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 using voltage, time, and temperature curves.
| Identifier | ExFiles\Box 24\2\ Scan220 | |
| Date | 21th February 1922 guessed | |
| 796 DIRECTIONS FOR DETERMINING THE ELECTRIC STRENGTH OF The electric strength shall be given in the form of a series of curves in which the actual breakdown voltage, R.M.S. value (0·707 of the maximum value), is plotted against time and temperature as shown in Figs. 1 and 2 respectively; and in volts per mil against thickness as shown in Fig. 3. In the case of tubes the voltage gradient across the layers adjacent to the inner electrode theoretically exceeds the voltage gradient across the other layers. On the assumption that the dielectric fails when the voltage gradient on the innermost layers exceeds the rupturing gradient of the material, the electric strength of the material is not that obtained by dividing the breakdown voltage by the thickness as in the case of flat sheet. To correct for the inequality in the voltage gradient across the various layers of the tube, the actual values obtained by test shall be multiplied by the ratio of the maximum stress to the mean stress for the appropriate ratio of outside diameter to inside diameter. The necessary values for this purpose are given in Table 1. Fig. 1.—Time-voltage curves at various temperatures. TABLE 1. Outside Diameter / Inside Diameter | Maximum Voltage Stress* / Mean Voltage Stress† 3 / 2 | 1·82 / 1·44 1·5 / 1·3 | 1·23 / 1·15 1·2 / 1·1 | 1·10 / 1·045 1·05 | 1·020 * Maximum stress = V / (r loge R/r) † Mean stress = V / (R-r) where V = breakdown voltage, R and r = outside and inside radii respectively. 6. TIME-VOLTAGE CURVES. (a) In carrying out electric strength tests it is important to obtain time-voltage curves showing the breakdown voltage over the time range from 0·5 minute to the time required for the breakdown voltage to become approximately independent of the time. Fig. 2.—Temperature-voltage curves for various thicknesses. NOTE.—When the dielectric is tested hot the time required for the breakdown to become sufficiently independent of the time of application of the voltage may be about 10 minutes. When, however, the tests are carried out at air temperature (about 20° C.) the time may be considerably longer. (b) In special cases transient voltage tests may be desirable. 7. TEMPERATURE-VOLTAGE CURVES. It is desirable that time-voltage curves should be carried out at the following temperatures :— 20, 60, 90 and 120 or 150° C. (see Fig. 1). The 120 and 150° C. are alternative, the latter being applicable to mica products and other heat-resisting materials (i.e. B.E.S.A. Class B insulation). | ||