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).
The method for testing the electric strength of fibrous insulating materials.
| Identifier | ExFiles\Box 24\2\ Scan226 | |
| Date | 11th December 1922 guessed | |
| 802 ELECTRIC STRENGTH OF FIBROUS INSULATING MATERIALS. 0·027 in. covered copper wire and the eureka to 3 ft. 8 in. of 0·018 in. covered eureka wire. From here copper flexible leads are connected to the instrument. Resistance, about 4·7 ohms at 20° C. A thermometer is bound to the cold junction with tape. An eureka series resistance can be inserted to increase the range, but this is not required, except for readings in the unstable range. Resistance of couple and series resistance = 8·65 ohms at 20° C. Thermocouple Instrument.—Microammeter by R.{Sir Henry Royce} W. Paul, London, N.1. 1 135. Resistance, 5·1 ohms at 20° C. Range, 0-300 microamps. by 5 (60 scale divisions). Temperature range: { Without series resistance : 0 to about 65° C. rise { With series resistance : 0 to about 90° C. rise Air Temperature.—By thermometer 3 in. from edge of specimen, with bulb on the same level. Felt Pads of coarse green felt, 4½ in. × 4½ in. × ⅛ in. Tinfoil 4 in × 3½ in. × 0·0015 in. thick. (b) Arrangement of Test. The material is cut into rectangles each 5 in. × 4½ in. and exposed to “Normal”* condition [Clause 3 (a)]. * If the material claims to have any special non-absorptive properties, tests should also be made on material which has been exposed to “Damp” or “Tropical” conditions [Clause 1 (c) and (d)]. After conditioning, sufficient of these to form a pad approximately 30 mils thick are carefully laid and pressed together so as to exclude entrapped air. On each side of the pad a tinfoil 4 in. × 3½ in. is pressed into contact. The specimen with its tinfoils is placed between two 4½ in. × 4½ in. pads of ⅛ in. coarse felt and mounted for test as shown in Fig. 9. A thermocouple in contact with the centre of the lower tinfoil measures the temperature attained. In series with the source of high-voltage A.C. are a D.C. battery of (usually) 52 volts and a microammeter by which unidirectional leakage current is measured. (c) Method of Test. An A.C. stress of moderate gradient is applied and maintained till temperature and leakage current attain steady values. The gradient is then increased and again held till results are steady. This procedure is repeated till a gradient is reached at which temperature and leakage current will not settle down to steady values, but rapidly increase to a destructive condition. At this stage, unless interrupted, charring and complete breakdown rapidly follow. The microammeter should be short-circuited or the voltage switched off before breakdown of the specimen takes place. (d) Method of Expressing Results Obtained. The values obtained should be shown graphically as illustrated in Fig. 10. The following should also be recorded :— (i) Maximum voltage and stress in volts per mil at which the material is in a stable condition. (ii) Maximum temperature-rise obtained in (i). | ||