Insulating oil deteriorates in use, and the maintenance engineer has to see that the working conditions are such as to minimize this loss of quality. The oil must be reconditioned when the necessity for this is revealed by a system of regular observations on the condition of the oil.Reference is made to points which should be watched in the design and installation of plant, and it is suggested that of the oils specified in B.S. 148-1933 only Grade B.30 oil should be retained. The revised Specification should include high-viscosity oils which are frequently used in electrical apparatus.Routine tests for sludge and moisture contamination are universal, but determination of acid value may be important and the flash-point has to be determined occasionally. Suggestions are made as to what values of these should be permitted before purification is considered necessary. The type of contamination likely to be present and the apparatus available for its treatment and various methods of using it are discussed.A Table showing suitable maintenance periods is given, and it is pointed Out that as it is difficult to handle oil and to work on oil-immersed gear without causing deterioration, these periods depend more on the immersed apparatus requiring attention than on the oil.