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Abstract

An electrical insulating oil composition which is excellent in low temperature characteristics and suitable for use in impregnating capacitors, and comprises (a) m-ethylbiphenyl, (b) p-ethylbiphenyl, (c) m-isopropylbiphenyl and (d) p-isopropylbiphenyl, and is characterized in that the total amount of the ethylbiphenyls is not less than 40% by weight and the proportion of solid phase in the composition at a temperature of -40.degree. C. is not more than 45% by weight, the solid quantity of each component being calculated according to the following equation of solid-liquid equilibrium: ##EQU1## wherein X.sub.1 is the equilibrium mole fraction of a component i in the liquid phase of the composition, .DELTA.H.sub.i.sup.f is the heat of fushion (cal.mol.sup.-1) of the component i, T.sub.i.sup.f is the melting point (K) of the component i, T is the temperature (K) of the system, and R is the gas constant (cal.mol.sup.-1.K.sup.-1).

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