SWITCHING EQUIPMENT 487 



is usually given in per cent overload of the rated capacity of the 

 circuit. 



The particular system of connections to be used depends 

 obviously on the conditions to be met, and each system must 

 be studied and an individual solution applied. There are, how- 

 ever, many points of similarity, and the solution in one case will 

 serve as a partial guide, at least in others. In any event, the 

 system as a whole should be carefully considered in deciding on 

 the connections, and the conclusions should not be based on the 

 condition in a generating station or a substation alone. The 

 characteristics of the customer's load conditions must be care- 

 fully investigated and future probable loads and additions pre- 

 determined as far as possible. 



It is especially essential to provide an uninterrupted service 

 for large and important customers, as the success of the project 

 depends in most cases entirely on the ability to maintain a satis- 

 factory service for these, but, on the other hand, the smaller cus- 

 tomers must also, of course, be considered and given the best 

 service possible. For this reason the power to important cus- 

 tomers is often supplied from two sources, such as from two sub- 

 stations or by means of double-line circuits, etc. Two such 

 sources of supply are, of course, the ideal arrangement, in which 

 case one of them would be automatically cut out in case of trouble 

 while the other would be kept in operation and continue to carry 

 the load. This, however, is not always possible for every cus- 

 tomer. 



In a general way the service of a large power system with its 

 transmission and distributing lines can be likened to a combined 

 express and local train service of a transportation company. The 

 transmission lines feeding the different substations on the system 

 correspond to the express trains and must be absolutely free from 

 interruption, for which reason such lines should be so arranged 

 that any substation is fed by two independent circuits. The 

 local train service would, on the other hand, correspond to the 

 distributing lines, and any interruptions which might be per- 

 mitted to occur, should be confined to these local circuits. Of 

 course, if the service demands, even these circuits can be installed 

 in duplicate. 



In a power transmission system the chief source of trouble is 

 always the transmission line and it can mostly be traced back to 



