Identification, Use and Maintenance of Cables and Wires

Ⅰ. How to identify the quality of the wire?

1. Characteristics of fake and inferior wires

(1) The resistance value doesn’t meet the national standard

Under the same current, the temperature of the wire with the resistance value exceeding the standard may be too high, which will accelerate the aging of the outer insulation layer and cause a short circuit to cause a fire. And because the resistance exceeds the standard, the wire will greatly increase the energy consumption of the power transmission process and increase the cost of electricity.

(2) The material of the raw material is poor

The first trick is to hit the idea of a copper conductor. Copper conductors account for 80% of the cost of wires, and bad manufacturers first reduce the quality of copper materials. They use much lower-priced impurity copper, recycled copper, and even local copper smelting. The copper content is extremely unstable, resulting in a large increase in the resistivity of the copper conductor. The second is to reduce the amount of copper used. Reduce the actual cross-sectional area of the copper core. The direct consequence of these two methods is that the wire will overheat during use, damage the insulating plastic, cause a short circuit and cause a fire.

The second trick is to play tricks on the plastic used for the insulation. The same degrades as copper, and the price difference between recycled insulating material and qualified PVC plastic powder is nearly 2,000 yuan per ton. However, the recycled insulating material has high impurity content, low mechanical strength, and decreased aging resistance and electrical insulation performance. This approach will directly lead to the leakage of electric wires easily.

(3) The cable length is not enough

The main method used by unscrupulous manufacturers is to manipulate the length of wires and cables. The actual length of the wire and cable is less than the nominal length, and the pricing and charging shall be carried out according to the nominal length. The most typical is the cloth wire packaged in rolls. According to the national standard, each roll is 100 meters, and the measurement error is not more than 0.5%, that is, the length of each roll is 100±0.5 meters. However, many manufacturers blatantly mark 100±20 meters on the qualified card, but the actual length is only about 80 meters. If its selling price is converted to per meter, it is not cheap. This kind of deception is easier to identify, one is to look at the qualified card; the other is to check the length of the sample. Because cloth wires are relatively soft, they are easy to measure.

2. Identification of fake and inferior wires

Whether the quality of the wire and cable is directly related to the safety of the circuit. Many fires and electric shock accidents are caused by poor quality wires and cables. So how do we distinguish the quality of wire and cable?

The "three-look and three-test" method can easily distinguish the quality of wires and cables. Below we will give you some suggestions on the purchase of wire and cable.

(1) Look at the appearance

For wires with qualified products and high quality, the following content shall be marked on the outer packaging: quality system certification, certificate of conformity, factory name, factory address, inspection stamp, production date, trademark, specifications, etc.

(2) Look at the position of the core

Look at the position of the core conductor, a good cable conductor must be located in the very center of the insulation layer. What is not centered is the eccentric phenomenon caused by the failure to meet the process requirements. It can meet performance requirements when using low-power electrical appliances. Once high power is used, it will cause the current-carrying temperature to rise and the thinner side is likely to be broken down by the current.

(3) Look at the conductor material

Check the conductor material of the wire. Generally, copper cores are used for wires and cables. However, the purity of copper is different, and its conductivity is also good or bad. It is best to be generally purple-red, with a little redness, less impurities, and soft texture. If the conductor is whitish and the color is lighter, it proves that there are more impurities in the copper. If the conductor is black and has a strong sense of hierarchy, this kind of copper is commonly known as "iron copper". It is very dangerous to use.

(4) Measurement of flame retardant

Take a lighter to ignite the wire, the qualified wire will show blue smoke, take the lighter away after the insulation layer is ignited, and it should extinguish itself within five seconds. Otherwise, it is a wire that is not up to the standard for fire protection. This kind of wire is easy to cause accidents such as leakage and fire.

(5) Measuring creases

We take a thread and bend it repeatedly by hand, and fold it in half several times. Those with soft hand feeling, good fatigue resistance, high elasticity of plastic or rubber hand feel and no cracks on the wire insulator are superior products. If the insulation layer is white, cracked or the conductor is broken, it proves that there is a problem with the quality of the wire.

(6) Measure the weight

Good quality wires are generally within the specified weight range. For example, the commonly used plastic insulated single-strand copper core wire with a cross-sectional area of 1.5 has a weight of 1.8 to 1.9 kg per 100 m. Poor quality wires are not enough in weight, or the length is not enough, or there are too many impurities in the copper core of the wire.

The above "three looks and three tests" can detect the quality of wire and cable products. When we buy wires and cables, we must look for national standard products and AXD channels, channel cables, sufficient cores, sufficient meters, and sufficient square meters, five-star quality, and national standard models.

Ⅱ. How to ensure the normal use of wires and cables?

Whether the cables can be used and run correctly is directly related to the safe operation of the entire power grid and the stability of the system. With the increasing number of cable lines and the growth of operating years, cable faults are also gradually increasing. The failures of the cable body, cable accessories, and cable accessory equipment during cable operation also increased significantly. Below we will introduce to you how to ensure the normal use of the cable.

