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).
Testing procedures and properties of insulating oils and various insulating materials like Xylolith, asbestos-slate and duro.
| Identifier | ExFiles\Box 24\2\ Scan110 | |
| Date | 25th January 1921 guessed | |
| - 23 - Alkalies and sulphur have harmful effects in addition to those produced by dampness and acid. No clouding of the liquid should result by passing pure oxygen through it at a temperature of 120° for 70 hours and the tar coefficient should not exceed 0.10. In order to determine the amount of alkalie contained a solution of the oil in alcohol and ether may be titrated with 1/10 of hydrochloric acid. If a piece of glowing iron is held in the oil and a spluttering noise is produced that is a sign of the presence of dampness. In this case copper sulphate that is free from water will show a blue coloring. The presence of sulphur may be tested by warming the oil with metallic sodium. If the product gives a blackish-brown spot on silver this shows the presence of sulphur. The determination of the tar coefficient, which indicates the quantity of asphalt materials in the oils, is carried out by treating the oils with alcoholic sodium lye, then acidifying it, and shaking it up with benzol. The test should be made with cold oil and also for the purposes of comparison, after heating it for a lengthy period. For use in switches mineral oils have the advantage over resinous oils, which have a high content of carbon that they do not carbonise so easily by contact with an arc. The oil should be as thin as possible so that it can fill up the cavity former by the arc as soon as possible. The best mineral oils have a puncture voltage about five times as great as that of the air. On the assumption that an air gap of 1 mm. breaks down at about 1700 volts, this corresponds to about 9,000 volts maximum puncture voltage for a layer of oil of 1 mm. thickness. Resinous oils after show a considerably higher puncture voltage. The thinner the oil the better is its capacity for hindering electrostatic discharges. For instance, puncture voltages of 17,000 to 26,000 volts have been measured for thickness of 1 mm. of petroleum. Xylolith, asbestos-slate and duro are sometimes mentioned as insulating materials, but this designation is erroneous as they cannot be reckoned amongst the insulators on account of the fact that they conduct current in the presence of humidity. Xylolith is a material formed of magnesium chlorite and sawdust. Asbestos slate consists of cement and asbestos fibre and duro is a compressed mixture of gypsum and fibrous materials with impregnated wood-wool. These materials are, however, often used as constructive materials for cells and other partitions walls and the first two for quenching sparks. contd. | ||