cable core

Matters needing attention during wire and cable stranding

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In order to improve the flexibility and integrity of wires and cables, two or more single wires are intertwined in the specified direction. This process is called stranding. The whole stranding process is divided into conductor stranding, cable forming, braiding, steel wire armor and winding. Any problem in any process will directly affect the cable quality.
What should we pay attention to in the process of stranding? Which links are prone to problems? Do you know all this? Next, let’s introduce the matters needing attention and easy problems in the middle process of wire and AAC Cable twisting.

18 questions and answers about cable stranding process
1. What are the quality requirements for stranded conductor cores?
Answer: the stranded wire core shall be tight, round, straight, without serious bending, strand skipping, scratch and flattening, the copper wire surface shall be free of oxidation, blackening, tin coating falling off or tin particles, uniform pitch, flat and firm joints, neat wire arrangement, AAAC Cable diameter shall meet the tolerance requirements, and length shall meet the specified requirements.
2. What preparations should be made before strand production?
Answer: (1) check whether all parts of the stranding machine are normal and whether there are obstacles in the operating parts（ 2) Check whether the electromechanical equipment, electrical switch and fence insurance are normal（ 3) Check whether the butt welding machine is normal（ 4) Check whether the tools and measuring tools are complete（ 5) Select and match the die according to the requirements of the dispatch list and process card, replace the pitch lap gear, replace the twist direction, measure the wire diameter, etc.
3. What are the main reasons for the scratch of stranded wire core and jumper?
Answer: (1) the main causes of abrasion are: the guide wheel is damaged or the conductor jumps out of the guide wheel, the threading pipe is damaged, the die is damaged or too small, and the conductor is crushed or abraded when the disc is rolling. Prevention method: replace the guide wheel to prevent the conductor from jumping out of the guide wheel, replace the threading pipe, select the die and replace the reel（ 2) The main reasons for jumper are: the die is too far from the distribution board, the setting out tension is inconsistent, the die is too large, etc. Prevention method: reasonably adjust the distance between the die and the branching board, adjust the base number of setting out tension, and reasonably select the die.
4. What is untwisting? Under what circumstances is the effect significant?
Answer: untwist stranding is to keep each reel equipped with a single line in a horizontal position through a special device (untwist mechanism), so as to ensure that each single line does not produce self twisting deformation in the stranding process. Only when the diameter of the single line is large, the pitch is large and the single line is hard, the untwisting will have a good effect.


5. Which two requirements must the structure of sector core meet?
Answer: (1) the single lines on the center warp of the central layer shall also be arranged on the center line of the sector. If this requirement cannot be guaranteed, when the core is bent, these single lines will be stretched or compressed, resulting in the uplift of the single line, resulting in the change of the shape of the sector core（ 2) Slip requirement: that is, the single line around the fan-shaped outer circumference shall be able to slide on the central layer. This is because the insulated fan-shaped line is not twisted back when it is twisted into the cable core, and each single line around the outer circumference tries to slide around the central plane. Therefore, if this requirement is not met, the single line will jump up and damage the shape of the fan-shaped line.
6. For stranded wires in concentric layers, the direction of adjacent layers is specified to be opposite. Why?
Answer: (1) the stranded wire is round. If the adjacent layers are twisted in the same direction, it is easy to embed the single wire of the outer layer into the inner layer, thus damaging the complete circular structure（ 2) When the stranded wire is under tension, the rotating torque generated by each layer is opposite, which can offset each other to prevent the loose strand caused by the single wire of each layer rotating in the same direction. At the same time, it can also avoid the looping phenomenon of the stranded ACSR Cables when it is not tensioned（ 3) In steel cored aluminum strand, AC impedance can be reduced, corona phenomenon and power loss can be reduced.
7. What are the two methods of twisting insulated cores together? What kind of communication cable is currently used in domestic cities?
Answer: twisted pair and star twisted pair. At present, twisted pair communication cables are used in domestic cities.
8. What’s the reason why the inlet of the die hole of the die used in twisting the cable is in the shape of a horn?
Answer: the horn shape at the inlet of the die hole has two advantages: (1) to gradually compress the twisted cable core（ 2) It will not scratch the cable core.


