Difficulties in locating cable faults

Due to changes in the environment in which Power Cables are laid and the application of new materials for cables and their accessories in the cables, it is increasingly difficult to locate faults on the power cable site.
The difficulty of field cable fault location is mainly reflected in the fault location of directly buried power cable lines. At present, despite the relevant classic technical literature and advanced fault detectors, it is still available for high resistance faults in power cable lines. However, when using a professional cable fault locator to locate on site, sometimes you will encounter some special and difficult faults that cannot be located. For example, using a professional cable fault locator, part of the creepage flashover fault on the insulation surface of the medium voltage cross-linked power cable terminals and intermediate joints, and accurately determining the metal short-circuit fault, often appears powerless or powerless.

For common cable faults, you can use a cable fault locator purchased on the market to determine the location of the fault point within minutes or hours. However, when you encounter special difficult faults and the detection is not stable, you may need to call multiple cable fault detectors with different functions to repeat the test, and take turns to detect, locate, compare and confirm. The types of these fault devices mainly include various cable fault detectors designed, manufactured and developed based on the principles of bridging method and wave method. In this way, it may take several days or even longer to locate the fault. In this way, if you are lucky, you can determine the location of the fault point. If you are unlucky, the location of the fault point is still uncertain.
In northern China, the ground freezes in winter, and the directly buried cables fail at this time. The actual fault detection and handling process is actually a difficult task. First of all, the cable fault locator used must have high accuracy, and secondly, the corresponding personnel must have a clear understanding of the actual cable laying route. Although some cable fault detection instruments are now equipped with cable path testers, they must also be equipped with cable path testers. Only on-site personnel who understand the approximate laying path of the cable can cooperate to improve the positioning accuracy. The actual handling of cable faults sometimes depends on man-made three points and machine-made seven points.


At present, there are many manufacturers selling cable fault detection instruments on the market, and there are many types of detection instruments, but in fact, it is impossible to locate all cable faults. In actual use, the instrument usually can only effectively locate one or several types of faults, but still cannot do anything about some faults. The current electric power department hopes to spend a lot of money to purchase a universal cable fault tester with complete functions and high positioning accuracy (including rough and precise measurement points) to quickly and effectively solve all actual cable faults. But it’s actually hard to buy. There are various updated cable fault detectors on the market. However, the actual on-site inspection will still encounter some technical problems that cannot be located using the cable fault table. I think the reasons are mainly from two aspects: First, the various insulation, filling and wrapping materials used in the cable and its accessories are constantly being developed and updated, which leads to continuous changes in the types of cable failures. The other is that the market demand for cable fault detectors is limited, and related R&D personnel are scarce, resulting in a delay in the start of portable, high-precision, intelligent and multi-functional cable fault detectors. It is believed that with the advent of the smart grid era and the rapid development of Aerial Bundled Cable ASTM B231 Standard fault detection technology, the location of cable faults will become very simple and easy.

Plastic Extrusion and Traction Speed of Cable Technology

The rate of plastic extrusion

According to the flow rate analysis of the viscous fluid in the material conveying and homogenizing section of the ABC conductor,  the flow rate of the plastic (that is, the extrusion speed) is proportional to the screw speed, and the screw speed is an important operating variable to characterize the extrusion speed in the extrusion process because of the convenience of adjustment. Therefore, in general, increasing the screw speed is an important means to improve the production capacity of modern plastic extruder and realize high-speed extrusion. But the analysis of the plastic melting length shows that the increase of the screw speed, on the one hand, increases the viscous dissipation heat due to the enhanced shear effect; On the other hand, in the absence of head pressure control, the screw speed increases, the flow rate increases, and the material stays in the machine time is shortened. Moreover, the influence of the latter is more than that of the former, and the normal extrusion process will be destroyed due to the extension of the melt length to the homogenizing section. Therefore, it is necessary to increase the screw speed to improve the extrusion speed, but also to increase the heating temperature or use the control of the head pressure to achieve the purpose.

