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).
Directions for determining the electric strength of materials, including conditioning samples and test apparatus.
| Identifier | ExFiles\Box 24\2\ Scan224 | |
| Date | 21th February 1922 guessed | |
| 800 DIRECTIONS FOR DETERMINING THE ELECTRIC STRENGTH OF To obtain some idea of the breakdown voltage of the material a test should be made by increasing the voltage as fast as is consistent with obtaining satisfactory readings of the measuring instrument. The material should then be subjected to two-thirds of the breakdown voltage obtained on a rapidly applied test, and the time required to produce breakdown noted. From the results of this test an approximate idea can be obtained of the voltage which will produce breakdown in about one minute; this estimated voltage should then be applied and the time required to produce breakdown noted. From this latter result, a still closer approximation can be made. In addition to the rapidly applied test, at least five breakdowns should be obtained on each sample, and the values plotted on a time-voltage curve, similar to Fig. 1, but for the time range round one minute only. From this curve, the voltage required to produce breakdown in one minute can be determined. (d) Other Factors. In conducting the abridged tests for electric strength, the conditions specified for the other factors entering into electric strength tests are the same as for the research tests dealt with in Part I. APPENDIX I. Details of Temperature, Time and Humidity required for the Various Conditions and Materials in Conditioning of Samples previous to the Electric Strength Test. Condition | Material | Temperature (° C.) | Time (Hours) | Humidity (Relative) ---|---|---|---|--- Normal | Treated papers and fabrics | 15-25 | 18-24 | 75 %* " | Pressboard | 15-25 | 18-24 | 75 %* " | Leatheroid and vulcanized fibre | 15-25 | 18-24 | 75 %* " | Varnish-paper boards and tubes | 15-25 | 18-24 | 75 %* Dry | Treated papers and fabrics | 75-80 | 2-3 | Low " | Pressboard | 75-80 | 18-24 | Low " | Leatheroid and vulcanized fibre | 75-80 | 18-24 | Low " | Varnish-paper boards and tubes | 75-80 | 18-24 | Low Damp | Treated papers and fabrics | 15-25 | 18-24 | 100 % " | Pressboard | 15-25 | 18-24 | 100 % " | Leatheroid and vulcanized fibre | 15-25 | 18-24 | 100 % " | Varnish-paper boards and tubes | 15-25 | 18-24 | 100 % Tropical | Treated papers and fabrics | 45-50 | 18-24 | 100 % " | Pressboard | 45-50 | 18-24 | 100 % " | Leatheroid and vulcanized fibre | 45-50 | 18-24 | 100 % " | Varnish-paper boards and tubes | 45-50 | 18-24 | 100 % * This figure is adopted provisionally, awaiting recommendations from the N.P.L. APPENDIX II. METHOD OF ASCERTAINING THE ELECTRIC STRENGTH OF MATERIALS WHEN SUBJECTED TO LONG APPLICATION OF A.C. STRESS AND POWER FREQUENCY. When in service in electrical machinery and apparatus, insulating materials have to withstand voltage stresses for long periods of time. As the insulating materials are usually in contact with metal parts, one of which is generally at a higher temperature than the insulation, the conditions are such that any heat generated in the insulation itself cannot be readily dissipated. To investigate the internal heating produced in fibrous materials when under A.C. stress, and to ascertain the highest A.C. stress which the material can endure without loss of insulating property as the result of excessive internal heating, the following test is recommended :— (a) Apparatus suggested. In Fig. 9 is shown a diagram of the connections of the necessary apparatus. Particulars of a set of instruments which have been used for these tests are given below :— A.C. Voltmeter.—Electrostatic, reading to about 120 volts, capable of being connected across various sections of a suitable potential dividing resistance, connected across the transformer secondary. Any other type of high voltage measuring equipment may be used. D.C. Voltmeter.—Reading up to 150 volts in 1 volt steps. Microammeter.—For leakage measurement. R.{Sir Henry Royce} W. Paul, of London, N.K. 1035. Resistance about 900 ohms at 20° C. Range, 0-24 microamps. by 0·2 (120 scale divisions). If the A.C. component is large an instrument of low resistance may be desirable. D.C. Battery.—Portable type, 25 lead cells or primary cells. Thermocouple.—Eureka-copper. 6¼ in. each of 0·008 in. covered wire and hard soldered to form couple. At the other end the copper is soldered to 3 ft. 8 in. of | ||