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 electrical properties and leakage tests conducted on phenol-formaldehyde mouldings.
| Identifier | ExFiles\Box 61\1\ scan0127 | |
| Date | 12th September 1930 guessed | |
| -3- Electrical Properties of Mouldings. Turning back now to the mouldings themselves the properties are usually given as follows :- Electrical breakdown strength (50~) 30,000 volts per mm. Insulation resistance ............ Infinity. Water absorption (24 hours) ....... 0.02% While no figure is given for insulation resistance, it is usually assumed to be infinity. The above figures appear to be quite satisfactory for the majority of purposes and yet with a factor of safety of as much as 20 : 1 trouble has occurred. Leakage Tests on Phenol-Formaldehyde Mouldings. A study of the leakage of mouldings under all conceivable conditions yielded much valuable information. The tests were conducted as follows :- A mould was constructed to make a 5" by 1/4" disc, and provision made to mould in two electrodes for the electrical test. In the centre of the disc was a spherical indentation, conforming to the E.R.A. standard, for electrical breakdown tests when desired; a rough sketch is reproduced in fig.1 and fig.2 is a diagrammatic representation of the testing set. To take the leakage between the two electrodes an electrically heated pot was used. The connections to the disc were made and it was then dropped into the pot and covered with transformer oil. A microammeter and a 600-v generator completed the apparatus. The procedure was as follows :- The current was switched on to the heating elements of the pot, 2.0 amps. gave a rate of temperature rise of approximately 5oC. per minute and was used up to 70 C. when it was increased to 2.5 amps. to maintain the rise. Readings on the microammeter were taken every minute and from them a graph was prepared. Three typical curves are shown (fig.3) and represent, respectively, fairly good, fairly bad, and medium mouldings. The graph is interesting because of the amount of useful information that can be obtained from it. First, there is the unusual amount of variation between a good and bad sample, in this connection it should be noted that samples with five to six times the leakage of C have been encountered, as also have samples better | ||