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.

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.

How to measure the temperature of cable terminal tail contact?

The contact resistance of the tail wire contact of AAAC Cable terminal (especially outdoor, sometimes connected by copper aluminum two different metals) will increase under the influence of long-term load and fault current, which will lead to overheating. When the line fails, the fault current flows through the contact, which will burn the contact. Measuring the contact temperature under the condition of no power failure, that is, to conduct live temperature measurement (preferably when the load is large), is an effective measure to monitor and check the contact condition. There are three main methods to check the contact temperature.


(1) Paste the temperature wax sheet: the temperature wax sheet is divided into three types: 60 ℃, 70 ℃ and 80 ℃, and are respectively expressed in different colors, and the commonly used ones are yellow, green and red. The temperature range can only be roughly checked by using the temperature wax plate. The reaction time is slow and the paste is not convenient. It is less used at present.
(2) Discoloration thermometer: this kind of color change thermometer is a surface measuring tool. It was used to monitor the temperature of heating working parts during welding in shipbuilding industry. It is used to indicate temperature according to the color changing characteristics of pigment in the pen at a certain temperature. The temperature measurement is quick and easy to use. Generally, the one with lower temperature can be selected for ABC Cable line. For example, the color changing temperature pen with color change temperature of 70 ℃ is selected, and it is marked as the original color. If the temperature is more than 70 ℃, it will become lake blue. The price of this color change thermometer is also cheaper. The temperature measurement reaction is fast (only 1-2s), and it is convenient to use.


(3) Infrared thermometer and thermal imager: the temperature wax plate and the color change thermometer shall be directly contacted with the live equipment, and the insulation rod shall be used for operation. Infrared thermometers and thermal imagers can measure temperature outside the measured point, so they are safe and reliable, and their measurement accuracy is much higher than the above two. The commonly used infrared thermometers or thermal imagers generally adopt portable type. See Fig. 9-4 and Fig. 9-5. The measuring distance is 5m, 10m, 100m, or even larger. The thermal imager can display the image of temperature distribution, and can store the image data in disk or computer, with good effect. But the price of this instrument is relatively expensive, and special personnel shall be responsible for the use and maintenance.

What is the national standard cable

National standard ACSR Cable, literally, is the wire and cable produced in strict accordance with national standards. China has clear regulations on the appearance, mechanical strength, insulation of sheath, withstand voltage strength and line resistance of cables. There are different national standards for different types of cables, mainly as follows:
·Executive standard for irradiation crosslinked power cable (0.6 / 1KV): GB / t12706.1-2008
·Executive standard for medium voltage XLPE cable (6 / 6kv-26 / 35kV): GB / t12706.2-2008
·Executive standard for high voltage XLPE cable (64 / 110kV): GB / t11017 – [/ b] 2002
·Executive standard of control AAAC Cable (450 / 750V): gb9330-88
·Executive standard for aluminum strand and ACSR (0.6 / 1KV): GB / t1179-1999


How to distinguish national standard cable?
I want to see it. See if there is quality system certification; Check whether the certificate is standard or not; Check whether there is factory name, address, inspection seal and production date; See if the trademark, specification, voltage, etc. are printed on the wire. But also look at the cross-section of the wire copper core, superior copper color bright, soft color, otherwise it is inferior.
Try. Take a wire head and bend it repeatedly by hand. Those with soft handle, good fatigue resistance, high elasticity of plastic or rubber handle and no crack on the wire insulator are excellent products.
weighing. Good quality wires are generally within the specified weight range. For example, the commonly used plastic insulated single strand copper wire with a cross-sectional area of 1.5mm2 weighs 1.8-1.9kg per 100m; The weight of 2.5mm2 plastic insulated single strand copper wire is 3-3.1kg per 100m; The weight of the poor wire is not enough, or the length is not enough, or the copper core of the wire is too much impurity.
Look at the copper. Qualified copper wire copper core should be purplish red, glossy and soft. The copper core of the fake copper wire is purple black, yellow or white, with many impurities, poor mechanical strength and poor toughness. It will break with a little force, and the wire is often broken. When checking, you only need to strip one end of the wire for 2cm, and then rub it on the copper core with a piece of white paper. If there is black substance on the white paper, it means that there are more impurities in the copper core. In addition, the insulation layer of fake wires seems to be very thick. In fact, most of them are made of recycled plastics. Over time, the insulation layer will age and leak electricity.
Look at the price. Because of the low production cost of fake wires, vendors often sell them at low prices under the guise of low price and good quality.


