From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Page discussing the properties and testing procedures for porcelain and asbestos electric insulators.
| Identifier | ExFiles\Box 24\2\ Scan106 | |
| Date | 25th January 1921 guessed | |
| - 19 - The smooth surface of the porcelain allows snow to slide off soon and this process is accelerated by the heat developed by the current. Heavy rains improve the insulation by clearing off dust, soot, and deposits of salt near the sea coast, but these impurities do not have any effect on the glaze. It is necessary to have regard to the fact that the voltage of high tension networks may be doubled by the formation of free oscillations or reach still higher values under the influence of resonance and the tests to which each high-tension insulator is subjected in the factory should take place at a considerably higher voltage than the transmission voltage. The tests are carried out by placing the insulators upside down in a vessel containing water, the inner chamber of the porcelain being also filled with water. It is not necessary to make the liquid conducting by adding acids or salts. The water should at least touch the edge of the neck of the insulator and should completely cover all surfaces of separation in the case of insulators that are cemented together. One of the electrodes is the liquid and the other must be movable. Furthermore, the separate parts of the insulators that are not burned together in the furnace, but are later cemented together with Portland cement and are thus made particularly strong mechanically should be tested at voltages that reach the spark-over voltage and often exceed the transmission voltage. The test voltage should be graduated according to the magnitude of the transmission voltage for which the insulators are to be used in order not to increase the cost of the insulation excessively. With high transmission voltage it is advisable to test twice this voltage, with moderate transmission voltages three to five times the transmission voltage may be used. As the discharges from the edge of the porcelain insulator have the main influence in determining its value, it is customary to imitate the effect of rain by carrying out our tests under sprays producing fine or large drops of water. The use of porcelain in the shape of the electric industry is extensive and thousands of shapes of all kinds are used every day which it is not necessary to describe more particularly. Asbestos is an important raw material for electric insulators. With the exception of the form steatite it is however, not possible to use this mineral alone for insulating purposes, as it soaks up liquid much as water and swells, owing to its fibrous texture. On this account asbestos is mixed with materials of good insulating qualities that penetrate its pores and fibres, and can be composed into an homogenous hard and strong texture together with the asbestos. Such materials are Contd. | ||