Q&A about Cable and Wire

1. What are the requirements for fixing AC single-core cables? why?
The clamp should have no iron to form a closed magnetic circuit. This is because when the cable core passes current, magnetic lines of force are generated around it. The magnetic line of force is proportional to the current through the core. If iron and other magnetic materials are used, Electromagnetic induction knows that eddy current will be generated in the iron to make the cable heat up and even burn the cable. Therefore, iron pieces cannot be used as fixing fixtures for single-core AC cables.

2. What are the biggest features and advantages of the following heat shrinkable cable head accessories?
The biggest feature of heat shrinkable accessories is the use of stress tubes instead of traditional stress cones. It not only simplifies the construction process, but also reduces the size of the joint terminal, which is convenient to install, save time and labor, has superior performance and saves metal. The heat-shrinkable cable accessory integrates the perfusion type and the dry pack type into one, which combines the advantages of these two accessories.

3. What inspection work should be carried out before cable laying?
(1) The bracket should be complete and paint complete.
(2) The cable type, voltage, and specifications conform to the design.
(3) The cable insulation is good. When there is doubt about the sealing of the oil-paper cable, the damp judgment should be made; the direct-buried underground cable and the small bottom cable should pass the DC withstand voltage test; the oil sample of the oil-filled cable should pass the test.
(4) The oil pressure of the oil-filled cable should not be lower than 1.47MPa.

4. what does AAAC stand for?
AAAC means Aluminum Alloy Conductor ,is a concentric-lay-stranded conductor consisting of aluminum-alloy wires available in both single layer and multi-layer constructions.
All Aluminum-Alloy Conductors (AAAC) can be used in Medium, High and Extra-High voltage transmission lines. AAAC offers better sag performance due to the high strength-to-weight ratio provided by the aluminum-alloy. In addition, AAAC provides a higher corrosion resistance than ACSR conductors.

5.What is Quad Moose conductor?
Moose is one specification of ACSR conductor ,under British Standard EN50182 (BS215), which structured by 54 strands aluminum wire and 7 strands steel wire, single diameter of each wire is about 3.53mm.

6. What are the advantages of cross-linked heat shrinkable cable accessories?
Answer: This is a new type of material. Compared with other types of accessories, it has the advantages of superior electrical performance, small size, small quality, easy installation, and matching materials. In addition, it has weather resistance, pollution resistance, flame retardant self-extinguishing, etc. ability.

7. When there are doubts about the sealing of oil-paper insulated power cables, how to check whether the cable insulation paper is damp in a simpler way?
Answer: Ignite the cable insulation paper or put it in the cable oil at about 150℃ to check, there is no “hissing” or white foam, indicating that it is not damp.

8. What should be indicated on the cable sign? What are the requirements for writing?
Answer: The cable circuit design number, cable model, specification and starting point should be indicated on the sign, and the cables used in parallel should have a sequence number. The handwriting is required to be clear and not easy to fall off.

9. Where should the azimuth signs of directly buried cables be set?
Answer: At both ends of the cable, the cable connector is at 50~100m of the straight section of the cable and the corner where the cable changes direction.

10. How to make the yellow wax ribbon, black glass lacquer tape, alkali-free glass ribbon, etc. used to make cable joints before construction?
(1) Constant temperature drying method: roll the insulating tape into a small roll with a diameter of 25~30mm, put it in a 110~120℃ constant temperature drying oven for 4~5h, cool and dry it out, and put it into a dry sealed cylinder.
(2) Oil immersion and moisture removal method: Put the small coil of insulation tape into the cable oil at a constant temperature of 120~130℃, keep a distance of 30mm from the bottom of the pot, after a certain period of time, the oil surface will no longer produce foam, then take it out and put it into storage In the barrel with cable oil, the oil level should exceed all the contents and seal it.

What’s The Difference Between ACSR and AAAC cable ?

Following are some Q&A people interested about wire and cables :

1. What types of commonly used wires and cables are classified by purpose?
Answer: According to the purpose, it can be divided into bare conductors , insulated wires, heat-resistant wires, shielded wires, power cables, control cables, communication cables, radio frequency cables, etc.

2.What is the difference between ACSR cable and AAAC cable ?
AAAC:This bare concentric-lay-stranded conductor, made from round aluminum alloy 6201 -T81 wires, is constructed with a central core surrounded by one or more layers of helically laid wires.
it provide better corrosion resistance than the one produced by the aluminum conductor steel reinforced(ACSR).
ACSR: This bare concentric-lay-stranded conductor is made from round aluminum 1350-H19 (extra hard) wires and round zinc-coated or aluminum-coated steel core wire(s) to be used as overhead electrical conductors.it provides better strength than AAAC cable.

3. What kinds of insulated wires are there?
Answer: There are the following types of insulated wires: PVC insulated wires, PVC insulated cords, nitrile and polyvinyl chloride compound insulated cords, rubber insulated wires, direct buried aluminum core plastic insulated wires for agricultural use, and rubber insulated cotton yarn Textile cords, PVC insulated nylon sheathed wires, PVC insulated cords for power and lighting, etc.

4. What kind of occasion is the cable tray suitable for?
Answer: The cable tray is suitable for the overhead laying of power cables and control cables indoors and outdoors in general industrial and mining enterprises, and can also be used for indoor and outdoor installations in telecommunications, radio and television departments.

5. What are the cable accessories?
Answer: Commonly used electrical accessories include cable terminal junction box, cable intermediate junction box, connecting pipe and terminal block, steel plate junction box, cable tray, etc.

6. What is the cable middle connector?
Answer: The device that connects the cable and the cable’s conductor, insulation shielding layer and protective layer to connect the cable line is called the cable intermediate joint.

