C-3 Insulation
This organic varnish coating (equivalent to ASTM Type C-3 coating) is applied over the natural oxide surface. It provides very high levels of surface insulation resistance as well as protection against rusting. C-3 insulation also has a beneficial effect on die life. While it is suitable at normal operating temperatures of electrical apparatus, it will not withstand the heat of stress-relief annealing. It withstands oil and is suitable for oil-immersed and air-cooled cores.