9. What are the main differences between a winch and a cage winch?
Answer: the stranding part of the cage winch is connected with the paying off device, while the stranding part of the winch is not connected with the paying off device, but with the traction device and take-up device.
10. In terms of structure, what are the wrapping forms of the package for cabling?
A: there are three types: simple type, plane type, tangent or semi tangent type. The simple wrapping head is a wrapping in which the wrapping reel is at an angle with the group or the cable core axis. The flat winding head is the winding in which the tape reel is perpendicular to the wire group or cable mandrel, and the tangent (half tangent) winding head is the winding in which the tape reel is parallel to the wire group or cable mandrel.
11. The trial fraction shows the relationship between the package noise pitch H (mm) and the forward speed V (M / min) of the cable core and the number of revolutions n (R / min) of the package head rotating around the cable core.
Answer: H = V / N = 1000 (mm)
12. What is the difference between unit stranding and concentric stranding?
Answer: the stranding direction of each layer of unit stranding cable core is the same, and the multiple of stranding distance is large, which is about 1.5 times that of concentric stranding. The unit quantity in each layer of cable core does not follow the rule of N + 6. The reason is that it is necessary to completely fill the cable core to cause deformation, so that each unit no longer maintains the original circle. The unit quantity of each floor basically follows the rule of N + 4.
13. What is a cable? How many deformations are there when the insulated core is cabled?
Answer: the process of twisting multiple insulated cores into cables according to certain rules is called cable. When the insulated core is cabled, there are two kinds of deformation: one is the bending deformation caused by the core around the cylinder, and the other is the torsional deformation caused by the direction of the spiral wire.
14. What is reverse twist? Why do we use untwist to form cables with circular cores?
Answer: when stranding, the method of reverse twisting of the wire is called untwisting. For the circular core cable, the untwisted cable is mostly used to reduce the core deformation, avoid damaging the insulation due to deformation, and make the cable have good flexibility.
15. What are the main qualities and requirements of auxiliary materials in the cable forming process of power cables?
Answer: the auxiliary materials in the cable forming process of power cable include: filler rope liner, tie belt and shielding belt, which shall have heat resistance, non water absorption, consistent cable working conditions and no corrosive insulating materials. The tie belt for filling and lining shall have considerable mechanical properties and meet the process requirements.
16. General selection principle of cable forming die for sector core? What should we pay attention to in the specific operation process?
Answer: the first mold is 1.0 ~ 3.0mm larger than the outer diameter of the cable, and the second mold is 0 ~ 0.6mm smaller than the outer diameter of the cable. The third die is 0 ~ 0.2mm smaller than the outer diameter of the finished cable. Pay attention to the actual use of the die. The cable cannot swing in the die, the cable core does not feel loose when rotating by hand, and the heat generated by the friction between the die and the insulated wire core should not be hot. The surface of the pressing die of the insulated wire core shall be smooth without damage and other marks.
17. After the completion of a certain specification of control cable, it is found that the conductor is thin or even broken, and the main causes are analyzed?
A: the following reasons lead to fine drawing and fracture: (1) excessive setting out tension（ 2) Wire clamp at wire nozzle（ 3) The insulation of the wire core is wrapped, and the part is thick（ 4) The wire joint is not firm（ 5) The take-up force is too large.
18. Causes and preventive measures of fan-shaped insulated wire core turning over?
Answer: the causes of fan-shaped insulated wire core turning over are: (1) insufficient pre torsion angle（ 2) When the pay off reel is on, the core row turns over, and the core is thick（ 3) The wire core is twisted back during the split footwall, resulting in turning over. The prevention methods are: (1) adjust the distance between the pre angle spring die and the core guide wheel（ 2) Adjust the pre twist angle, arrange the line on the pay off reel once, turn the reel if necessary, and the line shall be neat and not loose.

Current situation of power cable and accessories

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Power cables and accessories are the key products used for transmission and distribution of electric energy in the main line and distribution network line of power system, mainly including AC power cables and accessories with rated voltage U0 / u 0.6/1kv ~ 290 / kV. Among them, the rated voltage of 1.8/3kv and below is low-voltage power cable, 3.6/6kv ~ 26 / 35kV is medium voltage power cable, 66kV ~ 110kV is high-voltage power cable, and 220kV and above is ultra-high voltage AAC Cable. In recent years, China has promoted the development and application of flexible DC cables, including DC power cables and accessories of various voltage levels within the rated voltage uo10kv ~ 525kv.


Power cables are basically distinguished according to the types of insulating materials. Extruded solid insulated power cables represented by XLPE insulation materials have been widely used. At the same time, there are a small number of oil immersed paper insulated (hereinafter referred to as “Mi cable”) power cables and oil filled (hereinafter referred to as “of cable”) insulated power cables“ During the 13th Five Year Plan period, superconducting power cables and Gil gas insulated lines have also been continuously developed and applied in engineering. PP (polypropylene) thermoplastic insulation material with environmental protection characteristics is known as a new type of ACAR  Cable, which is being developed and applied in engineering demonstration in China.
Power cables are divided into land cables and submarine cables according to use scenarios. In recent years, submarine cables have been rapidly developed and widely used, especially the development of offshore renewable energy (wind power generation), which promotes a large demand for submarine cables.
6kV and above power cables need to be equipped with cable accessories of corresponding voltage level. Cable accessories include terminals and intermediate joints, including wrapping, molding, cold shrinkage, heat shrinkage, prefabrication, combination and other forms.


