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Abstract

An electrical insulating oil composition which comprises 45% by weight or more of at least 2 members selected from the group consisting of (a) m-ethylbiphenyl, (b) p-ethylbiphenyl, (c) o-benzyltoluene, (d) m-benzyltoluene, (e) p-benzyltoluene, and (f) 1,1-diphenylethane; and the remainder of non-condensed bicyclic aromatic hydrocarbons having not more than 17 carbon atoms besides the above 6 components; and is characterized in that, when the temperature of the composition is -40.degree. C., (I) the total quantity of solid phase obtained by calculating according to the following solid-liquid equilibrium equation is 45% by weight or less relative to the total quantity of the composition, and (II) in the liquid phase composition calculated by the following solid-liquid equilibrium equation, the total quantity of the compounds (a) to (f) is 40% by weight or more and the viscosity of this liquid phase is 500 cSt or lower, ##EQU1## wherein X.sub.i is the equilibrium mole fraction of a component i in the liquid phase; .DELTA.H.sub.i.sup.f is a heat of fusion (cal.mol.sup.-1); T.sub.i.sup.f is a meltinig point (K); T is the temperature (K); and R is the gas constant (cal.mol.sup.-1.K.sup.-1).

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