Pithy formula and interpretation of common cable selection

In our daily sales, we may encounter customers asking about ACSR Cable selection in the process of maintenance and repair. As a cable salesman, choosing the right cable for customers is the top priority of this work.
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First, the actual power of the load should be considered, at the same time, the maximum current to be borne by the cable should be calculated, and then the appropriate cable should be selected. The following is the relevant contents sorted out by the editor in AAAC Cable treasure. I hope I can provide some help for my colleagues.

[doggerel]: five out of ten, two out of one hundred, fifty-three out of four, and 75% off for buried casing.
[explanation]: according to the total current required by the insulated conductor, when the total current is less than 10a, the cross-sectional area of the conductor per square millimeter can pass 5A current, if it is more than 100A, it can only pass 2A current per square millimeter, 4A current per square millimeter between 10 ~ 50A and 3a current per square millimeter between 50 ~ 100A. After this calculation, In case of buried or casing laying, the current value that can be passed shall be multiplied by 0.75.
The calculation method given above obtains the allowable current per square millimeter. The selection of the actual specific conductor section should be: first determine the current required by the electrical equipment (including rated current and appropriate margin), then compare the current range in the doggerel according to the required current value, and then the conductor section that should be selected can be obtained by “allowable current per square millimeter” belonging to the range at the required current value.


For example, the rated current of an electrical equipment is 20A. Considering a certain margin, the required current is determined to be 22a. Then, find out that the current range of 22a current in the above doggerel belongs to 10 ~ 50a, and the available current per square millimeter is 4a. Finally, divide 22a by 4a to obtain the conductor section s that should be selected as:
S = 22 / 4 = 5.5 (mm2). According to this value, the conductor with corresponding section can be selected. If the calculated sectional area is not in the conductor section specification series, a slightly larger conductor section can be selected.

Shielded vs unshielded cable, how to choose the answer here!

Shielded cable vs unshielded cable

Shielded cable? Or unshielded cable—— This is a problem. The choice of shielded or unshielded cables mainly depends on the specific application requirements. Today, Xiaobian sorted out the factors that should be considered when using shielded cables on AAC Cable treasure.
01
PART
Shielded cable
Shielded cable is a transmission line that uses metal mesh braid to wrap the signal line. The braid is generally red copper or tinned copper; Shielded cables are mainly used to prevent interference signals from entering the inner conductor and are suitable for transformers and similar equipment.
The shielded cable complies with the national standard: gb12972.6-91. The long-term allowable working temperature of the conductor is 90 ℃. It is suitable for the connecting wires of control and monitoring circuits and protection lines with rated voltage of 450 / 750V and below. It is mainly used in places where electromagnetic wave interference is prevented and shielding is required.
In order to minimize the impact of electromagnetic and radio frequency interference, each component in the shielding system must be seamless and correctly installed and maintained. In addition, shielded cables and systems also need good grounding. Incorrect grounding can cause radiation and interference problems in the system.

Single layer shielding

double-layer screen
Shielded stray wire
Single layer shielding
double-layer screen
Shielded cable assembly
02
PART
Unshielded cable
When wiring near EMI / RFI interference sources is not involved, unshielded ACSR Cables shall be selected in most cases. Unshielded cable has the advantages of light weight, high flexibility, wide use, reliability and low price. It is widely used in IT applications and office networks.

Unshielded dispersion
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Unshielded cable assembly
However, it should be noted that when enterprise networks or SMB networks want to achieve high data rates such as 10Gbps or 40Gbps on copper wires, the use of shielded cables can significantly reduce or even eliminate external crosstalk (AXT) harmful to network performance. This problem is mainly aimed at high-speed networks using category 6 cables, and does not involve low-speed 10 / 100 / 1000 networks using category 5 and category 6 cables.
It can be seen that the type of cable selected depends on the physical location of the network and the technology adopted (such as 10Base-T). The best practice is to thoroughly evaluate the installation location and network technical requirements, and then select the appropriate cable according to the specific specification requirements.

How to skillfully use aluminum alloy cable in photovoltaic system?