7. What is electrical main wiring?
Answer: The electrical main wiring is the connection method of the main electrical equipment and the bus in power plants and substations, including the connection of the main bus and the auxiliary power system according to certain functional requirements.

8.When choosing the cross section of the power cable, which regulations should be followed?
Answer: The selection of power cables should follow the following principles:
(1) The rated voltage of the cable should be greater than or equal to the rated voltage of the power supply system at the installation point;
(2) The continuous allowable current of the cable should be equal to or greater than the maximum continuous current of the power supply load;
(3) The cross-section of the core must meet the requirements of stability of the power supply system when it is short-circuited;
(4) Check whether the voltage drop meets the requirements according to the cable length;
(5) The minimum short-circuit current at the end of the line should enable the protection device to operate reliably.

9. What are the advantages of XLPE cables and oil-paper cables?
Answer: (1) Easy to install, because it allows a small minimum bending radius and light weight;
(2) Not restricted by line drop;
(3) Good thermal performance, high allowable working temperature and large transmission capacity;
(4) The cable accessories are simple and all are dry structure;
(5) Simple operation and maintenance, no oil leakage problem;
(6) The price is lower;
(7) High reliability and low failure rate;
(8) There are few manufacturing procedures, simple process and remarkable economic benefits.


Structure Analysis of 10kv XLPE Power Cable

XLPE power cable has good electrical and corrosion resistance. It is easy to install and easy to operate and maintain. It is widely used in 10 kV rural distribution networks. However, once it fails, it is difficult to repair and find. Combining the actual operation and work, focus on the analysis of the principle and structure of the cross-linked cable.

1. Analysis of insulation principle of cross-linked polyethylene material (XLPE)
Polyethylene is used as the basic insulating material, and chemical and physical cross-linking methods are used to convert the high molecular compound polyethylene from a linear molecular structure to a three-dimensional network structure of cross-linked polyethylene. It completely maintains the high electrical and physical properties of polyethylene, such as: high breakdown strength, large insulation resistance, low dielectric loss tangent value, etc. At the same time, due to the cross-linking process, it has aging resistance, heat resistance, mechanical properties, The corrosion resistance has been greatly improved.

The XLPE cables used daily are mostly chemical cross-linking method-inert gas cross-linking. The polyethylene material with peroxide (commonly used dicumyl) crosslinking agent is used. After three-layer co-extrusion, it continuously and uniformly passes through a sealed crosslinking tube filled with high temperature and high pressure nitrogen. The peroxide is thermally decomposed to produce Free radicals, free radicals can combine with hydrogen atoms in polyethylene, and polyethylene molecules that have lost hydrogen atoms unite to form cross-linked polyethylene to complete the cross-linking process.

However, XLPE as a polymer also has its inherent defects. The macromolecular solid structure of the polymer makes it easy to accumulate “space charge” inside. Space charge is also called trap charge, that is, the part of the charge that stays in the medium after being trapped. , Can also refer to the polarization charge caused by uneven polarization. The formation of traps is due to pollution in the production process, introduction during mechanical processing or generated during application, but electrode injection is considered to be the main reason for the formation of space charges. The space charge is generally distributed in the medium impurities, physical defects and between polymer molecular chains. Factors such as residual charge in the production process, high electrode injection or polarization caused by impure materials will cause the accumulation of space charge, and these factors are inevitable in actual production. Space charge is very harmful to XLPE cable insulation.

In addition, due to the network molecular structure of XLPE, it has a greater water permeability problem. When the XLPE insulator invades a trace amount of moisture, it will cause the formation of water branches in the insulator, and at the same time cause a high electric field similar to the gas free process, causing insulation damage. The diameter of the water branches is generally only a few microns, and there are many microscopic small water drop gaps. composition. When voltage is applied to the cable, under the action of a strong electric field, water branches will evolve into electrical branches to induce insulation breakdown. It can be seen that the problem of water permeability is the flaw of the XLPE material, and XLPE is the main insulation, so it is necessary to strictly prevent the intrusion of the cable during installation and use.

2. Structural analysis of XLPE power cable

The cross-linked cable is mainly composed of a core conductor, an insulating layer and a protective layer. The core conductor is located in the center, and the insulation shielding layer of the “three-layer co-extrusion” process is on the periphery, which has excellent insulation shielding and heat resistance and heat dissipation performance. The outermost periphery is a protective layer, which is composed of an inner sheath, an armor, and an outer sheath to seal the conductor and the main insulation. This simple structure of cross-linked cable is based on high technology and craftsmanship. Each layer of the structure has special functions and requirements. If a certain layer of structure has a problem, the entire cable will be scrapped. In order to clarify The structural requirements of each layer of the cable, and the principle analysis of each structural part of the cross-linked power  cable are as follows.

(1).  Analysis of core conductor structure

The bare conductor of cable plays the role of transmitting electric energy. When the alternating current passes through the conductor, the skin effect makes the charge density near the surface of the conductor high. Since the electric field intensity on the surface of a conductor is directly proportional to its surface charge density and inversely proportional to the radius of curvature of the conductor surface, the radius of curvature at the edge or tip of the conductor is the smallest, the surface charge density is the largest, the space charge is most likely to accumulate, and the electric field strength is the highest. Local strong electric field discharge is most likely to occur, and this phenomenon is called “edge effect”.

If impurities invade the inner structure of the cable, the impurities will form tips and cause partial discharges, which will eventually lead to breakdown. This is another important reason why no impurities can penetrate into the cable structure.

In order to avoid the “edge effect” harm to the cable insulation, try to make the conductor electric field uniform and reduce the insulation requirements, we should make the conductor into a geometric shape with the largest radius of curvature-round, and make the conductor surface as smooth as possible to avoid The sharp electric field is strong. For this reason, the cross-linked cable core is a multi-core compact round stranded wire. It can be seen that during cable laying and installation, it is necessary to strictly avoid the behavior of damaging the flatness of the internal structure of the cable and destroying the uniform electric field.

