electrical insulating oil - high-quality oxidation-resistant oil refined to give long service as a dielectric and coolant for transformers and other electrical equipment. Its most common application is as a transformer oil. An insulating oil must resist the effects of elevated temperatures, electrical stress, and contact with air, which can lead to sludge formation and loss of insulation properties. It must be kept dry, as water is detrimental to dielectric strength. See gassing tendency, impulse strength, power factor.