632 ELECTRICAL EQUIPMENT 



ness of the insulation varying with the generator voltage. For 

 absolutely dry locations a good weatherproof braid may well 

 serve as a mechanical protection against abrasion, but the ducts 

 should nevertheless be provided with drains so that the cables will 

 under no circumstances lay in water which may be accumulated 

 from condensation. For damp localities, lead-covered cables 

 should always be used, and to be on the safe side the use of such 

 cables is always to be recommended. Endbells are always re- 

 quired for such cables. 



Exposed main wiring is generally considered out of date, but, 

 if used, the cables should be well supported and guarded and per- 

 fectly covered with a fireproof covering to prevent a fire from 

 spreading from one circuit to another. The installation of the 

 cables in ducts or conducts is much to be preferred. 



Fiber ducts should be used for all alternating-current cables, 

 although iron conduit is permissible if all conductors of one cir- 

 cuit are run in the same conduit. With single-conductor, lead- 

 covered cables, and preferably also for multi-conductor, fiber 

 conduits should be used. 



Whether single- or three-conductor cables are to be used de- 

 pends on the size, the length of run and the loss in the lead sheath. 

 Single-conductor cables are, as stated before, made in much larger 

 sizes than three-conductor and have, of course, a greater radiating 

 capacity, but on the other hand, especially for long runs, it is found 

 that three-conductor cables will be more economical, especially 

 for lead-covered cables. This is evident when one considers that 

 three lead sheaths, each, however, somewhat smaller, will be 

 required as compared to one. On the other hand, the eddy-cur- 

 rent losses in the lead sheath for a single-conductor cable is not 

 negligible, while with a multi-conductor cable they are entirely 

 neutralized. Lead sheaths are as a rule grounded at one end to 

 get rid of accumulation of static electricity and a ground of the 

 Isad sheath at the other end of the cable can very easily occur 

 without being noticed, resulting with single-conductor cables in 

 circulating currents in the lead sheath. These currents are only 

 limited by the resistance of the lead and the losses caused thereby 

 may be quite considerable. Of course, where the size is such that 

 two or more conductors per phase are required it is possible to 

 " nest " the conductors so as to neutralize the inductive effects. 



In selecting cables for generator leads, a larger factor of safety 



