notes11_Cables (1).doc - Notes 11 Cables 11.0 Introduction...

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Notes 11: Cables 11.0 Introduction Underground conductors are more expensive than overhead conductors because the entire length of each conductor must be insulated from the ground and from other conductors Therefore, cables are generally used for underground distribution. The most salient difference between cables and overhead conductors is that cables are insulated. In addition, to obtain a given ampacity, a cable requires a larger conductor than an overhead line because cables do not dissipate heat well because of 2 reasons: No wind Insulation confines the heat This further increases cable cost compared to overhead. 1
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And that is just for the wire. In addition, cable installation is more costly than overhead installation because of the variability of soil conditions. If the job requires trenching through the rich soil of Iowa farmland, installation cost is generally slightly higher. But if even in a relatively short distance of the overall installation, the soil is very rocky, or worse yet, underneath concrete or asphalt, costs can skyrocket. Some 1991 data indicates single-phase cable installation are 1.3 to 2.4 times the cost of equivalent overhead, whereas three-phase cable installation is 1.7-7.6 times the cost of overhead [1]. So what are the reasons for going underground? There are 4 main reasons: 1. Aesthetics: Only power engineers really like to look at overhead lines . 2
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2. Safety: Overhead is exposed and can fall. Other strange things can happen as well. Underground is not exposed. 3. Reliability: Overhead circuits typically fault about 90 times/100 miles/year. In contrast, underground faults less than 10 times/100 miles/year [1]. 4. Maintenance: The number 1 maintenance cost for overhead, tree- trimming, is entirely eliminated with underground. 5. Reach: With greater diameters to obtain desirable ampacity, reactance is decreased and therefore voltage drop is lower. 11.1 Cable construction There are seven distinguishing features in regards to cable construction. 1. Single phase vs. polyphase 2. Neutral 3. Conductor 4. Insulation 3
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5. Shielding 6. Jackets 7. Burial 11.1.1 Single phase vs. polyphase Cables may have 1, 2, 3, or 4 conductors.
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