(2). “Three-layer co-extrusion” analysis of the inner and outer semi-conductive layer and the main insulating layer

The semiconducting layer is a polymer material with a higher dielectric constant (it is a conductor under a high electric field), which makes up for the stranded cores that cannot be completely rounded, and the uneven electric field on the surface of the cores is uniform. At the same time, it can prevent manufacturing The local high electric field caused by the accidental stab of the core and the introduction of external impurities during the process. It contains substances that quickly capture moisture, which can effectively block moisture from intruding into the insulating layer from inside and outside, prevent moisture from spreading along the core, and prevent the generation of water branches. In addition, the inherent thermal resistance of the polymer can play a role in thermal shielding of separate temperature and increase the current carrying capacity of the cable.

The “three-layer co-extrusion” process is to tightly extrude the inner and outer semiconducting layers and the insulating layer, so that the three layers are tightly combined. This process avoids the local high electric field caused by the intrusion of external impurities (air, moisture, foreign particles, etc.), makes the electric field uniform and smooth, thereby increasing the initial free discharge voltage and greatly improving the insulation strength.

(3). Copper shield and armor

Between the outer semiconducting layer and the inner lining layer, two layers of annealed copper tape are spirally covered to form a cylindrical concentric conductor layer. This is the copper shielding layer, which has good contact with the shielded semiconducting layer, and For the equipotential. During installation, both ends of the copper shielding tape are grounded, so that the outer semi-conductive layer of the cable is always at zero potential, thereby ensuring that the electric field is evenly distributed in the longitudinal direction. When the cable fails, the protective device will act quickly through the ground current or short-circuit current on the copper shielding layer, thereby protecting the non-faulty part of the cable. Note that the length of the copper shielding layer not in contact with the outer semiconducting layer shall not exceed 2 cm after calculation.

The main function of armoring is to increase the longitudinal mechanical stress of the cable, reduce the influence of mechanical force on the cable, and at the same time, it also plays a role in uniform electric field and protection through fault current.

When the cable is subjected to insulation breakdown, lightning strike, operating overvoltage or large fault current flowing in the core, the induced voltage of the metal sheath may cause the inner sheath to break down and cause arcing, and the fault current at this time is not enough When the relay protection is activated, the breakdown phenomenon will gradually increase until the metal sheath is burnt into a hole, which further increases the fault. In order to eliminate this hazard, the metal sheath must be grounded at the terminal.

Main Components of Overhead Lines

Overhead lines mainly refer to overhead cable, erected on the ground, and are transmission lines that use insulators to fix transmission wires on poles that stand upright on the ground to transmit electrical energy. Overhead lines have low cost, strong mobility, and easy maintenance. However, overhead lines hinder traffic and construction and are easily polluted by debris in the air; moreover, overhead lines may collide with or get too close to trees and other tall facilities or objects, causing electric shocks, short circuits and other accidents.

The main components of overhead transmission lines are Low-voltage cables, insulators, cross arms, low-voltage poles, high-voltage suspension insulators, clamps, high-voltage cables, high-voltage poles,Lightning line.

1.Pole and Tower
The general term for supporting structures that suspend wires through insulators. Pole and tower are the general term for electric poles and iron towers.
The purpose of the tower is to support wires, lightning protection wires and other accessories. In order to keep a certain safe distance between the wires, the wires and the avoidance, the wires and the ground, and the crossing objects.

2. Overhead Wire and Cables
Classification: It can be divided into four types: single-stranded wire, multi-stranded wire, composite multi-stranded wire and insulated wire.
(1). Single-strand Cable: Due to the manufacturing process, when the cross-section increases, the mechanical strength of the single-strand wire decreases. Therefore, the cross-section of the single-strand wire is generally below 10 square millimeters, and currently it is widely used up to 6 square millimeters.
(2). The multi-stranded cable is twisted by multiple thin wires, and the twisting direction of the adjacent layers of the multi-layered wire is opposite to prevent twisting and twisting when paying off. Its advantages are high mechanical strength, flexibility, and flexibility ; And because the surface oxidation resistivity of the strands increases, the current flows along the strands, the skin effect is small, and the resistance is slightly reduced compared with the single-stranded wire of the same cross section.The common type have all aluminum cable (AAC) and all Aluminum Alloy Cable (AAAC ) etc.

(3). Composite multi-stranded wire refers to the multi-stranded wire of two materials. The common type are steel-cored aluminum stranded wire(ACSR cable) and ACAR Cable ,The core part is twisted by steel wire, and the external stranded aluminum wire is integrated. The mechanical properties of aluminum and the electrical properties of aluminum have become the most widely used overhead cable.
(4). Overhead power lines generally use multiple strands of bare conductors, but in recent years, 10kV overhead power distribution lines in urban areas have gradually switched to overhead insulated conductors. The operation proved that it has many advantages, the line failure is obviously reduced, the contradiction between the line and the tree is solved to a certain extent, the maintenance workload is reduced, and the safety and reliability of the line are obviously improved. 10kv mainly has copper core and aluminum core cross-linked polyethylene insulated wires, and low voltage mainly includes JV and JLV.

3. Lightning wire
The lightning protection wire is installed above the wire and is directly grounded for lightning protection to reduce the chance of lightning strikes to the wire, improve the lightning resistance level of the line, reduce the lightning trip rate, and ensure the safe power transmission of the line. Overhead lines have long distances and are exposed to the wilderness, and there are more chances of being struck by lightning. According to the voltage level of the network, the importance of the load, and the number of lightning days and investment conditions in the area, it can be along the whole line or only in the Install lightning protection wires at the entry and exit lines of the substation.

