Cable

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Cable

  1. 1. Cable 1. Introduction Electric wires and cables have become such an important part of everyday life that without them the world as we know it would simply not exist. For without wires and cables the existence and operation of conveniences such as electric lights, telephones, computers and a host of other household appliances would not have been possible. Moreover, as the standard of living rises, so does the demand for those types of products. Consequently, there has been an incredible increase in the demand for electric wire and cable. As developing nations around the world continue to develop, this demand will undoubtedly continue to rise. For example, Taiwan, the Republic of China, a country with a population of only 19 million, has more than one hundred factories successfully producing electric wire and cable to satisfy the needs of the domestic market. According to an estimate made in 1984,the total production of electric wire and cable had reached a level of 200,000 tons per year. Furthermore, the vast majority of this cable was purchased domestically by retailers, manufacturers, construction contractors, and the government owned power and telephone companies. Clearly, the establishment of an electric wire and cable making plant is a project worthy of investment. The wire and cable making plant described in this particular proposal is designed for the production of wire and low voltage (below 600V) power cable. It is not intended to be used for the production of telecommunication or high voltage power cable, as the plants capable of producing these types of cable are considerably more expensive and require a higher level of technical knowledge to set up. The types of wire and cable which can be produced by the machinery I. II. outlined Single wire: in this PVC proposal insulated are single classified copper as follows: conductors. Multiple wires: PVC insulated copper conductors consisting of 7-61 stranded wires. III. Flexible wire: single or twin core, PVC insulated cords consisting of 20-100 fine copper wires. IV. Flat twin-cord wire: twin core, PVC insulated single or multiple copper conductors, sheathed V. with PVC layers. Power cables: three of four cores of PVC insulated multiple copper conductors assembled
  2. 2. together. VI. Armour cable: power cables consisting of three or four round and shaped cores armored with steel wires and sheathed with PVC layers. 2. General Process Information 2.1. Process (1) Description Drawing Bulk copper is formed into wire of varying diameters by drawing it through a series of dies. (2) Annealing Since the drawing process causes the copper to become hard and brittle, it should be annealed. (3) Stranding Anywhere from 20-100 (very fine copper conductor wires) are twisted into cords which will be used in making flexible wire and cable. (4) Twisting Layers of wires (1+6+12+18+24 etc.) are stranded together to make copper conductors. The maximum nominal cross-section area of a power cable core is 500m㎡. (5) Insulating The copper conductors, whether they are single wire or multiple stranded wire, are covered by PVC for current insulation. (6) Lay-up Three or four of these PVC insulated copper conductors are assembled into power cables. (7) Sheathing Complete cables are formed by sheathing twin-core or multiple-core PVC insulated copper conductors with PVC.
  3. 3. (8) Armoring Special purpose power cables must be surrounded with steel wires in order to increase the cable structure strength. *For Special Purpose Power Cable Only. 2.2. Flow Chart
  4. 4. 3. Plant Description 3.1. Production According to investor’s different situations, we classify the plant within three sizes. Plant scale Copper consumption Production capacity capacity
  5. 5. Mini plant 30 Tons/month 45 Tons/month Small plant 60 Tons/month 90 Tons/month Medium plant 120 Tons/month 180 Tons/month 3.2. - Raw - PVC grain materials (30% of the product weight) - - 8.0mm or 2.6mm diameter copper rod (70% of the product weight) In order to save money for the beginning investors, it is more economical to purchase 8mm (or 2.6mm) diameter copper rod and PVC grains as raw materials. These raw materials can be supplied by Taiwan. For the detail information, please contact the supplier in Taiwan. 3.3. Manpower Manpower Required Job Classification Mini plant Small plant Medium plant Staff 2 4 6 Technician 2 3 4 Skilled operator 8 10 16 Unskilled labor 8 13 24 Total 20 30 50 3.4. Machinery and (1) Machinery Plant scale Name of Machinery Mini Small Medium 8mmØ copper rod breakdown machine 0 1 1 Medium Wire drawing machine 1 1 2 Fine Wire drawing machine 2 4 8 equipment
  6. 6. Annealing machine 1 1 2 Flexible wire twisting machine 1 2 4 7-Wire stranding machine 1 2 2 19-Wire stranding machine 1 0 1 37 wire stranding machine 0 1 1 3-4 core cable lay-up machine 0 1 1 70mm PVC insulation machine 1 2 2 90mm PVC insulation and sheathing machine 1 1 2 120mm PVC sheathing machine 0 1 (2) Accessory - 0 Copper wire Copper wire - drawing dies welding devices Fork - equipment Lift Air Cooling systems Compressor for drawing lubrication oil - Drums and bobbins (3) Accessory - equipment Lathe - Milling machine - Drilling machine - Grinding machine - Grinding machine - Welding machine
  7. 7. 3.5. Inspection and testing equipment (1) Mega meter (2) Tensile tester (3) Heat (4) deformation Dielectric tester withstanding tester (5) Spark tester (6) Aging tester (7) Drawing dies polishing machine 3.6. Utilities (1) Requirement of water (KL/month) Mini plant Small plant Medium plant 1,500 2,200 3,000 (2) Power requirement capacity (KW) Mini plant Small plant Medium plant 1,000 1,500 3.7. (1) 2,500 Plant Abundant electric site power (2) supply Abundant (3) Convenient (4) Lower humidity and far from the seashore 3.8. Area of land and plant building (square meter) Item Mini plant Small plant Medium plant Land 2,500 3,200 5,000 Plant building 1,500 2,000 3,000 planning and water supply labor transportation
  8. 8. 3.9. (1) Plant Mini size layout plant layout Legend: 1. QC room 2. Conference room 3. Lobby 4. Office 5. Manager 6. room Raw material 7. storage Product 8. storage Packing 9. area Annealing 10. machine Medium drawing machine 11. 19B stranding machine 12. Fine drawing machine 13. Twisting machine 14. 7B stranding 15. 16. Maintenance 70mm PVC room insulation machine 17. 90mm PVC insulation and sheathing machine (2) Small size plant layout Legend: 1. QC room 2. Conference room 3. Lobby 4. Office 5. 6. 7. Manager Raw room material Products storage storage 8. Packing area 9. Breakdown machine
  9. 9. 10. Annealing 11. Medium machine drawing 12. machine Twisting machine 13. Fine drawing machine 14. 7B stranding machine 15. 37B stranding machine 16. Cable lay-up machine 17. 70mm PVC insulation machine 18. 90mm PVC sheathing machine 19. Maintenance room (3) Medium size plant layout Legend: 1. QC room 2. Conference room 3. Lobby 4. Office 5. Manager 6. Raw 7. rooms materials Products 8. storage Packing 9. Break 10. are down Annealing 11. Medium 12. Fine 13. storage machine machine drawing drawing Twisting machine machine machine 14. 7B stranding machine 15. 19B stranding machine 16. 37B stranding machine 17. 120mm 18. 19. PVC Cable 70mm insulation lay-up PVC insulation machine machine machine
  10. 10. 20. 90mm PVC insulation & sheathing machine 21. Maintenance room 4. U Suppliers Gear Automatic 5F,No,10,Lane315,chungshan TEL:886-2-2249-0999 URL: E-mail: ugten@ms19.hinet.net Rd.,Sec.2, Information Machinery Chungho, Taipei Co.,LTD Hsien, Taiwan,R. . O. C. FAX:886-2-2240-5083 http://www.A1A1A.com
  11. 11. Introduction In order to transmit high voltage power, there is a need to use a cable that has the necessary qualities for the transmission of large quantities of electricity. Overhead power line entails transmission of electricity using towers. Moreover, another way to transmit electricity is through utility poles. Overhead transmission method tends to be the most commonly used way of transmitting high voltage power because most of the insulation is provided by air. This method is less costly especially when transmitting large quantities of electricity. In order to accomplish this obligation, the most efficient cable to use is aluminum. This paper explores aluminum as the best cable to use in high voltage transmission lines and the process used in making the cable. Aluminum Cable Properties of Aluminum Physical and chemical properties of aluminum Some of the physical properties include that aluminum is a slivery white metal. The metal is also reflective to heat. Moreover, aluminum metal can be produced to various different forms with the help of machines. This means that the metal can have various surface finishes. Another physical property of aluminum is that it is easily recyclable. One of the chemical properties of aluminum is that it is resistance to oxidation. The other chemical property is that the metal is created using an electronic method. Good conductor of electricity Aluminum cable tends to be the best to use in high voltage transmission lines compared to cables made from other metals. One of the advantages of using aluminum cable is that it is a good conductor of electricity (Warne, 2005). This means that the metal has a low electrical resistivity. The reason why aluminum is a good conductor of electricity is that it has 3+ charges. This means that aluminum has three delocalized electrons that tend to move freely in each atom of the metal. The relationship between of electricity and aluminum is that when there is an electrical field
  12. 12. applied on the metal, every loose electron is able to move freely. This translates that all the loose electrons will move towards the positive terminal where there is presence of an electric field. Eventually aluminum ends up being a good conduct of electricity hence eligible to use in transmission lines (Warne, 2005). The metal is also a very good thermal conductor. Light in weight The other significant importance that makes aluminum to be the best cable while using in high voltage transmission lines is its lightweight. Comparing aluminum with other metals like copper, nickel and brass, aluminum tends to be less in weight to about one third of the others. The other aspect in relation to weight is that aluminum has a specific gravity of 2.7. This means that the metal is very light in weight. Moreover, aluminum cables being of lightweight makes them most efficient for overhead transmission of electricity. Another significant importance of aluminum being lighter is that cables made from the metal require little support. Economical Another significant feature about aluminum is that it is economical. Aluminum cables poses as the most economical compared to other cables form other metals. Moreover, the production of aluminum is also economical. Most of the production sites of aluminum tend to be near the sources areas and therefore costs related to transportation are less. Another significant issue related to less cost in aluminum is that the metal is recyclable. Moreover, aluminum being of low cost enables cables made from this metal to move for a longer distance (Vargel, 2004). Corrosion resistant The other reason behind choosing aluminum as the best cable is that it is resistant to corrosion. Aluminum is able to resist corrosion because of presence of a thin layer on its surface. The thin layer lining is made of aluminum oxide. Moreover, using technology, this particular layer can be made stronger by anodizing the metal. Anodizing refers converting the metal to anode. This is done in electrolysis of dilute sulphuric acid. However, in order to accomplish this step, there is a need to first etch the aluminum with sodium hydroxide solution. This is done in order to remove