Power cables, whether land cables or submarine cables, whether AC cables or DC cables, are characterized by a wide range of voltage levels, from 1kV to 500kV. At present, the voltage level tends to be higher; Wide specification range, from 1.5 mm ² To 3500 mm ²， At present, the conductor section tends to be larger. Power cable is not only widely used in distribution network, but also an important foundation and indispensable product of power transmission in backbone network (Note: concentrated in urban power supply system). With increasing transmission capacity, operation safety and reliability are becoming more and more important. People not only pay attention to the quality improvement and technical improvement of power cables and accessories, but also study the operation reliability of AAAC Cable system, including electrical factors, mechanical factors, temperature and environmental factors, as well as mutual integration and coordination; It also pays more attention to the economy and long-term operation of power cable system, including transmission capacity, loss, maintenance cost and rapid recovery ability in emergency. Therefore, the power cable system is facing the challenges of quality improvement, operation economy, reliability and durability, involving material innovation, structure innovation and manufacturing process innovation. Facing the new requirements of social progress, urban development and environmental coordination, the challenge of long-distance transmission underground cable with large capacity, low loss, convenient laying, operation and maintenance and intelligent control, as well as the development and application of marine resources and new energy, and the challenge of safe power transmission reliability in complex environments such as seabed.


In recent years, China’s cable industry has made great progress in the above fields, but there is also a lot of room for improvement, improvement and promotion. In the manufacturing field, the integration of fine management, informatization, equipment automation and digitization of power cable manufacturing to realize high-quality intelligent manufacturing and development is still on the road. In the application field, there is still a big gap between the construction of high-voltage, large capacity and long-distance underground and submarine transmission networks and the development and application of corresponding materials and high-end equipment and the technical level of advanced countries.

Is the zero line current greater than the live line?

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In low-voltage distribution system, the current of zero line is smaller than that of live line. This is because when the three-phase load is balanced, the three-phase current is completely offset, and the zero line current is zero; However, when the AAC Cables consumption of three-phase load is unbalanced, the three-phase current cannot be completely offset, then the unbalanced current will return to the transformer through the zero line.
In three-phase four wire distribution system, the current of zero line is generally smaller than that of live line, which is the consensus of all people in the industry. However, more and more facts make people subvert this concept.
case
One day in class, a student called to ask me a question.
Some time ago, he received the installation of wall advertising lines in a building. All the advertising light boxes around the building are illuminated with fluorescent lamps, nearly 1200.
The power supply of the advertising light box is led from the distribution box by four wires (three 25 square meters and one 16 square meters), and 1200 fluorescent lamps are evenly distributed in three-phase lines.
After connection, the power on test is normal and the acceptance is passed. But after a while, it was found that the zero line was burnt out. At that time, I thought the zero line was too small, and then I replaced it with a 25 square one. ACSR Cables was like this for some time. Before long, the zero line burned out again.
The customer is very dissatisfied. If this problem is not solved again, he may not get the balance payment.
I calmed his emotions while thinking. According to my many years of experience, this situation is most likely due to excessive zero line current due to harmonics.
I asked him to measure the current of each wire, and finally measured it with a clamp meter. He was stunned!
The three-phase live line current is almost 90a per phase, but the zero line current reaches an amazing 158a.
In fact, the phenomenon of excessive zero line current is becoming more and more common. Why is there still current on the zero line when the three-phase power load is balanced, and the current reaches more than 150% of the phase line current?
This is caused by harmonics.
harmonic
We all know that the alternating current we use now is sinusoidal alternating current with frequency of 50Hz.
The waveform of sinusoidal alternating current is as follows:

sinusoidal alternating current
When sinusoidal alternating current is mixed with higher frequency clutter, this is called harmonic.
The figure mixed with harmonics is as follows:


The picture is a sinusoidal alternating current waveform mixed with harmonics
In the above figure, red is our normal sinusoidal AC waveform, in which green and blue are harmonic waveform.
AC non sinusoidal signals can be decomposed into linear combinations of sinusoidal components of different frequencies. When the frequency of the sine wave component is the same as that of the original AC signal, it is called the fundamental wave. Harmonic, in a narrow sense, refers to the electric quantity whose frequency is an integral multiple of the fundamental wave contained in the current. Generally, it refers to the Fourier series decomposition of periodic non sinusoidal electric quantity, and the electric quantity generated by other currents greater than the fundamental wave frequency. Broadly speaking, since the effective component of AC power grid is a single frequency of power frequency, any component different from power frequency can be called harmonic. When the frequency of the sine wave component is a non integer multiple of the frequency of the original AC signal, it is called fractional harmonic, also known as fractional harmonic or interharmonic. For any compound periodic vibration function y (T), it is expressed by Fourier series decomposition: the first term is called mean or DC component, the second term is fundamental wave or basic vibration, the third term is called second harmonic, and so on, or the following harmonics are collectively referred to as higher harmonic.
Harmonic generation
When the current waveform of three-phase alternating current is sine wave, they differ by 120 ° and have the same amplitude. The result of vector superposition on the zero line is that the sum is zero.
However, if the current on the phase line is pulsed and the difference is 120 °. Then, the pulses superimposed on the middle line are staggered, so they cannot be offset, and the current of the zero line is superimposed.
Since most modern electrical equipment contain rectifier circuits, even if the three-phase load is balanced, there will be a large current on the zero line.
The harm of excessive zero line current is very serious, mainly due to two reasons,
First, the cross-sectional area of the zero line is not larger than that of the phase line. The current exceeding the phase line will inevitably cause the zero line to overheat and burn out;
Second, when the zero line is disconnected, because the three phases are not completely balanced, the voltage of each phase will be inconsistent and the three-phase equipment can not work normally. The ABC Cable of one phase with less power will rise; The voltage of one phase with more electricity will be reduced. Single phase equipment on one phase of voltage rise will be damaged; Single equipment on one phase with reduced voltage cannot work normally.

