The best stability under all operating conditions is achieved by electrical rotary equipment (motors and generators) and static electrical equipment (transformers and sensors) if all the air trapped inside the wire windings is replaced with insulation material. This stage of the value-added chain is known as impregnation or secondary insulation. This commonly thermal impregnating process, i.e. chemical process under the action of heat, also increases the mechanical stability of the electrical winding and protects the enamelled wire during operation from environmental influences such as moisture. This impregnation can entail a trickling of the wire winding, a dip or dip-and-roll process. Today impregnation is increasingly carried out through the vacuum method to eliminate any air that might be trapped (VPI process). Trapped air bubbles reduce heat dissipation, since air is an excellent heat insulator, or they give rise to disruptive electrical charges in high-voltage applications, which can destroy the insulation. Besides, UV light curing might be employed, which creates very highly cross-linked, and thus impermeable, impregnating layers. The impregnating resin or varnish (impregnating material generally) consists of organic components (prepolymers) that react to cross-linked polymers during the curing process and thus form a further electrical insulation layer added to the wire enamel.