  13. 13. the existing oxide layer. After electrolyzing the aluminum article, a thick film of oxide is build up that is highly resistance to corrosion. Ductility and Malleability The other feature associated with aluminum is that it is highly ductile. Apart from being highly ductile, aluminum has a high aspect of malleability. Both ductility and malleability are properties related to how the possibility of deformation can occur on a particular metal. Aluminum holds both of these properties and therefore seems to be the best while using as electrical cables. In all metals, aluminum is the second malleable one. While in the aspect of being ductile, aluminum holds the sixth position in all metals. Both of the two properties are of significant importance when using in electrical cables. This is because; malleability refers to the ability of a metal to be deformed by compression. Moreover, this process ought to occur without cracking without or rupturing. This feature also translates that it is possible to roll aluminum into several sheets. Aluminum holds a high percentage on this specific feature. Ductile means that aluminum has the ability to be deformed plastically. This process ought to occur without fracture under tensile force. This feature also illustrates that aluminum can be easily drawn into wires. Aluminum also tends to have high percentage on this feature. Both of these properties make aluminum to be the perfect metal for drawing large cables that are recommendable for overhead lines. Others Aluminum has another significant property of being highly strength (Fraden, 2010). This makes aluminum cables to be the best in making overhead lines because the high strength helps the cables not to creep. Aluminum metal is also non-magnetic. This property makes aluminum to be the best in making cables because they cannot attach to each other in case they are swung by wind. Characteristics of Aluminum cables One of the characteristics of aluminum cables is that they tend to lose some of their strength during the high temperatures. However, even in during these periods, ductility of the metal remains the same as in low temperatures. This feature makes aluminum to the best in cabling
  14. 14. even in cold regions. The other characteristic of aluminum cables is that they are able to form a layer of oxide. The importance of these layers is that they are corrosion resistant. When repeatedly used, the cables made from aluminum tend to lose their strength. Therefore, they require extra care when handling. Process and Manufacturing the Power Transmission Lines Using Aluminum Cables High voltage electric transmission entails transfer of energy form an electric source to various substations. Most of the substations are located near residential places. This network of transferring electricity from the source to the final consumer is generally known as distribution system. The importance of using overhead line transmission is because the method is less costly. Most of the aluminum conductors were being manufactured from pure aluminum (ECAL) in the early period. In order to make the overhead transmission lines, most of the manufactures used wire rods. The wire rods were made through the process of hot rolling. The wire rods were also made through extrusion methods. However, nowadays, in order to process and manufacture transmission lines using aluminum cables, a set systematic system is used to make sure that the best product is made. Material and properties The most important materials include the recommended composite conductor. The conductor ought to have a number of zirconium strands that are made from aluminum. These specific strands ought to be of high temperatures. Another significant property required is reinforced composite wires that have to be of aluminum oxide. The reinforced wires are covered by the zirconium strands. The significant importance of both the composite wires that make the composite core and outer core aluminum-zirconium (Al-Zr) is that they help in making the transmission lines to have the overall conductor strength and conductivity. Composite core The composite core or the inner strands are made of aluminum composite wires. Most of the wires depend on the conductor size and the wire diameters. In most of the time, the wire diameters range from 0.073‖ (1.9mm) to 0.114 (209mm). One significant feature of the core
  15. 15. wires is that they have the stiffness and strength of steel. However, the core wires have little weight and higher conductivity compared to materials made from steel. This is an advantage to of the aluminum materials in making strong and reliable transmission wires. Each of the core wire is composed of a large numbers of aluminum oxide fibers. The fibers are small in diameters and they are of ultra-high strength. The other aspect of the composite core is that the ceramic fibers are continuous. They are oriented in the direction of the wire. The ceramic wires are also embedded with high-purity aluminum. The composite wires are different from aluminum itself in strength. Moreover, the wires tend to exhibit various mechanical and physical properties that are of more degree compared to that aluminum. Outer strands The outer strands compromise of a temperature resistance alloy. The specific alloy is aluminumzirconium. This particular alloy is of made of hard aluminum. The other significant feature is that this particular alloy is designed to maintain high strength. This in many cases occurs after high temperatures. The following figure shows how the outer strands of the transmission wire ought to be. Tensile strength After finding the necessary and recommended materials, the other process is making laboratory tensile tests. The test strength is made in a gauge facility that is 10ft in length. During this process, there is a need to take considerable care when handling the materials. Moreover, there is also significant advantage in cutting and preparing the materials in order to ensure that the wires
  16. 16. did not slacken. Disadvantage of slacken is that the wires might decrease their strength values. The other issue that is determined while checking the tensile strength is the breaking load. This is usually done by pulling a conductor to a 1000-lb load. Then the load is further loaded to failure at 10000 lbs/min. After the testing the tensile strength, the breaking load ought to reach the recommended Rated Breaking Strength (RBS). Stress strain behavior Another significant issue addressed during the manufacturing process of the transmission lines is the stress-strain behavior. The behavior is determined according to the set standards by the Aluminum Association. The stress-strain behavior is test is started at 1000lbs (4.4KN). During this process, the strain measurement is set at zero. The load is then incrementally increased to a percentage of between 30 and 75 of RBS. Moreover, curve fitting is then applied to the transmission lines (Kaufman, Rooy & American Foundry Society, 2004). Short Circuit Behavior This test is conducted in order to determine whether the aluminum cable is able to sustain the compression that might occur in case of short circuits. The following figure demonstrates the consequences that might occur during a short circuit while using cables other than aluminum. Axial Impact strength This is a test usually done to investigate whether there is slippage in conductor terminations. The other significant importance of this test is to investigate whether there is a possibility to sustain the high shock loads (>100% RBS). In most cases, the loads ought to be sustained by the 795-
  17. 17. kcmil Composite Conductor. Moreover, this is done under high rate axial loading. Through this particular process, the shock load is comparable to various loading rates. Some the loading rates include those experienced in certain situations like during ice jumps and galloping events. The following figure demonstrates an axial impact. Crush strength The crush strength test is usually done to test the full strength retention of the aluminum cable. The test is done on a 795-kcmil Composite Conductor. The main reason for conducting the test is to simulate the possible damage that might occur during the process of shipping and installation. An example of this process is crushing a section of the aluminum cable between 6-inch steel plates for a period of one minute. If the aluminum cable shows any detection of damage, then the cable it is not fit for application. Lightening Resistance This is a significant process usually undertaken during the process of manufacturing overhead transmission lines. A lightening arc is struck across the aluminum cable to determine whether it can be able to resist very severe strikes (Smith, 2008). The following figures demonstrate two types of cables where one is able to resist occurrence of lightening while the other one is not.
  18. 18. Conclusion After the aluminum cable, is able to pass through all the above manufacturing processes, then it eligible for application in overhead transmission lines. Aluminum tends to be the best metal for making transmission cables as illustrated above. Transmission of high-voltage electricity requires cables that are able to resist various manufacture and environmental unhealthy conditions. Through various developments in technology, it is possible to make aluminum cables in an easy way compared to the methods that were used in the past. Moreover, through technology there is a possibility of extra advancement in making the overhead aluminum cable lines. Aluminum cables poses to be the best compared with other metals.
  19. 19. Cable Construction Contents [hide] 1 Introduction 2 Cable Parts o 2.1 Conductor o 2.2 Conductor Screen o 2.3 Insulation o 2.4 Insulation Screen o 2.5 Conductor Sheath o 2.6 Filler o 2.7 Bedding / Inner Sheath o 2.8 Individual Screen (Instrument Cables) o 2.9 Drain Wire (Instrument Cables) o 2.10 Overall Screen (Instrument Cables) o 2.11 Armour o 2.12 Outer Sheath o 2.13 Termite Protection o 2.14 Conductor Protection (Appendix)  2.14.1 Non-Metallic  2.14.2 Metallic 3 Low Voltage Power and Control Cables 4 Low Voltage Instrumentation Cables 5 Medium / High Voltage Power Cables o 5.1 Teck Cables o 5.2 Shielded Cables o 5.3 Concentric Neutral Cables
  20. 20. o 5.4 Paper-Insulated Lead-Covered Cables (PILC) o 5.5 Submarine Cables o 5.6 Mining Cables o 5.7 Aluminum-Sheathed Cables 6 Trivia 7 References Introduction This article gives a brief exposition on the construction of typical low voltage, medium / high voltage and instrumentation cables. The focus is on thermoplastic and thermosetting insulated cables, however the construction of other cables are similar. Although there is more than one way to construct a cable and no one standard to which all vendors will adhere, most cables tend to have common characteristics. Low voltage power and control cables pertain to electrical cables that typically have a voltage grade of 0.6/1 kV or below. Low voltage instrumentation cables pertain to cables for use in instrument applications and typically have a voltage grade of 450/750 V or below. Medium / High voltage cables pertain to cables used for electric power transmission at madium and high voltage (usually from 1 to 33 kV are medium voltage cables and those over 50 kV are high voltage cables). Cable Parts Here, we will take a short overview of the main and the most typical cable construction parts: Conductor Usually stranded copper (Cu) or aluminium (Al). Copper is densier and heavier, but more conductive than aluminium. Electrically equivalent aluminium conductors have a cross-sectional
  21. 21. area approximately 1.6 times larger than copper, but are half the weight (which may save on material cost). Annealing – is the process of gradually heating and cooling the conductor material to make it more malleable and less brittle. Coating – surface coating (eg. tin, nickel, silver, lead alloy) of copper conductors is common to prevent the insulation from attacking or adhering to the copper conductor and prevents deterioration of copper at high temperatures. Tin coatings were used in the past to protect against corrosion from rubber insulation, which contained traces of the sulfur used in the vulcanising process. Conductor Screen A semi-conducting tape to maintain a uniform electric field and minimise electrostatic stresses (for MV/HV power cables). Insulation Commonly thermoplastic (eg. PVC) or thermosetting (eg. EPR, XLPE) type materials. Mineral insulation is sometimes used, but the construction of MI cables are entirely different to normal plastic / rubber insulated cables. Typically a thermosetting(eg. EPR, XLPE) or paper/lead insulation for cables under 22kV. Paper-based insulation in combination with oil or gas-filled cables are generally used for higher voltages. Plastics are one of the more commonly used types of insulating materials for electrical conductors. It has good insulating, flexibility, and moisture-resistant qualities. Although there are many types of plastic insulating materials, thermoplastic is one of the most common. With the use of thermoplastic, the conductor temperature can be higher than with some other types of insulating materials without damage to the insulating quality of the material. Plastic insulation is normally used for low- or medium-range voltage.
