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3. Outer protection layer

In the medium and high voltage power grid, cables are more and more widely applied. The outer sheath of power cables is the first line of defense for cables. Whether they are intact or not is directly related to the safety level of internal structures and the service life of cables. There are three main reasons for the failure of the cable outer shield.

(1) the damage of hard material or external force around the cable. The directly buried cable is directly exposed to the outer shield, especially on the road section with a long time. The hard point angle may pierce the outer protective layer and cause the internal structure to be damaged. In addition, the cable load changes, the thermal expansion and contraction of the cable itself and the uneven distribution of the damaged part of the electric field eventually lead to the damage of the insulating layer. When the pipe laying is laid, the outer layer or the inner wall of the pipe connection may cause the damage of the outer protective layer. In addition, the mechanical construction or pipe jacking around the cable path caused the damage of the outer protective layer.

(2) the defects and hidden dangers are left over in the construction. In the construction process of cable laying, the wound and cracking parts of the protective layer at home and abroad are in the pipe row, and the personnel can not be found in time. After running for a period of time, the cable parts of 110kV and above are cracked. The insulation of the outer sheath is reduced, the metal sheath is multiple grounded, and the circulating current increases, which eventually leads to insulation aging and breakdown.

(3) termites. Once there is a part of the termites, often on the cable line should have multiple parts of decay. We should pay enough attention, although termites harm, the northern grid is relatively rare.

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