How to Improve Pass Rate of Fire-resistant Cable Fire Test

As fire-resistant cables are now more and more widely used, many manufacturers are producing them, but often the quality cannot be guaranteed. Therefore, under normal circumstances, when companies develop fire-retardant cable products, they first try to manufacture a section of the product, send it to the relevant national inspection agency, and then start mass production after obtaining the test report. A few cable manufacturers have established their own fire-resistant test inspections. As everyone knows, the fire resistance test is an inspection of the process results of the cables produced. The same process plan and the performance of the cables produced in different periods have certain differences. For companies that produce fire-resistant cables, it is like the fire-resistant test of the fire-resistant cable. With a pass rate of 99%, the fire-resistant cable has a 1% safety hazard, which is 100% dangerous for users. So how to improve the pass rate of fire-resistant cable fire-resistant experiments, from the raw materials, conductor selection, production process control and other aspects to explain1. There are three types of mica, synthetic mica, phlogopite, and muscovite. The quality performance of each is the best synthetic mica and the worst muscovite. For small-sized power cables, synthetic mica tape must be selected for wrapping, and mica tape cannot be layered. Use, long-term storage of mica tape is easy to absorb moisture, so the temperature and humidity of the surrounding environment must be considered when storing mica tape.

2. When selecting mica tape wrapping equipment, it should be used with good stability. The wrapping angle is best to wrap around 300-400. The mica tape is wrapped evenly and tightly. All guide wheels and rods that touch the equipment must be smooth. The wiring is neat, the tension is not too large, and the side plate and cylinder of the take-up tooling wheel are flat and smooth.

3. For the circular core with axial symmetry, the mica tape is wrapped tightly in all directions, so the conductor structure of the fire-resistant cable should adopt a circular compact conductor. The reasons are as follows:

①Some users propose that the conductor is a stranded flexible cable, which requires the company to communicate with users from the reliability of the cable to a round compact conductor. The soft structure of the bundle and the twisting can easily cause damage to the mica tape as a fire-resistant Cable conductors are not desirable, but some manufacturers think that users need what kind of fire-resistant cables, and manufacturers should meet user needs. I think users do not fully understand the details of cables after all. Cables are closely related to human life. Yes, cable manufacturers must clarify related technical issues to users.

②Sector-shaped conductors are not suitable, because the pressure of the mica tape of the fan-shaped conductor is unevenly distributed. As shown in the figure, it can be seen that the pressure at the three fan-shaped corners of the fan-shaped core around the mica tape is the largest Yes, because mica is a sheet-like silicate polymer, the molecular attraction between the layers is much weaker than the s1-0 covalent bond in the crystal. The layers are easy to slide and adhere to silicon, but the bonding strength is also Low, it is easy to fall off and split when external force is scraped and squeezed, especially when the fan-shaped structure is adopted, the wrapped wire core passes through the guide wheel, the wire splitting rod and the wire to the edge of the side plate of the tooling wheel, and the rear track When the process extrudes the insulation into the core, it is easy to be scratched and bumped, which leads to a decrease in electrical performance. In addition, from the perspective of cost, the section perimeter of the sector-shaped conductor structure is larger than the section perimeter of the circular conductor, which increases the precious material mica Belt, although the outer diameter of the circular structure cable has increased. The amount of polyvinyl chloride sheathing material has increased, but the product material is compared with the total cost, and the overall cost of the circular structure cable is still saved. Based on the above description, from the technical and economic analysis, it is best to use a circular structure for the conductor of the fire-resistant power cable.

Treatment of Damp XLPE Power Cable

In urban power grid renovation projects, cables, especially XLPE insulated power cables, have been widely used. However, due to the particularity of the cable, there are special requirements for the installation, operation and maintenance of the cable. Moisture or water ingress of the cable reduces the insulation resistance of the cable, which is an important aspect of the operation of the cable line.

Causes and hazards of damp and water in cables:

(1) When the new cable is shipped from the factory, both ends of the cable are sealed with plastic sealing sleeves. However, after a section is used according to the actual situation at the construction site, the remaining part is simply wrapped with plastic cloth. Placed in the open air and poorly sealed, over time, water vapor will inevitably seep into the cable.

