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).
Placement of switches in low-voltage wiring systems with earth return to protect components.
| Identifier | ExFiles\Box 168b\2\ img156 | |
| Date | 25th September 1928 | |
| To R.{Sir Henry Royce} from EFC. c. E.{Mr Elliott - Chief Engineer} BJ. c. EY. X7560 EFC1/T25.9.28. WIRING CONNECTIONS. X7750 On a low voltage system with earth return in which there is no danger of shock, there would appear to be some amount of argument for putting apparatus which gives self-indication of being "on", e.g. indicating lamps, on the high pressure side, and the switch on the earthing or datum side. Should an earth occur on the intermediate wire (between switch and apparatus) it might be less disastrous to be reminded of the fact by the lighting of a lamp than to put the switch on with the likelihood of blowing the fuse. Where the switch consists of delicate contacts, e.g. contacts in our thermometer, or even green lamp switch contacts, it might be worth while protecting these from possible heavy currents by reducing the possibility of a short circuit current passing through them. We have three cases in our new wiring diagram :- (a) We have already done this in the case of the Klaxon and steering column switch, for reasons of mechanical convenience. (One wire up column in place of two). (b) On the water temperature lamp circuit we did have in the early experimental days cases of thermometer contacts being ruined which could only be explained by an earth at one of the terminals of the water temperature lamp. Now, however, there are protective covers on both thermometer and lamp terminals. (c) In the case of the green lamp circuit, the principal advantage would appear to be that it would reduce the green lamp switch to a three-terminal device, viz. common negative or earth relay and lamp circuit terminals. It is only our intention to suggest for consideration the query as to which way is really the better in the various cases. | ||