3. Railroad yards, railway-signal circuits and airport lighting
4. Lighting and power circuits for amusement parks, baseball and football fields, and industrial plants
5. Crossing under small lakes and streams
Both nonmetallic-armored cable and metallic- armored cable (parkway cable) are used for direct burial in the earth. The nonmetallic-armored types are lighter in weight. more flexible, and easier to splice and are not subject to rust, crystallization, induced sheath power loss, or trouble from stray currents. On the other hand, they do not give good protection against mechanical injury,.
Direct burial of power cable is normally accomplished with a backhoe digging a trench large enough to permit bed preparation. Whereas communications cable is laid in a small trench created by a chain type of mechanical trencher. Cables are installed at a minimum depth of 30 inches for power cables of 600 Vac or over and 18 inches for communications cables. Cables installed at 30 inches or greater will be protected against extreme mechanical hazard, such as at street intersections or under roadways. The powercables should be placed in a 3-inch-thick bed of sand. When backfilling a direct burial cable. you should place plastic streamers in the trench 12 to 18 inches above the cable. These streamers will alert future personnel conducting digging operations to the presence of the buried cable.
At intervals of 200 feet and at turns in the buried cable, you also should place small-concrete markers along the entire length. These precautionary signs should prevent some future human-related damages to the buried distribution systems. The marker should state the type of cable that is buried, such as power or communications, and voltage or number of pairs.
Types UF and USE cables are code-designated single-conductor or multiconductor cables suitable for direct burial in the earth. The NEC ® includes rules for the protection of underground conductors when the supply voltage exceeds 600 volts. These rules were introduced to minimize the hazards of "dig-ins." Section 710-3(b) of the NEC ® covers such rules.
For connection of underground distribution circuits at any location. the end must be prepared for termination. In earlier times this preparation was accomplished with a pothead. as shown in figure 4-44,
Figure 4-44. - Pothead.
but is now done with special kits which provide plastic molds to be placed over individually prepared phases. The molds are poured full of epoxy. The new way is much more efficient and clean. The new style is shown in figure 4-45.
The riser pole for underground distribution circuits should be inspected when overhead lines are inspected and maintained (fig. 4-46). The inspection should include the disconnect switches or fused cutouts, the lightning arresters, the operation of the arrester ground leads isolation devices, the riser cables and potheads or termination, support of the cables, conduit or U-guard, and identification of the circuit and pole conditions.
Figure 4-45. - Diagram of a modern single-phase cable end termination kit.Continue Reading