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 design considerations for porcelain insulators.
| Identifier | ExFiles\Box 24\2\ Scan105 | |
| Date | 25th January 1921 guessed | |
| - 18 - It would approach the ideal for an insulating material if it had not the drawbacks of being brittle. This property sets a limit to its use in many cases. The specific weight is about 2.3 to 2.4. Good porcelain insulators should not absorb water even before they are glazed, the fracture should be finely granulated and conchoidal and should be white and shining. Cavities and veins must be avoided at all costs, both in the interior of the substance and in the glaze. In low tension insulators those designs should be avoided in which the head and the bell are formed separately and burned together. The most perfect products are the insulators that ring clearly when they are hit, thus shewing the absence of fracture in the material. In porcelain insulators the form is very important. The dielectric strength of good products reaches, according to Wiecker, about 13,000 volts for a thickness of 1 mm, 53,000 volts for 5 mm, and 98,000 volts for 11 mm. This dielectric strength is generally sufficient, depending on the thickness of the material, but the current is liable to seek a path to the iron supports over the surface of the insulator. The insulators are best cemented into their supports by hemp combined with linseed oil and minium. During use in the open air the material is subjected to a number of influences by rainfall, loads of snow and accumulations of dust, which may have a very harmful effect. Rainfall causes the outer petticoats of the insulators to become conducting and as the drops that flow down to the lower edge have the same potential as the conductor they do not fall to the ground, but are attracted to the support that are oppositely charged. In this way form a conducting path over the surface of the porcelain. When the Hermsdorf Porcelain Factory realised this process, steps were taken to reduce the surface conductivity and to eliminate, as far as possible, the discharges off the edges by increasing the distance between the head and the support of the insulators and by employing several petticoats, one within the other, increasing their size and broadening them out in umbrella form. A further precaution consists in providing suspension insulators with wide-spread metal screens to protect them from raindrops. These screens constitute an increase in the upper insulating surface and cause the distribution of the field to be more uniform. By this means if a guard ring is provided on the support of the safety spark-gap is formed to protect the insulator against the effects of voltage rises and the danger of damaging Machine windings is thereby decreased. contd. | ||