An electrical insulation system (EIS) is defined by the IEC as an “insulating structure containing one or more electrical insulating materials (EIM) together with associated conducting parts employed in an electrotechnical device.” This is a rather simple definition for what can be a very complex combination of materials. An EIS is composed of two sets of components – major ground insulation components and minor components.
Major ground insulation components are EIM. That is, these materials are the electrically stressed components used to separate conducting parts at different electrical potentials. Typical examples of major components include magnet wires, varnishes, and flexible sheet materials used for core insulation, as high-low barrier insulation, or slot liners in motors.
Minor components are those materials used in combination with the major ground insulation for mechanical, heat transfer, decoration, or other non-electrically stressed applications. Typical examples of minor components include pressure-sensitive tapes, sleeving and tubing, lead wires, phase insulation, and potting compounds.
A more complete listing of components and their definitions can be found in Table 4.2 in UL 1446. Some of the finer points covered in the table include:
- A varnish may be a major or a minor component depending upon whether the original EIS was evaluated with a varnish or not.
- Layer insulation is defined as a minor component that serves as a mechanical barrier between successive layers of insulated conductor in the same winding, and does not serve as an electrical insulation.
- Interwinding insulation is defined as a major component that serves as electrical insulation between windings, for example, the sheet insulation between successive layers of foil in a strip-wound coil.
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