“Non standard / enterprise standard / market national standard” and “national standard of resistance protection”?
When many sales companies are asked whether they are GB cables, they often hear such words as “GB for market” and “GB for resistance protection”. Many people have little understanding. What do these vague words mean?
Resistance national standard: cable is used for power, and conductor resistance directly affects the conductivity. If the conductor resistance is too large, the conductivity is poor, when there is a large current through, it will produce a lot of heat, which will cause a fire. At the same time, because of the high resistance, it will also produce heat and inaction consumption of electric energy.
Resistance cable is to ensure that the resistance value of the cable is in line with the national standard, but it does not guarantee that the component materials of the cable are completely produced in accordance with the national standard, such as conductor square, insulation, sheath, etc. To put it bluntly, it means that through various technological innovations (or means), the manufacturer can make the cable reach the safe use condition on the premise of reducing the material cost.
Note that two words are used here: innovation and means. Innovation refers to the cost reduction through technological innovation, such as the current aluminum alloy conductor, special-shaped conductor, etc., which reduces the price on the premise of ensuring safe use. Some manufacturers reduce the cost through some illegal means, such as using second-generation materials, reducing the purity of conductor, etc. even if the resistance value of such cable is within the standard range, its use is extremely unsafe. When purchasing this kind of cable, it must be screened again and again.
“Enterprise standard / non-standard / market national standard”: these words are easy to understand, “enterprise standard / non-standard” is non-national standard cable, the price is extremely low, and it is also extremely unsafe“ Market national standard “generally refers to 9 fold cable (or 95 fold), we must not regard it as national standard, we must ask clearly. In addition, we should pay attention to the fact that the enterprise standard is higher than the national standard in accordance with the principle, but the domestic standard does come in the opposite direction!
In recent years, there are more and more fire accidents caused by unqualified wires and cables, many of which are purchased unqualified products due to the lack of cable knowledge. It is our duty and responsibility to produce and purchase GB cables, so as to ensure the safety of power supply and the healthy development of cable market.

Testing of sheath and corrosion protection

The intact cable sheath can effectively protect the insulation: it can prevent liquid water from immersing in the insulation, and the enclosed metal sheath can also prevent moisture from diffusing into the insulation, so it can obviously prevent the formation of water branches in PE / XLPE insulated cables. The sheath and anti-corrosion layer test which can provide scientific criteria can ensure that the cable will not fail in long-term operation; In addition, for oil filled AAAC Cable, it also makes an important contribution to environmental protection.


In general, the test is carried out with appropriate high DC voltage between the metal sheath or shield and the ground
Welded steel bushing for compressed air cable, 1kV
PVC sheath, 3KV (higher voltage may cause intact sheath breakdown due to moisture absorption of sheath)
Polyethylene sheath, 5 ~ 10KV
The duration of the test was about 1 min. Such a long time is enough to complete the charging process. The existing fault can be identified immediately by breakdown, and then the method described in Chapter 37 can be used for detection. Because this kind of test usually refers to fault detection, rather than insulation resistance test according to international electrical standard iec60229, longer test time is not recommended. For example, for PVC sheath, if the load is applied for a long time, it may cause breakdown of the innermost sheath.