The world’s largest copper mine enters the countdown to strike

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Union members of Escondida copper mine, the world’s largest copper mine in Chile, voted against the latest contract offer made by the mine owner and decided to go on strike. Many industries are worried that the global economy will continue to recover from the impact of the COVID-19 pandemic, which could lead to disruption in the supply of ABC Cables, a key metal.
Escondida copper mine is located in Atacama Desert in northern Chile. It is the largest copper mine in the world, and its output accounts for almost 5% of the total global copper supply. BHP Billiton currently manages the copper mine business and holds about 58% of the copper mine. Other investors include Rio Tinto and Mitsubishi Corp.


99.5% of union members reject BHP Billiton’s new contract
The union of Escondida copper mine said in a statement late Saturday that the union voted overwhelmingly against the new contract proposal of the mine owner (BHP Billiton). 2164 miners (about 99.5% of the total) rejected the proposal, and only 11 supported it.
The trade union said that the contract proposal did not include the terms previously required by the trade union: providing one-time bonuses for workers to maintain mine operations during the epidemic, performance-related remuneration and improved career development plans, and enabling workers’ children to enjoy the same educational benefits as those of supervisors.
BHP Billiton spokesman previously said in an e-mail statement on Friday that “the new offer proposed by the company improves the previous conditions and incorporates new benefits on issues that workers attach great importance to.” However, the trade union statement refuted that the so-called benefits in Escondida’s latest contract offer to the miners were actually based on extending working hours, increasing job requirements and other more demanding conditions for workers.
“We hope that this overwhelming vote will sound a decisive alarm and prompt BHP Billiton to start substantive negotiations to reach a satisfactory agreement – if they want to avoid the most costly long-term trade union conflict in Chile’s history,” the trade union statement said.


Historically, miners at Escondida copper mine also held a strike in 2017, which lasted for 44 days at that time.
Strike Countdown: last chance during government mediation
After the announcement of the AAC Cables of the union vote, BHP Billiton has said it will ask the government to intervene in mandatory mediation.
“The company’s interest is always to reach an agreement with employees, which is why we maintain an open dialogue and occupy all available space for this purpose,” the company said in a statement BHP Billiton did not comment on the Union’s specific request.
Analysts said that according to the local law of Chile, miners must continue to work during the government’s forced mediation, and the mediation period is up to 10 days. Therefore, although the strike has entered the countdown, it is not a certainty at present. In all walks of life, union voting to authorize strikes is often used as a negotiation strategy.
Eleni joannides, an analyst at wood Mackenzie, a commodity consulting firm, said before the union vote: “whether the market and prices will be significantly affected will depend on whether there will be a strike in about 10 days.” She also pointed out that “at current prices, Escondida’s negotiations seem to have affected the market.”
Will the strike push copper prices higher?
Copper is a key raw material for manufacturing wires and motors, and is widely used in many fields such as construction industry. Earlier this year, copper prices soared to a record high as the market expected that the global economy would continue to recover and the energy transformation would require a large amount of copper to produce electric vehicles and renewable energy.


Novel coronavirus pneumonia has been faced with severe challenges before mining operations. BHP Billiton said on July 20 that due to a number of epidemic prevention measures, the operation of several Chilean copper mines including Escondida has not been interrupted, but the number of employees has been reduced. The company reported that copper production at Escondida copper mine is expected to decline by 10% in the current fiscal year. BHP Billiton also said that due to the rising infection rate of the epidemic, the company is expected to face continuous operational challenges in Chile.
Although some analysts, including joannides, believe that the recent rise in copper prices has reflected people’s expectations of the interruption of mine operations, other analysts are still worried that the strike may push copper prices further to record highs. LME copper prices climbed to a six week high of $9924 last week after hitting an all-time high of $10747.50 in May.
Morgan Stanley said that the contract negotiation of Chilean copper mine is a key focus of short-term copper price trend. The bank said that although the copper market is expected to have surplus in 2022 due to output growth exceeding demand, the ACSR Cables will still face copper shortage this year.
Sabrin Chowdhury, an analyst at Fitch solutions in Singapore, also said that the strike risk of Escondida copper mine is driving up the short-term prospect of copper prices. Copper prices will also benefit from severe floods in Central China and the weakness of the US dollar.
Chowdhury pointed out, “whether the strike occurs or not, it will cause panic among investors, because copper stocks on the exchange have fallen sharply in recent weeks. The market is already tight, and the demand exceeds the supply that is trying to return to the pre epidemic level. If the strike does happen, copper prices will reach a new record high, higher than in May“

Choose the wire and cable, quickly identify the type!