4. Insulator
Insulators, also called porcelain bottles, are used to fix wires and insulate wires and poles. Therefore, the insulator should have sufficient electrical insulation strength and mechanical strength. There are two types of line insulators: high voltage and low voltage. The insulator is a kind of electrical isolation component, its purpose is to insulate between the wire and the wire and between the wire and the ground, support, suspend the wire, and fix it on the cross arm of the tower. The insulator should have good electrical and mechanical properties. . It should also have strong resistance to rain, snow, fog, wind, ice, sudden changes in temperature and the erosion of harmful substances in the atmosphere.

5. Hardware
The role of the fittings: used for suspension, fixation, protection, connection on overhead lines, connecting overhead lines or insulators, and metal devices used to connect the cables on the cable structure of the cable tower.
Basic requirements Electric power fittings are generally made of cast steel and malleable cast iron. It is required that the circuit fittings should have sufficient mechanical strength, and the part of the fittings connecting the conductors should have good electrical properties.

Main Types of Overhead Cable & Wire

Overhead power lines mainly refer to overhead electrical wires, erected on the ground, and are transmission lines that use insulators to fix transmission wires on poles standing upright on the ground to transmit electrical energy.

What type of wire is used for overhead?
The wires used in low-voltage overhead lines are divided into bare wires and insulated wires. According to the structure of the conductor, it can be divided into single-strand conductor, multi-strand conductor and hollow conductor; its common types are AAC/AAAC/ACSR/ACAR.
The bare wire is the main body of the overhead line and is responsible for transmitting electrical energy. Since the wires are erected on the poles, they must often bear the effects of self-weight, wind, rain, ice, snow, harmful gas erosion, and air temperature changes. Therefore, the wire is required not only to have good electrical conductivity, but also to have sufficient mechanical strength and good corrosion resistance.

(1). All Aluminum Conductor (AAC): This bare concentric-lay stranded conductor is constructed with a straight round central wire surrounded by one or more layers of helically layed wires. These wires are of aluminum 1350 and can be provided in different classes of stranding and tempers.

(2). All Aluminum Alloy Conductor (AAAC): This bare concentric-lay-stranded conductor, made from round aluminum alloy 6201 -T81 wires, is constructed with a central core surrounded by one or more layers of helically laid wires.
It was designed to attend the needs of an economic conductor for the applications on aerial circuit that require a larger mechanical resistance than the one of an All Aluminum Conductor (AAC), and a better corrosion resistance than the one produced by the aluminum conductor steel reinforced(ACSR). The conductors of Aluminum Alloy 6201-T81 are harder and have a better resistance to the abrasion than the conductors of aluminum 1350.

(3).Aluminum Conductor Steel Reinforced Conductor (ACSR) : This bare concentric-lay-stranded conductor is made from round aluminum 1350-H19 (extra hard) wires and round zinc-coated or aluminum-coated steel core wire(s) to be used as overhead electrical conductors.Used as bare overhead transmission cable and as primary and secondary distribution cable. ACSR offers optimal strength for line design. Variable steel core stranding enables desired strengthto be achieved without sacrificing ampacity.

(4).Aluminum Conductor Alloy Reinforced Conductor (ACAR)This bare concentric-lay-stranded conductor is made from round aluminum 1350-H19 (extra hard) wires and round aluminum alloy 6201-T81 core wires for use as overhead electrical conductors.It presents a higher mechanical resistance.

Overhead insulated cable is an overhead wire equipped with an insulating layer and a protective sheath. It is a special cable manufactured by a production process similar to that of a cross-linked cable. It is a new transmission method between overhead wires and underground cables.

Aerial bundled cables (also aerial bundled conductors or simply ABC) are overhead power lines using several insulated phase conductors bundled tightly together, usually with a bare neutral conductor. The conductor can be all aluminum, aluminum alloy or aluminum with a steel core, used for overhead power distribution as an alternative to bare conductor.


ABC cable used for low voltage overhead line transfer, structured by stranded aluminum conductor or aluminum conductor with steel core , both single core and multi-cores ,insulated by UV resistant XLPE.

Overhead ABC Cable Advantage:
ABC cable provide better level of safety and reliability ,lower power losses, easier to install ,less maintenance and operative cost.

  1. High reliability of power supply
    The use of overhead cables can greatly reduce various short-circuit faults (especially the common flashover faults of overhead bare wires). Compared with overhead bare wires, the failure rate is 4-6 times lower.
  2. Good power supply safety
    The use of overhead cables greatly reduces personal injury and death accidents due to electric shock.
  3. Convenient installation and maintenance
    Overhead cables can be erected on any kind of poles and towers, or along walls. Under special circumstances, they can also run through the bushes and be directly fixed on tree poles with hardware. It can be erected on a single circuit or multiple circuits on the same pole without requiring a wide “electrical corridor”.
  4. Reasonable economy
    Although the use of overhead cables is more expensive than the use of overhead bare wires, it is cheaper than ordinary underground cables. Therefore, although the one-time investment is slightly higher for the use of overhead cables, the operating cost will be significantly lower than that of overhead bare conductors based on other factors.

Laying method of overhead ABC cable 
A single conventional laying method. This erection method is to use the current conventional cement poles, iron accessories and ceramic insulator accessories with bare conductors, and erect according to the bare conductor erection method, which is more suitable for the area where the old line is reconstructed and the corridor is sufficient.
A special insulating bracket is used to suspend the wires for single laying. This method can increase the number of circuits erected, save the line corridors, and reduce the cost of the line unit.