Of course, the premise of reliable damage identification is to have a “corresponding electrode” with good conductivity to make the possible fault current return to the source point. For this purpose, the ABC Cable should be completely buried in the soil, and the water in the soil should be sufficient. Laboratory tests show that although the insulating layer in the sheath is still intact, it has no negative effect on the performance of the test. It has nothing to do with the structure of the isolation layer. The isolation layer either absorbs enough moisture (such as rubber inner cover and wrinkled paper), or forms a low resistance waterway at the overlapping position of the plastic film. That is to say, the expressiveness of the test mainly depends on the degree of moisture.
The graphite coating on the cable sheath can also improve the return circuit. However, this method is expensive and sensitive, so it can only be used in special cases according to the requirements. Before conducting DC voltage test on anti-corrosion protection, all grounding of metal sheath or shield and its connected accessories must be disconnected.

How to make a perfect cable identification?

The most commonly used label for ACSR Cable identification is covered with protective film. This kind of label is sticky and has a layer of transparent protective film outside the printing part, which can protect the label printing font from abrasion. In addition, single cable / jumper can also use non coated label, flag label and heat shrinkable sleeve label. Common material types include vinyl, polyester and polyfluoroethylene, as shown in the figure.

For bundled cables, it is recommended to use identification plates for identification. This kind of sign can be printed by printer, fixed with nylon tie or felt belt and AAC Cable bundle, and can be placed horizontally or vertically. The sign itself should have good tear resistance and conform to ROHS standards.

The most commonly used label for cable identification is covered with protective film. This kind of label is sticky and has a layer of transparent protective film outside the printing part, which can protect the label printing font from abrasion. In addition, single cable / jumper can also use non coated label, flag label and heat shrinkable sleeve label, as shown in the figure.
5.3 label of wiring panel / outlet panel
The wiring panel identification is mainly plane identification, which requires that the material can withstand the test of the environment, meet the environmental requirements of RoHS, maintain good image quality in various solvents, and can be pasted to various surfaces including low surface energy plastics. The label should be printed and not filled in by hand. It should be clear and easy to read. All labels shall be clear and complete and meet the requirements of the environment, as shown in the figure

Explore the longest 330kV cable laying site in China

Dancing a 700 meter long ultra-high voltage AAAC Cable that is thick at the mouth of the bowl and runs 3 kilometers underground, what are you doing? How many people will that take? How fierce is that? How hard is it to control?
On May 27, with a series of questions, the author came to the venue area of the 14th National Games in the northeast of Xi’an, Shaanxi Province, walked into the cable construction site of 330kV Jingxuan line I and II project of Shaanxi power transmission and Transformation Engineering Co., Ltd., and explored how to lay the longest 330kV UHV cable in China.
700 meter long cable “Jiaolong”


The 14th National Games is located in the northeast of Xi’an, and the ancient city ushers in the National Games for the first time. The relocation and landing of 330kV overhead transmission line in the northeast of Xi’an provides reliable power guarantee for the main stadium and surrounding area of Xi’an Olympic Sports Center. At the same time, by optimizing the 330kV line design and replacing the overhead line with the ground ABC Cable, the 330kV overhead line will be eliminated in the key areas of the National Games, creating a new sky landscape and showing the new style and features of the Millennium “charming ancient city” New weather.
The relocation and landing project of 330kV overhead transmission line in Northeast Xi’an involves 7 relocation and 2 new lines, 2 21km cable pipe corridors, 2 cable terminal stations and 100 km 330kV EHV cable lines, with a total investment of 8.274 billion yuan. After the completion of the project, it will be the largest 330kV and above UHV cable project in China.
The cable part of 330kV Jingxuan I and II line project is the longest and largest 330kV cable line in China. The first stage laying construction task is along the cable pipe gallery of Xingwei Road, with a total length of 6.3 km for single phase and 37 km for all cables. It is divided into nine sections, each section is 700 meters long, and each section contains six 700 meters long cables.
Green concept behind
There are 30 air vents in the underground high-voltage pipe gallery of Xingwei section, and 8 shafts for high-voltage cable laying. According to the conventional operation, each section of cable can enter the pipe gallery from the ground by selecting the nearest shaft, so as to minimize the cable transmission path. However, the actual construction scheme did not follow the conventional operation. Only two shafts were selected to enter the 9 sections of cables, and 6 of them need the whole cable to enter the pipe gallery and continue to transmit underground. After entering from shaft 24, the section with the longest transmission distance still needs to travel 3 kilometers underground, and it takes 6 hours to be in place.
Why choose this unconventional scheme of underground transmission?