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The main function of wire and cable is to transmit electric energy, signal and realize electromagnetic conversion. The transmission of electric power, such as power cable, overhead line, etc., is the same as shaft ABC Cable, and the wire with electromagnetic conversion is like paint wrapped wire. If you choose a wire or cable, you need to quickly identify the type and type to select the right cable to use.

According to the number of transmission lines of each other with each other which are fixed together, the insulated wires can be divided into single core wires and multi-core wires. Multi core wires can also fix multiple single core wires in one insulating sheath. The multi-core wires in the same sheath can be as many as 24 cores. The parallel multi-core line is indicated by “B”, and the twisted multi-core wire is indicated by “s”.
Insulated wires can be divided into single wire and multi-core wire according to the stock number of each transmission line. Generally, the insulated wires above 6 square mm are all multi-core wires. The insulated wires with 6 square mm and below can be single wires and can also be multi-core wires. We call the single wire of 6 square mm and the following as hard wire, and multi-core wire is called copper wire.
Hard wire is indicated by “B” and copper wire is indicated by “R”. The common insulating materials of ACSR Cable are polyethylene and high pressure polyethylene. The “V” of PE indicates that “Y” is used for high-pressure polyethylene.

Bv copper core polyethylene insulated wire; BLV aluminum core polyethylene insulated wire; BVV copper core polyethylene insulation layer polyethylene protective cable; Blvv aluminum core polyethylene insulation layer polyethylene protective cable; BVR copper core polyethylene insulation copper wire;
RV copper core polyethylene insulation layer is installed with copper wire; RVB copper core polyethylene insulation layer flat electrode connecting wire copper wire; BVS copper core polyethylene insulation layer twisted copper wire; RVV copper core polyethylene insulation layer polyethylene wire sheath copper wire;
BYR high voltage polyethylene insulated soft cable; Byvr high voltage polyethylene insulation layer polyethylene wire sheath copper wire; Ry high voltage polyethylene insulated copper wire; RYV high voltage polyethylene insulation layer polyethylene wire sheath copper wire;
BVVB copper core polyethylene insulation layer polyethylene protective wire sleeve flat cable; BLVVB aluminum core polyethylene insulation layer polyethylene protective wire sleeve flat cable; Bv-105 copper core high temperature resistant 105 ℃ polyethylene insulated wire.

For more information on cables, click Joy ’cable Blog

Is the cable conductor detected by the cable nose in accordance with the national standard

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The AAC Cable nose is often used for the end connection and continuation of the cable, which can make the connection between the cable and electrical appliances more firm and safer. Cable companies often receive customer complaints: with the corresponding specifications of the wire nose through the corresponding specifications of the conductor, found that the wire nose is larger than the outer diameter of the cable conductor! If it is suspected that the cable is a non-standard product, let the cable enterprise send someone to explain and deal with it on site immediately; In particular, with the use of “special-shaped wire stranded cable conductor and its production method” by cable enterprises, the compression coefficient of cable conductor has increased from 0.90 to more than 0.98, and the outer diameter of cable conductor has decreased by more than 5%. In this way, cable users worry about whether the cable is non-standard or wrong cable specification after receiving the goods? Today, I want to explain to you: why is the cable nose larger than the outer diameter of the cable conductor?

The reason why the cable nose is larger than the outer diameter of the cable conductor is mainly to reduce the contact resistance of the connection and strengthen the local heat dissipation capacity. For a cable, the resistance of the whole length is evenly distributed, but no matter how much pressure you use to install the connection of the cable nose, the resistance of the contact part is always greater than that of the non connection part of the cable, resulting in serious local heating, The solution is to increase the contact area and reduce the contact resistance. At the same time, the larger the nose, its heat dissipation capacity will be higher. Generally, the area of the cable nose is 30-50% larger than the corresponding specification of the cable conductor, because the cable conductor is sometimes rolled into a fan or tile shape, which is not a pure circle. Generally, it needs to be compressed by a hydraulic press and then turned into a circle. If the inner hole of the cable nose is designed according to the nominal outer diameter of the cable conductor, some special-shaped conductors cannot be penetrated.

