Methods of installing underground lines
There are two methods of installing underground lines. In the first method, underground raceway systems (ducts) are installed below grade and then cable is pulled through them. Ducts may or may not be provided with concrete encasement. The second method consists of underground cable systems installed directly in the ground.
Cables may be the direct-burial type cable assemblies in coilable plastic duct, or cable assemblies in metallic armor (in direct-burial rated sheath).
In this article, the word duct will be used rather than conduit:
Requirements for medium-voltage lines
Where underground systems are provided, the following standards will be followed:
- In industrial and densely populated areas, cables will be installed in underground duct lines with manholes. Ducts will be concrete encased.
- In lightly populated areas, cable may be placed in non-concrete-encased duct or buried directly.
- The use of direct-burial cable will be limited to long untapped runs in lightly populated areas where the reliability requirements are low; or the facilities served by the cables have a short-term life; or for other reasons which would justify the use of the more economical direct-burial installations.
Secondary distribution lines and service conductors
Where underground systems are provided, the following guidelines will be observed:
- In industrial and densely populated areas, cables be installed in underground duct lines (with manholes, handholes, or pullboxes as applicable). Ducts will be concrete encased.
- In lightly populated areas, cable in nonconcrete-encased duct or direct cable may be used.
- Low-voltage direct-burial cable will be restricted to applications where the load to be served is not anticipated to be increased; the underground cable can be replaced easily upon failure; and the cable system is not subject to disturbance or physical damage. The designer will coordinate burial requirements with the using or maintaining organization.