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CABLE TERMINOLOGY
SBEE Heavy Duty Cables : a) Letters used in Type Designations
:
A = Appearing as a first letter denotes Aluminium Conductor. Y =
(PVC) Insulation or (PVC) Sheath depending on the position in which
it appears. 2X = (Cross-linked Polyethylene) Insulation. W = Round
Steel Wire Armouring. WW = Double Round Steel Wire Armouring. F =
Formed Steel Wire (Strip) Armouring. FF = Double Formed Steel Wire
(Strip) Armouring. C = Metallic Screening (Usually of Copper). CE =
Metallic Screening (usually of Copper) over each individual core.
Gb = Holding Helix Tape (of Steel) Wa = Aluminium Round Wire &
Aluminium Formed Wire (Strip) Fa Armouring
b) Type Designations :
AYY Aluminium Conductor, Insulated, Outer Sheathed Heavy Duty
Cables. AYWY Aluminium Conductor, Insulated, Galvanised Round Steel
Wire Armoured and Outer Sheathed Heavy Duty Cables. AYFY Aluminium
Conductor, Insulated, Galvanised Flat Steel Wire (Strip) Armoured
and Outer Sheathed Heavy Duty Cable.
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CABLE TERMINOLOGY
AYCY Aluminium Conductor, Insulated, Metallic Screened and Outer
Sheathed Heavy Duty Cable. A2XCY Aluminium Conductor, TROPOTHEN-X
Insulated, Metallic Screened and TROPODUR Outer Sheathed Heavy Duty
Cable. AYCEFY Aluminium Conductor, Insulated, individual core
metallic screened, Flat steel Wire (strip) Armoured and Outer
Sheathed Heavy duty Cable. A2XCEFY Aluminium Conductor Insulated,
individual cores metallic screened, Flat Steel Wire (Strip)
Armoured and Outer Sheathed Heavy Duty Cable.
NOTE : If the first letter A is dropped in the above
designations (i.e. YY, YWY, YFY etc.) they will denote
corresponding types with Copper Conductors.
YWWGby(Mining) Copper Conductor Insulated, double round wire
armoured and Outer Sheathed Heavy Duty Mining Cable.
YFFGby(Mining) Copper Conductor Insulated, double flat wire
(strip) armoured and Outer Sheathed Heavy Duty Mining Cable.
Paper Cables :
a) Letters used in Type Designations :
A = Appearing as a first letter denotes Aluminium Conductor. P =
Paper Insulation L = Lead Sheath Ly = Lead Alloy Sheath
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CABLE TERMINOLOGY
S = Lapped bedding or serving of compounded fibrous materials
(when it comes below armouring in armoured cables, it is known as
bedding). Y = Extruded PVC Bedding or Serving. T = Double steel
tape armouring. Tg = Double steel tape armouring galvanised. W =
Single round steel wire armouring. WW = Double round steel wire
armouring. F = Single formed steel wire (strip) armouring. FF =
Double formed steel wire (strip) armouring.
b) Type Designations:
APLS Aluminium Conductor, Paper Insulated, Lead Sheathed and
Served Cable. APLyS Aluminium Conductor, Paper Insulated, Lead
Alloy Sheathed and Served Cable. APLy Aluminium Conductor, Paper
Insulated, Lead Sheathed and overall PVC Sheath protected Cable.
APLyY Aluminium Conductor, Paper Insulated, Lead Alloy Sheathed and
overall PVC Sheath protected Cable. APLSTS Aluminium Conductor,
Paper Insulated, Lead Sheathed, double steel tape armoured and
served Cable. APLySTS Aluminium Conductor, Paper Insulated, Lead
Alloy Sheathed, double steel tape armoured and overall PVC Sheath
protected cable.
NOTE :
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CABLE TERMINOLOGY
1. If the first letter A is dropped in the above designations
(i.e. PLS, PLSTS, PLSTY, etc) they will denote the corresponding
types with Copper Conductors.
2. If the cable is armoured with round steel wire armouring or
formed steel wire (strip) armouring replace T in above by W or F
respectively e.g. APLSWS, APLSFS, etc.
3. If the cable is armoured with double round steel wire
armouring or double formed steel wire (strip) armouring replace T
by WW or FF respectively e.g. APLSWWS, APLSFFS, etc.
4. If the cable is armoured with galvanised double steel tape,
replace T by Tg e.g. APLSTgS.
