Power cables are mainly composed of conductor cores, insulation layers, and sheaths. According to the regulations, power cables are divided into three categories: paper insulated power cables, rubber plastic insulated power cables (polyvinyl chloride insulated power cables, cross-linked polyethylene insulated power cables, ethylene propylene rubber insulated power cables), and capacitive oil filled cables, Their preventive tests
Insulation resistance measurement
Measuring the main insulation resistance of power cables can check whether the cable insulation is aging, damp, and any insulation defects exposed during voltage withstand tests When measuring cables below 1000V, use a 1000V insulation resistance tester, a 2500V insulation resistance tester for cables above 1000V, and a 5000V insulation resistance tester for cables above 6kV When the insulation resistance of power cables with plastic insulation is very low, a multimeter should be used to measure the DC resistance of the shielding layer to the armor and the armor layer to the ground through positive and negative wiring to check if they are damp. When the insulation is indeed damp, maintenance should be arranged When the cable is buried underground, the insulation resistance of the steel armor to the ground can be measured to check whether the outer sheath is damaged; Similarly, measuring the insulation resistance between the copper shielding layer and the steel armor can also check for damage to the inner sheath. These two measurements can be used to determine whether the insulation has become damp. When cables are laid on cable trenches or tunnel supports, the damage point of the outer sheath is not at the support point and is not immersed in water or placed in a particularly humid environment. Therefore, it is difficult to detect the operation of the outer sheath by measuring the insulation resistance. At this time, measuring the insulation resistance of the copper shielding layer to the steel armor is even more important Dirt and moisture on the surface of cable terminals or sleeves have a significant impact on insulation resistance. In addition to wiping it clean, a shielding ring should also be added, and the shielding ring should be connected to the "shielding" terminal of the KST-430A insulation resistance tester. When the cable is a three core cable, non measuring phase can be used as the connection between the shielding rings at both ends.
(a) Single core cable; (b) When the tested cable is long, the charging current is high, so the KST-430A insulation resistance tester starts to indicate a small value. This does not mean poor insulation, and it must be telemetry for a long time to obtain the correct results
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