• Electrical India
  • Jun 5, 2017

CABLE
Power Carrier

Cables are integral part of every technology and engineering field. Frequent research and development is required to compete with the fast growing infrastructure of other countries by improving the ratings and durability of cables so as to open new applications...

- Simmi Sharma


 Electric power is transmitted and distributed either by overhead system or by underground cables. Cables are mainly designed as per requirement. An electrical cable is an assembly consisting of one or more conductors with their own insulations, individual coverings, assembly protection and protective coverings. Electrical cables can be made more flexible by stranding the wires. Electrical cables are used for wiring in households for solving the purpose of lighting for power and control circuits that are permanently installed. Cables are securely fastened and organized by using cable trays, cable ties or cable lacing. In building construction, an electrical cable jacket material is a source of fuel for fires. To limit the spread of fire along cable jacketing use of cable coating materials with jacketing having inherently fire retardant is used. The plastic covering on metal clad cables is stripped off at installation to reduce the fuel source for fires. Inorganic coatings and boxes around cables also safeguard the nearby areas from the fire threat associated with unprotected cable jacketing. This fire protection also traps heat generated from conductor losses so the protection needs to be thin.

  At high frequencies current tends to run along the surface of the conductor. This is known as the skin effect. Electric cables connect two or more devices, thereby, allowing the transfer of electrical signals and power from one device to another. In this process, smaller individual wires are twisted to produce larger wires that are more flexible than solid wires of same size. Bunching of these small wires before concentric stranding provides more flexibility. Tinning is used to provide lubrication between strands. Tinning helps in removal of rubber insulation. Copper wires can be bare or they can be plated with a thin layer of another metal usually tin, gold, silver as these are less prone to oxidation than copper, which lengthens life of wire and makes soldering easier. In fiber optics, these cables contain one or more optical fibers in a protective jacket that supports the fibers. These network cables are bendable although they are made up of glass. They are useful in wide area network (WAN) installations where long distance underground or outdoor cable runs are required and also in office buildings that require a high volume of communication traffic.

  Optical fibre is a cable with following advantages:

• Light-weight
• Immune to noise
• Low attenuation
• Tolerates data rates on the order of 100 Mbps
• Bandwidth from tens of megahertz to several gigahertz.

  Undersea cables are used for long distance communication. Power cables are used for bulk transmission of alternating and direct current. Continuous-flex / flexible cables find utilization in moving applications.

  A current carrying conductor radiates an electromagnetic field and a cable picks up energy from this existing electromagnetic field around it. These effects are often undesirable in transmission of energy as it adversely affects nearby equipment and its parts. This can be prevented by keeping the cable length short as pickup and transmission are proportional to the length of the cable. A cable can cause unwanted pickup of noise which can mask the desired signal carried by the cable. The cable carrying power supply or control voltages even gets polluted to such an extent that it causes malfunctioning of the equipment. The preventive measure in regard is to route cables away from disturbance point. For this three principal design techniques are proposed:

• Twisted-pair Geometry
• Shielding Geometry
• Coaxial Geometry

  Shielding uses the electrical principle of the Faraday cage. The cable is encased for its entire length in a wire mesh and wires running inside this shielding layer are decoupled from external electrical fields. Simple shielding of this type is not useful in low-frequency magnetic fields. A grounded shield on cables operating at 2kV or more gathers leakage current and capacitive current thus protecting living beings from electric shock and equalizes stress on the cable insulation.

Cable Designs

1. Twisted pair has two wires of a cable twisted around each other. It is demonstrated by putting one end of a pair of wires in a hand drill and turning while maintaining moderate tension on the line. The interfering signal has a long wavelength compared to the pitch of the twisted pair. Alternate lengths of wires develop opposing voltages that cancel the interference effects.
2. Coaxial design is used to reduce low-frequency magnetic transmission and pickup. In this design the foil or mesh shield has a circular cross section and the inner conductor is placed at its centre to provide the voltages induced by a magnetic field between the shield and the core conductor to form two nearly equal magnitudes cancelling each other. They are used for radio frequency signals example in cable television distribution systems.
3. Paired cable consists of two individually insulated conductors that are generally used in DC or low-frequency applications.
4. Ribbon cable are required when many wires are to be used. This type of cable easily provides flex and it is designed to handle low-level voltages.
5. Twin-lead is a flat two-wire line. It is also called a 300 Ω line because the line has an impedance of 300 Ω. It is used as a transmission line between an antenna and a receiver like ones used in TV and radio. These cables are stranded to lower skin effects.
6. Hybrid optical and electrical cables are used in wireless outdoor fiber-to-the-antenna (FTTA) applications. The optical fibers carry information and the electrical conductors are used to transmit power. These cables are placed in several environments example to serve antenna mounted on poles or towers.

Some other types of cables are:

• Communications cable
• Direct-buried cable
• Flexible cables
• Non-metallic sheathed cable.
• Universal Serial Bus (USB) cables feature twisted pair wiring.
• Multicore cable
• Portable cord – Flexible cable for AC power in portable applications
• Shielded cable – Used for sensitive electronic circuits or to provide protection in high-voltage applications.
• Single cable
• Submersible cable
• Twinax cable
• Serial and parallel cables: used in older days.
• Cross over cables : The use of Ethernet crossover cables was especially common on older home networks years ago while connecting two PCs directly together.

  Earlier in homes, the electrical system was made up of single conductor wires. However, these days much of electrical wiring is done with multi conductor cables because of their ease of use. The purchase of wire is done by the foot or in spools that range from 50 to 500 feet. The most common multiconductor cable is made from metal or plastic and contains a neutral wire, a hot wire and a grounding wire. The neutral and hot wires are insulated by a thermoplastic material, the grounding wire can or can not be insulated. The wires are color-coded to guard for protection against connection errors. Neutral wires are white or gray, grounding wires are green, and hot wires are of any other colour.

  The cable are also designed according to number of conductors they contain and their size. The larger the diameter, the greater the wire’s capacity is to carry current.

  The mostly commonly used metal for wires is copper due to its efficiency as a conductor. Wire also comes in aluminum and aluminum clad with copper but these are not efficient in conducting current. So the wires are required to be of large size to achieve the same capacity.

  Power cables are mainly used for power transmission and distribution purpose. It is an assembly of one or more individually insulated electrical conductors, usually held together with an overall sheath. The assembly is used for transmission and distribution of electrical power. Electrical power cables can be installed as permanent wiring within buildings, buried in the ground and run overhead or exposed. Flexible power cables are used for portable devices, mobile tools and machinery. These are designed and manufactured as per voltage, current to be carried, operating maximum temperature and purpose of applications desired by customer. For mining, extra mechanical strength is provided to cable with double armouring. For wind power plant customers generally require flexible and UV protected cable with mechanical tough sheath. The underground cables have several advantages such as less liable to damage through storms, lightning, low maintenance cost, less chances of faults, smaller voltage drop and better general appearance. Voltage drop is also an important factor of cable design. As V=IR , COPPER is preferred over aluminium due to lesser voltage drop.

  A power cable consists of :

1. Conductor
2. Bedding
3. Armouring
4. Outer sheath
5. Insulation

  Another important factor is the mechanical and chemical properties of the connection technologies. In many applications cables withstand intense heat and welding sparks. Some of them are tabulated in Table 1.

Conclusion

  Cables are integral part of every technology and engineering field. Frequent research and development is required to compete with the fast growing infrastructure of other countries by improving the ratings and durability of cables so as to open new applications.


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