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).
Research tests on the electric strength of fibrous insulating materials.
| Identifier | ExFiles\Box 24\2\ Scan219 | |
| Date | 21th February 1922 guessed | |
| FIBROUS INSULATING MATERIALS. 795 gested, however, that for comparative purposes the latter figure should be obtained also. The Director of the E.R.A. will value any criticisms from those who have occasion to use any of the tests given in this report. Part I.—RESEARCH TESTS ON ELECTRIC STRENGTH. The following methods for the determination of the electric strength are recommended when the characteristics of the material are not known, and when a thorough investigation is required to ascertain the electric strength of the material under probable service conditions. 1. GENERAL. It is of primary importance in the case of fibrous insulating materials that the recognized electric strength be that of the material when hot and under long continued stress. It is desirable to test materials at temperatures appreciably above their intended working temperature. 2. PREPARATION OF MATERIAL PREVIOUS TO TEST. The ability of a material to withstand a damp atmosphere may be due to (a) The material itself being non-absorptive, (b) Additional protection given by a surface skin either developed during manufacture or added subsequently. In the latter case the specimens shall be prepared for test in the following manner :— (i) When determining the properties of the piece* as received the protective surface layers of (b) shall be retained. (ii) When determining the properties of the material itself it is necessary to exclude the additional feature (b) by making tests upon sections cut from the material and having the surface layers removed at all points. The results should in each case state how the specimen has been prepared for test. 3. CONDITIONING OF THE MATERIAL PREVIOUS TO TEST. As the electric strength of fibrous insulating materials is largely influenced by their moisture content, it is desirable that the characteristic curves referred to later should be obtained for as many as possible of the following conditions :— (a) “Normal” Condition. This is obtained by permitting the material to absorb its normal quantity of moisture by exposing it to the conditions stated in Appendix I for the period therein stated. (b) “Dry” Condition. This is obtained by removing from the material as much as possible of its free natural moisture, by means of the heat treatment recommended in Appendix I. * Such as a finished insulator, a varnished washer, etc. (c) “Damp” Condition. This is obtained by exposing the material to an atmosphere of 100 per cent relative humidity under the conditions stated in Appendix I. (d) “Tropical” Condition (for use when required). This is obtained by exposing the material to an atmosphere of 100 per cent relative humidity under the conditions stated in Appendix I. (e) “Recovered” Condition. This is only to be used when testing varnished silk, varnished cloth, varnished paper, or where specially called for. The specimens are to be dried at 75–80° C. for 8 to 15 hours and subsequently exposed to “normal” conditions for 1 week. NOTE (i).—If in (c) or (d) the material is removed from the atmosphere of specified humidity before testing, precaution must be taken to prevent appreciable change in electric strength from this cause. NOTE (ii).—When testing sheet material at the temperatures given in Clause 7, the electrodes should be raised to the high temperature before the material is removed from the atmosphere of specified humidity. 4. TESTS IN AIR OR OIL. It is often more convenient to immerse the sample under oil whilst the electric strength test is carried out. Under certain conditions the breakdown voltage obtained when the material is tested in oil is not the same as the breakdown voltage which would be obtained if the material were tested in air. Tests under oil are permissible under the following conditions :— (a) When the thickness in relation to the other dimensions is such that the specimen cannot be conveniently broken down in air. Materials which are liable to be dissolved or changed in any way (except by absorption as mentioned below) must not be tested in oil. (b) When the material is intended for use in oil-immersed apparatus. Materials which are capable of absorbing oil must not be tested in oil unless they are to be employed in oil-immersed apparatus. In the case of materials which are intended for use in air and also in oil-immersed apparatus it is desirable that the effect of a damp atmosphere [Clause 3 (c)] on the electric strength should be ascertained both in air and in oil. 5. METHOD OF EXPRESSING ELECTRIC STRENGTH. A single value for the electric strength of these materials is of practically no use unless the time of application of the voltage, the temperature and the thickness of the material are either stated or clearly indicated. Instantaneous values obtained on a rapidly applied test are usually very high and misleading, being out of all proportion to the electric strength obtained by the sustained conditions of practice. | ||