For more information on cables, click Joy ’cable Blog

How to use insulation megger to measure insulation resistance of power cable

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1. The performance of power cable insulation material decreases irreversibly with service time and environment, which is called insulation aging. The manifestations of insulation aging are as follows: breakdown strength decreases, dielectric loss tangent increases, local leakage and discharge increase, mechanical strength and other properties decrease.
2. In the aging process, the time required for the insulation performance to fall below the specified allowable range is usually called insulation life. Under normal service conditions, the service life of XLPE cable and oil immersed armored cable is 30 years and 40 years respectively. Under the action of power frequency AC voltage, partial discharge in insulation material is the main reason for insulation aging of power cable. In the oil immersed cable, the partial discharge leads to the decomposition of impregnant and paper fiber and the formation of waxy crystalline state. In XLPE cables, the insulation material is gradually damaged by partial discharge, forming micropores, cracks and dendritic buds, which gradually develop in depth in the insulation material until insulation breakdown.
3. The insulation aging degree of power cable can be measured by insulation resistance test and dielectric loss tangent tan δ Test, withstand voltage test and other electrical tests. Insulation resistance test shall be conducted for power cables below 6kV. 500 ~ 1000V grounding resistance tester (megohmmeter or megger) is used for cables with voltage below 1KV; 1000 ~ 2500V grounding resistance tester is used for 1kV and above cable.


The following items should be noted during the test.
① The method of measuring the insulation resistance is suitable for the cable which is not too long. Measure the insulation resistance and calculate the absorption ratio. Under the same test conditions, the better the insulation value is, the greater the absorption ratio is.
② Generally, the insulation resistance value of power cable is not specified, and the test value should be compared with the original record. Generally, the insulation resistance of 1 ~ 3KV cable should not be less than 200m Ω； 6 ~ 10kV cable should not be less than 400m Ω； 35kV Cable should not be less than 600m Ω； The unbalance coefficient of three-phase insulation shall not be greater than 2.5. During the test, the temperature, air humidity and other weather conditions shall be recorded for reference.
③ Before measurement, the cable shall be fully discharged, that is, the cable core and the metal sheath of the cable shall be grounded.
④ The surface of cable terminal bushing shall be wiped clean before testing. Use the other insulating core of the cable as the shielding circuit, connect the conductors at both ends of the insulated core to the bushing or insulation of the insulated core to be tested with flexible metal wires, wind them for several turns, and then connect them to the shielding terminal g of the grounding resistance tester.
⑤ After each test, the cable shall be grounded and discharged. The longer the cable line is, the better the insulation condition is, and the longer the grounding discharge time is, generally not less than 1min.

Several points for attention in extrusion of silicone rubber cable

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Abstract

This paper presents the application of silicone rubber in the acsr cable industry, some problems and solutions in the production of silicone rubber cable.
Key words: silicone rubber, mixing, extrusion, coke burning, foaming, technology

1、 Preface

Because of its high heat resistance and excellent cold resistance, silicone rubber has a long-term temperature range of – 90-250 ℃, and has excellent electrical insulation and aging resistance. Therefore, the development of silicone rubber in cable industry is very rapid.
In the past few decades, the silicone rubber industry has been facing the challenge of meeting the growing demand of modern extrusion products market. The wear resistance, cutting and penetrating resistance, chemical resistance, oil resistance and mechanical strength of silicone rubber have been improved. As a material with high temperature of thermal aging, silicone rubber has been widely used in manufacturers and users because of its value and reliability. Nowadays, the application of silicone rubber in the wire and cable industry is developing continuously, mainly used as insulation and sheath of ship cable, aviation wire, motor lead wire, heating wire and many special purposes (such as atomic energy industry, aerospace industry, metallurgy industry, etc.).
In recent two years, the company has received orders from customers about silicone rubber insulation and sheath cables. However, because such products are not the leading products of the company, the processing technology and production equipment are not perfect. Although we have encountered many difficulties in the production process, after the joint efforts of the family, the products are delivered to the customers on time, We have learned a lot from it.

2、 Problems encountered in the production of silicone rubber cable

1. after mixing and adding sulfur, the rubber material is easy to burn and contains a lot of impurities during extrusion, which leads to breakdown of insulation voltage.
2. loose shell and blister phenomenon occur during the extrusion process of sheath.

3、 Solutions

1. technical requirements for mixing and returning
Although the processing equipment of silicone rubber is not different from that of organic rubber, it is better not to use the same open mill to process organic rubber and silicone rubber. It is better to have a special operation room for silica gel and keep the environment clean, because the mechanical and electrical properties of the polluted silicone rubber should be reduced. If special processing equipment and processing site cannot be configured for silicone rubber, it is necessary to pay attention to completely isolate the polluting materials from the coordination aids of silicone rubber and silicone rubber, because most of the impurities are from the mixing of rubber.
Because of the characteristics of silicone rubber, the plastic properties of the rubber material which need to be recycled will change after the re refining, and it is easy to pack it on the roller with fast speed. Cooling water should be put into the roller of the mixer to avoid burning of the rubber. Special attention should be paid to the use of the rubber containing bis 2.4-dichlorobenzoyl peroxide as the curing system. Because the decomposition temperature of bis 2.4-dichlorobenzoyl is about 45 ℃, the decomposition products 2.4-dichlorobenzoic acid and 2.4-dichlorobenzene are not volatile, and the rubber materials are easy to burn. In order to obtain high quality products, the following basic steps must be followed when mixing silicone rubber:
(1) . carefully weigh each kind of coordination components (such as flame retardant, vulcanizing agent, color masterbatch, etc.).
(2) . after putting the pure rubber or reinforcing rubber on the open mixer, adjust the roll distance to make the silicone rubber wrap on the roller with fast speed and fully return to the refining. Pure silicone rubber can be filled with only a little reflexion or no return refining. However, due to the silica in the reinforced silicone rubber, it must be fully refined. As long as the rubber is wrapped on the roller with high speed, the degree of re refining is appropriate.


