When the decision is made to "Go Underground" on a new distribution system,there are many different areas to consider prior to construction. In the following paper, we would like to suggest some of these areas for your consideration in regards to equipment and procedures for underground cable installation.
The three most common methods of cable placement are Direct Buried, Cable in Conduit, and a Total Conduit System. Most Utilities are currently using one or more of these methods with various degrees of success.
The first system or method commonly undertaken is the Direct Buried method. As this system matures, a few questions come to the surface. How much care was taken in preparation of the trench? How was the cable placed in the trench? How does different cables react to different soils and ground conditions? How complicated is it to replace damaged or faulted cable? When a fault or damage caused by digging occurs, it is very time consuming to open the trench and make repairs. All these factors and many more affect the reliability and life of the conductor.
Due to these and other conditions, the Cable in Conduit method is considered in some areas. The cable is placed in a conduit during the manufacturing process, and this conduit protects the conductor as it lies in the trench, giving it longer life. However, most of the same questions occur when using the C.I.C. method as occurs when using the Direct Buried method.
The particular method we would like to discuss in this paper is the Total Conduit System.
1. System Preparation:
A. A good system must be installed properly with attention given to preparation of the trenches, placement of the conduit, and care used when backfilling the trenches.
B. Consideration should be given to selecting the size of inside diameter to ensure adequate fill ratios and schedules such as DB-120, SCH-40, or SCH-80 as required in specific areas.
C. The pulling cable has a great impact on the selection of the schedule of the conduit. Initially, everyone assumed if synthetic rope is good enough for overhead, it must be good enough for underground as well. However, we learned synthetic rope generates heat, causes friction, and burns through the PVC. The
most common response was to increase the wall thickness of the conduit or even go to steel 90's in order to stop this problem. Since the rope was causing the problem, most pullers today use steel pulling ropes. Steel ropes dissipate the heat over the length of the cable and slides around the 90's, instead of burning through them.
D. The maximum length between vaults depends on system designs, customer requirements, and pulling equipment available. The average pull normally runs between 500 and 1,000 ft., however, some systems require cable pulls over 2,000 ft.
E. How many 90-degree sweeps can one pull have before damage is done to the conductor? The rule of thumb in the industry is a maximum of 270 degrees. However, some designs or requirements demand more, but keep in mind every 90-degree sweep adds pulling tension to the pull. The cable manufacture normally recommends a maximum pulling tension for a specific size and type of conductor.
F. Remember a good conduit system is sized, installed, and tested (mandreled) properly prior to installing the conductor.