Modern Gasworks Practice

352 MODERN GASWORKS PRACTICE All contimiously-rated motors should be capable of developing an overload of at least 20 per cent, for one hour, and 50 per cent, for short periods. Totally-enclosed motors should be provided with terminal boxes so arranged that the conduit carrying the cable can be screwed right into the motors. (4) Cable should be of 2,500 megohm grade V.I.R. (5) Where cables are carried overhead stranded suspension wires should be used and the cable should be carried by leather or raw-hide suspenders. Where cable is run in tubing the tubing should be made continuous throughout. (6) All motor frames, starter cases, and tubing should be efficiently earthed. (7) All trenches for cables should be capable of easy inspection, and are prefci;ably covered with some form of grid. If this is not done, a small inspection chambcr should be built in at every point at which the direction of the cable or pipe is changed. PRECAUTIONS AGAINST BREAKDOWN If electrical plant is to operate with a minimum of trouble and derangement it is essential that it should not; be left to look after its own welfare. Breakdowns and temporary stoppages largely arise from insufficient attention being given to elementary considerations; and, in order to be safeguarded, the gas engineer should ensure that the following points are fully apprcciated and acted upon by those in charge of the plant:— (a) All parts of electrical machinery must be kept perf eetly clean. (b) Any traces of oil on the windings should be immediately removed. Oil, if permitted to remain, will eventually destroy the insulation. (c) The machines must be sufficiehtly large for their work. This point, requires special emphasis so far as gasworks are concerned, for small extra loads are so frequently added with out any thought being given to the original work for which the generating plant was installed. (<Z ) If in doubt as to the power required, most suitable type of plant, etc., leave the matter to the judgrnent of expert contractors, and see that your contract specifically binds them down so that you will get your work done as you require it to be done. On contracts for electrical machinery the balance of the purchase money should be kept in hånd until the plant has been in operation for three months. (e) Bear in mind that direct-coupled plant is the simplest, most compact, and probably the cheapest in the long run. (f) If a belt-driven plant is employed, endeavour to keep the belt horizontal with the slack side on top. The chief merit of a belt drive is that it provides a flexible connection ; and, in the event of an overload, it may give way and provide a “ safety valve.” The belts should be of the endless type, otherwise the joint may cause sparking. Sparking at the commutator gives rise to pitting, and if there is any sparking up to füll load the machine requires attention. A higli-class machine properly driven, should at all loads up to füll load be absolutely sparkless. (g) Extreme caution should always be exercised when a new dynamo or