“Mainly to make way for greening.” Wang Hailong, manager of Shaanxi power transmission and transformation construction project, said. High pressure pipe gallery in Xingwei section is mainly constructed by large-scale excavation except for shield construction in some sections. After the backfill treatment of the pipe gallery construction, the ground surface is planned as urban road and green belt according to the unified planning of the venue area of the 14th National Games“ Road construction, green belt planting, all need time, we occupy the site, we have to drag green back. At present, most of the greening of Xingwei road has been in good order.
Behind the posture is strength
Dancing “Jiaolong”
Following Wang Hailong into the underground high-voltage pipe gallery, he saw the sparse construction workers along the cable line, and was puzzled: “can such a few people dance the 700 meter cable?”
“Let’s call four or two thousand catties.” Wang Hailong squatted down, pointed to the roller under the cable and said, “this is the frequency conversion guide wheel, the main power of underground horizontal transmission of the cable.” A total of 900 electric frequency conversion rollers are arranged along the laying line, with a spacing of 5 meters, and a non powered roller support is arranged between the two. 900 variable frequency rollers are controlled by intelligent control box to start and stop synchronously. Although each variable-frequency roller intelligently generates a torque of 70kg, if 900 wheels rotate at the same time, the 700m “dragon” will move steadily along the passage at the speed of 6m per minute. In addition to the need for two people at the front end of the cable to lift the cable end to the next roller, the remaining 30 workers just need to watch the roller run well.
What should we do if there is any abnormality? There’s no cell phone in the tunnel. Moreover, the channel is too long and there are many turns, so the transmission distance of walkie talkie is greatly reduced.
“We have two call systems.” Wang Hailong said. They are equipped with 30 walkie talkies with coaxial cable signal relay. Another 50 walkie talkies are equipped with wireless chain relay. The two systems operate independently, which can realize the information synchronization of the whole cable laying line without delay, and ensure that the system can be shut down at the first time in case of abnormality.
“The pipe gallery is a limited space. In order to prevent accidents, we have gas masks every other section.” They did what they should have thought under special conditions.
“At the most spectacular time, there were three 700 meter long pipes running underground. The pipe gallery could not see the end at a glance, and the scene of more than 40 people dancing the 700 meter” dragon “could not be fully seen.” Wang Hailong added. Behind the grand scene is the courage, responsibility and courage of Shaanxi power transmission and transformation to seize the new challenges, explore new fields, start new engines, stimulate new power, help the new development of Shaanxi power grid, and serve the economic take-off of Shaanxi.

Factors affecting insulation resistance of wires and cables

Small insulation resistance is often encountered in AAC Cable production, which is affected by many factors. In fact, there are four main factors that have a great influence on the coefficient of insulation resistance.

1、 The influence of temperature
With the increase of temperature, the insulation resistance coefficient decreases. This is due to the increase of thermal motion and the increase of ion production and migration. Under the action of voltage, the conduction current formed by ion motion increases and the insulation resistance decreases.
The theory and practice show that the insulation resistance coefficient decreases exponentially with the increase of temperature, and the conductivity increases exponentially with the increase of temperature.
2、 Influence of electric field intensity
When the electric field strength is relatively low, the mobility of ions increases in proportion with the increase of electric field strength. The ionic current and electric field strength follow Ohm’s law. When the electric field intensity is relatively high, with the increase of electric field intensity, the mobility of ions gradually changes from linear relationship to exponential relationship. When the electric field is close to breakdown, a large number of electrons will migrate and the insulation resistance coefficient will be greatly reduced.
The withstand voltage test voltage of various wire and ACSR Cable products specified in the standard is in the stage of ion mobility increasing in proportion to the electric field strength, so the influence of electric field strength on the insulation resistance coefficient cannot be reflected. The effect of electric field on the insulation resistance coefficient is obviously reflected when the sample is subjected to breakdown test.