Five Main Types of Cable

1.What are the types of cable?
The wire products used to transmit electric (magnetic) energy, information and realize the conversion of electromagnetic energy are collectively referred to as wires and cables. According to different purposes, wires and cables can be divided into five categories:

(1). Bare conductors: products with only conductors without insulation, such as steel core aluminum stranded wire(ACSR cable), aluminum stranded wire, and copper stranded wire. Products are mainly used in suburbs, rural areas, user main lines, switch cabinets, etc.

(2). Winding wire (magnet wire): In the form of a winding, the magnetic line of force is cut in a magnetic field to induce current, or a wire that generates a magnetic field with current. It is mainly used in various motors, instruments and meters. Such as enameled wire.

(3). Power cables: Cable products used to transmit and distribute high-power electrical energy in the backbone of the power system, such as cross-linked polyethylene (xlpe) insulated power cables. The products are mainly used in the transmission of strong electric energy in power generation, distribution, transmission, transformation, and power supply lines, with large current (tens of amperes to several thousand amperes) and high voltage (220V to 500kV and above).

(4). Communication cables and optical cables: cables and optical cables that transmit telephone, telegraph, television, radio, fax, data and other telecommunication information.

(5). Wires and cables for electrical equipment: Wires and cables used to directly transmit electrical energy from the power distribution point of the power system to the power connection lines of various electrical equipment and appliances. Such as control cables, wiring wires, etc.

2. What are the application of Cable?
Power system: The wire and cable products used in the power system mainly include overhead bare wires, busbars (bus bars), power cables (plastic cables, oil-paper cables (basically replaced by plastic power cables), rubber-sheathed cables, overhead insulated cables), Branch cables, electromagnetic wires, electrical equipment wires and cables for power equipment, etc.
Information transmission system: The wires and cables used in the information transmission system mainly include local telephone cables, television cables, electronic cables, radio frequency cables, optical cables, data cables, electromagnetic wires, power communications or other composite cables.
Mechanical equipment, instrumentation system: Except for overhead bare wires, almost all other products have applications, but mainly power cables, electromagnetic wires, data cables, instrument cables, etc.

3. What is the basic structure of wire and cable?

Conductor: An object that conducts current. The specifications of wires and cables are expressed by the cross-section of the conductor.
The insulating material is wrapped on the outside of the conductor in different thicknesses according to the requirements of its withstand voltage.
Protective layer: the part that protects the cable.

Discussion about Low Voltage Concentric Power Cable

In the China wire and cable products, there is no very small structure with a concentric cable. Some developed countries abroad are committed to research in this area and put it into practice. In the British Standard, BS4553-92 copper core PE insulated cable for power supply, the same core layer can be used as The sexual conductor and the grounding conductor are made of unplated annealed copper wire in accordance with BS4109.If the number, diameter and resistivity of the copper wires in each specification cable are the same as the core layer, the right winding should be adopted.
So what does concentric neutral mean?
The Concentric Neutral is the current flowing from one end to the other, then into the ground and back into the wire at the starting position.

The wrapping pitch and the number of threads should make the minimum coverage rate reach 90%, and the wrapping tape that does not absorb moisture should be overlapped immediately outside the same core layer. The NYCY type in the German standard DIN57271-83 adopts the same core layer conductor and can be used as a neutral wire, a ground wire or a neutral wire, and it can be used as a shield wire at the same time. The number, diameter and resistivity of unplated annealed copper wires are also specified. Compared with the same type of cable, it has a compact structure and a round shape, which can save about 10% of the sheath cable material. Therefore, according to the above structure.
The characteristics, cross-sectional area requirements, equipment and production technology used to formulate the structural design principles of low power cable concentric conductors.

  1. In order to facilitate production and management, the wire diameter of the same core conductor should be unified as much as possible.
  2. According to the production equipment, the minimum wire diameter shall be specified under the condition that it is evenly distributed on the stranded insulating core around the cladding and the spacing is not greater than 4mm. The number of concentric conductors is as small as possible.
  3. In order to ensure the uniformity of the same-core conductors without relative sliding, and to ensure that the

The cable appearance is round. Using layered non-woven fabric or PVC wrapping method, that is, insulating core into cable wrapping tape + concentric conductor twisted spiral copper tape wrapping + non-woven fabric tape overlapping wrapping.

  1. There are three types of structures. The structure of three-phase four-wire cable is a structure, and the structure of three-phase five-wire cable is b and c.

 In the structure, the concentric conductor is wrapped outside the cable core. It can be N wire (neutral wire), PE wire (protective earth), or PEN (neutral wire and protective earth shared), and the material is copper Wire or copper tape, or copper wire plus copper tape. The carrying surface is the same or half of the electric phase core

  1. In the structure, the N wire is in the cable core, and the neutral wire is wrapped around the cable core as a PE wire. The neutral wire can be copper wire or copper wire plus copper tape. The cross section is half of the cable phase core
  2. In the structure, the N wire is wrapped outside the copper core, and the PE wire composed of copper wire is wrapped outside the N wire, and the copper wire is composed of copper tape to form a double-layer shielding structure. The cross-section is the same as the phase core of the cable.

Advantages of the same-core conductor and shielded neutral power cable

  1. When transmitting power, it will not cause interference to surrounding electrical equipment; it also eliminates the influence of external electromagnetic interference on electrical equipment connected to this cable
  2. It can avoid breakdown or damage caused by strong lightning electromagnetic effect voltage, and improve the lightning protection effect and reliability of high-rise buildings
  3. It can improve the sensitivity of the overcurrent protection device at the head end of the line, so that the cable and the electrical equipment connected to it can be effectively protected
  4. It has low and uniform forward (reverse) sequence and zero sequence impedance, which is conducive to improving the quality of copper electricity
  5. TN-C and TN-C-S system power supply suitable for coreless cables
  6. It can eliminate the ground fault caused by the insulation breakdown of one phase as soon as possible
  7. Convenient processing, high flexibility and easy construction
  8. Can reduce costs

Stranded and twisted prefabricated branch cable

With the acceptance of prefabricated branch cables by the majority of users, a variety of prefabricated branch cables have entered the market under the situation of rapid increase in usage. This situation has brought a certain degree of selection to building electrical designers. difficult. The choice of stranded type, also called twisted type, branch cable is introduced below.

