Encyclopaedic Dictionary of Physics: General, Nuclear, Solid State, Molecular, Chemical, Metal and Vacuum Physics, Astronomy, Geophysics, Biophysics, and Related Subjects - Vol. 2

Encyclopaedic Dictionary of Physics: General, Nuclear, Solid State, Molecular, Chemical, Metal and Vacuum Physics, Astronomy, Geophysics, Biophysics, and Related Subjects - Vol. 2

Encyclopaedic Dictionary of Physics: General, Nuclear, Solid State, Molecular, Chemical, Metal and Vacuum Physics, Astronomy, Geophysics, Biophysics, and Related Subjects - Vol. 2

Encyclopaedic Dictionary of Physics: General, Nuclear, Solid State, Molecular, Chemical, Metal and Vacuum Physics, Astronomy, Geophysics, Biophysics, and Related Subjects - Vol. 2

Excerpt

INSULATION, TEMPERATURE LIMITS OF. Most insulating materials used in electrical machinery and apparatus undergo gradual aging when heated. Some, though they soften or deteriorate electrically while hot will recover on cooling. Inorganic materials are superior to organic in this respect; but mica is generally used as built-up mica, and woven glass and asbestos textiles are mostly laminated or varnished, so that the bonds or varnishes determine the thermal endurance. Electrical porcelain used for bushing construction and transmission line insulators, and electrical toughened glass also used for the latter, are unaffected by normal service temperatures.Papers, particularly the traditional cellulosic types, and textiles become brittle when heated. Man-made fibres, such as polyethylene terephthalate, are an improvement and low alkali glass fibre fabrics have good thermal endurance. Vulcanized fibre, pressboard and many laminates may become brittle, films may soften, while the bonding of laminates may deteriorate causing cracking and distortion, loss of mechanical strength and increase of moisture absorption.The coating of enamelled wires and the film on varnished fabric tapes tend to become brittle. Some synthetics such as the oil modified alkyds have better thermal endurance than oleo-resinous varnishes, while suitable silicone resins have outstanding properties.Mouldings may yield under mechanical stress and therefore have to be restricted to lower temperatures for moving parts than for static apparatus.The deterioration of properties affected by heat aging such as tensile, tearing, bursting or cross breaking strength, impact resistance and loss of flexibility can be assessed. This is used for acceptance and comparison purposes and reference should be made to the appropriate National Standard Specifications for details, but such tests only check the effect after a limited time on a few properties and are not always representative of working conditions.The functioning of an insulating material is also governed by vibration and differential thermal expansion, while atmospheric moisture, dirt, oil and other contaminants play their part.The acceptable life of apparatus depends upon service conditions and the relative importance of weight, reliability and other economic factors; as extreme examples long life may be required, or a device may be regarded as expendable.The quantitative assessment of the thermal endurance of a material as related to its long term performance presents difficulties and reference should be made to Publication 85 of the International Electrotechnical Commission. This document represents the consensus of international opinion, insulation being grouped into classes, each associated with a standard temperature which, when adopted as the maximum permissible temperature, will ensure an economic life for the insulation of that class in a wide range of machinery and apparatus. An appendix provides a general guide to the classification of a number of particular materials.Work to secure more quantitative data is proceeding in many countries . . .
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