3、 Influence of humidity
Due to the large conductivity of water, the size of water molecules is much smaller than that of polymer molecules. Under the action of heat, the polymer macromolecules and the constituent chain move relatively, so that water molecules can easily penetrate into the polymer, increase the conductive ions in the polymer, and reduce the insulation resistance.
The standard specifies the immersion test of various wires and cables. For example, the rubber specimen is immersed in water for 24 hours before the insulation resistance is measured. The purpose is to meet the influence of moisture and water on electrical performance during use.
Insulation resistance is one of the main electrical properties of insulating materials, and it is also an important index of wire and cable products or materials. Generally, the insulation resistance should not be lower than a certain value. If the insulation resistance value is too low, the leakage current along the wire and cable line will inevitably increase, resulting in the waste of electric energy. At the same time, electric energy will be turned into heat energy to prepare for thermal breakdown and increase the possibility of thermal breakdown.
4、 Influence of material purity
The impurity mixed into the material increases the conductive particles in the material and reduces the insulation resistance. Therefore, the insulation resistance of a certain rubber plastic material will reflect the purity of the material and verify whether it meets the standard.
In the process of wire and cable production, the process does not strictly comply with the operating procedures, mixed impurities and materials Blister due to moisture, insulation core deviation or outer diameter size is less than the standard, insulation delamination or crack, insulation scratch, etc., It will reduce the insulation resistance of the product.
Therefore, in order to check the insulation resistance, it is necessary to check whether there are any problems in the process operation. During the use of wires and cables, measuring the change of insulation resistance can also check the insulation damage and prevent accidents.

For more information on cables, click Joy ’cable Blog

High voltage cable intelligent anti breaking system

01
Project background

If the city grid is given life, the AAAC Cable is his blood vessel, which injects the energy of the power grid.
However, with the development, the construction frequency is high, which leads to the explosion of the risk of cable external force damage. Please see, this is the scene after the cable has been destroyed. Then, once the underground cable is damaged by the external force of construction machinery, it may cause chain reaction and lead to large-scale power failure.
If the power outage occurs in hospitals, transportation hub, government agencies and other important places, how much loss will be suffered to the people’s lives and property, and the indirect economic losses caused by the power failure are immeasurable; On the other hand, cable damage may also lead to the construction of personnel casualties. In view of this, it is urgent to prevent external damage!


Traditional cable inspection mainly relies on manual field inspection, which makes it difficult to find out the cable external breakage due to various mechanical construction on the ACSR Cable path in time and accurately. There are few operation and maintenance personnel, and the inspection mode mainly depends on the drawing and oral communication to confirm the cable location. Inspection efficiency is low and maintenance cost is high.

02
Function and characteristics of the scheme

With the national policy support for the digital transformation of state-owned enterprises and the promotion and application of 5g technology, our team seized the opportunity to break through the bottleneck, combined smart IOT technology with cable anti-external breaking work organically, and independently developed “intelligent brain – high voltage cable intelligent external breaking system based on artificial intelligence”.


Function 1
The full time visual monitoring device is applied to improve the real-time visibility of the equipment channel. The visual monitoring device is installed on the smart light pole in key cable channels and construction intensive areas, which can automatically identify the hidden dangers outside the cable channels such as large construction machinery.
The real-time data collected by camera is compared with the hidden danger image database of cable channel, and all kinds of external damage hazards on cable channel are automatically studied and identified.
At the same time, the system has the ability of autonomous learning, and with the deep use of the system, the recognition efficiency is improved.
Function 2
The fiber vibration technology is applied to improve the state perception of the equipment. The abnormal vibration caused by external force is automatically captured, and the warning is given before the external force endangers the power cable. According to the external vibration of optical fiber, the external force failure signal is automatically identified and the location of the hidden danger of external failure is accurately located.