1. Safety requirements

(1) The distance between the cable and the foundation of the building should be able to ensure that the cable is buried outside the scattered water of the building. When the cable is led into the building, it should be protected by wearing a pipe, and the protective pipe should also exceed the scattered water of the building.

(2) There should be a distance of 0.25~0.5m between the cables directly buried in the ground and the grounding of the general grounding device. The burial depth of the cables directly buried in the ground should generally not be less than 0.7m, and should be buried under the frozen soil layer.

(3) When the cables cross each other, the high-voltage cables should be under the low-voltage cables. If one of the cables is protected by a pipe or separated by a partition within 1m before and after the intersection point, the minimum allowable distance is 0.25m.

(4) When the cable is close to or crosses the thermal pipeline, if there are heat insulation measures, the minimum distances for parallelism and crossing are 0.5m and 0.25m respectively.

(5) When the cable crosses the railway or road, it should be protected by wearing a pipe, and the protective pipe should extend beyond the track or road surface by 2m.

2. Storage method

If the cables are to be stored for a long time, the following considerations should be made according to the location of the cables:

(1) External wall. Avoid direct sunlight on the wall and man-made damage.

(2) Lay directly in the underground cable trench. This environment is the one with the least amount of control. The installation of the cable trench should be regularly inspected for dryness or humidity.

(3) Underground pipelines. In order to facilitate future upgrades, cable replacement and isolation from surface pressure and the surrounding environment, auxiliary pipelines are installed to isolate them. Auxiliary pipeline is a better method. But we don't hope that the pipe will stay dry forever, which will affect the choice of cable type.

(4) Inside the pipe (plastic or metal). For example, in the pipeline, we must pay attention to the damage of the plastic pipeline and the heat conduction of the metal pipeline.

(5) Under the roof. Cables Standard LAN cables can be used as long as they are not exposed to direct sunlight or extreme heat. We recommend using pipes.

(6) Suspension applications/overhead cables. Which bundling method we use should not only consider the sag and pressure of the cable, but also whether the cable is exposed to direct sunlight.

Ⅲ. What are the tasks of daily cable maintenance?

1. Temperature monitoring

The operation of cable lines will be affected by environmental conditions and heat dissipation conditions, and there will be local temperature rises in the early stage of cable line faults. Therefore, it is necessary for us to monitor the temperature of the cable line.

We use various instruments to measure the temperature of the cable sheath, cable joints and other parts, in order to prevent the cable insulation from exceeding the allowable maximum temperature and shorten the cable life, and prevent cable accidents in advance.

Measuring the cable temperature should be carried out in summer or when the cable load is the largest. We should choose the most densely arranged cables or the worst heat dissipation conditions and line segments affected by external heat sources. When measuring the temperature of direct buried cables, we should measure the soil temperature. The distance between the installation point of the thermocouple thermometer for measuring soil temperature and the cable is not less than 3m.

The newly put into operation of the cable cold head should be tracked and inspected with an infrared temperature tester. During power outage maintenance, we arrange personnel to fasten the conductive connection part. Since the connection of the cable cold head depends on mechanical force such as double-headed screws to crimp the conductive contact surface, the working load current and fault short-circuit current all rely on the crimping surface for energy transfer, and thermal expansion and contraction and electromagnetic vibration may cause poor contact.

2. Online monitoring

With the wide application of XLPE power cables, the on-line monitoring technology of its operating conditions has also been developed, and has been widely used abroad (such as Japan). On-line monitoring of the operation status of XLPE power cables is mainly based on parameters such as voltage, current, partial discharge or operating temperature, water content, etc., to monitor the operation status of the main insulation and outer sheath of XLPE power cables.

The main methods currently used are:

(1) DC superposition method

(2) Low frequency superposition method

(3) Partial discharge on-line monitoring method

(4) DC component method

(5) Moisture online monitoring method

(6) Cable temperature online monitoring method

(7) Ground wire current method

3. Cable and trench fire protection

Whether the cable fire accident is caused by an external fire source or its own fault, it has the characteristics of fierce fire, fast spread, difficult rescue and serious loss. There are many reasons for cable fire, and it is difficult to fundamentally avoid it. Therefore, in order to avoid the serious loss of cable fire accidents, on the one hand, we must actively try to eliminate the hidden dangers of cable fire; on the other hand, we must attach great importance to the countermeasures to effectively prevent the cable from igniting and burning.

At present, the more common method of cable fire prevention is to use fireproof materials to prevent flame delay. Existing fireproof materials include fireproof coatings, fireproof plugs, and fillers.

(1) Fireproof coating

The main feature of the intumescent fire retardant coating is that the thinner covering layer has better fire prevention and flame retardant effects, and it hardly affects the current carrying capacity of the cable. Since the coating expands many times at high temperature than at normal temperature, it can give full play to its heat insulation effect and is more conducive to fire and flame retardancy, but it will not hinder the normal heat dissipation of the cable.

This paint has the advantage of being easy to apply by brush and spray, and can be applied even in narrow tunnels. However, for large cross-section cables, the thermal expansion and contraction coating film of the cable may not be able to adapt. Fireproof coatings are mostly used in medium and low voltage cables, but not suitable for large cross-section high-voltage cables.