Plastic extrusion speed or the quality of plasticizing is related to the use of plastic material and temperature control, the plasticizing temperature of various plastics is different. If you want to quickly extrude plastic, only the material is good, the temperature is appropriate, to achieve. In addition, the extrusion speed is closely related to the extrusion thickness. In the normal extrusion process, the amount of glue is large and the extrusion speed is slow. On the contrary, the extrusion speed is fast, on the premise of ensuring the quality, the extrusion speed can be appropriately improved.

Traction speed

Extruded products are dragged through the machine head by traction device, in order to ensure the quality of products, the traction speed is required to be uniform and stable, in coordination with the screw speed, to ensure the uniformity of extrusion thickness and outer diameter of products. If the traction speed is not stable, the extrusion layer is easy to form bamboo joints, and the extrusion thickness is large when the traction speed is too slow, and the phenomenon of glue stacking or empty pipe occurs. When the traction speed is too fast, it is easy to cause extruding and thinning, or even the phenomenon of degumming and leakage. Therefore, in the normal extrusion process, we must control the traction speed.

There are Six Materials for the Cable

We are commonly used are super five types of lines or six types of lines, although they are all cable, but the cable material is also divided into several kinds, most of them are in line with the national conditions of production!When buying a line, don’t be a pit!Don’t be greedy for cheap!

First, four iron four aluminum wire

Four iron four aluminum is one of the worst quality cable, transmission quality is poor, it is easy to appear attenuation.Resistance is about 50 Ω 100 m, up to transmit 60 meters.

The differences are as follows:

  1. Aluminum mesh wire, the inner layer of the wire skin without luster, A pull will be broken.
  2. The easiest way to do this is to use a magnet to attract the wire. If it is attracted to the wire, it proves that there is iron in it.
  3. The four wire cores of iron and aluminum are made of different materials. Compared with all copper, materials such as copper-clad aluminum are much harder.
  4. With fire, the hardness of iron is very high, with fire, will not be soft soon.

Two, copper clad copper wire

Copper coated copper wire refers to the bronze outside and coated with a layer of oxygen free copper, so it is also called bronze wire.This kind of wire is generally ignored by everyone, not many people understand it, the conductor of the bronze wire belongs to the secondary furnace copper, which contains more impurities.Resistance is about 100 meters, around 40 Ω can transfer about 80 meters, the oxidation resistance is strong, long service life.

The resistance of bronze is much higher than that of ordinary copper-aluminum and oxygen-free copper, so the effective propagation distance of copper-bronze is much less than that of copper-aluminum.However, the physical properties of copper is still relatively stable, not easy to oxidize, so the service life is longer, because the surface is plated with a layer of oxygen-free copper, the surface does not see any difference, but with fire, the oxygen-free copper layer on the surface of the copper will soon melt away.Generally speaking, 0.4 all-copper wire can only reach 80 meters, 0.45 all-copper wire can reach 90 meters, and 0.5 all-copper wire can reach 100 meters.

Three, copper coated aluminum wire

The material of copper cladding wire is an oxygen-free copper layer on the surface of the copper core of aluminum or aluminum/steel alloy core.Because aluminum conducts electricity less well than copper, the DC resistance of copper-clad aluminum conductors is greater than that of pure copper conductors.If used as a power supply, copper-clad aluminum conductors will incur additional power consumption and a much lower voltage.100 meters would be about 28 Ω resistance, to sell cheap and transmission can be roughly 100 meters, poor oxygen resistance, long service life.The tensile strength of copper-clad aluminum is lower than that of copper conductor.Do not pull the cable body vigorously during use.Why is copper-clad aluminum mesh wire more easily oxidized?Because copper and aluminum are two metals with different electrodes, in a humid environment, copper and aluminum also have a small amount of water vapor containing impurities to form a galvanic cell.Aluminum is the negative electrode, copper is the positive electrode, the metal aluminum is corroded, so it can not be placed in the place of high air humidity.So in the choice of cable, how to see their use is copper aluminum cable?Fire can be used, if the core of the wire is quickly softened (this identification method is very practical), it proves that the copper core of the wire contains aluminum.

