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).
The properties, regulations, and testing of insulating materials for switchboards and electrical installations.
| Identifier | ExFiles\Box 24\2\ Scan093 | |
| Date | 25th January 1921 guessed | |
| - 6 - If such materials were used for switchboards or the like, and the heat were developed through the production of a bad contact, the insulating material would be jeopardised by the use of an inferior dielectric. The Regulations of the Institution, that came into force on the 1st, July 1915, stipulate that switchboards and distribution boards must be built of materials which can resist fire, and meter boards, that contain switches or fuses besides the measuring instruments are to be regarded as distribution boards. Materials that can resist fire are such that cannot be ignited or cannot continue to burn if ignited. A material is to be regarded as heat resisting if it does not show any change which influences its use at the highest temperature that can arise. The temperature limit has not yet been definitely fixed in the new Regulations. Hitherto it has been prescribed that insulating materials must withstand a temperature of 175 degrees without deformation, but it is probable that the limit will be raised. It is important that at high temperatures no breaking up, cracking or softening should take place, but the solid structure of the insulating material must remain unaffected, and should not destroy the insulation and the mechanical qualities of the dielectric. Dr. Passavant gives a simple method for testing the value of an insulating material for switch and distribution boards and supports of parts carrying current. He uses a burning match. If the material will not stand this test and shows softening, crackling or burning, further tests are not required and the material should not be used on heavy current installations. Great progress has been made in recent years in the manufacture of insulating materials. It has not only been possible to improve the insulating qualities of the materials, but a great improvement has been reached in their resistance to high temperatures. Many present day insulators are mechanically strong enough to compete in strength with iron. Formerly natural insulators, such as marble were used very frequently. These materials have the drawback that their safety is qualified by the possible inclusion of veins and enclosures of conducting material. These are also porous and in consequence liable to condense water vapour. At high temperatures they become conducting, just in the same way as glass, porcelain and asbestos, which will be referred to later. Similar phenomena then appear as are associated with the glowing filaments of the Nernet lamp. In order to be able to use plates of stone in electrical installations they must be impregnated and polished, or painted with an insulating varnish or enamel, particular attention being paid to the back of marble boards. Such treatment of holes bored in the material is often forgotten. contd. | ||