(2) When laying cables, they need to cross roads, bridges and culverts frequently. Due to weather or other reasons, a lot of water often accumulates in the cable trenches. During the laying process, it is inevitable that the cable heads will be immersed in water. , Because the plastic cloth is not wrapped tightly or damaged, so that water enters the cable; in addition, the outer sheath or even the steel armor cable is sometimes scratched when pulling and threading. This phenomenon is particularly prominent when using mechanical traction.

(3) After the cable is laid, the cable head cannot be made in time due to the constraints of the site construction conditions, so that the unsealed cable fracture is exposed to the air for a long time, or even immersed in water, causing a large amount of water vapor to enter the cable.

(4) In the process of making cable heads (including terminal heads and intermediate joints), due to the negligence of the construction personnel, the newly processed cable ends may accidentally fall into the stagnant water on site.

(5) In the normal operation of the cable, if a breakdown or other fault occurs for some reason, the water in the cable trench will enter the cable along the fault point; in civil construction, especially when using large construction machinery It is not uncommon for cables to be damaged or broken down due to various human factors on construction sites in China. When such an accident occurs, the cable insulation is severely damaged, and water can enter the cable.

After the cable enters the water, under the action of the electric field, the aging phenomenon will occur, and finally the cable will break down.

Five Main Types of photovoltaic power systems cable

1. Power cable: NH-VV
NH-VV copper core PVC insulated PVC sheathed fire-resistant power cable. Suitable for rated voltage 0.6/1KV.
Use characteristics: Long-term allowable working temperature is 80℃. Allowable bending radius during laying: single-core cables are not less than 20 times the cable outer diameter, and multi-core cables are not less than 12 times the cable outer diameter. When the cable is laid, the ambient temperature is not lower than 0 degrees Celsius without pre-heating. Voltage laying is not limited by the drop.
Laying: Suitable for occasions with fire resistance requirements, and can be laid indoors, in tunnels and trenches. Note that it cannot withstand external mechanical forces and can be buried directly in the ground.

2. Control cable: ZRC-KVVP
ZRC-KVVP copper core PVC insulated PVC sheathed braided shielded control cable. It is suitable for control, monitoring circuit and protection circuit of AC rated voltage 450/750V and below.
Features: Long-term allowable use temperature is 70℃. The minimum bending radius is not less than 6 times the outer diameter.
Laying: Generally, it is laid in indoor, cable trench, pipeline and other fixed places that require shielding and flame retardancy.

3. Communication cable: DJYVRP2-22
DJYVRP2-22 polyethylene insulated PVC sheathed copper wire braided shielded armored computer special soft cable, suitable for electronic computers and automation connection cables with high requirements for interference prevention at rated voltages of 500V and below.
Features: DJYVRP2-22 cable has the characteristics of oxidation resistance, high insulation resistance, good voltage withstand, and low dielectric coefficient. While ensuring service life, it can also reduce crosstalk and external interference between loops, and has high signal transmission quality. The minimum bending radius is not less than 12 times the cable outer diameter.
Laying: The cables are allowed to be fixedly laid and used under the conditions of ambient temperature of -40℃~50℃. Laying indoors, cable trenches, pipelines and other places requiring electrostatic shielding.

4. Communication cable: RVVP
Copper core PVC insulated PVC sheathed insulated and shielded flexible cable RVVP, also known as electrical connection anti-interference flexible cable, is a communication cable suitable for anti-interference, safe and efficient data transmission such as alarms and security.
Features: Rated working voltage 3.6/6KV middle voltage cable , long-term working temperature of cable conductor is 90℃, minimum allowable bending radius is 6 times of cable outer diameter. It is mainly used as a communication cable for anti-interference. Laying: The RVVP cable cannot be exposed to sunlight, and the bottom core must be well grounded. The weak current circuit communication cables that need to suppress the intensity of electrical interference should be laid in steel pipes and boxes. When laying parallel to the power cable, the distance should be kept away as far as possible.

5. RF cable: SYV
Solid PE insulated PVC sheathed radio frequency coaxial cable SYV.
Features: The commonly used video cables in monitoring are mainly SYV75-3 and SYV75-5. If you want to transmit video signals within 200 meters, you can use SYV75-3, and if you want to transmit video signals within 350 meters, you can use SYV75-5.
Laying: It can be laid through a pipe.