The cost of control system is very important in the era of photovoltaic affordable Internet access. Over the past decade, the price of modules and inverters has decreased by 90%, which has made a great contribution to the overall cost reduction of photovoltaic system, but the cost of cables has not decreased. In large projects, the proportion of cables in the system reaches 10%, which is higher than that of inverters. In fact, as long as the design and installation are proper, on the premise of ensuring the normal operation and safety of the system, some AC cables adopt aluminum alloy cables, which can reduce some costs.
The cables of photovoltaic power station are divided into DC cables and AC cables. The DC cables between components and between components and inverter are generally required to use photovoltaic special DC cables, while the cables from inverter to AC distribution cabinet and from distribution cabinet to transformer are not required to use any ACSR Cables.


What is aluminum alloy cable: aluminum alloy cable is a new material power cable with aa8030 series aluminum alloy material as conductor. The resistivity of aluminum alloy is between aluminum and copper, higher than copper and slightly lower than aluminum. Under the premise of the same current carrying capacity, the weight of aluminum alloy conductor with the same length is only half of that of copper.
When the cross-sectional area of aluminum alloy conductor is 1.5 times that of copper, aluminum alloy ACSR Cable and copper cable achieve the same current carrying capacity, resistance and line loss.
Service life: different metal oxides are formed after the metal surface reacts with oxygen. Aluminum oxide can form a dense surface protective film with a certain hardness. Copper oxide, commonly known as copper green, is a toxic substance between the above two.
Economically, this is the most important aspect. The cost of aluminum alloy cable is about 25% ~ 50% of that of copper core cable. The cost advantage of aluminum alloy cable is relatively large. We can make a comparison. In a 400KW village level poverty alleviation power station, the distance from the AC distribution cabinet to the step-up transformer is 1500 meters, and the maximum output current of the 400KW power station is 580A. If copper cables are used, two 150 square meters are required for each phase and six for three phases, with a total length of 9000 meters and a price of 864000; If you use two 240 square meters of aluminum alloy, the total length is 9000 meters, and the price is 450000 yuan, 414000 yuan can be saved, and the loss of 240 square meters of aluminum wire is lower than that of 150 square meters of copper wire.


There is also a rare earth aluminum alloy cable, which is made of high elongation aluminum alloy material. By adding rare earth trace element materials such as boron to pure aluminum and treated by rolling technology and special annealing process, the cable has good flexibility. When its surface is in contact with air, it can form a thin and firm oxide layer, which can withstand all kinds of corrosion. Even when overloaded or overheated for a long time, it can ensure the stability of the connection, and the cost is about 10% higher than that of ordinary aluminum alloy cable.
Application scope of aluminum cable: aluminum cable can be considered for long-distance overhead line, underground cable trench with large space and places with reliable fixed bridge.
Precautions for use of aluminum cable:
The melting point of copper is 1080 ℃, while that of aluminum and aluminum alloy is 660 ℃, so copper conductor is a better choice for fire-resistant cable. At present, some aluminum alloy AAC Cable manufacturers claim that they can produce fire-resistant aluminum alloy cables and have passed the relevant national standard tests, but there is no difference between aluminum alloy cables and aluminum cables. If they are in the fire center, that is, when the temperature is higher than the melting point of aluminum alloy and aluminum cables, no matter what heat insulation measures are taken, the cables will melt in a very short time and lose their conductive function, Therefore, aluminum alloy should not be used as fire-resistant cable conductor, nor in densely populated urban distribution network, buildings, factories and mines.


Compared with aluminum core cable, the copper core is flexible, the allowable bending radius is small, and it is not easy to break after repeated bending. Therefore, aluminum cable should not be used when there are many bends, many pipes or complex lines.
As the terminals of electrical switchgear are made of copper, copper and aluminum are directly connected, and the chemical reaction of primary battery will occur after power on: aluminum with higher activity will accelerate oxidation, resulting in higher resistance and lower current carrying capacity at the joint. Therefore, some measures need to be taken when copper and aluminum are connected, such as using copper aluminum transition terminal or copper aluminum transition terminal block, Eliminate electrochemical reaction.

What are the advantages of overhead cables over underground cables?

