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    FEATURED QUESTIONS                                                                                                    ..// updated : 16th November '05

 
  1. What are different elements of a Cable?
  2. Basic about cable components & constructional details.
  3. What are the type designations of cable?
  4. Nomenclature of the cables.
  5. What are different cable types and categories?
  6. Comparison of XLPE cables with PVC cables.
  7. What is screening in cable and why it is required?
  8. What is FRLS H cable?
  9. What are the general Packing practices followed?

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  • Conductor

    Conductor is a current carrying element made up of Aluminium or Copper with a specific cross section for the assigned rating, having resistance / km within certain specified limits.

     
  • Electrical Insulation (dielectric)

    Insulation is provided over the conductors to electrically isolate them from one another. Various types of insulating materials used are : PVC, XLPE (Cross Linked Polyethylene), Rubber, Impregnated Paper, etc.

    An insulated conductor is termed as Core. A cable may have one or more cores. In a multiple core cable , the cores are helically stranded together.
    In twin, three & multi-core cables, the cores are laid up together with a suitable lay; the outer most layer has right-hand lay and the successive layers are laid with opposite lay direction.

     
  • Inner sheath (bedding)

    Cables with stranded cores are provided with Inner Sheath applied either by extrusion or by wrapping. It is ensured that the shape is as circular as possible. Inner Sheath is so applied that it fits closely on the laid up cores & it should be possible to remove it without damage to the insulation. Thickness of Inner Sheath is specified in relevant standard and is based on calculated diameter over laid-up cores.

     
  • Armour

    A wire, strip or a tape applied helically over the cable, to protect the cable from penetration by sharp objects, crushing forces, and damage from rodents or boring insects, is termed as Armour.
    Armour is applied over the insulation in case of single core cables & over inner sheath in case of twin, three & multi core cables.

     
  • Outer Sheath

    Outer sheath /Jacket is usually an extruded plastic cover over the laid-up or armoured core. PVC is a common sheathing material. Outer sheath provides mechanical, thermal, chemical and environmental protection. No electrical function is assigned to Outer sheath. The colour of outer sheath is normally black.
     

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Cables can be divided into large number of types based on a combination of classifications as follows:

  • Based on the voltage ratings as low voltage, high voltage, extra high voltage cables, etc.
  • Based on the conductor material, Copper conductor or Aluminium conductor.
  • Based on the insulating material as Paper Insulated, PVC insulated, Rubber insulated, XLPE insulated, etc.
  • Armoured or Unarmoured cables.
  • Based on the sheathing material, as PVC Sheathed, Rubber Sheathed, Lead Sheathed, Aluminium Sheathed, etc.
  • Based on the number of cores as single core, two core, three core, three-and-a-half core, four core, multicore, etc.
  • Based on the cross-section of the conductor.
  • Based on the type of conductor, solid, stranded, sector shaped, etc.

From above it is evident that the types of cables will differ widely depending upon various factors involved. To facilitate identification and description of the type of cable it is general practice to adopt some form of type designations. With these designations it becomes easier to convey in a few words the right type of cable.

The details of various cable types can be checked in catalogues.

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Comparison of XLPE cables with PVC cables

The important difference is the extra toughness of insulation and, in particular, the ability to withstand much high temperature without deformation due to mechanical pressure. The better physical properties of XLPE enable the insulation thickness to be reduced and hence overall size of the cable. The continuous temperature rating is increased from
70C to 90C and the temperature for short circuit ratings for the cable from 160C to 250C.

In general, XLPE insulated cables are competitive alternative to PVC cables for industrial use and Paper insulated cables for public supply systems.

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Screening in Cable

Electrical Screening is necessary only for cables with phase voltage > 1 kv and fulfills the following functions.

  • Potential grading and limiting of electrical fields
  • Conduction of charge and discharge currents

To satisfy these functions, the screening normally comprises a combination of conducting layers with metallic elements.

The magnitude of electric stress and the degree of sensitivity of the insulation material against partial discharge govern the type of screening of the insulation with conducting layers.

Above certain voltages, as a means of containing the electrical field within the insulation, semiconducting screens are applied over the conductor & insulation. Screens are provided to achieve symmetrical dielectric fields within the cable structure & carry current during short circuit. By this it is possible to eliminate any electrical discharges arising from air gaps adjacent to the insulation.

The coefficient of expansion of polyethylene & EPR is approximately ten times greater than that of either Copper or Aluminium, and when the conductor is at its maximum operating temperature of 90C a sufficiently large gap is formed between the insulation and the surface of the conductor to enable electrical discharge to occur. This discharge site & any others which are formed around a conductor when conductor is bent can be eliminated by applying a semiconducting layer over the conductor. Similarly, any discharges arising from air gaps between laid-up cores can be nullified by the use of a screen over the insulation.

During the early 1960s semiconducting tapes were applied the conductor but these have since been superseded by an extruded layer. This has the advantage of providing both a smoother finish and, as it fills the interstices between the wires, a circular envelop around the conductor. By reducing the concentration of the flux lines around the individual wires, the electrical stress around conductor is reduced by between 10% to 15%. The semiconducting layer is compatible with, and bonds to, the insulation and a nominal thickness of 0.7 mm is typical

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FRLS H cable

Flame Retardant Low Smoke and Low Halogen cables have improved flame resistant characteristics and emit lower smoke and toxic gases.

Special features

Reduced flame propagation
Low smoke emission
Low acid gas generation

Formulation for the FRLS compounds, which are mainly for the sheathing materials, require special ingredients. These cables may have insulation of PVC or XLPE, but sheath is PVC based, suitably compounded to meet FRLS H requirements.

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General Packing Practices

Cables are generally received on wooden/steel drum. There is an arrow painted on the flanges of the drum which indicate the direction in which the drum should be rolled. The cable will unwind and become loose if the drum is rolled in the opposite direction. All drums should be stored in such a manner as to leave sufficient space between them for air circulation. In no case should the drums be stored ' on the flat' i.e. with flange horizontal.
 

 

 

 

2005-06 Copyright Ecko Cables Pvt Ltd. All rights reserved.

 
 

 2005-06 Copyright Ecko Cables Pvt Ltd. All rights reserved.