the solid insulation transformer has a rectangular core covered with a compressible closed - cell foam to eliminate stress during curing of the cast dielectric material surrounding the core and during operation . heat pipes are placed between the inner coil and the core to extract heat before the temperature builds up . for safety and to eliminate the need for a separate enclosure , an outer multi - layer casing having an incorporated grounded conductive layer is provided to cover the sides of the cast body . the outer casing prevents explosion if dielectric break down and arcing occur , and reduces the danger of electric shock .