5. In the case of H type cable with common Lead Sheath, add
suffix (screened) e.g. APLSTS (screened) APLSTY (screened) etc.
6. In the case of HSL type cable with separately Lead Sheathed
cores, add suffix (screened SL) e.g. APLSTS (screened SL), APLSTY
(screened SL) etc.
For completely specifying the cable, the type designation is to
be followed by indication of number of cores, nominal cross-section
of conductor, construction of conductor and voltage rating of cable
as shown in the following examples :
1. AYY 3 1/2 x 50/25 sm 0.65/1.1 kV
2. AYFY 3 x 185sm 3.8/6.6 kV (E)
3. AYCY 1 x 400 rm 6.35/11 kV (E)
4. AYCEFY 3 x 240 rm/v 6.35/11 kV (E)
5. A2XCEFY 3 x 300 rm/v 12.7/22 kV (E)
6. YWWGbY 3 x 16 rm/12 (Mining) 0.65/1.1 kV
7. YFFGbY 3 x 70 sm/35 (Mining) 1.9/3.3 kV
8. APLS 1 x 630 rm 3.8/6.6 kV (E)
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CABLE TERMINOLOGY
9. APLySWY 3 x 240 sm 6.35/11 kV (E)
10. APLSTS 3 x 400 sm 11 kV (UE)
11. APLSTS (Screened) 3 x 150 sm 12.7/22 kV (E)
12. APLSTgY (Screened) 3 x 300 rm 19/33 kV (E)
NOTE :
i. The nominal cross-section of conductors indicated are in
sq.mm.
ii. In Item (1) the cross-section indicated as ' 50/25 ' mean
that the main conductors are of nominal cross-section 50 sq.mm. and
the neutral conductor is of nominal cross-section 25 sq.mm.
iii. The construction of conductor is indicated as follows :
re : circular, solid conductor rm : circular, multiwire stranded
conductor rm/v : circular, compact multiwire stranded conductor sm
: shaped, multiwire stranded conductor
In Items (6) and (7) the examples Mining Cables the figures ' 16
rm/12 ' & ' 70 sm/35 ' denote that the nominal cross-section of
conductor is 16 sq.mm. (circular, multiwire stranded), 70 sq.mm.
(shaped, multi-wire stranded) and the minimum effective
cross-sections of armouring are equivalent to 12 sq.mm. of copper*
and 35 sq.mm. of copper* respectively.
(*or Aluminium in case of Aluminium conductor Mining cables)
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CABLE TERMINOLOGY
Rubber Cable :
For Cables other than Ship Wiring :
Constituent Code Letter Aluminium Conductor A Elastomer (Rubber)
Insulation R Copper wire braiding or helical screening of Copper
wires
C
Semi-conducting extruded Screening over & below
Insulation
Cg
Copper wire braiding or helical screening of Copper wires on
individual cores
CE @
Glass Fibre Yarn braiding C Textile Braiding B Synthetic Yarn
Braiding S Textile Braiding on individual cores BE @ Synthetic Yarn
Braiding on individual cores SE @ Steel round wire armouring W
Double steel round wire armouring WW Steel strip armouring F Double
steel strip armouring FF Non-magnetic round wire armouring Wa
Non-magnetic strip armouring Fa Steel wire braiding Wb Steel wire
pliable armouring Wp Elastomer (Rubber) Outer Sheath R PVC Outer
Sheath Y
Normal flexible conductor is denoted as 'rf' & special (as
per client requirement) flexible conductor is denoted as "rff".
When the conductor material is Copper no Code Letter is requried
for conductor @ In case only some of the cores are individually
screened/braided the number of such cores shall be indicated as
suffix to the Code Letter e.g. CE4, BE4, etc
For Ship Wiring Cables :
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CABLE TERMINOLOGY
Constituent Code Letter Ship Wiring Cable (Marine use) M
Elastomer (Rubber) Insulation G Braiding of Steel/Copper/Annealed
Tinned Copper
C
Elastomer (Rubber) Outer Sheath G PVC Outer Sheath Y
NOTE :
Material of braiding shall be indicated in brackets at the end
as follows :
Plain Copper (CU) and Annealed Tinned Copper (ATC)
Examples for the above code letters :
1) Annealed tinned Copper flexible conductor of nominal
cross-section 25Sqmm insulated with Rubber Compound, provided with
Copper wire braiding on individual cores, for cores stranded
together, inner sheathed, provided with steel wire braiding and
overall sheathed with Rubber compound.