(1) The power transmission is not easily affected by the surrounding environment and has high reliability.
(2) The insulation distance between wires is small, the land occupation is small, and there is no interference with radio waves.
(3) When laying underground, it does not occupy the ground and space, which is not only safe and reliable, but also not easy to expose the target.
Therefore, in densely populated urban areas, large factories, power plants, traffic congestion areas and power grid intersection areas require small floor area, safety and reliability, reduce the impact of power grid on transportation and urban construction, and generally use AAC Cable power supply; In severely polluted areas, cables are often used to improve the reliability of power transmission; For the river crossing and river crossing lines with large span, it is not suitable to erect overhead lines, or cables are also used to avoid the interference of overhead lines to ship navigation or radio; Some national defense and military projects use cables to avoid exposing targets; Some also use cables due to the needs of architectural beauty.
Whenever encountering stormy weather, the high-voltage lines exposed in the air will be affected, resulting in power failure, and the power grid staff will nonstop start to repair and maintain the “injured” power grid. Therefore, some people will ask: since it is easy to be damaged when erected in the air, why not “hide” them underground?

Because if all the high-voltage lines are buried underground, once there is a fault, the power grid staff should first apply for the formalities of digging the ground channel, then work overtime to dig the channel, check the fault section by section, and finally find the fault point before repair. The whole process is time-consuming and labor-consuming. So, is it more appropriate to lay the high-voltage line underground or in the air?

Cost

Underground cables: AAAC Cables need to be wrapped in expensive and thick insulating shells.
Overhead wire: high enough to expose the conductor.

technically

Underground cable: difficulty in manufacturing, construction and maintenance.
Overhead wire: it can quickly find and solve faults.

Safety

Underground cable: if electric leakage occurs, it will spread around and cause accidents. Therefore, the voltage level will be limited when laying.
Overhead wire: the exposed wire can be erected in the air according to a certain safe distance, because the air is a natural insulating medium. In this way, it can not only reduce the consumption in transportation, but also reduce the risk of electric shock.

To sum up, overhead high-voltage lines have obvious advantages! Therefore, the use of overhead lines in remote mountainous areas has the advantages of fast construction and low cost. However, there is no space for building overhead lines in many parts of the city, so we can only go underground, which saves space and gives consideration to the beauty of the city.

Causes of blackening of copper wire of heavy rubber sheathed flexible cable

2.1 catalytic aging of copper is an important reason for rubber hair adhesion
The experiment of the former Soviet Union Institute of AAAC Cable science proved that copper infiltrated into the insulating rubber from the contact with rubber during vulcanization, and the thickness of 1.0-2.0mm contained 0.009-0.0027% copper. As we all know, trace copper has a great damage to rubber, that is, heavy metal is the catalytic aging of rubber. During the process of insulation vulcanization, qiulanm precipitates some free sulfur to react with copper to form active copper containing groups: CH3 ■ ch2-ch-c-ch2- ■ ■ SS ■ ■ cucu, when aging, weak s-s-bond breaks, forming active copper containing base: cu-s-, which acts with rubber, and acts with oxygen, breaks down long bond molecules of rubber, making rubber soft and sticky, which is a combination of low molecular chain. The French Academy of rubber also pointed out that if there are harmful metals in rubber, such as copper, manganese and other heavy metal salts, the rubber viscosity will occur regardless of the type of promoter.

2.2 sulfur migration to the surface of insulating rubber and copper wire in rubber sheath cable
The possibility of sulfur diffusion in cable sheath rubber was confirmed by the use of radioisotope by former Soviet scientists. The diffusion coefficient of free sulfur is about 10-6cm2 / s at 130-150 ℃ in the vulcanized rubber based on natural rubber. In the continuous vulcanization factory, when vulcanizing sheath rubber, the temperature is between 185-200 ℃, and the diffusion coefficient is greater. Because of the diffusion of free sulfur in rubber sheath, the structure of the colum rubber is changed, and the polysulfide bond may be formed. These polysulfide compounds migrate through chemical decomposition and chemical combination, namely “chemical diffusion”. Due to the migration, not only the structure of the insulating rubber can be changed, the heat resistance of the rubber can be reduced, but the reaction between sulfur and copper surface will result in copper sulfide and cuprous sulfide, which leads to the blackening of copper wire. In turn, copper sulfide and cuprous sulfide accelerate the aging of rubber, and lead to the occurrence of adhesion.

