The intact cable sheath can effectively protect the insulation: it can prevent liquid water from immersing in the insulation, and the enclosed metal sheath can also prevent moisture from diffusing into the insulation, so it can obviously prevent the formation of water branches in PE / XLPE insulated cables. The sheath and anti-corrosion layer test which can provide scientific criteria can ensure that the cable will not fail in long-term operation; In addition, for oil filled AAAC Cable, it also makes an important contribution to environmental protection.
In general, the test is carried out with appropriate high DC voltage between the metal sheath or shield and the ground
Welded steel bushing for compressed air cable, 1kV
PVC sheath, 3KV (higher voltage may cause intact sheath breakdown due to moisture absorption of sheath)
Polyethylene sheath, 5 ~ 10KV
The duration of the test was about 1 min. Such a long time is enough to complete the charging process. The existing fault can be identified immediately by breakdown, and then the method described in Chapter 37 can be used for detection. Because this kind of test usually refers to fault detection, rather than insulation resistance test according to international electrical standard iec60229, longer test time is not recommended. For example, for PVC sheath, if the load is applied for a long time, it may cause breakdown of the innermost sheath.
Of course, the premise of reliable damage identification is to have a “corresponding electrode” with good conductivity to make the possible fault current return to the source point. For this purpose, the ABC Cable should be completely buried in the soil, and the water in the soil should be sufficient. Laboratory tests show that although the insulating layer in the sheath is still intact, it has no negative effect on the performance of the test. It has nothing to do with the structure of the isolation layer. The isolation layer either absorbs enough moisture (such as rubber inner cover and wrinkled paper), or forms a low resistance waterway at the overlapping position of the plastic film. That is to say, the expressiveness of the test mainly depends on the degree of moisture.
The graphite coating on the cable sheath can also improve the return circuit. However, this method is expensive and sensitive, so it can only be used in special cases according to the requirements. Before conducting DC voltage test on anti-corrosion protection, all grounding of metal sheath or shield and its connected accessories must be disconnected.