The coil- and-core assembly of the distribution transformer is completely immersed in insulating oil to keep operating temperatures low and to provide additional insulation for the ________.

• A. windings
• B. secondary bushings
• C. grounding strap
• D. primary bushings

Answer: (A)

Insulating oil around the coil- and-core assembly mainly serves to insulate and cool the windings. The windings are the hot, layered conductors wrapped around the core, and surrounding them with oil provides a dielectric medium that both carries away heat and increases the dielectric strength between turns and layers. This helps prevent insulation breakdown inside the winding. While bushings and other parts have their own insulation systems, the key reason for oil immersion of the coil- and-core assembly is to insulate the windings.