  22. 22. The designators used with thermoplastics are much like those used with rubber insulators. The following letters are used when dealing with NEC type designators for thermoplastics: T - Thermoplastic H - Heat-resistant W - Moisture-resistant A - Asbestos N - Outer nylon jacket M - Oil-resistant Paper has little insulation value alone. However, when impregnated with a high grade of mineral oil, it serves as a satisfactory insulation for extremely high-voltage cables. The oil has a high dielectric strength, and tends to prevent breakdown of the paper insulation. The paper must be thoroughly saturated with the oil. The thin paper tape is wrapped in many layers around the conductors, and then soaked with oil. Enamel: the wire used on the coils of meters, relays, small transformers, motor windings, and so forth, is called magnet wire. This wire is insulated with an enamel coating. The enamel is a synthetic compound of cellulose acetate (wood pulp and magnesium). In the manufacturing process, the bare wire is passed through a solution of hot enamel and then cooled. This process is repeated until the wire acquires from 6 to 10 coatings. Thickness for thickness, enamel has higher dielectric strength than rubber. It is not practical for large wires because of the expense and because the insulation is readily fractured when large wires are bent. Mineral-insulated (MI) cable was developed to meet the needs of a noncombustible, high heatresistant, and water-resistant cable. MI cable has from one to seven electrical conductors. These conductors are insulated in a highly compressed mineral, normally magnesium oxide, and sealed in a liquidtight, gastight metallic tube, normally made of seamless copper.
  23. 23. Silk and Cotton: in certain types of circuits (for example, communications circuits), a large number of conductors are needed, perhaps as many as several hundred. Because the insulation in this type of cable is not subjected to high voltage, the use of thin layers of silk and cotton is satisfactory. Silk and cotton insulation keeps the size of the cable small enough to be handled easily. The silk and cotton threads are wrapped around the individual conductors in reverse directions. The covering is then impregnated with a special wax compound. Insulation Screen A semi-conducting material that has a similar function as the conductor screen (ie. control of the electric field for MV/HV power cables). Conductor Sheath A conductive sheath / shield, typically of copper tape or sometimes lead alloy, is used as a shield to keep electromagnetic radiation in, and also provide a path for fault and leakage currents (sheaths are earthed at one cable end). Lead sheaths are heavier and potentially more difficult to terminate than copper tape, but generally provide better earth fault capacity. Filler The interstices of the insulated conductor bundle is sometimes filled, usually with a soft polymer material. Bedding / Inner Sheath Typically a thermoplastic (eg. PVC) or thermosetting (eg. CSP) compound, the inner sheath is there to keep the bundle together and to provide a bedding for the cable armour. Individual Screen (Instrument Cables)
  24. 24. An individual screen is occasionally applied over each insulated conductor bundle for shielding against noise / radiation and interference from other conductor bundles. Screens are usually a metallic (copper, aluminium) or semi-metallic (PETP/Al) tape or braid. Typically used in instrument cables, but not in power cables. Drain Wire (Instrument Cables) Each screen has an associated drain wire, which assists in the termination of the screen. Typically used in instrument cables, but not in power cables. Overall Screen (Instrument Cables) An overall screen is applied over all the insulated conductor bundles for shielding against noise / radiation, interference from other cables and surge / lightning protection. Screens are usually a metallic (copper, aluminium) or semi-metallic (PETP/Al) tape or braid. Typically used in instrument cables, but not in power cables. Armour For mechanical protection of the conductor bundle. Steel wire armour or braid is typically used. Tinning or galvanising is used for rust prevention. Phosphor bronze or tinned copper braid is also used when steel armour is not allowed. SWA - Steel wire armour, used in multi-core cables (magnetic), AWA - Aluminium wire armour, used in single-core cables (non-magnetic). When an electric current passes through a cable it produces a magnetic field (the higher the voltage the bigger the field). The magnetic field will induce an electric current in steel armour (eddy currents), which can cause overheating in AC systems. The non-magnetic aluminium armour prevents this from happening. Outer Sheath
  25. 25. Applied over the armour for overall mechanical, weather, chemical and electrical protection. Typically a thermoplastic (eg. PVC) or thermosetting(eg. CSP) compound, and often the same material as the bedding. Outer sheath is normally colour coded to differentiate between LV, HV and instrumentation cables. Manufacturer’s markings and length markings are also printed on the outer sheath. Termite Protection For underground cables, a nylon jacket can be applied for termite protection, although sometimes a phosphor bronze tape is used. Conductor Protection (Appendix) Wires and cables are generally subject to abuse. The type and amount of abuse depends on how and where they are installed and the manner in which they are used. Cables buried directly in the ground must resist moisture, chemical action, and abrasion. Wires installed in buildings must be protected against mechanical injury and overloading. Wires strung on crossarms on poles must be kept far enough apart so that the wires do not touch. Snow, ice, and strong winds make it necessary to use conductors having high tensile strength and substantial frame structures. Generally, except for overhead transmission lines, wires or cables are protected by some form of covering. The covering may be some type of insulator like rubber or plastic. Over this, additional layers of fibrous braid or tape may be used and then covered with a finish or saturated with a protective coating. If the wire or cable is installed where it is likely to receive rough treatment, a metallic coat should be added. The materials used to make up the conductor protection for a wire or cable are grouped into one of two categories: non-metallic or metallic. Non-Metallic The category of non-metallic protective coverings is divided into three areas. These areas are: (1) according to the material used as the covering,
  26. 26. (2) according to the saturant in which the covering was impregnated, and (3) according to the external finish on the wire or cable. These three areas reflect three different methods of protecting the wire or cable. These methods allow some wire or cable to be classified under more than one category. Most of the time, however, the wire or cable will be classified based upon the material used as the covering regardless of whether or not a saturant or finish is applied. Many types of non-metallic materials are used to protect wires and cables. Fibrous braid is by far the most common and will be discussed first. Fibrous Braid Fibrous braid is used extensively as a protective covering for cables. This braid is woven over the insulation to form a continuous covering without joints. The braid is generally saturated with asphalt, paint, or varnish to give added protection against moisture, flame, weathering, oil, or acid. Additionally, the outside braid is often given a finish of stearin pitch and mica flakes, paint, wax, lacquer, or varnish depending on the environment where the cable is to be used. Woven Covers Woven covers, commonly called loom, are used when exceptional abrasion-resistant qualities are required. These covers are composed of thick, heavy, long-fibered cotton yarns woven around the cable in a circular loom, much like that used on a fire hose. They are not braids, although braid covering are also woven; they are designated differently. Rubber and Synthetic Coverings Rubber and synthetic coverings are not standardized. Different manufactures have their own special compounds designated by individual trade names. These compounds are different from the rubber compounds used to insulate cable. These compounds have been perfected not for insulation qualities but for resistance to abrasion, moisture, oil, gasoline, acids, earth solutions, and alkalies. None of these coverings will provide protection against all types of exposure. Each covering has its own particular limitations and qualifications.
  27. 27. Jute and Asphalt Coverings Jute and asphalt coverings are commonly used as a cushion between cable insulation and metallic armour. Frequently, they are also used as a corrosive-resistant covering over a lead sheath or metallic armour. Jute and asphalt coverings consist of asphalt-impregnated jute yarn heli-wrapped around the cable or of alternate layers of asphalt-impregnated jute yarn. These coverings serve as a weatherproofing. Unspun Felted Cotton Unspun felted cotton is commonly used only in special classes of service. It is made as a solid felted covering for a cable. Metallic Metallic protection is of two types: sheath or armour. As with all wires and cables, the type of protection needed will depend on the environment where the wire or cable will be used. Metallic Sheath Cables or wires that are continually subjected to water must be protected by a watertight cover. This watertight cover is either a continuous metal jacket or a rubber sheath molded around the cable. Lead-sheathed cable is one of three types currently being used: alloy lead, pure lead, and reinforced lead. An alloy-lead sheath is much like a pure lead sheath but is manufactured with 2percent tin. This alloy is more resistant to gouging and abrasion during and after installation. Reinforced lead sheath is used mainly for oil-filled cables where high internal pressures can be expected. Reinforced lead sheath consists of a double lead sheath. A thin tape of hard-drawn copper, bronze, or other elastic metal (preferably nonmagnetic) is wrapped around the inner sheath. This tape gives considerable additional strength and elasticity to the sheath, but must be protected from corrosion. For this reason, a second lead sheath is applied over the tape. Metallic Armour
  28. 28. Metallic armour provides a tough protective covering for wires and cables. The type, thickness, and kind of metal used to make the armour depend on three factors: (1) the use of the conductors, (2) the environment where the conductors are to be used, and (3) the amount of rough treatment that is expected. 1. Wire-braid armour Wire-braid armour, also known as basket-weave armour, is used when light and flexible protection is needed. Wire braid is constructed much like fibrous braid. The metal is woven directly over the cable as the outer covering. The metal used in this braid is galvanized steel, bronze, copper, or aluminum. Wire-braid armour is mainly for shipboard use. 2. Steel tape A second type of metallic armour is steel tape. Steel tape covering is wrapped around the cable and then covered with a serving of jute. There are two types of steel tape armour. The first is called interlocking armour. Interlocking armour is applied by wrapping the tape around the cable so that each turn is overlapped by the next and is locked in place. The second type is flat- band armour. Flat-band armour consists of two layers of steel tape. The first layer is wrapped around the cable but is not overlapped. The second layer is then wrapped around the cable covering the area that was not covered by the first layer. 3. Wire armour Wire armour is a layer of wound metal wire wrapped around the cable. Wire armour is usually made of galvanized steel and can be used over a lead sheath (see view C of the figure above). It can be used with the sheath as a buried cable where moisture is a concern, or without the sheath when used in buildings. 4. Coaxial cable
  29. 29. Coaxial cable is defined as two concentric wires, cylindrical in shape, separated by a dielectric of some type. One wire is the center conductor and the other is the outer conductor. These conductors are covered by a protective jacket. The protective jacket is then covered by an outer protective armour. Coaxial cables are used as transmission lines and are constructed to provide protection against outside signal interference. Low Voltage Power and Control Cables Low voltage power and control cables pertain to electrical cables that typically have a voltage grade of 0.6/1 kV or below.
