Wire and cable common sense questions and answers

1. What are the requirements for cable piping?

Answer: (1) The distance from the top of the pipe to the ground is 0.2m in the workshop, 0.5m under the sidewalk, and 0.7m in general areas;

(2) Pipe pits should be installed at the change direction and branch, and pits should also be added when the length exceeds 30mm;

(3) The pit depth is not less than 0.8m, and the manhole diameter is not less than 0.7mm;

(4) The drain pipe should have a drainage slope of 0.5% to 1% inclined to the pit.

2. What are the requirements for the resistance of the cable conductor connection point?

Answer: The resistance of the connection point is required to be small and stable. The ratio of the resistance of the connection point to the conductor of the same length and the same cross-section should not be greater than 1 for the newly installed terminal head and intermediate head; This ratio should not be greater than 1.2.

3. What requirements should the design of cable joints and intermediate head meet?

Answer: The requirements that should be met are:

(1) High compressive strength and good conductor connection;

(2) High mechanical strength, low medium loss;

(3) Simple structure and strong sealing.

4. What is a cable fault? There are several common types?

Answer: Cable failure refers to the failure of the cable’s insulation breakdown during the preventive test or during the operation, which forces the aerial bundled cable 33-209 standard  to power out due to insulation breakdown, wire burnout, etc. Common faults include ground faults, short-circuit faults, disconnection faults, flashover faults and mixed faults.

5. How to deal with the single-phase ground fault of the cable line?

Answer: Generally speaking, the damage to the cable conductor is only partial. If it is a mechanical damage and the soil near the fault point is relatively dry, local repairs can generally be carried out and a fake connector is added, namely Without sawing the cable core, only the fault point is insulated and sealed.

6. What tests and inspections should be carried out on power cables before laying?

Answer: Before laying, check whether the type, specification and length of the cable meet the requirements and whether there is external force damage. Low-voltage cables use a 1000V megohmmeter to remotely measure the insulation resistance, and the resistance is generally not less than 10MΩ, and high-voltage cables are measured with a 2500V megger. The resistance is generally not less than 400MΩ.

7. What should be paid attention to when laying cables in the main workshop?

Answer: When laying cables in the main factory building, generally pay attention to:

(1) All control cables leading to the centralized control room should be laid overhead;

(2) 6KV cables should be laid in tunnels or pipes, and the high groundwater level can also be laid overhead or pipes;

(3) For 380V cables, tunnels, trenches or pipes should be used when the two ends of the cable are at zero meters. When one end of the equipment is on the top and the other is on the bottom, it can be partially overhead laid. When the local water level is high, it should be overhead.

8. Where are the inner and outer shielding layers of power cables? What material is used? What’s the effect?

Answer: In order to make the insulation layer and the cable conductor have better contact and eliminate the increase in the electric field strength of the conductor surface caused by the unevenness of the conductor surface, the conductor surface is generally covered with an inner shielding layer of metalized paper or semiconductor paper tape. In order to make the insulating layer and the metal sheath have better contact, generally the outer surface of the insulating layer is covered with an outer shielding layer. The material of the outer screen layer is the same as that of the inner screen layer, and sometimes copper tape or braided copper ribbon is tied outside.

9. Briefly describe the composition and performance of epoxy resin compound.

Answer: The epoxy resin compound is composed of epoxy resin added with hardener, filler, toughening agent and diluent. Has the following properties:

(1) Have sufficient mechanical strength;

(2) Excellent electrical performance;

(3) Stable electrical performance;

(4) It has sufficient adhesion to non-ferrous metals;

(5) Good corrosion resistance;

(6) When used outdoors, it is resistant to rain, light, and humidity.

