What are different elements of a Cable?
Basic about cable components & constructional details.
What are the type designations of cable?
Nomenclature of the cables.
What are different cable types and categories?
Comparison of XLPE cables with PVC cables.
What is screening in cable and why it is required?
What is FRLS H cable?
What are the general Packing practices followed?
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
An insulated conductor is termed as Core. A cable may have one or
more cores. In a multiple core cable , the cores are helically
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.
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.
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
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.
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
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,
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.
details of various cable types can be checked in catalogues.
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Comparison of XLPE cables with PVC cables
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
70°C to 90°C and the temperature for short circuit ratings for the
cable from 160°C to 250°C.
general, XLPE insulated cables are competitive alternative to PVC
cables for industrial use and Paper insulated cables for public
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
satisfy these functions, the screening normally comprises a
combination of conducting layers with metallic elements.
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.
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.
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 90°C 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.
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
FRLS H cable
Retardant Low Smoke and Low Halogen cables have improved flame
resistant characteristics and emit lower smoke and toxic gases.
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.
General Packing Practices
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.