Four, high conductivity aluminum mesh wire

High conductivity aluminum wire, its composition is not just copper silver composition, in fact, is a kind of copper aluminum silver alloy of high performance wire, usually also known as copper coated silver.Has a resistance is small, the characteristics of the transmission is far, compared with the copper, iron and aluminum cable, resistance would be about 20 Ω 100 meters, can transmit 120 meters, but the oxygen resistance is poor, life is not long.There are three main kinds of high conductivity aluminum mesh wire: copper coated aluminum mesh wire (aluminum in the middle, the surface is thin oxygen-free copper layer), high conductivity aluminum mesh wire (imported aluminum, copper coated surface), copper coated silver mesh wire (imported aluminum, full copper coated layer)

 

Five, copper coated silver cable

The main material of copper coated silver wire is imported aluminum, which is higher in purity than copper coated aluminum wire and high conductive aluminum wire. The inner and outer leather is made of new materials.Resistance would be about 15 Ω 100 meters, which can transmit 150-180 meters, but the oxygen resistance is poor, life is not long.Copper coated silver because it can be transmitted far, we can generally be used in ADSL, computer room, supermarket, warehouse such occasions, the general network transmission is no problem.Compared with the copper coated silver wire, the transmission speed is not so long, but it is not far away. It can also be used in supermarkets, warehouses, and families in small and medium-sized areas.There is also a small amount of aluminum in the copper – silver – coated wire, and aluminum is easily oxidized.With four or five years will probably be oxidized, so in the use of choice, to consider this factor.

We can see from the copper core wire skin, generally speaking, all copper wire is the outer new material, and oxygen free copper is inside and outside the new material.You can see from the luster of the leather and cut a section of the leather to see if it can be drawn. The bright luster is the new material, and the new material can be drawn.But now all copper wire also tends to inside and outside the new material, so it can only be used as a basic basis.

You can also use a knife to scratch a surface or cross section:

Oxygen-free copper wire: scrape off the surface of a layer, the color is basically no difference, the cross section color is uniform, no difference.Bronze wire: scratch off a layer of surface, surface and interior color difference, cross section has a little white spots.You can also use fire, copper surface coated with a layer of oxygen-free copper, in the burning process, you will find oxygen-free copper outer layer will melt, and then wipe with paper, will find that the copper core is difficult to change back to the original brass color;And oxygen-free copper does not contain impurities, in the burning process, only the surface oxidation, after wiping with paper, or restore yellow.

According to the standard, the oxygen content in the oxygen-free copper wire is not more than 0.003%, the total impurity content is not more than 0.05%, and the purity of copper is more than 99.95%.Therefore, oxygen-free copper has high conductivity, good processing performance, welding performance, corrosion resistance and low temperature performance, and is suitable for high quality wire material.Oxygen-free copper cable 10 Ω resistance is about 100 meters of less than, can transfer about 120-150 meters, the signal transmission distance, packet loss rate is low, so in high-definition monitor network engineering, network engineering and home decorating go dark lines are suggested to use oxygen free copper, network stability is guaranteed, the oxidation resistance is strong, long service life.

So here’s the problem:

The above mentioned 6 materials of network cable, two of which have a relatively high transmission distance, theory can transmit about 150 meters, copper coated silver and oxygen free copper, then which one is better?Under normal use environment, the service life of the copper coated silver wire is much shorter than that of the oxygen free copper wire. The antioxidant of the copper coated silver wire is very poor, and the longest can be used for 4-5 years. The oxygen free copper wire can be used for more than 10 years due to its high purity and high antioxidant.When using copper coated silver cable, it should be noted that whether to use copper coated silver or not depends on the service life.

A final note:

Referring to various material cable transmission distance, before mentioned their resistance, cable transmission distance has much to do with the resistance, the smaller the resistance, the smaller the loss, transport the farther the distance, can be seen from the above, oxygen-free copper cable resistance minimum, and resistance to oxidation, it is also the reason why we should choose in the actual project it.

How to choose the anti-theft alarm cable

The alarm system based on the professional bus form is currently the most widely used, and the aerial bundled cable SANS 1418  is also the most complicated, so I will focus on the discussion here.