1. Defects of twisted type
The so-called twisted type and twisted type are multiple single-core power cables twisted together. For the core wires of power cables, the purpose of twisting multiple copper cable or aluminum wires is to increase the cross-section of the wires. Because they are all in the same phase and the same electromagnetic angle, there will be no problems with the distribution and change of capacitance and inductance due to any difference in electromagnetic phase angle. However, the influence of heat dissipation and the decrease of current-carrying value have also appeared. Therefore, the larger the cross-section, the lower the unit current intercept value. And it is still on the special equipment, according to the specified pitch and twisting force, the whole strip is finished at one time. This kind of twisting is consistent in terms of torque, pitch and uniformity, and there will be no change in pitch or uneven elasticity. The following only takes the round conductive core as an example, and the stranding rules are as follows:
(1). The center is generally a single wire, and the second layer has six single wires. In the future, each layer will increase by six more than the original number of the inner layer. The single wire adopts the same wire diameter;
(2). The stranding direction of each layer of single wires should be opposite to the previous layer, and generally the outermost layer is twisted to the left.
Stranded and twisted prefabricated branch cables, that is, multiple single-core power cables are twisted and used as three-phase three-wire, three-phase four-wire or three-phase five-wire power supply circuits. First of all, let’s start with “stranding”. This kind of “stranding”, whether in the production plant of prefabricated branch cables or the completion of the “stranding” process at the construction and installation site, is a special “stranding” equipment that has neither specialized , There is no corresponding pitch, strength, tightness, and uniformity regulations or requirements to follow. What is even more difficult to deal with is the branch cables and branch connectors of various sizes and locations with no rules, which are enough to determine the “multi-core cable” after the “stranding”, whether it is in terms of torque, pitch, density, The tightness, uniformity, and external dimensions will all be unknown. Therefore, it is determined that this “stranded” “multi-core cable” has become the most basic parameters and indicators in terms of the distribution of inductance and capacitance, and the electromagnetic theory of electromagnetic vibration, attraction and repulsion. The chaotic “Brownian motion”.

In the actual operation after construction and installation, first, the distribution, pitch, and tightness of inductance  and capacitance are chaotic, which will inevitably lead to chaos in the entire power supply circuit and some local heat dissipation and temperature rise. Even for several power cables without branches and without branch connectors, the power cables that are twisted under the conditions of strict control of their twisting strength, tightness, and pitch, are affected by heat dissipation and temperature rise. The actual allowable current carrying capacity cannot be compared with single-core power cables laid horizontally or vertically in accordance with the specifications.

Secondly, under the operating state of balanced or unbalanced current, when the load current, temperature, environment change or influence, the resistance between each cable and each cable itself in terms of absorption, repulsion, extension, contraction, up and down, etc. Irregularities in force and movement. This phenomenon is extremely detrimental to the stability and reliability of the entire power supply loop, and it may even cause damage to the sheath and insulation at the location where the cable is installed and tightened, which may cause an accident.

Thirdly, this kind of “twisted” power cables in a chaotic manner is really unsightly when placed in cable wells or cable channels. At the same time, it will also bring a defect in the design and construction of electrical system drawings, electrical system construction and engineering quality that fail to comply with the “horizontal, horizontal and vertical” regulations.

2. Relevant regulations in the electrical design code for civil buildings

2.1 Regulations for parallel laying of cables
When cables with the same voltage in the “Code for Electrical Design of Civil Buildings” promulgated and implemented by the Ministry of Construction in 1993, the net distance of the cable should not be less than 35 mm and should not be less than the outer diameter of the cable. It is stipulated in the article that when cables are laid side by side, a certain distance should be kept between cables to ensure the safe operation of the cables and the needs of maintenance and overhaul, so as to avoid the burning of adjacent cables when the cables fail. Reduce current carrying capacity, affect maintenance and cause mechanical damage. In addition, the distribution and influence of inductance and capacitance are also important reasons for increasing operating temperature and decreasing current carrying capacity. Obviously, “twisted type” and “twisted type” cannot meet the requirements of this article.

2.2 Regulations for cable selection
The electrical design code of civil buildings stipulates: “The power cable used in the three-phase four-wire system should be a four-core cable.” In the article description, it is stipulated in the three-phase four-wire system that an additional three-core cable is used. Conductor or single-core cable is used as the neutral line. When the three-phase system is unbalanced, it is equivalent to the operating state of single-core cable, which is easy to cause power frequency interference. For bare armored cables, it will also accelerate the corrosion of the metal sheath and armor layer, so four-core cables are required. “This article explains that although the distribution and change of inductance and capacitance are not clearly raised, it also indirectly pointed out their influence. It is not possible for a three-core power cable to be used as a neutral line, let alone four or five single cables. “Twisted” and “twisted” of core power cables? Obviously, “twisted” and “twisted” types cannot meet the requirements of this article.

2.3 Regulations for laying of power cables
The laying of power cables, whether indoor or outdoor, whether it is in the “Code for Electrical Design of Buildings”, “Code for Construction of Electrical Construction”, the “Codes” for the design and construction of various power systems, and whether it is the “Codes” of China or IEC standards, NEC “regulations”, Japanese standards, and standards of other countries all have a clear stipulation: whether it is installed vertically or horizontally, it must meet the requirements of “horizontal, horizontal and vertical” in order to ensure the acceptance of the project quality. Obviously, “twisted type” and “twisted type” can not meet the requirements of this regulation.