 

Photovoltaic cable: PV1-F1

Photovoltaic cable: PV1-F1*4mm2
The cable from the string to the combiner box is generally used: photovoltaic cable PV1-F1*4mm2.
Features: The photovoltaic cable has a simple structure. The polyolefin insulation material used in it has excellent heat resistance, cold resistance, oil resistance, and ultraviolet resistance. It can be used under harsh environmental conditions and has a certain mechanical strength.
Laying: It can be protected by passing through the pipe, and the component bracket is used as the channel and fixing for cable laying to reduce the impact of environmental factors.

Power cable: ZRC-YJV22
Steel tape armored flame-retardant cross-linked cable ZRC-YJV22 is widely used in: combiner box to DC cabinet, DC cabinet to inverter, inverter to transformer, transformer to power distribution device, power distribution device to the power grid connecting cables.

The nominal cross-sections of ZRC-YJV22 cables commonly used in photovoltaic power generation systems are: 2.5mm2, 4mm2, 6mm2, 10mm2, 16mm2, 25mm2, 35mm2, 50mm2, 70mm2, 95mm2, 120mm2, 150mm2, 185mm2, 240mm2, 300mm2.
Features:

 

(1) The texture is hard, the temperature resistance grade is 90°C, it is easy to use, has the characteristics of low dielectric loss, chemical corrosion resistance and no drop limit for laying.
(2) It has high mechanical strength, good environmental stress resistance, good thermal aging performance and electrical performance.

Laying: It can be buried directly, suitable for fixed laying, and adapt to the needs of different laying environments (underground, water, trench pipes and tunnels)

The selection of cable path should meet the following requirements

(1) Avoid the cable from being exposed to mechanical external force, overheating, corrosion and other hazards.
(2) Make the cable shorter when meeting safety requirements.
(3) Easy to install and maintain.
(4) Avoid places that will be excavated
(5) The cable should meet the requirements of the allowable bending radius of the cable in any way of laying and all path conditions of the cable.

What are the differences between YJV3+2 and YJV4+1 cables?

The YJV power cable  is composed of one or several soft wires, and the outer covering is covered with a light soft sheath; the cable is composed of one or several insulated wires, and the outside is covered with a tough outer layer made of metal or rubber.

As we all know, there are single-core cable and multi-core distinctions in wire and cable. Single-core cable means that there is only one conductor in an insulating layer, and multi-core means that there are multiple conductors in an insulating layer. Then there will be doubts from users who don’t know. 3+2 cores and 4+1 cores are both 5 cores. What is the difference between them? YJV3+2 and YJV4+1 cables are wires and cables.  In fact, these two kinds of wires and cables have their own characteristics.

The 3+2 and 4+1 on the YJV power cable indicate the number of cores of a cable. There are 2 or 3, 3+2, 4+1, and 5 wires that are insulated from each other in a cable, respectively Called 2-core or 3-core, 3+2 core, 4+1 core, 5 core wire.

What is the difference between YJV3+2 cable and YJV4+1 cable in the model name?

1.YJV 3+2 core

3 of the 3+2 cable is the three phase wires (L1, L2, L3 phase wires, the diameter of these three wires is large), the diameter of the wires is the same, 2 is the neutral wire and the PE wire, and one wire is used to connect the neutral wire (N in The diameter of this wire is smaller than L1, L2, L3), and one wire is used to connect to the grounding wire (PE grounding protection wire, this diameter is also smaller than L1, L2, L3).

2.YJV 4+1 core

The 4 of the 4+1 cable means that the diameter of the three phase wires is the same as the diameter of the neutral wire, 1 means that the PE ground wire has a single diameter; 4+1 means that a cable consists of 4 wires, one of which is used to connect to the live wire (L1 phase wire). ), one wire is used to connect to the neutral wire (N neutral wire), and one wire is used to connect to the ground wire (PE grounding protection wire).

The above is about the structure difference between YJV3+2 cable and YJV4+1 cable, and the difference in naming. We all know that cables are named based on models, and different models of cables are different in nature, and each model of cable has its own characteristics.

Heating reason of power cable in operation

When the power cable passes through a certain load current, it will be heated. With the increase of the load current, the cable surface temperature will be higher. If it is not handled in time, the consequences can be imagined. For example: PVC cable, the core temperature of 70 degrees is considered as the upper limit, and the surface temperature will be 5-10 degrees lower. Therefore, the cable surface temperature below 60 ℃ is basically safe. Considering the power supply maintenance, of course, the lower the temperature, the better.

