On the core breaking of flame retardant copper wire and cable

With the development of economy, the industry of wire and cable is also changing and developing. The application demand of wire and cable is also increasing. Meanwhile, the flame retardant and fire-resistant wires have emerged. How to detect the quality of these wires? So, let’s talk about the detection of the core breaking of flame retardant wires and cables!
1: The method of electrifying capacitance
Some aaac cable factories usually use capacitance comparison to find out the approximate position of broken wires. The efficiency of the method is very low, which is to use continuous power on to accurately break the cable. Generally, it takes about 1 and a half hours to find a broken line point. This method is very unlikely to find the broken core for the flame retardant cable which has been completed in the shielding, armouring and sheath process. If it is not made properly, the flame retardant cable will be discarded. This method is basically no longer used.

2: The capacitance method and the induction method are combined
This method is used for core breaking, which requires that there is no metal layer and no extrusion cover outside the core. In the search, we first find out the approximate position of cable break with capacitance, and then use inductive voltage method to find the broken point accurately. This method is very simple and fast, and it is also the most used method at present. The induction voltage method is to connect 650V AC voltage at one end of the broken core of the cable, and the other end and other cores to ground, and then test with an inductive pen which can send light signal. When the probe slips out of the core, the signal will change, so that the cable break point can be accurately found.

3: Combination of constant current source and bridge method
This method is used for completed cables or cores covered with metal layers. The method of finding is to burn and break down the insulation at the broken core of flame retardant cable with constant current source, and then to locate the fault point accurately by bridge fault location instrument, which is also a common method.

Cable Insulation Resistance Decreases Due to Moisture

The insulation resistance of directly buried power cables is reduced and the phenomenon of cable line failures often occurs, especially when the cable insulation is damp, it is easy to cause the insulation resistance of directly buried power cables to decrease and substandard

1. The cable raw materials are damp

The raw materials used for cable insulation and sheathing are mainly plastic and rubber materials, and many kinds of materials with special functions are derived from this modification. When manufacturing materials, material manufacturers go through the processes of compounding, mixing, granulation, cooling and drying, as well as during the transportation and storage of the materials, dampness of varying degrees often occurs, resulting in varying degrees of material content. Moisture. Therefore, the cable manufacturer must dry the material before squeezing the material on the cable conductor. The extrusion unit is equipped with a material drying device to prevent the extruded insulation layer and sheath from happening. No defects such as bubbles and blisters, and no bubbles on the surface. This is the rigid process regulation of the cable manufacturer, otherwise the finished cable will not pass the factory withstand voltage test.

2. the cable manufacturing process is damp

In the insulation extrusion process, the insulation layer is scratched, causing holes or degumming of the insulation layer, and the insulation core enters the cooling water tank, which causes the insulation resistance to drop. Or when the protective layer is squeezed, the protective layer is damaged and water enters, so that the insulating layer is damp and the insulation resistance decreases. When manufacturing a multi-core cable, even if the insulation layer is extruded intact, when the insulated core is twisted into a cable, and when the sheath is extruded, damage may occur and the water may enter and become damp, so the finished cable cannot pass the factory withstand voltage test .

3. Moisture during cable construction

In the process of direct buried cable construction, if the cable trench is excavated, the cable burying operation, the cable intermediate joint and the terminal joint are not made standardly, it is very likely to damage the cable sheath and insulation layer. If the soil is wet or the cable trench is filled with water, water in the cable will definitely occur. After the insulation is damp, the surface resistance of the cable insulation will decrease and the surface leakage current will increase, and the insulation resistance will decrease, which will also cause the electric field distortion between the conductor and the insulation layer. The uneven electric field distribution in the insulation will cause free discharge inside the insulation and even lead to cable breakdown. After-sales service practice has proved that more than 95% of the direct-buried cable insulation resistance drop accidents are caused by improper construction.


As we all know, water and damp in the process of cable manufacturing and laying operation are the main factors that endanger the electrical performance and service life of the cable. Both cable manufacturers and users attach great importance to this.
Practical experience has proved that the main reasons for the ingress of water and moisture in the cable are as follows.

1) Material purity
If the cable insulation material is mixed with impurities, especially metal impurities, even the pigments of different colors used by China Wire and Cable manufacturer  will directly affect the electrical performance of the insulation and reduce the insulation resistance. The reason is that the non-metallic impurities in the insulating layer will absorb moisture when the cable is damp, and form numerous conductive points; second, the m

etallic impurities in the insulating layer are directly conductive points. Under the combined action of the conductor operating temperature and the external ambient temperature, these conductive points form conductive channels in the insulating layer, resulting in a decrease in insulation resistance and an increase in leakage current, which in turn leads to insulation breakdown.

2) The material is damp
If the cable insulation material has been damp and is not dried before being squeezed on the conductor, there will be quality defects such as a large number of pores in the insulation layer, an unsmooth extrusion surface, reduced mechanical strength, and even cracking. Therefore, when extruding the cable insulation layer, the cable manufacturer must dry the material. When extruding low-smoke and halogen-free materials, more attention should be paid to drying. These are the basic technical knowledge of cable manufacturers.