Throughout the development history of power cables, from the advent of single-core power cables to the present, we have fully realized in theoretical research and use practice: “twisting” and “twisting” four or five single-core power cables together , As a three-phase four-wire, three-phase five-wire system power supply circuit inside the building facilities, it is not advisable, nor feasible. my country has always followed the practice and regulations of parallel laying, which is based on its theory and practice. If in order to reduce or compress its laying and use space, just “twisting” or “twisting” can meet all requirements from theory to practice, then there is no need for multi-core power cables to come out. Throughout the domestic and international “standards”, “standards”, “regulations” and examples of outstanding well-known buildings, there are no regulations or allow four or five single-core power cables to be “twisted” or “twisted” as Precedents for the use of three-phase three-wire, three-phase four-wire, and three-phase five-wire power supply circuits. In irregular, small and sporadic construction facilities, use “twisted” or “twisted” multiple single-core power cables as three-phase three-wire, three-phase four-wire, three-phase five-wire power supply circuits exception.

In foreign power supply circuits embedded in the soil, it is allowed to use three single-core power cables twisted and embedded. Its purpose is: the twisted cable has a certain degree of flexibility, and it can play a certain buffering role when the cable is damaged or impacted by the local changes and land subsidence. But it must be twisted in strict accordance with the specified pitch and strength inside the factory, and no branches are allowed.

Single-core Parallel Type and Multi-core Twisted Type Cable

The advent of power cables has greatly improved the safety of power generation, transmission, transformation, supply, distribution, and use of electricity. Single-core power cables appeared first. As there are more and more occasions for three-phase four-wire power supply, three-phase five-wire power supply and multi-loop power supply in actual use, the requirements for occupied space and laying occasions are also getting higher and higher. When multiple and multi-layer laying are required, and the space occupied and laying conditions are limited, single-core power cables cannot be used conveniently. Therefore, multi-core power cables have been developed and quickly entered the field of power applications, and are accepted and used by the majority of users.

With the rapid growth of power cable usage, even if single-core power cables are used in actual use, the joints and branches shall be stripped and insulated on site, and the branches or joints shall be crimped before using epoxy resin insulation The method of material encapsulation treatment still has disadvantages such as large site occupation, long construction time, high cost, multiple equipment, high technical requirements, and high difficulty, especially the joints or branches after the completion of the on-site construction, and their insulation Strength, reliability, and consistency are difficult to guarantee. Therefore, the busway was developed, and after the development was put into the market, it was quickly accepted and used by a large number of users.
With the increase in user usage, it is found that the bus duct also has some defects, such as too many parts connected by * screws, complicated installation and construction, and large maintenance and high maintenance costs. In the process of operation, it often encounters the influence of electromagnetic vibration, thermal expansion and contraction, expansion coefficient, external force and other factors, which will cause the loosening of the screw. If a screw is loose, there will be heat and high temperature at the fault point, which will affect the stability of the entire bus duct. In particular, the improper use of the five-wire bus duct will also cause the contact resistance of the PE wire to increase. It violates the basic requirements for the continuity of the PE wire that is clearly stipulated in the building electrical design code and construction code. However, bus ducts still have their own advantages in the case of large capacity. Because when the current reaches thousands of amperes, if a cable is used, even a single-core cable must be laid in multiple, otherwise the corresponding large current capacity will not be reached, and the busway will show its own advantages at this time.

2. Prefabricated branch cable
With the development of technology and the increasing market demand, prefabricated branch cables have been developed and developed from single-core prefabricated branch cables to multi-core prefabricated branch cables, which also include flame-retardant, fire-resistant, and armored prefabricated branch cables. That is, in the factory, in accordance with the cable specifications, models, cross-sections and specific locations of the branches specified in the architectural design drawings, special production equipment and molds are used on the professional production line, which can be completed in one time. Its advantage lies in the high insulation strength, the encapsulated injection-molded branch joint connector and the outer insulating sheath of the cable are tightly bonded together, and it has excellent water tightness and air tightness. Because it is made of factory-specific equipment and molds, It has excellent reliability and consistency, and it is extremely convenient for users to install and use. When installing vertically, only the main cable can be evenly fastened to the corresponding bracket. In horizontal, pre-buried, overhead, tunnel, airport runway, port Construction and installation under environmental conditions such as docks, mines, and modern industrial plants are simpler and more convenient. The requirements for site, equipment, technical level of construction personnel, and costs during the installation and construction process are much lower than those for handling cable branches or cables at the construction site. Busway: It saves a lot of maintenance costs during operation and reduces the power outage time. In some cases, it can achieve the effect of maintenance-free. In the case of small cable shafts and cable channels, it can show its unique advantages. Therefore, branch cables are ideal products to replace bus ducts in medium and small capacity power supply occasions.

3. Stranded and twisted prefabricated branch cables
With the acceptance of prefabricated branch cables by the majority of users, a variety of prefabricated branch cables have entered the market under the situation of rapid increase in usage. This situation has brought a certain degree of selection to building electrical designers. difficult. The choice of stranded type, also called twisted type, branch cable is introduced below.