The Type Designation shall be RCEWbR 4 x 25 rf
2) Annealed tinned Copper (2.5 sq.mm) flexible conductor,
insulated with silicon rubber, overall braided with Glass Fibre
Yarn and varnished cable. The type designation shall be RG 1 x 2.5
rf.
3) Annealed tinned copper flexible conductor of nominal
cross-section 2.5 sq.mm insulated with Rubber Compound, 2 cores
provided with protective screen of annealed tinned Copper wire
braiding, 2 screened cores and 14 un-screened cores laid up
together in one layer, inner sheathed, provided with annealed
tinned Copper wire braiding and overall sheathed with Rubber
Compound with reinforcement of Synthetic Yarn Braid.
The Type Designation shall be RCE2CSR (14 + 2) x 2.5rf
4) Stranded conductor of annealed tinned Copper of nominal
cross-section 2.5 sq.mm, insulated with Rubber Compound, 3 cores
stranded together, inner sheathed, provided with
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CABLE TERMINOLOGY
plain Copper/annealed tinned copper/steel wire braiding and
overall sheathed with Rubber/PVC Compound.
The Type Designation shall be :
With Plain Copper Wire : MGCG (Cu) 3 x 2.5 rm Braiding : MGCY
(Cu) 3 x 2.5 rm
With annealed tinned : MGCG (ATC) 3 x 2.5 rm Copper Wire
braiding : MGCY (ATC) 3 x 2.5 rm
CABLE GLOSSARY
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Ampacity: Current carrying capacity of a cable is known as its
ampacity.
This property is unique for each wire/ cable & is influenced
by
Conductor material, number of conductors & cross- sectional
area
Thermal conductivity & Thickness of insulation
Ambient temperature
Ability of construction to dissipate heat after installation
Armouring (Or Armour): A metal covering usually applied in the
form of tape or wire, intended to protect a cable from mechanical
damage.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
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CABLE TERMINOLOGY
Bobbins: Metal spools used for taking up drawn wire and
subsequently used for payoff packages in cabling and stranding
equipment
Braid : A fiberous or metallic group of filaments interwoven in
cylindrical form to form a covering over one or more wires.
Braider: a machine used to apply a woven fiberous or metallic
braid over a cable.
Braiding: The plaited protective covering of a cable.
Barrier Joint
: A cable joint between two mass-impregnated cables in which the
impregnating compound in each cable is separated from that in the
other.
Bedding (of an armoured cable): A layer or layers of material
applied to a cable beneath the armour.
Bunched Stranding : A term applied to a number of wires twisted
together in one direction in one operation without any regard to
their geometric arrangement.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Cable: An assembly of one or more conductors, ether solid or
stranded, each covered with a layer of insulating material
throughout its length, the whole being provided with a common
protective covering.
Cable Filler : A material used in a multiple-core conductor
cable to occupy space and thus round up the cable, sometimes used
to accomplish conductor spacing.
Cable Terminal Box: A box fitted at the end of a cable in order
to facilitate the connection, and sometimes the quick disconnection
of other conductors.
Calculated Effective Sectional Area: The area of a solid
conductor of the same resistivity and having the same resistance as
that of any equal length of the conductor. In the case of a split
conductor cable, the calculated effective area is the sum of the
cross sections of each of the two or more sections into which the
conductor is divided.
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CABLE TERMINOLOGY
Compacting : Compact stranding, are conductor constructions that
can add little more ampacity into a tight space.
Conductor: A body or substance, which offers a low resistance to
the passage of an electric current.
Continuous Vulcanisation (CV) : A continuous, in-line process
whereby a wire has an extruded covering applied, is then passed
through a tube containing such temperature and pressures as are
necessary to complete vulcanisation.
Control Cable : A multi-conductor cable made for operating in
control or signal circuits, usually flexible, relatively small in
size, and with relatively small current rating.
Core: Assembly comprising a conductor and its own
insulation.
Corona Discharge: A phenomenon called corona discharge may occur
in high voltage transmission line , resulting in formation of
ozone, a highly reactive form of oxygen, and in ionisation of
oxygen in the surrounding air. The insulation may be attacked by
ozone and by corona
Corona partial discharge may also occur in a void within
insulation system where the voltage gradient is sufficiently
high.
Cross-Sectional Area: The sum of the cross-sectional area of the
component wires of the conductor of a cable, the cross sectional
area of each wire being measured perpendicular to its individual
axis.