The heating reasons of power cable in operation are as follows:

1. Cable conductor resistance does not meet the requirements, resulting in cable heating in operation.

2. Improper selection of cable leads to too small conductor cross-section of the cable used and overload phenomenon in operation. After long-term use, the imbalance of heating and heat dissipation of the cable causes heating phenomenon.

3. If it is too close to the cable, it may cause heat dissipation when the cable is too close to the cable.

4. Poor joint manufacturing technology and loose crimping will lead to excessive contact resistance at the joint and also lead to cable heating.

 

5. The insulation performance of cable phase to phase is not good, resulting in low insulation resistance and heating phenomenon during operation.

6. The partial sheath damage of armored cable will cause slow damage to the insulation performance after water inflow, resulting in the gradual decrease of insulation resistance, and also the heating phenomenon during the operation of the cable.

If the cause is not found out and the fault is eliminated in time, the insulation thermal breakdown will occur after the power cable is continuously energized. This will lead to phase to phase short-circuit tripping of cables, which may cause fire in serious cases.

Treatment method of PVC power cable affected by damp

Cables, especially PVC insulated power cables, have been widely used in urban power grid reconstruction projects. However, due to the particularity of the cable, the installation, operation and maintenance of the cable have special requirements. Cable insulation resistance is reduced by damp or water intrusion, which is an important aspect of cable line operation accidents.

Cause and harm of cable damp and water inflow

PVC power cable affected by damp  (1) When the new cable is delivered from the factory, both ends of the cable are sealed with plastic sealing sleeve. However, after a section of cable is used at the construction site according to the actual situation, the remaining part is simply wrapped with plastic cloth. As it is usually placed in the open air and the sealing is not good, water vapor will inevitably seep into the cable after a long time.

(2) During power cable laying, it is necessary to often cross roads, bridges and culverts. Due to weather or other reasons, a lot of water is accumulated in the cable trench. During the laying process, it is inevitable that the cable head will be immersed in the water, and the water will enter the cable due to loose wrapping or damage of plastic cloth. In addition, the outer sheath and even steel armor will sometimes occur during traction and pipe threading Scrape phenomenon, when using mechanical traction, this phenomenon is particularly prominent.

(3) After the completion of cable laying, due to the limitation of site construction conditions, the cable head can not be made in time, so that the cable fracture without sealing treatment is exposed to the air for a long time, even immersed in water, so that a large amount of water vapor enters the cable.

(4) During the manufacturing process of cable head (including terminal head and intermediate joint), due to the negligence of construction personnel, the newly treated cable end sometimes falls into the water on site.

(5) In the normal operation of the cable, if the breakdown and other faults occur for some reason, the water in the cable trench will enter the cable along the fault point; in the civil construction, especially in the construction site using large construction machinery, the cable damage or breakdown accidents caused by various human factors are also common. When this kind of accident occurs, the cable insulation will be seriously damaged, which will also cause cable water intrusion.

 

After the cable enters water, the aging phenomenon will occur under the action of electric field, which will eventually lead to cable breakdown.

WTTEZ cable performance characteristics

First, fire prevention, fire resistance is very superior
In order to ensure the normal power supply of the cable in flame, that is, to keep the cable structure stable under fire conditions, a layer of coating with fire resistance and expansion performance was added in the cable structure design. The volume expansion of the coating layer under flame conditions makes the cable insulation structure more stable. Therefore, this kind of cable can continue to maintain power supply in the case of fire near the melting point of copper, which is a real sense of fire prevention cable.
Continuous production with long length, large section and good bending performance
1. Long continuous production length
The production length of the cable is not limited by the processing technology and production equipment, and completely meets the demand of power supply length. The longest can be produced up to 2000 meters.
2. Large section
The cable conductor is made of multi-strand single wire twisted, which can greatly increase the flexibility of the conductor. In addition, the twisted conductor has good production continuity and long processing length, which can reduce the number of joints. The section of large single-core cable can reach 800mm2.
3. Good bending performance
The copper sheath is processed to increase the softness and bending performance of the cable.


4. Heavy load flow
When the cable is normal, the conductor temperature can reach 180℃. Under the premise of the same conductor cross section, its carrying capacity is much higher than that of conventional cable.
5. Waterproof
The cable is protected by sealed non-magnetic metal tube, and its waterproof property is very superior.
6. Explosion-proof
Because the cable adopts copper sheath, and after the cable burns, the filler layer expands and compacts the cable core tightly, the flammable gas and flame cannot reach the electrical equipment connected with the cable, so the cable has explosion-proof characteristics.
7. Good corrosion resistance
Due to the good corrosion resistance of copper, no additional protection is required under normal use environment. In special environmental conditions, such as the environment with strong corrosion effect on copper, the corrosion resistance can be achieved by adding a layer of low-smoke halogen-free outer protection on the cable.
8. No electromagnetic interference
Under the shielding of copper sheath, the cable will not disturb the signal and control line when it is laid together with information and control line.