4. Multi-core prefabricated branch cable
The multi-core power cable is the core wire conductors are individually insulated, and are collectively and parallelly enclosed in the same outer sheath. Inside the outer sheath of the entire power cable, whether it is armored or unarmored, all power cable core wires that have been independently insulated and encapsulated are parallel and tightly encapsulated in the outer insulation. Set of interior. During the entire encapsulation process of the outer insulating sheath, no part or any power cable is allowed to “cross”, “displace”, or “twist” inside the sheath. Of course, “twisting” is not allowed. If any of the above phenomena occurs, the entire power cable will be judged as “unqualified” and not allowed to leave the factory. Therefore, the multi-core power cable itself is a qualified product that uses special production equipment in a professional production plant and strictly follows the relevant standards. Regardless of the distribution of inductance and capacitance and the basic requirement that the vector sum of any part is equal to zero, good consistency can be guaranteed. This cannot be achieved by any “stranded conductor type” or “twisted type”, and is a basic requirement that must be guaranteed on the power distribution circuit. Therefore, prefabricated multi-core branch cables made of qualified multi-core power cables can ensure that various technical parameters and basic requirements will not be destroyed, and can ensure the stability and reliability of operation. Of course, compared with single-core prefabricated branch cables, multi-core prefabricated branch cables are much more complicated and difficult regardless of the required technical level, the complexity of the manufacturing process, and the production equipment and investment.

In summary, the use of “twisted” or “twisted” prefabricated branch cables in the three-phase three-wire, three-phase four-wire, and three-phase five-wire power supply loops is used as the power supply loop to ensure the safety of its operation. And stability are extremely disadvantageous. The author also consulted related materials, regulations, specifications, standards such as IEC, NEC, etc., which clearly stipulated the distance between cables and cables when laying in parallel, and the correction coefficient of the current carrying capacity of cables at various distances. The author believes: This is also enough to explain the use of power cables. The series of processes from design to laying must fully consider the distribution of capacitance and inductance during operation, electromagnetic attraction, repulsion, vibration, etc.; the temperature rise during operation (capacitance And the distribution of inductance, electromagnetic attraction, repulsion, vibration, etc. are all part of the’temperature rise’) and heat dissipation environment and conditions. Therefore, only stipulating the different correction factors for the ampacity at different distances is sufficient to explain the nature and essence of the problem. What’s more, these are facts proved by theory and practice

Cable Broken Core Detection Method

The broken core fault point has the following methods to detect:

1.1. Induction method
It can be used with induction pen and digital multimeter; suitable for cables without metal armor and steel tape shielding; attention should be paid to prevent electric shock, the test place and the terminal connection when the equipment is rewinded.

specific methods:
1) Hang the conductor core of the cable in the air, and ensure that it will not cause road and electric shock accidents; at the same time, ensure that the cable is as far away as possible from the grounding body (such as the ground, equipment, etc.);
2) Select a good insulation core in the cable, connect the 220VAC phase wire (live wire), not the ground wire;
3) If you use an induction pen, touch your finger to the induction contact on the pen, and test whether the pen is normal outside the insulating layer of the charged body. If you use a digital multimeter, put the multimeter in the 20 or 200mV range, put a thinner plastic insulating sleeve on the red test lead, and hold the black test lead with your hand; test it outside the insulating layer of the clearly charged body, and take the reading; then remove it The charged body is far away, and read; compare the difference between the two readings, usually on the charged body, there should be a higher reading, such as 0.4mV, while the farther away from the charged body is lower, such as 0.15mV; remember this feature, you can Start testing.
4) Test close to the cable along the cable. When the indicator light of the induction pen is dimmed or the reading of the multimeter drops significantly, the change is the breakpoint.
5) The test is completed, pay attention to discharge!  


1.2. Capacitance method

When there is a metal armor layer such as copper or steel tape armored cable outside the cable, the induction method cannot be used for detection. At this time, the capacitance method is used; it is suitable for all cables; when using the capacitance method, first understand the principle of capacitance testing-when testing capacitance, The AC/pulse signal is used in the test loop, that is, measuring the AC partial voltage or charging and discharging the capacitor (two mutually insulated metal poles) to test the accumulated electricity on the capacitor and convert it to the reading of the capacitance.

Capacitance method may affect the accuracy due to the inductance formed by winding the cable into a circle, the insulating cores being wound together, the resistance of improper conductors (such as steel strips), and the stray capacitance between conductors; among them The inductance is very small and can be ignored; the resistance has little effect on the measured capacitance, but the difference in capacitance between the bare conductor and the steel strip and the unconnected capacitance is not big, and it can be ignored; but the stray capacitance has a greater impact, so I did an experiment : The capacitance between the intact core and the steel strip is 117nF. Connect the other cores to the steel strip. The measured result is still 117nF, while there is 72nF between the two cores.

For the convenience of description, suppose the cable is a 2-core steel tape armored cable with 1 core with 1 break point; the specific methods are as follows:

1) Hang all insulated cable core conductors and armor layers at both ends of the cable;
2) Measure the capacitance value of the intact insulating core and the broken core insulating core to the steel strip (or the third intact insulating core) at both ends, and record the value; at this time, the corresponding intact insulating core is measured at both ends The capacitance value should be very close; the sum of the capacitance value at both ends of the same broken core should be slightly greater than the capacitance value of the intact insulated core at the same position, it means there is only one breakpoint, or multiple breakpoints but very close; if both ends of the same broken core The sum of the capacitance values ​​is less than the capacitance value of the intact insulating core at the same position, indicating that there are at least two breakpoints;
Note: Theoretically, if there is only one breakpoint or multiple breakpoints but very close, the sum of the capacitance values ​​at both ends should be greater than the capacitance value of the intact insulating core at the same position. The amount varies with different cables. See the following theoretical analysis.
3) Compare and calculate the capacitance value of the broken core insulation core and the intact insulation core, and obtain the lengths of the two ends respectively. At this time, the length may be different from the actual length. The next step is to re-calibrate; but the two-core unarmored cable cannot be Make corrections.
4) If the sum of the calculated length is greater than the actual length, the extra length will be negative, and if it is less than the actual length, it will be positive; then use the capacitance value of the broken core insulating core to allocate the difference and divide the long segment to correct the long segment , The short segment is corrected for the short segment (see the following theoretical analysis), and the actual position of the breakpoint is obtained.