  30. 30. Armoured FAS Cable An important item that is under the grouping known as 'Low Voltage Cables', is Type FAS (Fire Alarm & Signal Cable). This 300-volt cable, is specifically designed for the interconnection of security system elements, including fire protection signalling devices such as smoke and fire detectors, fire alarms, and two-way emergency communications systems. Fire alarm installations in non-combustible buildings require mechanical protection, consisting of interlock armour, metallic conduit, non-metallic conduit embedded in concrete or installed under-ground. Armoured FAS Cable provided with an interlocking aluminum armour, may be expected to have an appreciable cost advantage, compared with cables installed in rigid conduit. Other common cables are LVT (Low Voltage Thermoplastic) and ELC (Extra Low Voltage Control), which are frequently used in residential installations for such items as door bells and thermostats. Low Voltage Instrumentation Cables
  31. 31. Instrumentation cables Low voltage instrumentation cables pertain to cables for use in instrument applications and typically have a voltage grade of 450/750 V or below. Instrumentation Cables rated at 300 volts have copper conductors 0.33 mm2 (#22 AWG) to 2.08 mm2 (#14 AWG), while those rated at 600 volts have 0.82 mm2(#18 AWG) to 5.26 mm2(#10 AWG), and unarmoured and armoured types are available. The cables may be an assembly of single conductors, pairs, triads or quads. The conductors are stranded seven-wire tinned or bare copper. The insulation is usually a PVC compound chosen dependant on the environment for which it is intended. Insulated conductors are paired with staggered lays to prevent electromagnetic coupling and crosstalk. When individual shielding is specified, each pair is aluminum/polyester shielded with drain wire to eliminate electrostatic interference. Armoured cables have an interlocked aluminum or galvanized steel armour. The armouring is applied over an inner PVC jacket, followed by a PVC outer jacket. Armoured cables are suitable for installation on cable trays in dry, damp and wet locations, or direct earth burial. Unarmoured Instrumentation Cables are intended for installation in raceways/conduit (except cable trays) in dry, damp or wet locations, or direct earth buried. Unarmoured Cable with Type TC (Tray Cable) designation, may be installed in cable trays. Thermocouple Extension Cables
  32. 32. Thermocouple Extension Cables have a 300 volt rating, and are of similar construction to Instrumentation Cables, but the metals/alloys used for the conductors are different. Thermocouples measure temperature using the electric current created when heat is applied to two dissimilar metals/alloys. The cable assembles may consist of as many as 50 pairs, depending on the number of locations being temperature monitored. Medium / High Voltage Power Cables Medium or High Voltage power cables have voltage grade greater than 1 kV. Medium voltage usually goes up to 46 kV and High voltage is considering all voltage levels above 46 kV.
  33. 33. Medium Voltage distribution systems begin at substations and supply electricity to a wide spectrum of power consumers. When selecting a cable, the basic aim is to safely provide adequate electrical power, with continuous, trouble-free operation, in a system that is able to withstand unexpected demands and overload conditions. Each installation has particular requirements that must be considered. There are distinct benefits from specifying a copperconductor cable that has been manufactured under rigid specification and quality control procedures. It will provide maximum performance with minimum maintenance. There are seven types different by construction for medium voltage copper power cables in the 1 kV to 46 kV range. Most are available in single- and multi-core configurations. There are ranges of sizes and design variations for each type. The MV cable types are: Teck Cables, Shielded Cables, Concentric Neutral Cables, Paper-Insulated Lead-Covered Cables, Submarine Cables, Mining Cables, Aluminum-Sheathed Cables. In the cable descriptions a number of insulation and sheath (jacket) materials have been abbreviated as follows: Cross-Linked Polyethylene - XLPE, Ethylene-Propylene Rubber - EPR, Polyvinyl Chloride - PVC, Polyethylene - PE, Tree-Retardant Cross-Linked Polyethylene - TR-XLPE. Teck Cables
  34. 34. Teck Cables were originally developed for use in mines, but they are now widely used in primary and secondary industries, chemical plants, refineries and general factory environments. They are also used in multi-storey and commercial buildings. They are flexible, resistant to mechanical abuse, corrosion resistant, compact and reliable. A modified Teck Cable construction may be used for vertical installations, such as in mine shafts and multi-storey buildings, where the armour is locked-in-place to prevent slippage of the inner core. There are many different combinations of conductor size, voltage rating, armour type and so forth, available in Teck Cables to meet the requirements of particular installations. Annealed, bare, copper is used for the conductor (s), and they are usually compact stranded to reduce diameter. In multi-conductor cables, the insulated conductors are cabled together, including the bare copper bonding (grounding) conductor. In shielded multi-conductor cables, the bonding (grounding) conductor is positioned to contact the copper shields. A PVC outer jacket which may be colour-coded depending on the rating of the cable is applied. Shielded Cables Shielded Power Cable may be single-or three-conductor. The basic construction begins with a conductor of annealed, bare, solid or concentric-stranded copper, which may be compact or compressed. This is followed by a semi-conducting conductor shield, insulation, and then a semiconducting insulation shield. Metallic shielding follows, which is usually either gapped or lapped copper tape. Other types of metallic shielding are available, including concentric wires and longitudinally corrugated copper tape. The outer jacket is either PVC or PE. Concentric Neutral Cables These power cables may be used in dry or wet locations, for a wide variety of types of installations, and may be single- or three-conductor. The two standard constructions are Unjacketed and Jacketed, the latter being most frequently used. The conductor is typically annealed, bare, stranded copper, but tin-coated wire and solid conductors are also available. The concentric neutral conductor, from which the cable derives its name, is bare or tin-coated copper wire, applied helically over the insulation shield. These wires act as the metallic component of the shield and the neutral, at the same time.
  35. 35. Paper-Insulated Lead-Covered Cables (PILC) PILC cables are used in power distribution and industrial applications, and they may be installed exposed, in underground ducts or directly buried. Their design begins with annealed, bare copper conductor(s) which may be round, concentric, compressed or compact stranded, compact sector, and in larger sizes … Type M segmental stranded. An example of compact sector conductors is shown in the illustration. The insulated cable core is impregnated with a medium viscosity polybutene-based compound. The combination of the excellent electrical and mechanical characteristics of the liquid and the paper has resulted in a reliable and economic insulation, which now claims a history of almost 100 years. It is little wonder why so many utilities and power-consuming industries, still continue to specify PILC. To prevent the ingress of moisture, a seamless lead-alloy sheath is applied. The outer jacket may be PVC or PE, and if required by the application, armour is available. Submarine Cables For submarine installations, usually Self-Contained Liquid-Filled Cables (SCLF), or Solid Dielectric Cables are selected, depending on voltage and power load. SCLF Cables are capable of handling very high voltages. However, for medium-voltage installations, a Solid Dielectric Cable can easily fulfil the electrical demands of the system. A submarine Solid Dielectric Cable is shown in the illustration. Its construction begins with a compact stranded, annealed, bare copper conductor, followed by a semi-conducting conductor shield. A copper tape shield is helically applied, followed by a lead-alloy sheath. Due to the severe environmental demands placed on submarine cables, a lead-alloy sheath is often specified because of its compressibility, flexibility and resistance to moisture and corrosion. The sheath is usually covered by a number of outer layers, comprising a PE or PVC jacket and metal wire armouring. Mining Cables A number of different types of cables are used in mines. There are fixed mining cables and portable mining cables, the latter being described here. The key requirements of portable cables are flexibility, and resistance to mechanical abrasion and damage. Due to the additional demands put on portable mining cables used for reeling and dereeling applications, special design may be
  36. 36. required. There are many types of portable mining cables. They are available in ratings up to 25 kV, and may have as many as five conductors. An example of SHD-GC Cable, is shown in the illustration. It has three insulated, shielded conductors, two bare ground wires, a ground check wire, and an overall jacket. The conductors for this cable are annealed, bare or tinned copper wires. The braided shield may be tin-coated wires, or a tin-coated copper wire/textile composite. The grounding conductor(s) annealed, bare or tinned, stranded copper wires, and the ground check conductor is annealed, bare, stranded copper wires with EPR insulation and nylon braid, elastomeric jacket holds the conductor assembly firmly in place, to minimize snaking and corkscrewing during reeling and dereeling. Aluminum-Sheathed Cables These power cables are used for exposed and concealed wiring, in wet and dry locations, and where exposed to the weather. They may be installed in ventilated, unventilated and ladder-type cable-troughs, and ventilated flexible cableways. Aluminum-Sheathed Power Cables may be single-,two-,three- or four-conductor, the conductor(s) being annealed, bare, compressed-round stranded copper. The insulated core is enclosed in a liquid- and vapour-tight solid corrugated aluminum sheath, covered by a PVC jacket.
  37. 37. PaperInsulated Shielded Cable Teck Cable overview overview Concentric Neutral Cable overview Submarine Mining Lead- Cable Cable Covered overview overview Cable overview AluminumSheathed Cable overview
  38. 38. A high-voltage cable - also called HV cable - is used for electric power transmission at high voltage. High-voltage cables of differing types have a variety of applications in instruments, ignition systems, AC and DC power transmission. In all applications, the insulation of the cable must not deteriorate due to the high-voltage stress, ozone produced by electric discharges in air, or tracking. The cable system must prevent contact of the high-voltage conductor with other objects or persons, and must contain and control leakage current. Cable joints and terminals must be designed to control the high-voltage stress to prevent breakdown of the insulation. Often a high-voltage cable will have a metallic shield layer over the insulation, connected to earth ground and designed to equalize the dielectric stress on the insulation layer. Segments of high-voltage cables High-voltage cables may be any length, with relatively short cables used in apparatus, longer cables run within buildings or as buried cables in an industrial plant or for power distribution, and the longest cables are often run as submarine cables under the ocean for power transmission. Contents 1 Construction 2 AC power cable o 2.1 Quality 3 HVDC cable 4 Cable terminals 5 Cable joints 6 X-ray cable 7 Testing of high-voltage cables 8 See also 9 Sources and notes o 9.1 Notes 10 External links Construction
  39. 39. A cross-section through a 400 kV cable, showing the stranded segmented copper conductor in the center, semiconducting and insulating layers, copper shield conductors, aluminum sheath and plastic outer jacket. Like other power cables, high-voltage cables have the structural elements of one or more conductors, insulation, and a protective jacket. High-voltage cables differ from lower-voltage cables in that they have additional internal layers in the insulation jacket to control the electric field around the conductor. For circuits operating at or above 2,000 volts between conductors, a conductive shield may surround each insulated conductor. This equalizes electrical stress on the cable insulation. This technique was patented by Martin Hochstadter in 1916;[1] the shield is sometimes called a Hochstadter shield. The individual conductor shields of a cable are connected to earth ground at the ends of the shield, and at splices. Stress relief cones are applied at the shield ends. Cables for power distribution of 10 kV or higher may be insulated with oil and paper, and are run in a rigid steel pipe, semi-rigid aluminum or lead sheath. For higher voltages the oil may be kept under pressure to prevent formation of voids that would allow partial discharges within the cable insulation. Sebastian Ziani de Ferranti was the first to demonstrate in 1887 that carefully dried and prepared paper could form satisfactory cable insulation at 11,000 volts. Previously paper-insulated cable had only been applied for low-voltage telegraph and telephone circuits. An extruded lead sheath over the paper cable was required to ensure that the paper remained absolutely dry. Vulcanized rubber was patented by Charles Goodyear in 1844, but it was not applied to cable insulation until the 1880s, when it was used for lighting circuits.[1] Rubber-insulated cable was used for 11,000 volt circuits in 1897 installed for the Niagara Falls Power Generation project. Mass-impregnated paper-insulated medium voltage cables were commercially practical by 1895. During World War II several varieties of synthetic rubber and polyethylene insulation were applied to cables.[2] Modern high-voltage cables use polymers or polyethylene, including (XLPE) for insulation.