3. Reasons for processing technology
3.1 reasons for processing rubber
In the insulation formula based on the combination of natural rubber and SBR, the plastic of rubber needs to be improved by plastic refining. In order to produce, some factories use internal mixer to improve plasticity by adding a small amount of chemical plasticizer, accelerator M. If the temperature of plastic refining and rubber filtration are not well controlled, high temperature above 140 ℃ appears. When the raw rubber is put on the opening mill, it passes through the drum slowly. Because the rubber is affected by hot oxygen and accelerator M, it will be found that the rubber surface seems to be coated with oil. In fact, rubber molecules are more serious in promoting the chain breaking under the promotion of chemical plasticizer, A relatively soft and sticky rubber with smaller molecular weight was produced.
Although the rubber was mixed with SBR and then mixed with insulating rubber, these small molecular weight natural rubber were evenly dispersed in the rubber. After the rubber was extruded on the copper wire for continuous vulcanization, there might be no problem at that time, but a hidden danger was buried for the rubber copper ABC Cables. That is, the local copper wire sticking phenomenon will appear in the first place for these small molecular weight natural rubber.

The process of adding vulcanizing agent and accelerator to insulating rubber is also very important. Some small factories add vulcanizing agent on the mixer, that is, pour the pot containing vulcanizate into the middle of the drum, with many in the middle, and less on both sides. When the vulcanizate into rubber, the number of turning triangle was less, which would make the vulcanizate distributed unevenly in the rubber. In this way, copper wire blackening is easy to appear in many places with vulcanizing agent when extrusion is continuous vulcanization. In the blackening place for a long time, the phenomenon of rubber adhesive copper wire will appear.
3.2 reasons for vulcanization of insulating rubber
In order to pursue the production, some enterprises only have 60 meters long continuous vulcanization tubes, 1.3mpa steam pressure, and the vulcanization speed is 120 m / min. thus, the residence time of insulating rubber in the pipe is only 30 seconds. Rubber itself is a bad conductor of heat. The surface temperature of the insulating core is more than 190 ℃, and when the temperature is transferred to the inner rubber contacting with copper wire, it is also absorbed by copper wire. When the copper wire is heated to close to the inner rubber temperature, the vulcanized rubber wire core has been discharged from the vulcanizing tube. So the temperature of the inner rubber is relatively low, about 170 ℃, and the vulcanization tube will be left only a few seconds. When it enters the cooling and winding, the insulation rubber will not be vulcanized enough.
In order to achieve sufficient vulcanization, the amount of promoter TMTD (as vulcanizing agent) is up to 3.4%. The excess of vulcanizing agent also releases more free sulfur during the curing process. Besides the crosslinking rubber molecules, there are also excess free sulfur. This is the reason why the copper wire surface is blackened.
In short, it is still difficult to solve the problem of copper wire blackening. Every process from copper wire to rubber should be taken seriously to achieve better results. The key to the problem is the choice of rubber species and the adoption of vulcanization system. The solution to this problem needs to go through the test of time.

What are the effects of temperature on cable selection?

Whether our colleagues in electrical design or factory electrical maintenance can not open the topic of cable selection, we often see that the AAC Cable selection size of the same equipment under the same power will be different in the work, some may be less than one or two levels, and some friends may be confused by which right? Why do you choose different? Today we will learn how to choose the cable and how to consider it from? What is the effect of temperature on the selection of cables?


1、 Cable selection
(1) Select cable according to temperature rise
We all know that when the conductor passes through the load current, the temperature of the conductor will rise, the conductor has insulation layer, and the conductor temperature shall not exceed the maximum temperature allowed for a long time by the conductor insulation, and the conductor will be damaged after exceeding.
(2) According to mechanical strength
When laying, the conductor has a minimum section allowed according to the laying mode and the distance between the support point and the distance.
(3) Select according to economic conditions
That is, choose the most economical way.
(4) Select by line voltage drop
We all know that when the current passes through the conductor, the voltage drop will be generated in the line due to the existence of line impedance. If the voltage drop is too large, the voltage at the end of the line, that is, the load end is too low, which will cause the equipment to fail to work or affect the equipment life. Therefore, it is required that the voltage drop cannot be too large, so the cable section should be selected reasonably.

(5) Cable selection shall meet the requirements of load protection
When the circuit is in fault, the protective device should act according to the specified time, whether it can operate reliably or not has a great relationship with the cable section. Therefore, we should consider the requirements of reliable action when choosing the ACSR Cable.
(6) Select cable according to the requirements of thermal stability
In case of failure of cable or electrical equipment, the cable shall meet the requirements of thermal stability, and the size of cable section shall be considered in order to meet this requirement. The maximum cross section is the final result by combining the above conditions.