Function 3
The application of “four-dimensional” visualization technology can enhance the visual operation and maintenance capability of underground cables. Realize the operation and maintenance personnel to stereoscopic view the cable lines and auxiliary facilities in VR form on site through PDA terminal, and guide the accurate operation.

For more information on cables, click Joy ’cable Blog

How to distinguish the rigidity or flexibility of mineral insulated cable

The structure of mineral insulated ABC Cables with different names is convenient for us to design and select better, and there is no confusion about many naming rules.

What are (rigidity) and (flexibility)? For a long time, the domestic mineral insulated cables are divided into two types: rigidity and flexibility.
The internationally used magnesium oxide mineral insulated cable (IEC 60702-1:2002) uses copper solid conductor instead of stranded conductor or soft conductor which is used in our common common cable.
Later, the following MI cables were developed by domestic production enterprises:
(1) It meets the industry standard: yttw series of jg/t 313 metal sheathed inorganic mineral insulated cable and terminal with rated voltage of 0.6/1kv and below (the release and update time of the standard is 2011 and 2014);
(2) It meets national standards: RTT series of gb/t 34926 mica tape mineral insulated corrugated copper sheath cable and terminal with rated voltage of 0.6/1kv and below (the standard release time is 2017);
(3) There are also some mica tape mineral insulated cables that meet the enterprise standards: non solid conductor.
All the above cables are generally classified into flexible mineral insulated cables in the industry, only to distinguish them from rigid MgO mineral insulated AAC Cables.
1. What form of (MI) cable is identified by the specification
The code for fire protection design of buildings GB 50016-2014, article 10.1.10 of the article description states that:
Mineral insulation non combustible cable is composed of copper core, mineral insulation material, copper and other metal sheath. Besides good conductivity, mechanical and physical properties and fire resistance, it also has good non flammability. Under fire conditions, this cable can ensure the fire power consumption within the fire duration.
That is to say, mineral insulation cable recognized by the specification must be made up of copper core, mineral insulation material, copper and other metal sheath as the basic composition.
BTT series rigid mineral insulated cable structure form
According to this requirement, we will see whether the structural forms of various rigid cables and flexible cables meet the requirements.
2. Rigid cable with unique naming rule (BTT)
BTT series mineral insulated cable is the only mineral insulated cable recognized internationally. It is a kind of cable with seamless copper tube sheath and dense compacted magnesium oxide crystal powder in the middle as the insulation material. The conductor is a single copper rod cable.
The reason for being recognized internationally depends on the material used is the real high temperature resistant material, among which the melting point of copper conductor and copper sheath is 1083 ℃, and the melting point of MgO mineral insulation layer reaches 2800 ℃, so that the line can still operate safely under fire temperature of more than 1000 ℃.
Because all the materials used are inorganic materials, and the structure is dense and hard, the product has the electrical, mechanical and environmental withstand properties that can not be compared with the conventional VV, YJV, yjy and other (organic cables).
It has high temperature resistance, fire prevention, explosion-proof and non combustion. It can run continuously for a long time at 250 ℃ and can also maintain 180min short-time operation under the limit state of 1000 ℃.
For BTT series Mi cables, the domestic implementation standard is gb/t 13033-2007 mineral insulated cables and terminals with rated voltage of 750V and below, and has not been updated so far. It can be seen from the release time of the specification that the stable state of product standards can be said to be the most mature Mi cable in technology.
BTT series rigid cables have good technical performance advantages, but also have problems such as difficult construction, short manufacturing length and complex intermediate joint processing technology. In order to solve the above problems, a variety of flexible cables have been developed in China.
This also brings us a lot of difficulties in the selection of engineering design.
3. Flexible cable named (bbtrz)
When the mineral insulated cable was introduced to China in the early stage, the copper sheath was often cracked because of the immature technology. Therefore, magnesium hydroxide was used as the sheath instead of copper sheath.
So (T) is changed to (b), and B is mineral.
The product structure consists of copper strand, mica tape insulation, irradiation crosslinked polyethylene water barrier, magnesium hydroxide fire protection sheath and low smoke halogen-free sheath.

