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).
Specification for testing the electric strength and toughness of hard composite dielectric materials.
| Identifier | ExFiles\Box 24\2\ Scan211 | |
| Date | 21th February 1922 guessed | |
| HARD COMPOSITE DIELECTRICS (INSULATING MATERIALS). 569 500 volts, and the surface resistivity shall be expressed in megohms for a centimetre square. The materials shall be tested in accordance with conditions (a), (b), (c), (e), (f), and (g) given in Clause 2 of this Specification. 5. Electric Strength (Puncture Strength). The electric strength of material in moulded or sheet form shall be tested on a specimen of the shape and dimensions shown in Fig. 6. The test shall be carried out by applying an alternating potential difference between a spherical and a flat surface. Contact with the flat surface of the specimen shall be obtained with a flat disc, 40 mm diameter, which shall make good contact with the surface of the specimen. The middle of the disc shall be opposite the thinnest part of the specimen. Contact with the spherical surface shall be obtained by the use of shredded lead, mercury-tin amalgam, or other suitable material, pressed into intimate contact with the surface of the specimen. Similar materials shall be used for the flat surface if required by its nature to obtain effective contact. The frequency of the alternating potential difference shall be about 50 periods per second, the wave-form being approximately sine shaped. FIG. 6.—Specimen for Electric Strength Test. * For materials of low electric strength which will not flash over. For a material having a very high electric strength this dimension (2 mm) must be increased so that the specimen will puncture at a potential difference not exceeding 80 000 volts. For coarse materials this dimension may be increased to a maximum of 5 mm by arrangement. The potential difference shall be applied without shock, and shall be increased at such a rate that the specimen is punctured in about 20 seconds. The potential difference shall be expressed in volts per millimetre (R.M.S.) computed from the following formula* :— X = V / Tf{T. Fisher} where X = breakdown volts per mm (electric strength), V = volts required to puncture specimen, T = minimum thickness of specimen in mm, f = function depending on the thickness T. With one electrode consisting of a sphere 25 mm radius and the other a disc as specified, the values of f for various thicknesses of specimen are as given in the following table :— Minimum Thickness of Specimen T, mm Approximate Value of f 0·5 1·01 1·0 1·03 2·0 1·05 3·0 1·08 4·0 1·11 5·0 1·14 If the electric strength of the material is so high that the specimen cannot be punctured in air the test may be carried out under oil. The materials shall be tested in accordance with conditions (a), (d), (e), (f), (g), and (h), given in Clause 2 of this Specification. MECHANICAL TESTS. 6. Toughness (Absence of Brittleness). Toughness shall be proved by an Impact Test carried out with a machine of the pendulum type, at a temperature from 15° C. to 20° C. The form and dimensions of the specimen for moulded material shall be as shown in Fig. 7. The notch shall be moulded in. FIG. 7.—Moulded Specimen for Impact Test. The form and dimensions of the specimen for sheet or rod material shall be as shown in Fig. 8. The notch shall take the form of a drilled 1 mm hole radius opened out to the side of the specimen by a saw cut. When it is impracticable to obtain sheet 15 mm thick, the * This formula is adopted from “A Treatise on the Theory of Alternating Currents,” vol. 1, by Dr. A.{Mr Adams} Russell. FIG. 8.—Specimen (machined from Sheet or Rod) for Impact Test. | ||