RVV2*0.3 (signal cable) and RVV4*0.3 (2-core signal + 2-core power supply) cables are generally used between the front-end detector and the alarm controller, and the two aerial bundled cables ASTM B231 are generally used between the alarm controller and the terminal security center. As for the core signal wire, as for the use of shielded wire or twisted pair or ordinary sheathed wire, it needs to be determined according to the requirements of various brands of products. The thickness of the wire diameter is determined according to the distance and quality of the alarm controller and the center. First of all, determine the location of the security center and the distance between each alarm controller. The farthest distance cannot exceed the length specified by various brands, otherwise it will not meet the requirements of the bus; the entire alarm area is relatively large, the bus must not meet the requirements Under the conditions, the alarm can be divided into several areas. In each area, determine the installation location of the sub-control center to ensure that the bus in the area meets the requirements, and determine the location of the general management center and the sub-management center, and determine the sub-control center to the general management The communication method of the center is to adopt RS232-RS485 conversion transmission or adopt RS232-TCP/IP to use the integrated wiring system of the community for transmission or the management software of the sub-management center to use TCP/IP network to forward to the general management center.The power supply of the alarm controller generally adopts local power supply instead of centralized power supply in the control room. The line is relatively short. Generally, RVV 2×0.5” or more specifications can be used. Configure according to the actual line loss. Perimeter alarm and other public area alarm equipment power supply Generally, the centralized power supply mode is adopted and the line is longer. Generally, the specifications above RVV2*1.0″ are adopted, and the configuration is based on the actual line loss. The grounding of all power cable must be unified.

Different types of alarms (such as perimeter alarm, public area alarm bus and household alarm bus separately) should not use the same bus. The installation location of the junction box should be easy to operate, and the high-quality junction interface is used to handle the connection between the bus and the bus, which is convenient for maintenance. And debugging; it is recommended that the bus and other lines take charge of the wiring. The bus weakening bridge should be kept away from other lines according to the weak current standard to avoid causing unshielded non-twisted pair audio lines and other high and low frequency interferences such as video intercom systems.

Wire and cable common sense questions and answers

1. What are the requirements for cable piping?

Answer: (1) The distance from the top of the pipe to the ground is 0.2m in the workshop, 0.5m under the sidewalk, and 0.7m in general areas;

(2) Pipe pits should be installed at the change direction and branch, and pits should also be added when the length exceeds 30mm;

(3) The pit depth is not less than 0.8m, and the manhole diameter is not less than 0.7mm;

(4) The drain pipe should have a drainage slope of 0.5% to 1% inclined to the pit.

2. What are the requirements for the resistance of the cable conductor connection point?

Answer: The resistance of the connection point is required to be small and stable. The ratio of the resistance of the connection point to the conductor of the same length and the same cross-section should not be greater than 1 for the newly installed terminal head and intermediate head; This ratio should not be greater than 1.2.

3. What requirements should the design of cable joints and intermediate head meet?

Answer: The requirements that should be met are:

(1) High compressive strength and good conductor connection;

(2) High mechanical strength, low medium loss;

(3) Simple structure and strong sealing.

4. What is a cable fault? There are several common types?

Answer: Cable failure refers to the failure of the cable’s insulation breakdown during the preventive test or during the operation, which forces the aerial bundled cable 33-209 standard  to power out due to insulation breakdown, wire burnout, etc. Common faults include ground faults, short-circuit faults, disconnection faults, flashover faults and mixed faults.

5. How to deal with the single-phase ground fault of the cable line?

Answer: Generally speaking, the damage to the cable conductor is only partial. If it is a mechanical damage and the soil near the fault point is relatively dry, local repairs can generally be carried out and a fake connector is added, namely Without sawing the cable core, only the fault point is insulated and sealed.

6. What tests and inspections should be carried out on power cables before laying?

Answer: Before laying, check whether the type, specification and length of the cable meet the requirements and whether there is external force damage. Low-voltage cables use a 1000V megohmmeter to remotely measure the insulation resistance, and the resistance is generally not less than 10MΩ, and high-voltage cables are measured with a 2500V megger. The resistance is generally not less than 400MΩ.

