Underground Power Cables

Figure 4-46.—Riser pole for underground distribution circuit. UNDERGROUND POWER CABLES Underground cables have various types of insulation and sheaths. Because higher voltages generate more heat, the amount of voltage carried determines the composition of the insulation. Cables rated at 15 kilovolts and below usually have rubber or varnished cambric insulation and a PVC or rubber sheath. Those rated at 600 volts to 425 kilovolts have oil-impregnated paper insulation and a PVC sheath. Cables rated at 5 kilovolts and above have metallic tape shields between the insulation and sheath for mechanical protection. Exceptions to this requirement are for single-conductor (1/0) cable with a PVC sheath and three-conductor (3/0) belted type of cable. Much of the new cable being installed is cross- linked-polyethylene (XLP) or ethylenepropylene rubber (EPR). These are called solid type of insulations. The size and number of conductors in the cable depend on the job requirements. UNDERGROUND COMMUNICATIONS CABLES The most common types of underground communications cables in use today are steel-armored with plastic insulation (STELPATH), plastic- insulated with aluminum armor (PIC), and the new shielded fiber-optic cables. PULLING CABLE When installing a new run of duct, you pull in “pulling wire,” usually a lo-gauge iron wire. With this wire, you pull in a wire rope to which you attach the cable for pulling in. Sometimes, when the duct has been in the ground a long time, the original iron pulling wire may be rusted so that it is not strong enough to pull the wire rope through. Also for a 400- to 500-foot run, it would be difficult to push a fish tape through the duct. The job can be simplified by using an air compressor to blow a chalk line cord through the duct. To do this, take a small cloth and tie the chalk line end to the four comers, so the cloth functions like a small parachute. 4-31