  40. 40. AC power cable High voltage is defined as any voltage over 1000 volts. Cables for 3000 and 6000 volts exist, but the majority of cables are used from 10 kV and upward.[3] Those of 10 to 33 kV are usually called medium voltage cables, those over 50 kV high voltage cables. Figure 1, cross section of a high-voltage cable, (1) conductor, (3) insulation. Modern HV cables have a simple design consisting of few parts. A conductor of copper or aluminum wires transports the current, see (1) in figure 1. (For a detailed discussion on copper cables, see main article: Copper wire and cable.) Conductor sections up to 2000 mm2 may transport currents up to 2000 amperes. The individual strands are often preshaped to provide a smoother overall circumference. The insulation (3) may consist of cross-linked polyethylene, also called XLPE. It is reasonably flexible and tolerates operating temperatures up to 120 °C. EPDM is also an insulation. At the inner (2) and outer (4) sides of this insulation, semi-conducting layers are fused to the insulation.[4] The function of these layers is to prevent air-filled cavities between the metal conductors and the dielectric so that little electric discharges can arise and endanger the insulation material.[5] The outer conductor or sheath (5) serves as an earthed layer and will conduct leakage currents if needed. Most high-voltage cables for power transmission that are currently sold on the market are insulated by a sheath of cross-linked polyethylene (XLPE). Some cables may have a lead or aluminium jacket in conjunction with XLPE insulation to allow for fiber optics. Before 1960, underground power cables were insulated with oil and paper and ran in a rigid steel pipe, or a semi-rigid aluminium or lead jacket or sheath. The oil was kept under pressure to prevent formation of voids that would allow partial discharges within the cable insulation. There are still many of these oil-and-paper insulated cables in use worldwide. Between 1960 and 1990, polymers became more widely used at distribution voltages, mostly EPDM (ethylene propylene diene M-class); however, their relative unreliability, particularly early XLPE, resulted in a slow uptake at transmission voltages. While cables of 330 kV are commonly constructed using XLPE, this has occurred only in recent decades. Quality During the development of HV insulation, which has taken about half a century, two characteristics proved to be paramount. First, the introduction of the semiconducting layers.
  41. 41. These layers must be absolutely smooth, without even protrusions as small as a few µm. Further the fusion between the insulation and these layers must be absolute;[6] any fission, air-pocket or other defect - of the same micro-dimensions as above - is detrimental for the breakdown characteristics of the cable. Secondly, the insulation must be free of inclusions, cavities or other defects of the same sort of size. Any defect of these types shortens the voltage life of the cable which is supposed to be in the order of 30 years or more.[7] Cooperation between cable-makers and manufacturers of materials has resulted in grades of XLPE with tight specifications. Most producers of XLPE-compound specify an ―extra clean‖ grade where the number and size of foreign particles are guaranteed. Packing the raw material and unloading it within a cleanroom environment in the cable-making machines is required. The development of extruders for plastics extrusion and cross-linking has resulted in cable-making installations for making defect-free and pure insulations. The final quality control test is an elevated voltage 50 or 60 Hz partial discharge test with very high sensitivity (in the range of 5 to 10 picoCoulombs) This test is performed on every reel of cable before it is shipped. An extruder machine for making insulated cable HVDC cable A high-voltage cable for HVDC transmission has the same construction as the AC cable shown in figure 1. The physics and the test-requirements are different.[8] In this case the smoothness of the semiconducting layers (2) and (4) is of utmost importance. Cleanliness of the insulation remains imperative. Many HVDC cables are used for DC submarine connections, because at distances over 30 km AC can no longer be used. The longest submarine cable today is the NorNed cable between Norway and Holland that is almost 600 km long and transports 700 megawatts, a capacity equal to a large power station. Most of these long deep-sea cables are made in an older construction, using oil-impregnated paper as an insulator. Cable terminals
  42. 42. Figure 2, the earth shield of a cable (0%) is cut off, the equipotential lines (from 20% to 80%) concentrate at the edge of the earth electrode, causing danger of breakdown. Terminals of high-voltage cables must manage the electric fields at the ends.[9] Without such a construction the electric field will concentrate at the end of the earth-conductor as shown in figure 2. Equipotential lines are shown here which can be compared with the contour lines on a map of a mountainous region: the nearer these lines are to each other, the steeper the slope and the greater the danger, in this case the danger of an electric breakdown. The equipotential lines can also be compared with the isobars on a weather map: the denser the lines, the more wind and the greater the danger of damage. Figure 3, a rubber or elastomer body R is pushed over the insulation (blue) of the cable. The equipotential lines between HV (high voltage) and earth are evenly spread out by the shape of the earth electrode. Field concentrations are prevented in this way. In order to control the equipotential lines (that is to control the electric field) a device is used that is called a stress-cone, see figure 3.[10] The crux of stress relief is to flare the shield end along a logarithmic curve. Before 1960, the stress cones were handmade using tape—after the cable was
  43. 43. installed. These were protected by potheads, so named because a potting compound/ dielectric was poured around the tape inside a metal/ porcelain body insulators. About 1960, preformed terminations were developed consisting of a rubber or elastomer body that is stretched over the cable end.[11] On this rubber-like body R an shield electrode is applied that spreads the equipotential lines to guarantee a low electric field. The crux of this device, invented by NKF in Delft in 1964,[12] is that the bore of the elastic body is narrower than the diameter of the cable. In this way the (blue) interface between cable and stress-cone is brought under mechanical pressure so that no cavities or air-pockets can be formed between cable and cone. Electric breakdown in this region is prevented in this way. This construction can further be surrounded by a porcelain or silicone insulator for outdoor use,[13] or by contraptions to enter the cable into a power transformer under oil, or switchgear under gas-pressure.[14] Cable joints Connecting two high-voltage cables with one another poses two main problems. First, the outer conducting layers in both cables shall be terminated without causing a field concentration,[15] similar as with the making of a cable terminal. Secondly, a field free space shall be created where the cut-down cable insulation and the connector of the two conductors safely can be accommodated.[16] These problems have been solved by NKF in Delft in 1965 [17] by introducing a device called bi-manchet. Photograph of a section of a high-voltage joint, bi-manchet, with a high-voltage cable mounted at the right hand side of the device. Figure 4 shows a photograph of the cross-section of such a device. At one side of this photograph the contours of a high-voltage cable are drawn. Here red represents the conductor of that cable and blue the insulation of the cable. The black parts in this picture are semi-conducting rubber parts. The outer one is at earth potential and spreads the electric field in a similar way as in a cable terminal. The inner one is at high-voltage and shields the connector of the conductors from the electric field. The field itself is diverted as shown in figure 5, where the equipotential lines are smoothly directed from the inside of the cable to the outer part of the bi-manchet (and vice versa at the other side of the device).
  44. 44. Field distribution in a bi-manchet or HV joint. The crux of the matter is here, like in the cable terminal, that the inner bore of this bi-manchet is chosen smaller than the diameter over the cable-insulation.[18] In this way a permanent pressure is created between the bi-manchet and the cable surface and cavities or electrical weak points are avoided. Installing a terminal or bi-manchet is skilled work. Removing the outer semiconducting layer at the end of the cables, placing the field-controlling bodies, connecting the conductors, etc., require skill, cleanness and precision. X-ray cable X-ray cables [19] are used in lengths of several meters to connect the HV source with an X-ray tube or any other HV device in scientific equipment. They transmit small currents, in the order of milliamperes at DC voltages of 30 to 200 kV, or sometimes higher. The cables are flexible, with rubber or other elastomer insulation, stranded conductors, and an outer sheath of braided copperwire. The construction has the same elements as other HV power cables. Testing of high-voltage cables There are different causes for faulty cable insulations. Hence, there are various test and measurement methods to prove fully functional cables or to detect faulty ones. One needs to distinguish between cable testing and cable diagnosis. While cable testing methods result in a go/no go statement cable diagnosis methods allow judgement of the cables current condition. In some cases it is even possible to locate the position of the fault in the insulation. One of the favorite testing methods is VLF cable testing. Using a very low frequency voltage with frequencies in the range of 0.1 to 0.01 Hz protects the device under test from deteriorating due to the test itself, as it used to be with DC testing methods in the older days. Depending on the sort of treeing in the insulation two cable diagnostics methods are common. Water trees can be detected by tan delta measurement. Interpretation of measurement results yield the possibility to distinguish between new, strongly aged and faulty cables and appropriate maintenance and repair measures may be planned. Damages to the insulation and electrical treeing may be detected and located by partial discharge measurement. Data collected during the measurement procedure is compared to measurement values of the same cable gathered during the acceptance-test. This allows simple and quick classification of the dielectric condition of the tested cable.