7. What should be paid attention to when laying cables in the main workshop?

Answer: When laying cables in the main factory building, generally pay attention to:

(1) All control cables leading to the centralized control room should be laid overhead;

(2) 6KV cables should be laid in tunnels or pipes, and the high groundwater level can also be laid overhead or pipes;

(3) For 380V cables, tunnels, trenches or pipes should be used when the two ends of the cable are at zero meters. When one end of the equipment is on the top and the other is on the bottom, it can be partially overhead laid. When the local water level is high, it should be overhead.

8. Where are the inner and outer shielding layers of power cables? What material is used? What’s the effect?

Answer: In order to make the insulation layer and the cable conductor have better contact and eliminate the increase in the electric field strength of the conductor surface caused by the unevenness of the conductor surface, the conductor surface is generally covered with an inner shielding layer of metalized paper or semiconductor paper tape. In order to make the insulating layer and the metal sheath have better contact, generally the outer surface of the insulating layer is covered with an outer shielding layer. The material of the outer screen layer is the same as that of the inner screen layer, and sometimes copper tape or braided copper ribbon is tied outside.

9. Briefly describe the composition and performance of epoxy resin compound.

Answer: The epoxy resin compound is composed of epoxy resin added with hardener, filler, toughening agent and diluent. Has the following properties:

(1) Have sufficient mechanical strength;

(2) Excellent electrical performance;

(3) Stable electrical performance;

(4) It has sufficient adhesion to non-ferrous metals;

(5) Good corrosion resistance;

(6) When used outdoors, it is resistant to rain, light, and humidity.

Analysis and Engineering Application of Fireproof Performance of Mine Cable

Due to the increase of wires and cables, the concentration of laying, the poor quality of construction, etc., the danger of wire and cable fires has increased. Therefore, to prevent aerial bundled cable astm b231 fires in actual engineering applications, it is necessary to start with controlling dangerous factors, apply relevant regulations, and take corresponding fire prevention measures.

1 Fire causes and characteristics of wires and cables

Mainly because of overload, short circuit, excessive contact resistance and external heat source. Causes of fire caused by wires and cables under fault conditions such as short circuit, local overheating and external heat. Insulation materials decrease in insulation resistance, lose insulation ability, or even burn, which may cause a fire. The main characteristics of wires and cables in fire are:

In case of fire, the fire temperature is generally 800℃~1000℃. Wires and cables will quickly lose their insulating ability, which will cause secondary electrical accidents such as short circuits and cause greater losses;

2 The conductor cable has a larger overload capacity under the specified allowable current carrying capacity;

The wire and cable will cause the insulation material to melt and burn in an instant, 3 short-circuit conditions. And ignite the surrounding combustible materials.

2 Analysis of fire performance of aerial bundled cable (abc) astm b231 standard 

2.1 Analysis of fire protection mechanism

2.1.1 Flame retardant mechanism

The flame retardant in the condensed phase decomposes and absorbs heat, and is under the heat of the combustion reaction. Slow down the temperature rise in the condensed phase and delay the thermal decomposition rate of the material;

The chain reaction free radical blocker is released, and the flame retardant is decomposed by heat. Interrupt the branches of flame and chain reaction, and slow down the speed of gas phase reaction;

The formation of the coking layer or foam layer strengthens the effect of these layered hard shells in hindering heat transfer; 3 catalyzes the thermal decomposition of the solid phase products of the condensed phase.

The flame retardant has an endothermic phase change, under the action of 4 heat. Physically prevent the temperature rise in the condensed phase.

2.1.2 Fire resistance mechanism

To reduce the heat generated by the polymer, a certain additive is added to the insulation and sheath materials of the wire and cable. Prevent polymer decomposition or promote the carbonization of insulation and sheath materials to form a maintenance layer;

After the insulation and sheath layer is eroded by fire, a layer of mica glass ribbon and other inorganic insulating materials are added to the 2 cores. *The mica refractory tape wrapped around the conductor is maintained and continues to be energized, so as to maintain normal operation for a certain period of time when a fire occurs.

