The cable sheath should not be too thin or too thick, how can it be qualified?

In the announcements of many State Grid notification suppliers, we often see such a notification from cable companies: the thickness of the insulation layer of the power cable produced is unqualified. How does the unqualified thickness of the specific insulating layer affect the cable? What kind of sheath is qualified? How do we make qualified cables in production?

The hazards of unqualified sheath thickness

1. Reduce the service life of wire and cable products

This is easy to understand. After long-term operation, especially if it is directly buried, immersed in water, exposed to the open air or in an environment prone to corrosion, due to long-term corrosion by external media, the insulation level and mechanical level of the thinnest point of the sheath will decrease. will all fall.

Coupled with routine sheath test detection or a ground fault in the line, the thinnest point may be broken down. In this way, the protective effect of the cable sheath will be lost. In addition, the internal consumption cannot be ignored, and the long-term power supply of wires and cables will generate a lot of heat.

A little common sense is added here: the allowable operating temperature of the conductor is 70°C, and the long-term use temperature of polyvinyl chloride should not exceed 65°C. Therefore, wire and cable are actually in a situation of "internal and external troubles".

2. Increased difficulty in the laying process

With the development of global industry, more and more environmental requirements require that high-voltage cable products must have a small outer diameter. During the laying process, it is necessary to consider leaving gaps so that the heat generated after the wires and cables are energized can be dissipated. The thickness of the sheath Excessive thickness will increase the difficulty of laying, so the thickness of the sheath must strictly meet the relevant standards, otherwise it will not be able to protect the wires and cables. Nor can we blindly pursue its thickness.

What kind of sheath is qualified?

Based on the above two analyses, it is not difficult to see that the first characteristic of product quality is reflected in the appearance quality of the product. No matter what kind of product or semi-finished product, appearance must be paid attention to in production. Quality, strict control and inspection.

The sheath is the appearance of the cable, and its appearance is required to be smooth and round, with uniform luster, no eccentricity (not exceeding the specified deviation), no mechanical damage, flattening, no visible sundries, bubbles, sand holes, obvious particles, bamboo Section shape, twist shape, etc. In addition to meeting the above quality requirements for the sheath, the thickness of the sheath also has a certain influence on the quality of the cable.

It is well known that the function of the cable sheath is to protect the insulated core of the cable from damage under the action of external force and play a protective role. If the thinnest point of the cable sheath does not meet the requirements, the cable sheath will be damaged before reaching the normal maximum external destructive force.

If the sheath thickness of the cable in production is lower than the standard requirement, it is unqualified, and the thickness exceeds the standard requirement. For example: the cable model is DLD-KYJV22 3*1.5mm2, and the measured average thickness of the sheath is 1.7mm. If this model refers to the requirements of the GB9330-88 standard, the thickness should be 1.2mm.

Several points that need to be paid attention to during sheath production

1. Calculate the thickness of the control sheath according to the standard Calculation formula: D (outer diameter before extrusion) × 0.035+1

2. Online measurement of sheath thickness

Sheath thickness = (perimeter after extrusion sheath - circumference before extrusion sheath)/2π or Thickness of the sheath = (circumference after squeezing the sheath - circumference before squeezing the sheath) × 0.1592

3. The thinnest point of single-core sheath: nominal value × 85%-0.1

4. The thinnest point of multi-core sheath: nominal value × 85%-0.1

5. Increase the temperature of the low-density polyethylene sheathed extruder to increase the stress-cracking resistance, because the extrusion temperature is too high, it is easy to cause the plastic to burn, or "slip" phenomenon; in addition, the shape stability of the extruded cladding is poor , the shrinkage rate increases, and even causes discoloration and bubbles in the extruded plastic layer.

6. Do a good job of extruding machine body and screw cooling system to eliminate frictional overheating, so as to maintain the heat balance in the extrusion process, stabilize the extrusion pressure, promote the uniform mixing of plastics, and improve the quality of plasticization.

To sum up, in the production process, we only need to carefully operate the equipment and strictly control the thickness of the sheath according to the standard requirements. This will not only save resources for the enterprise, reduce material consumption, increase profits, but also ensure the quality of the cable and create high-quality cables. Inexpensive product.