What are the maintenance items of bridge cable and special cable bridge?

Project Name: design and processing of high voltage ACSR Cable bridge
Innovative ideas: there are three 10kv High-voltage incoming lines for 112201 fully mechanized coal mining face, which are laid from 11 panel substation to equipment train of working face. The high-voltage cable enters the rubber conveying chute section and is suspended along the coal wall side. From the coal wall side to the cable storage vehicle, it needs to cross the belt.

Therefore, the design and processing of the bridge will cross the high-voltage cable belt into the cable storage vehicle.

Design principle: I-steel is used to process the bridge, the lower part is fixed with the equipment train pole, the high voltage cable is laid from the I-steel groove, and the protective skin is padded at the lower part of the cable.
Expected effect: after the processing, installation and use of the cable bridge, the three high-voltage cables inside the belt are smoothly led across the belt to the high-voltage cable car, and the cable bridge is integrated with the equipment train, each time the equipment is pulled and moved, there is no need to re hang the high-voltage cable, saving working hours.
(1) Check whether the cables on both sides of the bridge are under large tension.

(2) check whether there are cracks, oil leakage and corrosion on the cables on both sides of the bridge.
(3) whether the AAAC Cable, protective tube and groove are damaged by collision or external force.
(4) paint bracket and exposed protective pipe and groove.
(5) check the cable armour cover.

Introduction of conductor calculation formula of wire and cable

1、 Wire and AAC Cable material consumption
The conventional calculation method of copper weight without conversion: sectional area * 8.89 = kg / km
For example: 120 * 8.89 = 1066.8kg/km
1. Quantity of Conductor:
(Kg/Km)=d^2 * 0.7854 * G * N * K1 * K2 * C /
D = diameter of copper wire, g = specific gravity of copper wire, n = number of wires, K1 = twisting rate of copper wire, K2 = twisting rate of core wire, C = number of insulated core wires
2. Insulation amount:
(Kg/Km)=(D^2 – d^2)* 0.7854 * G * C * K2
D = outer diameter of insulation d = outer diameter of conductor g = specific gravity of insulation K2 = twisting rate of core wire C = number of insulation core wires
3. Dosage of external coating:
(Kg/Km)= ( D1^2 – D^2 ) * 0.7854 * G
D1 = finished outer diameter d = upper process outer diameter g = insulation specific gravity


4. Amount of wrapping tape:
(Kg/Km)= D^2 * 0.7854 * t * G * Z
D = outer diameter of upper process t = thickness of cladding g = specific gravity of cladding z = overlap ratio (1 / 4lap = 1.25)
5. Winding amount:
(Kg/Km)= d^2 * 0.7854 * G * N * Z
D = copper wire diameter n = number of wires g = specific gravity z = twist in rate
6. Weaving amount:
(Kg/Km)= d^2 * 0.7854 * T * N * G / cos θ
θ = Atan (2 * 3.1416 * (D + D * 2)) * mesh / 25.4/t
D = diameter of braided copper wire t = number of ingots n = number of bars per ingot g = specific gravity of copper
Specific gravity of material:
89; Cu -8; 50; Ag -10; Aluminum – 2.70; Zinc-7.05; 90; Ni -8; 30; tin-7; Steel -7.80; Lead-11.40; Aluminum foil mylar-1.80; 35; Myra -1.37
PVC-1.45; LDPE-0.92; HDPE-0.96; PEF (foaming) – 0.65; FRPE-1.7; Teflon(FEP)2.2; Nylon-0.97; PP-0.97; PU-1.21
55; cotton belt -0; PP rope -0.55; Cotton yarn-0.48
2、 Calculation formula of material outside conductor
1. Sheath thickness: outer diameter before extrusion × 035 + 1 (for power cables, the nominal thickness of sheath of single core ABC Cable shall not be less than 1.4mm, and that of multi-core cable shall not be less than 1.8mm)
2. On line measurement of sheath thickness: sheath thickness = (perimeter after sheath extrusion – Perimeter before sheath extrusion) / 2 π
Or sheath thickness = (perimeter after sheath extrusion – Perimeter before sheath extrusion) × zero point one five nine two