  45. 45. Cable Construction & Manufacturing Process NATIONAL INSTITUTE OF INDUSTRIAL ENGINEERING, MUMBAI CABLE CONSTRUCTION & MANUFACTURING PROCESS UNDER THE GUIDANCE OF Dr. KVSS NARAYANA RAO Submitted By
  46. 46. PRASENJIT HOJAI HIREMATH ROLL NO-66 SARVESH ROLL NO-88 Cable Construction & Manufacturing Process fig-power cable parts Power Cable mainly subdivided into parts1. Conductor 2. Insulation 3. Metallic Sheath 4. Bedding 5. Armouring 6. Outersheath Conductor Copper or Aluminium used for the Conductors obtained in the form of rods. The 8.0 mm Copper or 9.5mm aluminium rods. After testing, rods are drawn into wires of required sizes. These wires are formed into final Conductor in the stranding machines under strict Quality Assurance Program. Insulation Cross linked polyethylene compound or PVC is insulated over Conductor by Extrusion process. XLPE insulated cores are cured by steam curing in vulcanizing chamber to provide thorough cross-linking.
  47. 47. The raw materials & thickness of Insulation are maintained under strict Quality Control and conform to B.S. 5467 / IEC 60502 Part-1 or B.S. 6346 / IEC 60502 Part-1 Standards for XLPE & PVC cables respectively. Laying Up The insulated cores are laid up with a right hand, or alternating left & right hand, direction of lay in the sequence of the core numbers or colours. Where ever necessary nonhygroscopic PP / PVC Fillers & binder tape are used to form a compact and reasonably circular cable. Bedding All armoured cables have extruded PVC bedding. The PVC used for bedding is compatible with the temperature of Insulation material. Armour When armouring is required, the armour consists of single layer of Galvanised steel wire. The armour is applied helical, with a left hand direction. We also provide other armours such as steel strip, tape or tinned copper. Single core cables are armoured with Aluminium or copper wires. OuterSheath The standard cables are manufactured with Extruded black PVC Type-9 of B.S. 7655 or ST-2 of IEC 60502. Outer sheath is embossed or printed with the information required by the related standards. Special FR, FRLS compounds are used for outer sheathing of cables, to suit customer’s specification requirements.
  48. 48. Cable Manufacture An extruded cable production line is a highly sophisticated manufacturing process that must be run with great care to assure that the end product will perform reliably in service for many years. It consists of many sub processes that must work in concert with each other. If any part of the line fails to unction properly, it can create problems that will lead to poorly made cable and will potentially generate many metres of scrap cable. The process begins when pellets of insulating and semiconducting compounds are melted within the extruder. The melt is pressurised and this conveys material to the crosshead where the respective cable layers are formed. Between the end of the screw and the start of the crosshead.it is possible to place meshes or screens, which act as filters. The purpose of these screens was, in the earliest days of cable extrusion, to remove particles, or contaminants that might be present within the melt. While still used today, the clean characteristics of today’s materials minimize the need for this type of filter. In fact, if these screens are too tight, they themselves can generate contaminants in the form of scorch or precross linking. Nevertheless, appropriately sized (100 to 200-micron hole size) filters are helpful to stabilize the melt and protect the cable from large foreign particles that most often enter from the materials handling system. The most current technology uses a method called a true triple extrusion process where the conductor shield, insulation and insulation shield are coextruded simultaneously. The cables produced in this way have been shown to have better longevity (Figure 3) [7]. After the structure of the core is formed the cable is crosslinked to impart the high temperature performance. When a CV tube is used fine control of the temperature and residence time (linespeed) is required to ensure that the core is crosslinked to the correct level. Jackets In most MV, HV and EHV cable applications, the metal sheath/neutral is itself protected by a polymeric oversheath or jacket. Due to the critical performance needed from the oversheath, there are a number of properties that are required, such as good abrasion resistance, good processability, reasonable moisture resistance properties, and good stress cracking resistance. Experience has shown that the material with the best composite performance is a PE-based oversheath, though PVC, Chlorosulfonated Polyethylene and Nylon have been used as jacket materials. Tests on XLPE cables retrieved after 10 years of operation show that the mean breakdown strength falls by almost 50% (from 20 to 11 kV/mm – HDPE & PVC, respectively) when PVC is used as a jacket material. Many utilities now specify robust PE based jackets as a result. The hardness of PE is also an advantage when protection is required from termite damage. Jackets extend cable life by retarding the ingress of water and soluble ions from the ground, minimizing cable installation damage and mitigating neutral corrosion. Ninety three percent of investor-owned utilities in the USA specify a protective jacket. The semiconductive
  49. 49. jacket or oversheath is recommended for high lightning incident areas or joint-use trenches where telecommunications cables co-exist with power cables. Inspection and Test Plan for Power Cable The inspection and test plan for power cable article provides you information about power cable test and power cable inspection in manufacturing shop. Witnessing voltage and insulation resistance tests or alternative spark tests. For 33Kv cable, witness dielectric power factor voltage test. Dimensional checking on sample off-cut i.e. construction consistency, insulation thickness, external sheath screen, armours and mans of main components. Visual checking in respect of cable formation, core and external sheath colors, marking legibility. Testing on material sample i.e. conductor coating, insulation external jacket for elongation, heat strocle blending and characteristics of armour, metal and sheath components including zinc coating. Third Party Inspection for Power Cable Configuration Third party inspector checks the power cable configuration in accordance to drawing and datasheets. Following items is taken in account: Cable type Number of conductors Conductor size Conductor color coding Insulation type and size Fillers Water stoppers Armour Shield Outer diameter Other specified elements/dimensions Cable identification
  50. 50. References:Electrical Power Cable Engineering by Bruce S. Bernstein and William A. Thu L.V. Power and Control Cables by Oman Cables Industry TR-101670 ―Underground Transmission Systems Reference Book: 1992 Edition,‖ Electric Power ResearchInstitute http://en.wikipedia.org http://ieeexplore.ieee.org/Xplore/home.jsp
  51. 51. Technical specifications of cable work 1.1 SCOPE This chapter covers the requirements for the selection, installation and jointing of power cables for low, medium and high voltage applications upto and including 33KV. For details not covered in these Specifications, IS:1255-1983 shall be referred to. All references to BIS-Specifications and codes are for codes with amendments issued upto date i.e. till the date of call of tender. 1.2 TYPES OF CABLES 1.2.1 The cables for applications for low and medium voltage (upto and including 1.1KV) supply shall be one of the following: (i) PVC insulated and PVC sheathed, conforming to IS:1554 (Part-1)- 1988 (ii) Cross linked polyethylene insulated, PVC sheathed (XLPE), conforming to IS: 7098 (Part-1)- 1988. 1.2.2 The cables for applications for high voltage (above 1.1KV but upto and including 11KV supply) supply shall be one of the following: (i) PVC insulated and PVC sheathed, conforming to IS:1554 (Part-2)- 1988. (ii) Paper insulated, lead sheathed (PILCA) conforming to IS:692-1973 (iii) Cross linked polyethylene (XLPE) insulated, PVC sheathed conforming to IS:7098 (Part-2)- 1985. 1.2.3 The cables for applications above 11KV but upto and including 33KV supply shall be one of the following: (i) Paper insulated lead sheathed (PILCA) conforming to IS: 692-1973. (ii) Cross linked, polyethylene insulated (XLPE) conforming to IS:7098 (Part-2)-1985. 1.2.4 The cables shall be with solid or stranded aluminium conductors, as specified. Copper conductors may be used, only in special applications, where use of aluminium conductors is not technically acceptable. 1.2.5 Where paper insulated cables are used in predominantly vertical situation, these shall be of non-draining type. 1.3 ARMOURING AND SERVING 1.3.1 All multicore cables liable for mechanical damage and all HV cabkes (irrespective of the situation of installation) shall be armoured. Where armouring is unavoidable in dingle core cables, either the armour should be made of nonmagnetic material, or it should be ensured that the armouring is not shorted at terminations, thus preventing the flow of circulating currents therein. 1.3.2 Short runs of cables laid in pipes, closed masonary trenches and similar protected or secured enclosures need not be armoured. 1.3.3 PVC and XLPE cables, when armoured, shall have galvanized steel wires (flat or round) for armouring. 1.3.4 Paper insulated cables shall have for armouring, a double layer of steel tape for normal applications. Steel wire armouring is preferred where the cables are liable to tensile stresses in applications such as vertical runs, suspended on brackets or laid in soil that is likely to subside. 1.3.5 Serving over armouring in paper insulated cables shall consist of a complete layer or layers of suitable compounded Hessian materials.
  52. 52. 1.4 SELECTION OF CABLE SIZES 1.4.1 The cable sizes shall be selected by considering the voltage drop in the case of MV (distribution) cables and Current carrying capacity in the case of HV (feeder) cables. Due consideration should be given for the Prospective short circuit current and the period of its flow, especially in the case of HV cables. 1.4.2 While deciding upon the cable sizes, derating factors for the type of cable and depth of laying, grouping, ambient temperature, ground temperature, and soil resistivity shall be taken into account. 1.4.3 Guidance for the selection of cables shall be served from relevant Indian Standards such as IS:3961 (Part-1)-1967 for paper insulated lead sheathed cables, IS: 3961 (Part-2)-1967 for PVC insulated and PVC sheathed heavy duty cables, IS: 58191970 for recommended short circuit ratings of high voltage PVC cables, IS: 1255-1983 on code of practice for installation and maintenance of power cables upto and including 33KV rating etc. 1.5 STORAGE AND HANDLING 1.5.1 Storage (i) The cable drums shall be stored on a well drained, hard surface, so that the drums do not sink in the ground causing rot and damage to the cable drums. Paved surface is preferred, particularly for long term storage. (ii) The drums shall always be stored on their flanges, and not on their flat sides. (iii) Both ends of the cables especially of PILCA cables should be properly sealed to prevent ingress/ absorption of moisture by the insulation during storage. (iv) Protection from rain and sun is preferable for long term storage for all types of cables. There should also ventilation between cable drums. (v) During storage, periodical rolling of drums once in, say, 3 months through 90 degrees shall be done, in the case of paper insulated cables. Rolling shall be done in the direction of the arrow marked on the drum. (vi) Damaged battens of drums etc. should be replaced as may be necessary. 1.5.2 Handling (i) When the cable drums have to be moved over short distances, they should be rolled in the direction of the arrow marked on the drum. (ii) For manual transportation over long distances, the drum should be mounted on cable drum wheels, strong enough to carry the weight of the drum and pulled by means of ropes. Alternatively, they may be mounted on a trailer or on a suitable mechanical transport. (iii) For loading into and unloading from vehicles, a crane or a suitable lifting tackle should be used. Small sized cable drums can also be rolled down carefully on a suitable ramp or rails, for unloading, provided no damage is likely to be caused to the cable or to the drum. 1.6 INSTALLATION 1.6.1 General (i) Cables with kinks, straightened kinks or any other apparent defects like defective armouring etc. shall not be installed.