2.1.3 Mechanism of mineral insulated cables

AlOH3 is 34.6% under high temperature operation, and the absorption effect of metal hydrate makes the cable flame-retardant. For example: use AlOH3 and MgOH as flame retardants. MgOHz is 31%, see Reaction Equation 1 and Reaction Equation 2, the reaction is decomposed into an endothermic reaction, which can inhibit the burning of polymers. 2A IOH3–*A lz03+3H20-2648KJ1MgOH2–MgO+H20-93.3KJ2

2.2 Classification of burning characteristics of wires and cables and their standard tests

It can be divided into ordinary wire and cable, flame-retardant wire and cable, fire-resistant wire and cable, halogen-free low-smoke wire and cable, and mineral insulated cable. Wire and cable according to their own burning characteristics.

1 Flame-retardant wires and cables refer to wires and abc cables b230 that are difficult to catch fire and have the ability to prevent or delay the spread of flame. The commonly used standard test is GBT18380. 3 is equivalent to IEC60332-1999

Wire and cable that can still maintain the integrity of the circuit. The commonly used standard test is GBT12666. 6 which is equivalent to IEC60331-21-19992 fire-resistant wire and cable means that it burns under a flame at a specified temperature and time.

When burning, it produces less smoke and has wires and cables that prevent or delay the spread of flames. Commonly used standard tests are GBT17650. 2 is equivalent to IEC60754-2, GBT17651. 2 is equivalent to IEC61034-2 and GBT18380. 3 is equivalent to EC60332-3. On the basis of the above, the flame-retardant and fire-resistant type also need to meet the requirements of maintaining the integrity of the line. 3 There are two types of halogen-free and low-smoke wires and cables. Flame-retardant type means that the material does not contain halogen. At the same time, the commonly used standard test adds GBT12666. 6 is equivalent to IEC60331

Wire and cable products are mainly divided into five categories

1. Bare wires and bare conductor products

The main features of this category of products are: pure conductive metal, no insulation and sheath, such as steel core aluminum stranded wire, copper-aluminum busbar, electric locomotive wire, etc.; processing technology is mainly pressure processing, such as melting, rolling, drawing The products are mainly used in suburbs, rural areas, user main lines, switch cabinets, etc.

2. Power cable

The main features of this type of product are: squeeze (wind) the insulation layer outside the conductor, such as overhead insulated cables, or stranded several cores (corresponding to the phase, neutral and ground wires of the power system), such as overhead insulated cables with more than two cores , Or add a sheath layer, such as plastic/rubber wire and cable. The main process technologies include drawing, stranding, insulation extrusion (wrapping), cable formation, armoring, sheath extrusion, etc. The different process combinations of various products have certain differences.

The products are mainly used in the transmission of strong electric energy in power generation, distribution, transmission, transformation, and power supply lines, with large current (tens of amperes to several thousand amperes) and high voltage (220V to 500kV and above).

, Wire and cable for electrical equipment

The main features of this type of products are: a wide range of varieties and specifications, a wide range of applications, with more voltages of 1kV and below, and constantly deriving new products in the face of special occasions, such as fire-resistant cables, flame-retardant cables, low smoke and halogen free Smoke and low halogen cables, termite-proof, mouse-proof cables, oil-resistant/cold-resistant/temperature-resistant/wear-resistant cables, medical/agricultural/mining cables, thin-walled cables, etc.

4. Communication cables and optical fibers (a brief introduction)

With the rapid development of the communications industry in the past two decades, products have also developed at an astonishing speed. From the simple telephone and telegraph cables in the past, it has developed to thousands of pairs of voice cables, coaxial cables, optical cables, data cables, and even combined communication cables.

The structure size of this kind of product is usually small and uniform, and the manufacturing precision is high.

5. Magnet wire (winding wire)

Mainly used in various motors, instruments, etc.

Derivatives/New Products of Wire and Cable

Derivatives/new products of wire and cable are mainly due to different application occasions, different application requirements, convenience of equipment and requirements of equipment cost reduction, etc., and the use of new materials, special materials, or changing product structure, or improving process requirements, or different Varieties of products are combined.

Use different materials such as flame-retardant cables, low-smoke zero-halogen/low-smoke abc cables, termite-proof, mouse-proof cables, oil/cold/temperature-resistant cables, etc.;

Change the product structure such as: fire-resistant cable, etc.;

Improve process requirements such as: medical cables, etc.;

Combination products such as OPGW, etc.;

It is convenient to install and reduce equipment costs, such as prefabricated branch cables.