Insulating oil is typically some form of ________.

• A. Mineral oil
• B. Vegetable oil
• C. Synthetic ester
• D. Water

Answer: (A)

Insulating oil must provide reliable electrical insulation while also helping to carry away heat, stay chemically stable, and be cost-effective. Mineral oil fits this role in most transformers and electrical equipment because it has strong dielectric strength, good thermal properties for cooling, and excellent chemical stability with cellulose insulation. It’s inexpensive, readily available, and well understood in terms of aging and filter-reconditioning practices, which keeps maintenance predictable over long service lives. Other oils—vegetable oils or synthetic esters—offer environmental or fire-safety advantages, but they can be more expensive, may age differently, and can require special handling or system adjustments. Water is not used because it conducts electricity and would ruin insulation.