  53. 53. (ii) Cables shall not be bent sharp to a small radius either while handing or in installation. The minimum safe bending radius for PVC/XLPE (MV) cables shall be 12 times the overall diameter of the cable. The minimum safe bending radius for PILCA/XLPE (HV) cables shall be as given in Table-II. At joints and terminations, the bending radius of individual cores of a multi core cable of any type shall not be less than 15 times its overall diameter. (iii) The ends of lead sheathed cables shall be sealed with solder immediately after cutting the cables. In case of PVC cables, suitable sealing compound/tape shall be used for this purpose, if likely exposed to rain in transit storage. Suitable heat shrinkable caps may also be used for the purpose. 1.6.2 Route Before the cable laying work is undertaken, the route of the cable shall be decided by the Engineer-in-Charge considering the following. (i) While the shortest practicable route should be preferred, the cable route shall generally follow fixed developments such as roads, foot paths etc. with proper offsets so that future maintenance, identification etc. are rendered easy. Cross country run merely to shorten the route length shall not te adopted. (ii) Cable route shall be planned away from drains and near the property, especially in the case of LV/MV cables, subject to any special local requirements that may have to be necessarily complied with. (iii) As far as possible, the alignment of the cable route shall be decided after taking into consideration the present and likely future requirements of other services including cables enroute, possibility of widening of roads/lanes etc. (iv) Corrosive soils, ground surrounding sewage effluent etc. shall be avoided for the routes. (v) Route of cables of different voltages. (a) Whenever cables are laid along well demarcated or established roads, the LV/MV cables shall be laid farther from the kerb line than HV cables. (b) Cables of different voltages, and also power and control cables shall be kept in different trenches with adequate separation. Where available space is restricted such that this requirement cannot be met, LV/MV cables shall be laid above HV cables. (c) Where cables cross one another, the cable of higher voltage shall be laid at a lower level than the cable of lower voltage. 1.6.3 Proximity to communication cables Power and communication cables shall as far as possible cross each other at right angles. The horizontal and vertical clearances between them shall not be less than 60cm. 1.6.4 Railway crossing Cables under railway tracks shall be laid in spun reinforced concrete, or cast iron or steel pipes at such depths as may be specified by the railway authorities, but not less than 1m, measured from the bottom of the sleepers to the top of the pipe. Inside railway station limits, pipes shall be laid upto the point of the railway station limits, pipes shall be laid upto a minimum distance of 3m from the center of the nearest track on either side. 1.6.5 Way Leave Way leave for the cable route shall be obtained as necessary, from the appropriate
  54. 54. authorities, such as, Municipal authorities, Department of telecommunication, Gas Works, Railways, Civil Aviation authorities, Owners of properties etc. In case of private property, Section 12/51 of the Indian Electricity Act shall be complied with. 1.6.6 Methods of laying The cables shall be laid direct in ground, pipe, closed or open ducts, cable trays or on surface of wall etc. The method(s) of laying required shall be specified in the tender schedule of work. 1.6.7 Laying direct in ground 1.6.7.1 General This method shall be adopted where the cable route is through open ground, along roads/lanes, etc. and where no frequent excavations are likely to be encountered and where re-excavation is easily possible without affecting other services. 1.6.7.2 Trenching (i) Width of trench The width of the trench shall first be determined on the following basis (Refer figure 1) (a) The minimum width of the trench for laying a single cable shall be 35cm (b) Where more than one cable is to be laid in the same trench in horizontal formation, the width of the trench shall be increased such that the inter-axial spacing between the cables, except where otherwise specified, shall be at least 20cm. © There shall be a clearance of at least 15cm between axis of the end cables and the sides of the trench. (ii) Depth of trench The depth of the trench shall be determined on the following basis (Refer figure 1): (a) Where the cables are laid in a single tier formation, the total depth of trench shall not be less than 75cm for cables upto 1.1KV and 1.2m for cables above 1.1KV. (b) When more than one tier of cables is unavoidable and vertical formation of laying is adopted, the depth of the trench in (ii) a above shall be increased by 30cm for each additional tier to be formed. © Where no sand cushioning and protective covering are provided for the cables as per 2.6.7.3(i)(b), 2.6.7.3(vii)(c) and 2.6.7.3(ix)(d) below, the depth of the trench as per (ii)(a) and (b) above shall be increased by 25cm. (iii) Excavation of trenches (a) The trenches shall be excavated in reasonably straight lines. Wherever there is a change in the direction, a suitable curvature shall be adopted complying with the requirements of clause 2.6.1(ii). (b) Where gradients and changes in depth are unavoidable, these shall be gradual. © The bottom of the trench shall be level and free from stones, brick bats etc. (d) The excavation should be done by suitable means-manual or mechanical. The excavated soil shall be stacked firmly by the side of the trench such that it may not fall back into the trench. (e) Adequate precautions should be taken not to damage any existing cable(s),
  55. 55. pipes or any other such installations in the route during excavation. Wherever trickd, tiles or protective covers or bare cables are encountered, further excavation shall not be carried out without the approval of the Engineer-in-Charge. (f) Existing property, if any, exposed during trenching shall be temporarily supported adequately as directed by the Engineer-in-Charge. The trenching in such cases shall be done in short lengths, necessary pipes laid for passing cables therein and the trench refilled in accordance with clause 2.6.7.4. (g) It there is any danger of a trench collapsing or endangering adjacent structures, the sides may be left in place when back filling the trench. (h)Excavation through lawns shall be done in consultation with the Department concerned. 1.6.7.3 Laying of cable in trench (i) Sand cushioning (a) The trench shall then be provided with a layer of clean, dry sand cushion of not less than 8cm in depth, before laying the cables therein. (b) However, sand cushioning as per (a) above need not be provided for MV cables, where there is no possibility of any mechanical damage to the cables due to heavy or shock loading on the soil above. Such stretches shall be clearly specified in the tender documents. © Sand cushioning as per (a) above shall however be invariably provided in the case of HV cables. (ii) Testing before laying All the time of issue of cables for laying, the cables shall be tested for continuity and insulation resistance (See also clause 2.8.1) (iii) The cable drum shall be properly mounted on jacks, or on a cable wheel at a suitable location, making sure that the spindle, jack etc. are strong enough to carry the weight of the drum without failure, and that the spindle is horizontal in the bearings so as to prevent the drum creeping to one side while rotating. (iv) The cable shall be pulled over on rollers in the trench steadily and uniformly without jerks and strain. The entire cable length shall as far as possible be laid off in one stretch. PVC/XLPE cables less than 120sq.mm. size may be removed by “Flaking” i.e. by making one long loop in the reverse direction. Note: - For short runs and sizes upto 50sq.mm. of MV cables, any other suitable method of direct handing and laying can be adopted without strain or excess bending of the cables. (v) After the cable has been so uncoiled, it shall be lifted slightly over the rollers beginning from one and by helpers standing about 10m apart and drawn straight. The cable shall then be lifted off the rollers and laid in a reasonably straight line.
  56. 56. (vi) Testing before covering The cables shall be tested for continuity of cores and insulation resistance (Refer clause 2.8.1) and the cable length shall be measured, before closing the trench. The cable end shall be sealed /covered as per clause 2.6.1 (iii) (vii) Sand covering Cables laid in trenches in a single tier formation shall have a covering of dry sand of not less than 17cm above the base cushion of sand before the protective cover is laid. In the case of vertical multi-tier formation, after the first cable has been laid, a sand cushion of 30cm shall be provided over the base cushion before the second tier is laid. If additional tiers are formed, each of the subsequent tiers also shall have a sand cushion of 30cm as stated above. Cables in the top most tiers shall have final sand covering not less than 17cm before the protective cover is laid. Sand covering as per (a) and (b) above need not be provided for MV cables where a decision is taken by the Engineer-in-Charge as per sub clause (i)(b) above, but the inter tier spacing should be maintained as in (b) above with soft soil instead of sand between tiers and for covering. Sand cushioning as per (a) and (b) above shall however be invariably provided in the case of HV cables. (viii) Extra loop cable (a) At the time of original installation, approximately 3m of surplus cable shall be left on each terminal end of the cable and on each side of the underground joints. The surplus cable shall be left in the form of a loop. Where there are long runs of cables such loose cable may be left at suitable intervals as specified by the Engineer-in-Charge. (b) Where it may not be practically possible to provide separation between cables when forming loops of a number of cables as in the case of cables emanating from a substation, measurement shall be made only to the extent of actual volume of excavation, sand filling etc. and paid for accordingly. (ix) Mechanical protection over the covering (a) Mechanical protection to cables shall be laid over the covering in accordance with (b) and (c) below to provide warning to future excavators of the presence of the cable and also to protect the cable against accidental mechanical damage by pick-axe blows etc. (b) Unless otherwise specified, the cables shall be protected by second class brick of nominal size 22cmX11.4cmX7 cm or locally available size, placed on top of the sand (or, soil as the case may be). The bricks shall be placed breadth-wise for the full length of the cable. Where more than one cable is to be laid in the same trench, this protective covering shall cover all the cables and project at least 5cm over the sides of the end cables.
