Cable Thermal Resistance Problems
When installing cables for the transmission and distribution of power or radio frequency, temperature is an important consideration. Temperature can affect the transmission capability of both underground and above-ground cables. When the temperature exceeds the thermal resistance of the cable, transmission problems can occur. Therefore, before installing cables, study the proposed location for year-round temperature variations and weather conditions, including rain and snow.
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How Cables Generate Heat
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With underground installations, cables generate heat using one of two methods. Dielectric heating results from the charging and discharging of the insulating dielectric at 50 to 60 times per second. If the cable has a constant transmission, in the long-term, this heat will degrade and damage the cable. AC resistance heating, on the other hand, results from the current that flows through the cable. The phase conductors, sheath or skid wires, concentric metallic shields and cable system pipes cause this type of resistance, depending on the installation type.
Underground Installation
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Different factors also affect the thermal resistance around the cable in an underground installation. Backfill surrounds the native soil in the trench. The type of backfill, solar radiation, rainfall and the external environment can affect the resistance. The ambient temperature of the soil and the depth of the cable will also impact the resistance. Additionally, the underground temperature, while generally more consistent, does not disperse heat loads. Therefore, the thermal resistance for underground installations should remain high.
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Site Considerations
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Installing cables underground in underground mainlines requires different considerations than underground cables installed in soil. The tunnel of the mainline can dissipate any heat created by the cables more effectively the soil. However, a shallow water table in a soil installation will also help dissipate some of the heat. The lack of moisture in most underground soil installations can dry and crack the cable creating air pockets. This can lead to problems because the temperature will not decrease in the air pockets.
Cable Quality
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Choosing the right cable can help to prevent problems in the future. Cable installations are not cheap and using low-grade cable will result in shorter life spans for the installation. Using high-quality cables with a high thermal resistance rating, on the other hand will help to increase the longevity of the cables and ensure a reliable transmission.
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