The advantage of fireproof tape is that it can make up for the shortcomings of coatings, and is suitable for high-voltage cables with large cross-sections, and has the protective effect of strengthening mechanical strength. Its construction is simpler than that of paint, and the winding thickness can be accurately grasped, and the quality is easy to be guaranteed.

(2) Fireproof plugging and packing

Many cable fire accidents at home and abroad have shown the serious consequences when the holes where cables run through walls or floors are not blocked. Under the fiery spread of the cables, it affects the equipment in the control room or switch room, causing serious damage to the panels and cabinets. Oil plugging of cable penetrations has received widespread attention.

4. Load monitoring

The monitoring of the cable load can grasp the change of the load of the cable line, control the load of the cable line in principle, and analyze the operation status of the cable line. Since the overload is very harmful to the cable, we should always measure and monitor the load of the cable. The load of the cable line can be measured with a clamp ammeter to keep the cable line running under the specified allowable continuous current carrying capacity.

In order to prevent premature aging of the cable insulation, the line voltage must not be too high. The voltage should generally not exceed 15% of the rated voltage of the cable.

5. Corrosion monitoring

Cable corrosion generally refers to the corrosion of cable metal lead or aluminum sheath, which can be divided into chemical corrosion and electrolytic corrosion.

The cause of chemical corrosion is generally that the soil near the cable line contains acid and alkali solutions, chlorides, organic humus, and ironmaking furnace ash. The main source of electrolytic corrosion is caused by stray currents flowing into the earth from DC tram tracks or electric railways.

(1) Methods to prevent chemical corrosion:

① For cables laid outdoors overhead, paint them with anti-corrosion paint regularly.

② Collect soil data for chemical analysis to judge the erosion degree of soil and groundwater. We take steps to change paths, partially replace bad soil, or run cables in corrosion-resistant pipes, etc.

③ When the cable is found to be corroded, or when there is leakage of chemicals on the cable line, we dig out the soil to check the cable, and conduct a chemical analysis of the nearby soil to determine the degree of damage.

(2) Methods to prevent electrolytic corrosion:

① Install a shielding tube.

② Strengthen the insulation between the cable sheath and nearby huge metal objects.

③ Device drainage or forced drainage, polarity drainage equipment, set cathode station, etc.

6. Prevention of external damage

External damage accidents mainly occur in the cable line itself. After the cable is damaged by external force, due to the damage of the seal, it sometimes takes a certain period of time to run before the insulation resistance drops due to the tide and causes an operation failure. The existence of hidden dangers of external force constitutes a potential threat to the safe operation of cables, which is more harmful, unpredictable and sudden. It brings certain unfavorable factors to the operation of the cable.

Cause analysis of cable line external force failure:

(1) Internal reasons

People do not pay enough attention to cable operation management. The aftermath of many projects is not meticulous, the drawings and materials are seriously lacking, and there are many hidden dangers in the lines, which affect the safe operation of the cables. So this is a very important factor that causes external force accidents.

Ineffective operation and management have resulted in insufficient checks on operating personnel. There is no clear control and assessment measures, which makes the operation and management work seem more chaotic. The cable relocation at the construction site was not timely enough, and the coordination was not effective. Due to the lack of close cooperation between various departments, different work priorities, and poor coordination, many good opportunities for relocation and cable protection were missed.

(2) External reasons

The construction environment is more complex. Mechanized construction is becoming more common and poses a greater threat to power cables. Just cleaning up the site in the early stage will damage the cable and cause an external force accident. This is also an important reason for external force accidents on power cables.

(3) Other reasons

Another important reason that makes external damage difficult to control is the lack of strict and effective protection measures and management methods.

7. Precautions

To prevent external force damage to the cable, the following work should be done:

(1) Establish a system and strengthen publicity;

(2) Strengthen cable protection and construction supervision;

(3) Strengthen the inspection work of the line;

(4) Explore effective management methods for the operation of power cables.

Ⅳ. How do cables work?

Electrical cables work by providing a low resistance path for the current to flow through. Electrical cables consist of a core of metal wire offering good conductivity such as copper or aluminium, along with other material layers including insulation, tapes, screens, armouring for mechanical protection, and sheathing.

Ⅴ. How are wires and cables classified?

The power cable is divided into low voltage cable, medium voltage cable, high voltage cable and so on. According to the insulation condition, it is divided into plastic insulated cable, rubber insulated cable, mineral insulated cable and so on. The overhead cable is also very common.

Ⅵ. What type of wire is used in homes?

Often called “Romex” after one popular brand name, nonmetallic (NM) cable is a type of circuit wiring designed for interior use in dry locations. NM is the most common type of wiring in modern homes.

Ⅶ. What are the cable accessories?

Commonly used electrical accessories include cable terminal junction boxes, cable intermediate junction boxes, connecting pipes and terminals, steel plate junction slots, cable trays, etc.

Ⅷ. What is the importance of cables?

Electrical cables are used to connect two or more devices, enabling the transfer of electrical signals or power from one device to the other. Long-distance communication takes place over undersea communication cables.