Underground Residential Distribution (URD) is common power distribution system for modern city in this world. If you are visiting some of the greatest city you may not find any overhead distribution cable. Generally a classic underground residential distribution circuit system is an underground circuit in a loop type arrangement that fed at each end from an overhead circuit. This loop arrangement system allows utilities crew to restore power for customers more quickly after they find the faulted section and they can reconfigure the loop system and isolate any failed section of cable. This system returns power to all valuable customers. This system also allow to utility crews to delay any replacing or fixing the cable until with more convenient time or when suitable equipment arrives at faulted location. But all URD is not configured in a loop system. Sometimes utilities use purely radial circuits or circuits with radial taps or branches. In this system pad-mounted step down transformers step voltage down for delivery to customers and provide a sectionalizing point.
Normally elbow connectors on the cables which attach to bushings on the power transformer to maintain a dead front and not exposed the energized conductors. By this way crew can open a section of cable just by simply pull an elbow off from the transformer bushing and place it on a parking stand and which is an elbow bushing intended for holding an energized elbow connector. But elbows and the other terminations are available with continuous current at the ratings of 200 or 600 A according by the IEEE Standard. Load break elbows are designed and manufactured to break load and these types of elbows are only available in 200A ratings. If there is no load-break capability then crews should only disconnect the elbow if the cable is de-energized. Normally all elbows have a test point where crews can check if the cable is live or not. Elbows can also tested to withstand ten cycles of fault current with 200A elbows tested at 10 kA and for 600A elbows tested at 25 kA.
Interface between the overhead circuit system and the URD circuit system is the riser pole. At the riser pole cable terminations provide the interface between the insulated cable and the bare overhead conductors or cable. Pothead terminations grade the insulation to prevent any kind of excessive electrical stress on the insulation. Potheads also keep away water to entering the cable and it is so critical for cable reliability. At riser pole are expulsion fuses, normally in cutouts. The areas with high short circuit current may also have current limiting fuses for better protection. But we cannot save our underground distribution system from lightning without protection. For this reason to keep lightning surges from damaging the cable, the riser pole should have lightning arresters right across the pothead with as little lead length as possible.
In history, underground designs for residential developments expanded dramatically in the 1970s. But that time political pressure coupled with the technology improvements were the driving forces behind underground distribution. In addition to improving the visual landscape and underground construction improves reliability. But it’s true that underground residential distribution system had some difficulties, especially high cable failure rates. Then in the late 1960s and early 1970s, given the durability of plastics, the polyethylene cables installed at that time were thought to have a life of at least 50 years. But in practical, cables failed much higher rate than expected and it was enough so that many utilities had to replace large amounts of this cable. But now this underground residential distribution is one of the best ways to distribute power to customer.
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