THE UNDERGROUND INSTALLATION OF UTILITIES 367 



it houses a fairly standard transformer in a below-surface vault, con- 

 cealed by the base of a street light. 



If street lighting is not used, the vault can be covered at ground 

 level by an almost invisible street grating. 



Looking further into the future, we see the possibility of a direct- 

 buried transformer, with which several utilities and manufacturers 

 are experimenting at the present time. 



Let me emphasize that I have been talking about underground 

 distribution for new residential subdivisions, in which the use of 

 underground construction is economically feasible. Replacement 

 of existing overhead powerlines with an underground system is still 

 too expensive to receive anything but token consideration, simply 

 because of the tremendous costs involved in taking down existing 

 lines and attempting to install underground lines in built-up areas 

 with paved streets, concrete sidewalks and a tangle of existing water- 

 lines, sewers and buried construction of various sorts. 



Also, while the cost-gap between underground and overhead 

 residential power distribution has narrowed to a point of practicality, 

 the cost of underground transmission is prohibitively expensive for 

 the vast majority of high-voltage, high-capacity transmission line 

 application. Present 220 and 345-kv. underground cable trans- 

 mission is many times the cost of overhead transmission, even though 

 in many situations it must be used because of prohibitions against 

 going overhead through congested metropolitan centers. 



Cable circuits have much lower limits of power-carrying capability 

 than overhead circuits. For example, a 345-kv. cable can carry up 

 to 500,000 kv.-a. effectively for distances of up to 15 miles. For 

 greater distances, additional equipment is required. Underground 

 systems of the future may require transmission capabilities in excess 

 of 2 million kv.-a. for 25 or more miles. 



One solution is to improve the power-carrying capacity of the 

 cable. Another possible solution, looking to the future, is suggested 

 by our engineers who are exploring the use of pressurized gas as the 

 insulating medium with the conductor being supported inside a pipe 

 by appropriate cylindrical insulators. 



Three 12- to 14-inch pipes could operate at 345,000 volts and 

 carry up to 1,500,000 kv.-a. for distances up to 300 miles. This 

 would provide three times the carrying capacity now possible with 

 conventional cable systems, for 20 times the distance. We have 

 labeled this the Pressure Insulated Piped Electrical System, and we 

 have already made an urban substation layout utilizing this system 