3. Thinnest point of insulation thickness: nominal value × 90%-0.1
4. Thinnest point of single core sheath: nominal value × 85%-0.1
5. Thinnest point of multi-core sheath: nominal value × 80%-0.2
6. Steel wire armor: number=
{π ×( Outer diameter of inner sheath + diameter of steel wire) ×λ)
Weight = π × Wire diameter? ×ρ× L × Number of roots ×λ
7. Weight of insulation and sheath = π ×( Pre extrusion outer diameter + thickness) × thickness × L ×ρ
8. Weight of steel strip = {π ×( Outer diameter before wrapping + 2 × Thickness – 1) × two × thickness ×ρ× L}/(1+K)
9. Weight of tape = {π} ×( Outer diameter + number of layers before wrapping × Thickness) × Number of layers × thickness ×ρ× L}/(1±K)
Where: K is the overlap rate or gap rate, if it is overlap, it is 1-k; In case of gap, it is 1 + K
ρ Is the specific gravity of the material; L is the length of cable; λ Stranding coefficient

Maintenance contents of cable channel

1. Replace the damaged well cover, cover plate and protection plate, and complete the missing well cover, cover plate and protection plate.
2. Maintain the working shaft stop.
3. Clean up the water and debris in the passage.
4 stairs (ladder) for tunnel maintenance personnel entering and leaving the shaft

5. Maintain the ventilation, lighting, drainage and low voltage power supply system in the tunnel.
6. Maintain fire prevention and isolation facilities and fire fighting facilities in cable trench and tunnel.
7 prune and cut branches and vines with insufficient safety distance around AAAC Cable terminal tower (pole) and T-joint platform.
8 repair the grounding down lead with loose connection, poor grounding, corrosion and other defects.
9 replace the missing, faded and damaged stakes, warning signs and signs, and timely correct the inclined stakes, warning signs and signs.
10. Carry out anti-corrosion treatment for corroded cable support, and replace or repair the missing, damaged and severely corroded support parts.
11. Bailey frame, I-beam and other facilities can be used to protect the cable trench in operation, and the suspension and support protection shall be well done. When the suspension protection is carried out, the cable trench body or row pipe shall be protected as a whole, and direct suspension of bare cables is prohibited.


12. If the cable passage in the green belt or sidewalk is changed to slow lane or fast lane, it should be relocated. Before the relocation, the relevant parties should be required to take reinforcement measures according to the load-bearing road standard to protect the working shaft, drainage pipe and cable trench.
10.2.13 when there are excavators, cranes and other large machinery passing through the non load bearing AAC Cable channel, the relevant parties shall be required to take protective measures such as setting steel plates above, and the protective measures shall prevent noise from disturbing the residents.
10.2.14 if the elevation of the working shaft or trench body is inconsistent with the surrounding due to the change of the environment of the cable passage, the elevation of the working shaft or trench body shall be adjusted by prefabricated shaft or cast-in-place method.

What are the cable maintenance items?

1. Manhole and drainage pipe
(1) water samples were taken for chemical analysis.
(2) remove the water in the well and remove the sludge.
(3) paint the iron parts such as cable bracket and hook.

(4) check the well cover and ventilation, and check the well body for settlement and cracks.
(5) dredge the spare pipe hole.

(6) check the condition of ACSR Cables and joints in the manhole to see if there is any oil leakage, whether the insulation gasket on the bracket is in good condition and whether the grounding is good.
(7) check the circuit name plate.
(8) check whether there is electric erosion, and measure the potential and current distribution of cables in the manhole.
2. Cable trench and tunnel
(1) check whether the door lock is normal and whether the access is smooth.
(2) check whether there is water seepage and ponding in the tunnel, remove the ponding and repair the leakage.

(3) check whether the ABC Cable bracket falls.
(4) check whether the insulation gasket between the metal sheath of the cable and connector and the bracket is intact, and whether there is any damage on the bracket; Whether the bracket has fallen off.
(5) check whether the fireproof tape, coating, blocking material and fireproof box are in good condition, whether the fireproof equipment and ventilation equipment are perfect and normal, and record the room temperature.

(6) check whether the grounding condition is good, and measure the grounding resistance if necessary.

(7) clean cable trench and tunnel.
(8) check the cable and cable joint for oil leakage.
(9) check cable tunnel lighting.