(3) . if necessary, flame retardant, color master material, etc. shall be added to the adhesive. Some fillers fall into the receiving tray through the roller during mixing, and they should be collected and added to the rubber before the next filling is added. Rubber scraper is often used to scrape the filler from the receiving plate. It is better not to use brush, because some brush hairs may fall off the brush and mix them into the rubber. It is especially important to note that all fillers can not be added to the rubber at one time, but should be added in batches in 2-3 times. Every batch of filler is added, the rubber should be fully turned over. This can ensure the packing is evenly distributed and avoid the formation of hard packing clots. Reasonable roll distance can ensure the best mixing speed and quality of rubber.
(4) . finally, the vulcanizing agent is added to the rubber. Because the vulcanizing agent we currently use is 2.4-dichlorobenzoyl peroxide, when the rubber is too hot (no more than 4O ℃), no vulcanizing agent is added, otherwise, partial pre curing will occur, which will lead to the loss of the rubber or vulcanizer. Enough cooling water is put into the roller to avoid overheating of rubber. Finally, in order to make the vulcanizate disperse evenly, the whole roll of rubber material should be thin through several times.
2. correct use of filter screen and rubber filter pad
The filter screen is usually composed of 20-40 mesh filter plate and 60-100 mesh stainless steel filter with fine mesh. Some manufacturers do not like to extrude directly with the filter screen, because this can improve the extrusion speed and sometimes eliminate the possibility of heat generation and coke burning near the rubber filter plate. However, the use of the filter screen can not be ignored, because it plays a significant role in removing impurities and dispersed filler particles in the rubber. At the same time, the filter screen can also exclude the air in the rubber material during mixing and back mixing, especially for the softer rubber.
Because silicone rubber is only slightly thermoplastic and is not easy to be subjected to flow stress or shear strain in its unvulcanized state, the design of filter plate is not the key. Most filter plate designs are suitable for silicone rubber processing.
3. selection of extrusion equipment
In terms of equipment selection, we have no choice because we have no special silicon rubber cable production equipment at all. Among the three continuous vulcanization production equipments we currently use, the equipment that can be used for the production of silicone rubber is as follows: Φ 65/ Φ The 90 steam continuous vulcanization and extrusion machine and PLCV salt bath continuous vulcanization and extrusion machine.
4. process exploration and improvement
Because silicone cable is not our regular product, we always feel the stone across the river when we produce silicone cable. We used the insulation extrusion separately Φ 65/ Φ 90 steam continuous vulcanization and extrusion machine and PLCV continuous vulcanization extruder. But because Φ 65/ Φ The expansion pipe cannot be connected to the head of the 90 steam continuous rubber extruder, because the temperature of the head will rise rapidly after the expansion pipe is connected to the head under the steam heating, which leads to the vulcanization of silicone rubber in advance during extrusion. So in actual production, we do not connect expansion pipe to the head, but use steam in open mode, which will cause the steam pressure can not be applied too much, which affects the production speed and waste a lot of steam. When PLCV unit is in insulation extrusion, under the condition of controlling extrusion temperature, extrusion is not too big, only 0.2MPa pressure or no pressure is needed to meet the product requirements.
We are producing a number of single core cables with silicone rubber insulation and rubber sheath. Because the customers require that the insulation and sheath can be easily stripped, this means that the double layer co extrusion process we usually adopt in the production of single core cables can no longer be used. When the insulation extrusion is ready for the sheath extrusion, the problem arises: the silicone rubber insulation layer in the inner mold of the insulating wire core is easy to be driven up, which blocks the inner mold, and the bubble opening of the sheath after extrusion of the sheath. After repeated tests, we have adopted a layer of non-woven fabric in the insulation core to provide a storage space for the gas released by the silicone rubber insulation during secondary vulcanization. Under the condition of pipe pressure of 0.2-0.3mpa, the problem of being caught and blistering of sheath in the inner mold of insulation is solved.
In September, 2006, a customer customized a silicon rubber insulated silicone rubber sheath frequency conversion cable. Because the core of the insulating wire is formed, the copper tape is wrapped as the shielding layer. In order to prevent water from entering the sheath during the extrusion of the sheath, we first adopt the method of sealing the finished product head and then pressurizing, but the problems of sheath loose shell, blistering and explosion are also appeared. It is proved that we also use the method of wrapping a layer of non-woven cloth outside the shield layer of copper tape, and pressing when the head enters the vulcanizing tube 5m, which solves the problems of sheath loose shell, blistering and explosion.
In view of the success of the last few times, we blindly believe that non-woven fabric can solve the problems in the production of silicone rubber cable, but it is not all silicone rubber cables can produce qualified products by wrapping the non-woven fabric on the insulation. When the sheath is extruded after the multi-core cable is formed, although the non-woven cloth is wrapped, it is often because the cable is not filled enough, the melt pressure is not enough during the extrusion of the sheath, and the silicone rubber cannot be squeezed into the gap of insufficient filling, but the phenomenon of luller appears after the vulcanization tube is pressurized and vulcanized. Therefore, when the sheath is extruded after the multi-core cable is formed, we do not agree to add filling and wrapping non-woven cloth when the cable is formed, because the cable core has enough clearance to store the gas released by the silicone rubber insulation during secondary vulcanization. As long as the pipe pressure is applied properly, the loose shell and blister and the dew phenomenon of the sheath can be avoided completely.