Insulated cable

At present, there are some cables with better temperature characteristics in China, such as:
1. Rubber insulated cables:
The long-term operating temperature of WYJHDP wire core for rubber insulated cables in the field shall not exceed -45 ~ 50℃.
WYJHQ, a mobile rubber insulated cable used in the field, operates at -45 ~ 50℃. The operating temperature of the core shall not exceed 65℃ for a long time.
Rubber insulated cold – resistant cable for field use YHD operating environment is -50 ~ 50℃. The operating temperature of the core shall not exceed 65℃ for a long time.
2. Fluoroplastic insulated cables:
The service environment of AF-200 of fluoro-46 insulated installation line is -60 ~ 200℃.
Copper conductor outdoor PVC insulated wire BVW (NC-BV), laying temperature not less than -20℃.
Ff4-2, FF4H3-2 and other silver-plated copper ptFE insulated wire and cable are fixed and perfunctory, with rated voltage of 600V; Rated temperature: 200℃; The minimum operating environment is -65℃.
Silver plated copper core polyfluoroethylene propylene insulated wire and cable such as FF46-2, FF46P21H6-2, rated voltage of 600V; Rated temperature: 200℃; The minimum operating environment is -65℃.


3. Polyethylene cables
Polyethylene insulated coaxial RF cables, such as SYV-75-1.5-1 and SYV-75-2.5, are used at temperatures ranging from -40 ~ +65℃. Rated voltage: 300V; The temperature of installation and laying should not be lower than -15℃.
BV, BYR, BYR, BYP, BYVP (for fixed laying) RY, RVP, RYVP (for mobile use) can be used under 98% relative humidity at -40 ~ +60℃.
4. PVC cable
Avp-1 and AVP-3 are used for connecting telecommunication equipment with PVC insulated and shielded installation cables. The ambient temperature is -40 ~ 70℃ and the installation temperature is not lower than -15℃.
The lowest operating temperature of VV and VCY of copper conductor PVC insulated and sheathed power cable is -40℃, and the highest allowable operating temperature is 70℃. The ambient temperature for cable laying should be no less than 0℃.

Cable test methods and procedures

The fan Angle needs to be adjusted by the yaw driver according to the wind direction. Power, control and communication cables bend along the horizontal axis or rotate along the vertical axis. This makes the torsional flexure more demanding and requires more attention. Although there are currently no standards or regulations regarding torsional flexure, end users generally seek to have the cable tested in some way before it is put into service.
The following is a general test method used by end users in the cable industry.
(1) Torsional stress test of a single cable at low temperature (-40℃) :
The top of a 10-meter long vertical suspension cable sample is secured, and the bottom is bound to a rotating device. First, turn the cable clockwise 4 times (+1440o), then counterclockwise 4 times to return it to its original position. Then turn the cable counterclockwise 4 times (-1440o) and clockwise 4 times to return it to its original position. The whole process was repeated 5,000 times to simulate 20 years of use. If the cable is not broken or cracked after 5 minutes at 2.5U0, the cable passes the test.
Note: U0 can be 600, 1000, or 2000V depending on the voltage class of the cable.


(2) Torsional stress test of a bundle of cables
The test procedure is the same as (1) except that the cable bundles are replaced.
Wind turbines operate in harsh environments, which generally have a wide temperature range (about -40℃ to 50℃) and are exposed to extremely strong ultraviolet radiation. Therefore, to achieve the expected service life, the special cable used needs to be able to withstand the low temperature of -40℃ and can resist ultraviolet radiation. For the moving parts in the fan, the cable should have excellent torsion and bending flexibility, and have a small bending radius. Cables also need to be resistant to fuel, refrigerant, oil, corrosive chemicals and wear. If the wind field is onshore or offshore, the cables must also be resistant to erosion by highly saline water. For safety reasons, in addition to the above requirements, the cable is also required to be flame retardant. In some cases, other properties such as low smoke, zero halogen (LSZH) materials, and EMI protection are also required.