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).
Letter to The Cambridge Instrument Co. Ltd. discussing various instrument failures due to earthing and insufficient clearances.
| Identifier | ExFiles\Box 175\4\ img084 | |
| Date | 27th September 1927 | |
| X3875 Ref. RDY13/MG27.9.27. Your ref. JLO/MW Sept. 27th 1927. Messrs. The Cambridge Instrument Co,Ltd., Cambridge. Dear Sirs, Reverting to your communication of the 8th instant, with you, we certainly trust we shall not find any other instruments made in the early stages of this development earthed as was Instrument No.87545, which was returned to you against our Order R.5865, and hope the further testing arrangements you have instituted for some time past will obviate any further trouble under this heading. Meantime we are returning, under separate cover, against our Order R.5975, another instrument which has failed, the failure being due to a contact being made between the main Bourdon tube and the bottom end of one of the insulated terminal screws. This allowed the 12-volt current to pass through the hair spring of the mechanism and get to earth via the fault contact, thus damaging the spring - due, probably, we should say, to insufficient clearances. We would further draw your attention to the fact that on this instrument there is a very small clearance between the teeth of the sector and the base block of the mechanism, which is normally insulated. Our Experimental Department advise that we are not ascribing all the failures we have had with this instrument to you, and admit that we have had failures which we cannot reasonably ascribe to any fault on your part, but which we can only put down to the terminals, which are exposed, having been accidentally earthed while the thermometer has been on contact To obviate this contingency, it is our intention to provide insulating covers for the terminals of both the thermometer and the bare terminal of the warning lamp which is lighted thereby. We have further carried out a number of tests in our own Laboratory, and confirm your previous contention that the ordinary current of the lamp cannot under any circumstances fuse the contact. Contd. | ||