component group A comprises at least two of the components X, Y, and Z or wherein at least the second component group B comprises at least two of the components X, Y, and Z, - wherein the at least three components X, Y, and Z are assigned to the at least two component groups A and B in such a way that a weighted average value M A of a molecular mass of the component or components in the first component group A differs from a weighted average value M B of a molecular mass of the component or components in the second component group B, - deriving the group concentrations c A and c B of the component groups A and B by using the measurement variables (ρ, p, T, λ, η, c S) and at least one relating equation, which is the same or different for each of the component groups A and B, and - deriving the operating parameter P using the group concentration c A and the group concentration c B. - measuring by means of at least one sensor a plurality of measurement variables (ρ, p, T, λ, η, c S), wherein the measurement variables (ρ, p, T, λ, η, c S) are indicative of at least a pressure (p) and a temperature of the insulation fluid, - assigning the at least three components X, Y, and Z to at least a first component group A with a group concentration c A and to at least a second component group B with a group concentration c B, wherein at least the first component group A comprises at least two of the components X, Y, and Z or wherein at least the second component group B comprises at least two of the components X, Y, and Z, - wherein the at least three components X, Y, and Z are assigned to the at least two component groups A and B in such a way that a weighted average value M A of a molecular mass of the component or components in the first component group A differs from a weighted average value M B of a molecular mass of the component or components in the second component group B, - deriving the group concentrations c A and c B of the component groups A and B by using the measurement variables (ρ, p, T, λ, η, c S) and at least one relating equation, which is the same or different for each of the component groups A and B, and - deriving the operating parameter P using the group concentration c A and the group concentration c B.