  57. 57. © Where bricks are not easily available, or are comparatively costly, there is no objection to use locally available material such as tiles or slates or stone/cement concrete slabs. Where such an alternative is acceptable, the same shall be clearly specified in the tender specifications. (d) Protective covering as per (b) and (c) above need not be provided only for MV cables, in exceptional cases where there is normally no possibility of subsequent excavation. Such cases shall be particularly specified in the Tender specifications. (e) The protective covering as per (b) and (c) above shall, however invariably be provided in the case of HV cables. 1.6.7.4 Back filling (i) The trenches shall be then back-filled with excavated earth, free from stones or other sharp ended debris and shall be rammed and watered, if necessary in successive layers not exceeding 30cm depth. (ii) Unless otherwise specified, a crown of earth not less than 50mm and not exceeding 100mm in the center and tapering towards the sides of the trench shall be left to allow for subsidence. The crown of the earth however, should not exceed 10 Cms so as not to be a hazard to vehicular traffic. (iii) The temporary re-statements of roadways should be inspected at regular intervals, particularly during wet weather and settlements should be made good by further filling as may be required. (iv) After the subsidence has ceased, trenches cut through roadways or other paved areas shall be restored to the same density and materials as the surrounding area and –re-paved in accordance with the relevant building specifications to the satisfaction of the Engineer-in-Charge. (v) Where road beams or lawns have been cut out of necessity, or kerb stones displaced, the same shall be repaired and made good, except for turfing /asphalting, to the satisfaction of the Engineer-in-Charge and all the surplus earth or rock shall be removed to places as specified. 1.6.7.5 Laying of single core cables (i) Three single core cables forming one three phase circuit shall normally be laid in close trefoil formation and shall be bound together at intervals of approximately 1m. (ii) The relative position of the three cables shall be changed at each joint at the time of original installation, complete transposition being effected in every three consecutive cable lengths. 1.6.7.6 Route markers (i) Location Route markers shall be provided along the runs of cables at locations approved by the
  58. 58. Engineer-in-Charge and generally at intervals not exceeding 100m. Markers shall also be provided to identity change in the direction of the cable route and at locations of underground joints. (ii) (a) Plate type marker Route markers shall be made out of 100mm X 5mm GI/ aluminium plate welded / bolted on 35mm X 35mm X 6mm angle iron, 60cm long. Such plate markers shall be mounted parallel to and at about 0.5m away from the edge of the trench. (b) CC marker Alternatively, cement concrete 1:2:4 (1 cement:2 coarse sand: 4 graded stone aggregate of 20mm in size) as shown in figure 2 shall be laid flat and centered over the cable. The concrete markers, unless otherwise instructed by the Engineer-in-Charge, shall project over the surrounding surface so as to make the cable route easily identifiable. (c) Inscription The words „CPWD-MV/HV CABLE‟ as the case may be, shall be inscribed on the marker. 1.6.8 Laying in pipes / closed ducts 1.6.8.1 In locations such as road crossing, entry in to buildings, paved areas etc. cables shall be laid in pipes or closed ducts. Metallic pipe shall be used as protection pipe for cables fixed on poles of overhead lines. 1.6.8.2 (i) Stone ware pipes, GI, CI or spun reinforced concrete pipes shall be used for cables in general; however only GI pipe shall be used as protection pipe on poles. (ii) The size of the pipe shall not be less than 10cm in diameter for a single cable and not less than 15cm for more than one cable. (iii) Where steel pipes are employed for protection of single core cable feeding AC load, the pipe should be large enough to contain both cables in the case of single phase system and all cables in the case of poly phase system. (iv) Pipes for MV and HV cables shall be independent ones. 1.6.8.3 (i) In the case of new construction, pipes as required (including for anticipated future requirements) shall be laid alongwith the civil works and jointed according to the CPWD Building Specifications. (ii) Pipes shall be continuous and clear of debris or concrete before cables are drawn. Sharp edges if any, at ends shall be smoothened to prevent damage to cable sheathing. (iii) These pipes shall be laid directly in ground without any special bed except for SW pipe which shall be laid over 10cm thick cement concrete 1:5:10 (1 cemtnt:5coarse sand:10 graded stone aggregate of 40mm nominal size) bed. No sand cushioning or tiles need be used in such situations. 1.6.8.4 Road crossings (i) The top surface of pipes shall be at a minimum depth of 1m from the pavement level when laid under roads, pavements etc. (ii) The pipes shall be laid preferably askew to reduce the angle of bend as the cable enters and leaves the crossing. This is particularly important for HV cables.
  59. 59. (iii) When pipes are laid cutting an existing road, care shall be taken so that the soil filled up after laying the pipes is rammed well in layers with watering as required to ensure proper compaction. A crown of earth not exceeding 10cm should be left at the top. (iv) The temporary re-instatements of roadways should be inspected at regular intervals, particularly after a rain, and any settlement should be made good by further filling as may be required. (v) After the subsidence has ceases, the top of the filled up trenches in roadways or other paved areas shall be restored to the same density and material as the surrounding area in accordance with the relevant CPWD Building Specifications to the satisfaction of the Engineer-in-Charge. 1.6.8.5 Manholes shall be provided to facilitate feeding/drawing in of cables with sufficient working space for the purpose. They shall be covered by suitable manhole covers. Sizes and other details shall be indicated in the Schedule of work. 1.6.8.6 Cable entry into the building Pipes for cable entries to the building shall slope downwards from the building. The pipes at the building end shall be suitably sealed to avoid entry of water, after the cables are laid. 1.6.8.7 Cable-grip / draw-wires, winches etc. may be employed for drawing cables through pipes / closed ducts. 1.6.8.8 Measurement for drawing/ laying cables in pipes/ closed duct shall be on the basis of the actual length of the pipe / duct for each run of the cable, irrespective of the length of cable drawn through. 1.6.9 Laying in open ducts 1.6.9.1 Open ducts with suitable removable covers (RCC slabs or chequered plates) are generally provided in sub-stations, switch rooms, plant rooms, workshops etc. for taking the cables. The cable ducts should be of suitable dimensions for the number of cables involved. 1.6.9.2 (i) Laying of cables with different voltage ratings in the same duct shall be avoided. Where it is inescapable to take HV & MV cables same trench, they shall be laid with a barrier between them or alternatively, one of the two (HV &MV) cables may be taken through pipe(s). (ii) Splices or joints of any type shall not be permitted inside the ducts. 1.6.9.3 (i) The cables shall be laid directly in the duct such that unnecessary crossing of cables is avoided. (ii) Where specified, cables may be fixed with clamps on the walls of the duct or taken in hooks/brackets/troughs in ducts. 1.6.9.4 Where specified, ducts may be filled with dry sand after the cables are laid and covered as above, or finished with cement plaster, specially in high voltage applications. 1.6.10
  60. 60. Laying on surface 1.6.10.1 This method may be adopted in places like switch rooms, workshops, tunnels, rising (distribution) mains in buildings etc. This may also be necessitated in the works of additions and/or alterations to the existing installation, where other methods of laying may not be feasible. 1.6.10.2 Cables may be laid in surface by any of the following methods as specified: (a) Directly clamped by saddles or clamps, (b) Supported on cradles, (c) Laid on troughs/trays, duly clamped. 1.6.10.3 (i) The saddles and clamps used for fixing the cables on surface shall comply with the requirements given in Table-III. (ii) Saddles shall be secured with screws to suitable approved plugs. Clamps shall be secured with nuts on to the bolts, grouted in the supporting structure in an approved manner. (iii) In the case of single core cables, the clamps shall be of non-magnetic material. A suitable non-corrosive packing shall be used for clamping unarmoured cables to prevent damage to the cable sheath. (iv) Cables shall be fixed neatly without undue sag or kinks. 1.6.10.4 The arrangement of laying the cables in cradles is permitted only in the case of cables of 1.1KV grade of size exceeding 120sq.mm. In such cases, the cables may be suspended on MS flat cradles of size 50mmX5mm which in turn shall be fixed on the wall by bolts grouted into the wall in an approved manner at a spacing of not less than 60cm. 1.6.10.5 All MS components used in fixing the cables shall be either galvanized or given a coat of red oxide primer and finished with 2 coats of approved paint. 1.6.11 Laying on cable tray 1.6.11.1 This method may be adopted in places like indoor substations, air-conditioning plant rooms, generator rooms etc. or where long horizontal runs of cables are required within the building and where it is not convenient to carry the cable in open ducts. This method is preferred where heavy sized cables or a number of cables are required to be laid. The cable trays may be either of perforated sheet type or of ladder type. 1.6.11.2 Perforated type cable tray (i) The cable tray shall be fabricated out of slotted/perforated MS sheets as channel sections, single or double bended. The channel sections shall be supplied in convenient lengths and assembled at site to the desired lengths. These may be galvanished or painted as specified. Alternatively, where specified, the cable tray may be
  61. 61. fabricated by two angle irons of 50mmX50mmX6mm as two longitudinal members, with cross bracings between them by 50mmX5mm flats welded/bolted to the angles at 1 m spacing. 2mm thick MS perforated sheet shall be suitably welded/bolted to the base as well as on the two sides. (ii) Typically, the dimensions, fabrication details etc. are shown in figure 3A,B and C. (iii) The jointing between the sections shall be made with coupler plates of the same material and thickness as the channel section. Two coupler plates, each of minimum 200mm length, shall be bolted on each of the two sides of the channel section with 8mm dia round headed bolts, nuts and washers. In order to maintain proper earth continuity bond, the paint on the contact surfaces between the coupler plates and cable tray shall be scraped and removed before the installation. (iv) The maximum permissible uniformly distributed load for various sizes of cables trays and for different supported span are given in Table IV. The sizes shall be specified considering the same. (v) The width of the cable tray shall be chosen so as to accommodate all the cables in one tier, plus 30 to 50% additional width for future expansion. This additional width shall be minimum 100mm. The overall width of one cable tray shall be limited to 800mm. (vi) Factory fabricated bends, reducers, tee/cross junctions, etc. shall be provided as per good engineering practice. (Details are typically shown in figure 3). The radius of bends, junctions etc. shall not be less than the minimum permissible radius of bending of the largest size of cable to be carried by the cable tray. (vii) The cable tray shall be suspended from the ceiling slab with the help of 10mm dia MS rounds or 25mmX5mm flats at specified spacing (based on Table III). Flat type suspenders may be used for channels upto 450mm width bolted to cable trays. Round suspenders shall be threaded and bolted to the cable trays or to independent support angles 50mmX50mmX5mm at the bottom end as specified. These shall be grouted to the ceiling slab at the other end through an effective means, as approved by the Engineer-in-Charge, to take the weight of the cable tray with the cables. (viii) The entire tray (except in the case of galvanized type) and the suspenders shall be painted with two coats of red oxide primer paint after removing the dirt and rust, and finished with two coats of spray paint of approved make synthetic enamel paint. (ix) The cable tray shall be bonded to the earth Terminal of the switch bonds at both ends. (x) The cable trays shall be measured on unit length basis, along the center line of the cable tray, including bends, reducers, tees, cross joints, etc. and paid for accordingly. 1.6.11.3 Ladder type cable tray (i) The ladder type of cable tray shall be fabricated of double bended channel section longitudinal members with single bended channel section rungs of cross members welded to the base of the longitudinal members at a center to center spacing of 250cm.

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