4、 Concluding remarks

In conclusion, through the analysis of the problems encountered in the production process, we can not grasp the special points, and find out the causes of the problems fundamentally through the phenomenon and adopt practical technology and technical measures to solve the problems. We believe that through our efforts, the production process of silicone rubber cable will be mature, the quality of products will be improved, and the silicone rubber cable will gradually become our leading product, and the market competitiveness of rubber sheath cable will be enhanced.

Laying and installation of pre branched cables

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1、 Laying and installation of pre branch cable
Prepare for laying and installation according to the requirements of design drawings and be familiar with the laying direction and location of cables.
(1) Formulate construction plan and organize professional construction personnel.
(2) Prepare for installation of tools and equipment.
(3) Verify the abc cable model and specification and packing sequence.
(4) Verify the accessories and assign the installation site of accessories to the construction party.
(5) Place cable tray on the cable release frame.
(6) When the cable is installed vertically, the cable laying frame is lower than the floor, and the cable is lifted by winch or pulley block through the rope. Professional construction personnel shall be required for each floor. As for the terminal, hang the cable on the installed hook
(7) When the cable is installed horizontally, the cable setting out frame shall be set by the professional construction personnel (one person every two meters and the commander) as for the receiving position.


(8) Fix the intermediate part of the main and branch lines of prefabricated branch cables as required.
(9) Connect the main line and branch line with the electrical control device in phase sequence.
(10) After installation, clean the site, measure the insulation resistance of each circuit connected to each phase of prefabricated branch cable.
(11) Fill in the construction record.
2、 Precautions during laying and installation
(1) When laying, it is necessary to use the vertical hoisting as far as possible. When the construction site is restricted or special requirements are required, reverse installation can be used. No matter which setting method, the branch line shall not be released in advance in the process to prevent the branch body from scratching when passing through the hole and avoid being subjected to excessive mechanical external force.
(2) During lifting and hoisting, the cable with a weight of more than 4 times the strength must be selected. After laying, the upper and lower installation and fixing fixture shall be carried out first.
(3) The bending radius of cable shall not be less than 25d during laying and installation.
(4) Metal clamps are not allowed when fixing single core prefabricated branch cables.
(5) When connecting the main line and branch line with the receiving and electric measuring apparatus, the metal clamp must be used and the metal type of the clamp shall be selected correctly.

(6) The spreader must be installed on the load-bearing wall.

Advantages of BTTZ rigid mineral insulated cable

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Due to the special structure of BTTZ rigid mineral insulated cables, it has some characteristics different from traditional cables; it is also the most superior fire resistance characteristic of mineral insulated cables.
Advantages of BTTZ rigid mineral insulated cable
1) Completely fireproof BTTZ cable itself does not burn at all, and it will not cause a fire source at the same time. Even in the case of flame barbecue, as long as the flame temperature is lower than the melting point of copper, the cable can be used without replacement after the flame is eliminated. In the case of being grilled by flames, no toxic smoke and gas will be produced.


2) Strong overload protection capability When the line is overloaded, the cable will not be damaged as long as the heat does not reach the melting point of copper. Even if the breakdown occurs instantaneously, the magnesium oxide crystal at the breakdown will not form carbides. After the overload is eliminated, the cable performance will not change, and normal use can still be continued.
3) High working temperature Since the melting point of the insulating layer of magnesium oxide crystal is much higher than the melting point of copper, the maximum normal working temperature of the cable can reach 250°C, and it can continue to operate at a temperature close to the melting point of copper of 1083°C in the short term.
4) Good anti-corrosion and explosion-proof performance. As the seamless copper tube is used as the sheath, the BTTZ cable has the performance of waterproof, moisture, oil and some chemical substances. The copper tube has considerable mechanical strength, so it has better explosion-proof performance.


5) Long service life BTTZ cables are all made of inorganic materials, so there is no insulation aging, and the service life can reach more than 3 times that of ordinary cables.
6) Greater flexibility in laying BTTZ cables can be laid on the bridge with ordinary cables, and can also be laid with special brackets, which is more flexible than ordinary cable laying. It also saves the investment in cable bridges and reduces the overall cost of the project.