564 ELECTRICAL EQUIPMENT 



per cent of the total cost, very little is added by substituting a 

 face brick. With this type of construction the compartment 

 shelves are generally made of concrete or soapstone, from 2 to 3 

 inches thick, depending on the size of the compartment. 



Concrete, although more costly, has gained in favor over 

 brick work, and therefore the majority of bus and switch compart- 

 ments nowadays are built of concrete, especially for the larger sta- 

 tions. In some cases complete forms are made, usually of wood, 

 and the whole compartment poured, giving a very substantial 

 construction. It is more often the case, however, that concrete 

 slabs are used, set in cement. 



The general dimensions of bus and switch compartments are 

 determined by the minimum distance allowable between conductors 

 and ground (see table LII, page 627), the brick or concrete being 

 considered as ground. The switching apparatus also governs to 

 a great extent the dimensions of the compartment, although even 

 here it is generally a matter of ground distance in the apparatus. 

 For mechanical reasons and accessibility the distances are gen- 

 erally increased somewhat; this also to guard against joints, 

 clamps or bolts acting as spillways at times of abnormal voltage 

 rises on the system. Low-voltage compartments, where relatively 

 heavy copper is used, should have proportionally more liberal dis- 

 tances than those for equal capacities but of higher voltages, with 

 connections of smaller size. 



Removable doors are recommended for all openings of compart- 

 ments to prevent accidental contact with live parts, and in the 

 case of oil circuit breakers, to prevent the scattering of oil should 

 it be forced out of the oil vessels. Compartment doors should 

 be made of light, fireproof material and swung from the top to 

 allow free movement in case of explosion in the compartment. 

 Asbestos lumber with a light wood frame has proved to be the most 

 satisfactory construction for compartment doors. Compartment 

 doors should be considered as ground, that is, in respect to all live 

 parts. 



The arrangement of switch and bus structures varies consider- 

 ably, depending not only on the system of connections, but also 

 on the different designs of the circuit breakers. It is therefore 

 impossible to give any definite recommendations that will meet 

 all conditions. In addition to the illustrations shown in the sec- 

 tion on " Arrangement of Apparatus," page 175, Figs. 353 to 357 



