What are the main causes of insulation breakdown in low-voltage cables?
The first is mechanical damage. Damage to the insulation can occur due to heavy objects hitting the cable, accidental damage caused by excavators, excessive bending during installation, severe compression during shipping, and damage to the insulation and protective layer. Directly buried cables can also experience excessive tension due to subsurface subsidence. These factors can all lead to insulation damage and even cable breakage. To prevent mechanical damage, use overhead cables. Cables laid along walls should be covered. Underground cables should be clearly marked, and soil removal near the cable lines should be prohibited.
The second is improper construction. Poor construction methods and low-quality materials can cause weak links in cable heads and intermediate areas to fail, leading to insulation breakdown. To prevent this, the installation quality of cable heads should be improved. During the manufacturing and installation process, the insulation bag should be tightly sealed, with no gaps. Before applying epoxy resin and quartz powder, the cable head should be thoroughly dried to prevent air bubbles and moisture from entering. Furthermore, the insulation around the lead sheath should be strengthened.
Overhead insulated cables
Third, insulation moisture. Poor cable head construction allows moisture to penetrate the cable, or damage to the cable's inner sheath allows moisture to enter. Lead-sheathed cables installed near earthquake sources can experience fatigue cracking due to prolonged vibration. Corrosion of the cable sheath can cause cavitation. Poor manufacturing quality can lead to small holes or cracks in the lead sheath. To address these issues, the cable's outer layer should be protected by regularly applying a layer of asphalt to the outer sheath.
Fourth, overvoltage. Insulation breakdown can occur due to atmospheric overvoltage or internal overvoltage. In particular, overvoltage within a system can cause multiple cables to breakdown simultaneously. Lightning arresters should be installed to improve the system's active protection technology.
Fifth, insulation aging. Long-term operation of cables can lead to poor heat dissipation or overload, causing the electrical and mechanical properties of the insulation material to deteriorate, resulting in brittle or cracked insulation. If this occurs, the incoming cables should undergo periodic preventive voltage withstand tests. If the insulation deteriorates and no longer meets safe operation requirements, the cables should be replaced.
In principle, the primary methods for controlling copper metal used in cables include physical isolation from moist air, cathodic protection (oxidation-reduction) to prevent oxidation, chemical protection by forming a passivation film on the surface of the copper conductor, or spraying a special liquid on the conductor surface for protection. Taking the production of general-purpose rubber-sheathed cables as an example, the primary methods for controlling oxidation in each process are as follows.
1. Transportation, inspection, and storage of copper rods before entry: Most cable companies in my country purchase copper from external suppliers. Therefore, high-quality copper rods should be selected and supplier transportation and delivery procedures and specifications should be standardized. Upon entry, copper rod inspection should be conducted in accordance with GB/T 3048.2 or the cable company's corporate standards. For storage, copper rods can be covered with plastic sheeting or film, a relatively simple physical isolation method to prevent contact with moist air. When receiving copper rods from the workshop, each coil must be visually inspected for any blackening, ensuring control at the source.
2. Control of the Copper Rod Wire Drawing Process: 0.4mm single wire is generally drawn using continuous annealing copper large and medium drawing machines, and requires the following processes: pay-off, drawing and annealing, cooling, drying, and take-up. First, an appropriate die should be selected; it must not be too small, otherwise it will force the copper lattice to change and aggravate the sharp rise in metal temperature. Second, the pH of the emulsion should be checked before starting the machine to ensure it is an alkaline solution. Antioxidants should also be added to the drawing oil to form a passivation film on the copper conductor surface to prevent oxidation. When paying off the wire, the pay-off tension should be maintained stable and uniform, and excessive vibration should be avoided. During the drawing process, the operator should maintain the correct water level to ensure uniform annealing and avoid under-annealing or over-annealing. When take-up, there should be no residual liquid on the copper conductor surface. A piece of dry felt (replaced frequently) can be placed in front of the take-up to ensure the dryness of the single wire. Finally, after drawing the wire, seal it with a transparent plastic film and store it in a dry environment until it is circulated. Otherwise, the conductor surface temperature will be high and it will be easily oxidized by humid air.
