The electrical insulation system for

wire Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm A wire is a flexible strand of metal. Wire is co ...

s used in generators, electric motors,

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

s, and other wire-wound electrical components is divided into different classes by

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on ...

and temperature rise. The electrical insulation system is sometimes referred to as ''insulation class'' or ''thermal classification''. The different classes are defined by NEMA,

Underwriters Laboratories The UL enterprise is a global safety science company headquartered in Northbrook, Illinois, composed of three organizations, UL Research Institutes, UL Standards & Engagement and UL Solutions. Established in 1894, the UL enterprise was founded as ...

(UL), and IEC standards. For complete electrically operated appliances, the "insulation system" is the overall design of electrical insulation of the energized components to ensure correct function of the device and protection of the user from

electric shock Electrical injury is a physiological reaction caused by electric current passing through the body. The injury depends on the density of the current, tissue resistance and duration of contact. Very small currents may be imperceptible or produce a ...

Temperature classes

The maximum hot-spot

operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...

is reached by adding the rated ambient temperature of the machine (often 40 °C), a temperature rise, and a 10 °C hot-spot allowance. Electrical machines are usually designed with an average temperature below the rated hot-spot temperature to allow for acceptable life. Insulation does not suddenly fail if the hot-spot temperature is reached, but useful operating life declines rapidly; a rule of thumb is a halving of life for every 10 °C temperature increase. Older editions of standards listed materials to be used for the various temperature classes. Modern editions of standards are proscriptive, only indicating that the insulation system must provide acceptable life at the specified temperature rise. In large machines, different systems may be used according to the predicted temperature rise of the machine; for example, in large hydroelectric generators, stator windings may be Class B but the more difficult to cool rotor winding may be Class F.

Categories of insulation

In IEC standards, the insulation system is a classification based on the level of electrical shock protection given to a user. Functional insulation is that required to prevent short circuits within the equipment. Basic insulation is any material added to protect a user from accidental contact with energized parts. Supplemental insulation is rated to withstand 1500 volts AC. Double insulation is a design concept where failure of one insulation system will not expose the user to a shock hazard due to the presence of a second independent layer of insulation. Reinforced insulation is a supplemental insulation system that is strong enough to effectively perform as if a double insulation system was present. Selection of the insulation system is coordinated with the choice of appliance class.

Temperature classes

Categories of insulation

See also

References

Further reading