Medium and low voltage cable accessories are currently used more products

The main types of products that are currently used for medium and low voltage cable accessories are heat shrinkable accessories, prefabricated accessories, and cold shrinkable accessories. They have the following characteristics:

1. Heat shrinkable accessories

The material used is generally a blend of polyethylene, ethylene-vinyl acetate (EVA), and ethylene-propylene rubber. This kind of product mainly uses stress tube to deal with the problem of electric stress concentration. That is, the parameter control method is used to relieve the electric field stress concentration. The main advantages are light weight, easy installation, good performance and low price.

The stress tube is a kind of heat-shrinkable tube with special electrical parameters with moderate volume resistivity (1010-1012Ωcm) and large dielectric constant (20-25). The electrical parameters are used to force the stress at the abc cable insulation and shielding fracture to evacuate. The stress tubes are more evenly distributed. This technology is generally used in cable accessories of 35kV and below. Because the stress tube will heat up when the voltage level is high and cannot work reliably.

The key technical issues in its use are:

To ensure that the electrical parameters of the stress tube must reach the values ​​specified in the above-mentioned standards, it can work reliably. In addition, attention should be paid to filling the air gap at the fracture of the cable insulation semi-conductive layer with silicone grease to eliminate gas and achieve the purpose of reducing partial discharge. Cross-linked cables will shrink greatly during operation due to poor internal stress handling, so when installing accessories, pay attention to the stress tube and the insulation shield to cover not less than 20mm to prevent the stress tube from separating from the insulation shield during shrinkage. Due to the small elasticity of heat-shrinkable accessories, air gaps may occur at the interface during thermal expansion and contraction during operation. Therefore, the sealing technology is very important to prevent moisture intrusion.

2. Prefabricated accessories

The material used is generally silicone rubber or ethylene propylene rubber. The geometric structure method is mainly used to deal with the stress concentration problem. Its main advantages are excellent material performance, easier and faster installation, installation without heating, good flexibility, and greatly improved interface performance. It is the main form adopted by low-voltage and high-voltage cables in recent years. The disadvantage lies in the high requirements for the outer diameter of the cable insulation layer. The usual interference is 2-5mm (that is, the outer diameter of the cable insulation is larger than the inner hole diameter of the cable accessory by 2-5mm). The interference is too small, and the cable The accessories will malfunction; the interference is too large, and the installation of the cable accessories is very difficult (high process requirements). Especially in the middle joint, the problem is prominent, the installation is not convenient, and it often becomes the point of failure. In addition, the price is more expensive.

The key technical issues in its use are:

The size of the accessory and the size of the cable to be installed must meet the specified requirements. In addition, it is necessary to use silicone grease to lubricate the interface to facilitate installation and fill the air gap of the interface. Prefabricated accessories generally rely on their own rubber elasticity to have a certain sealing effect, and sometimes sealants and elastic clamps can be used to enhance the sealing.

3. Cold shrinkable accessories

The material used is generally silicone rubber or ethylene propylene rubber. Cold-shrinkable accessories generally adopt geometric structure method and parameter control method to deal with the problem of electric stress concentration. The geometric structure method that uses the stress cone to relieve the concentrated electric field distribution is better than the product of the parameter control method.

Like prefabricated accessories, it has excellent material properties, can be installed without heating, and has good flexibility, which greatly improves the interface performance. Compared with prefabricated accessories, its advantage is that it is more convenient to install and only needs to be in the correct position. The installation can be completed by pulling out the inner lining core tube of the cable accessory. The material used is better than prefabricated accessories in terms of mechanical strength, and the requirements for the outer diameter of the cable insulation layer are not very high, as long as the inner diameter of the cable accessories is less than the outer diameter of the cable insulation 2mm (the data is like this, this The type attachment requires a deviation of 2~5mm (Editor) to fully meet the requirements. Therefore, the construction and installation of cold shrinkable accessories are more convenient.

Its biggest feature is that the installation process is more convenient and quicker. After the installation is in place, its working performance is the same as the prefabricated accessories.

The price is equivalent to that of prefabricated accessories, and slightly higher than heat shrinkable accessories. It is the most cost-effective product.