Bbtrz, as a series of flexible mineral insulated cable models, has been highly recognized by many design institutes, owners and users, and has been widely used in various large-scale building fire lines in China.
Although the fire resistance tests of C, W and Z of the circuit integrity of bs-6387 can be passed, the mineral insulated cable does not use metal sheath such as () copper).
With the development of national comprehensive level, the structure of non combustible cable of mineral insulation is officially defined as “composed of copper core, mineral insulation material, copper and other metal sheath” in the code for fire protection design of buildings (GB 50016-2014) promulgated and implemented in 2015.
Because the series does not contain metal sheath, it has gradually withdrawn from the stage of fire protection application, and is no longer classified as “mineral insulation non combustible cable”.
4. Flexible cable named after [bttrz, yttw, rttz]
In fact, the three named Mi flexible cables have the same structure, all of which are called flexible rolled copper sheathed mica tape mineral insulated cables.
The product structure consists of copper pipe sheath composed of multi strand copper strand, mica tape mineral insulation wrapping, alkali free glass fiber dense filling and copper strip longitudinal welding.
picture
Before the relevant domestic standards were not issued, the bttrz flexible cable, which was developed and named by the manufacturer, was mainly different from bttz rigid cable, which highlighted (soft), so it was added (R) on the basis of bttz naming.
Later, the domestic construction industry standard: jg/t 313 was promulgated and implemented in 2011, and bttrz named by the manufacturer was changed to yttw. However, this naming rule is not in line with the domestic cable naming method, and many people can not accept it.
Later, the national standard was promulgated and implemented by the National Standardization Commission in 2017: gb/t 34926, which named the former flexible cable rttz.
So bttrz = yttw = rttz. In addition, before the relevant standards were issued, the flexible cables were named by different manufacturers: bttw, Btte and btwtz, which can be described as various.
It is obvious that the flexible cables meet the requirements of gb50016-2014, and the fire resistance type test meets the most stringent requirements of C, W and Z of bs-6387.
5. Flexible cable named (ng-a, btly)
Ng-a and btly series cables belong to the same category. They use aluminum as the main material metal pipe extrusion instead of copper tube pull sheath, which not only simplifies the production process and improves the efficiency, but also reduces the product cost (aluminum is only 1/10 of the comprehensive cost of copper).
The product structure consists of copper strand, mica tape insulation, seamless aluminum tube sheath, oxygen insulation layer filling, isolation refractory layer and low smoke and halogen-free sheath.
picture
The reason why aluminum tube can replace copper pipe is not melted under high temperature flame, which is due to the expanded refractory layer extruded by the outer part of the aluminum pipe.
It is foamed and solidified under the fire, forming a thick barrier to block the direct effect of flame on aluminum pipe. The insulation stability of mica tape can be improved (the insulation resistance will decrease with the increase of temperature).
According to the structure form and type test, it also meets the requirements of grade C, W and Z of gb50016-2014 and bs-6387. The insulation performance of mica tape inside the insulation layer (Mg (OH) 2 or Al (OH) 2 mineral material) on the outer layer of aluminum sheath has been improved.
However, ng-a (btly) series cables are not flexible cables specified in jg/t 313 and gb/t 34926 standards, and only enterprise standards are supported at present.
6. Summary
The above Mi cables are simply combed as follows:
(1) Rigid cable: BTT, structure form and product performance are in accordance with international and domestic specifications.
(2) Flexible cable: RTT and yttw, the fire resistance performance of the product meets the most stringent level tests of C, W and Z of bs6387, and the structure form meets the national specifications.
(3) Flexible cable: ng-a and btly, the fire resistance performance of the product meets the most stringent level tests of C, W and Z of bs6387, and the structure form meets the national standards, but only [enterprise standard] is supported.
From the above situation, the performance of flexible cable meets the requirements of the specification, and the length of a single cable is long and the construction is relatively easy. We will surely think, “can flexible cable replace rigid cable?”