Current Carrying Capacity: The current a conductor of a given
size is capable of carrying safely without exceeding its own
temperature limitations, at a defined set of conditions.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Direction of Lay: Defined as right-hand or Left-hand these terms
have the same meaning as specified for screw threads. It is said to
be right-handed if, when assembled with a fixed mating thread and
twisted in a clockwise direction, it moves away from the operator;
and left-handed if, when assembled with a fixed mating thread and
twisted in a clockwise direction, it approaches the operator. The
right-hand lay is also know as Z-lay and left hand lay as
S-Lay.
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CABLE TERMINOLOGY
Distributor: The portion of any underground cable with which a
service line is, or is intended to be, immediately connected.
Dividing Box: A box fitted to one end of a two, three, four core
or multicore cable for termination.
Dielectric : A material with good electric insulating
characteristics, insulating medium.
Dielectric Constant : A term used to define the degree of
insulating characteristics possessed by a dielectric.
Typical Values of Dielectric Constant
Material Dielectric Constant PVC 3.4 - 8.0 EPR 2.5 - 3.5 PE 2.5
- 2.6 XLPE 2.3 - 6.0
Drain wires : A number of small gauge bare wires applied
concentrically about the insulation shield of a high voltage cable
for the purpose of a fault current return path.
Drawing : The process of reducing a cylindrical rod or wire to a
desired diameter by pulling the wire through a die or series of
dies thus stretching the wire.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
EPR : Ethylene Propylene Rubber
Extrusion : The application of a semi-solid plastic or rubber
material by forcing it on a wire passing through the extruder in a
continuous fashion.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Feeder: A line, which supplies a point of distribution network
without being tapped at any intermediate point.
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CABLE TERMINOLOGY
Fillers (of a cable): The material used to fill the interstices
between the cores of a two-, three-four-core or multicore
cable.
Fully-Impregnated Insulation: Mass-impregnated insulation where
no attempt has been made to remove free compound after
impregnation.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Galvanisation : A coating of some metal part (usually steel or
iron) with zinc by dipping or electroplating.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Insulation : A non-conductive material usually surrounding or
separating two or more conductive materials.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Jacket : A covering put around an insulated conductor for the
purpose of protection and/or resistance.
Properties of Jacketing Material
Units PVC PE
Continuous Service Temperature of conductors
deg. C 70 75
Installation Temperature (min) deg. C -10 -40 Tensile Strength
(min) PSI 1500 1400 Elongation (min) % 100 350 Specific Gravity -
1.43 0.93
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CABLE TERMINOLOGY
Joint Box: A box to protect the insulation of a cable from air
or moisture at a cable joint.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Lay : The distance taken to complete one revolution of helically
laid strand of wire around a central core.
Lay Direction : A simple means of determining the direction of
lay is that- when looking along a strand, the individual wires
disappear in the forward direction to the left, the strand is said
to left handed and if to the right, right handed.
Lay Ratio: The ratio of the axial length of complete turn of the
helix formed by the core of a cable or the wire of a stranded
conductor, to the mean diameter of the helix.
Length of lay(LAY): The axial lenth of one complete turn of the
helix formed by the core in the case of a cable, or of the wire in
the case of a stranded conductor.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Mass-Impregnated and Drained Insulation: Mass- impregnated
insulation from which free impregnation compound is removed by
draining at a temperature in excess of the maximum working
temperature.
Mass-Impregnated Insulation: Insulation in which the paper tapes
are applied un-impregnated, the complete insulation being
subsequently dried and impregnated with compound as a whole.
Mass-Impregnated Non-Draining Insulation- Insulation in which
the impregnating compound has a sufficiently high viscosity at
maximum working temperature to preclude migration of compound or
the draining of compound under service conditions.
Mean Diameter of a wire: The mean of two measurements taken at
right angels at the same cross sections.
Mains, Underground: All underground cables used for the
transmission and distribution of electrical energy and includes
feeders, distributors and pilot cables.
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CABLE TERMINOLOGY
Messenger : A bare cable used for its strength characteristics
to support power conductors and insulated power cables. A messenger
can be used as a conductor, partial conductor, or
non-conductor.
Milliken conductors : With alternating current there is a
tendency for more of the current to be carried on the outside of
the conductor than in the centre (skin effect), and to overcome
this problem the larger sizes of conductor are frequently of
Milliken construction. Such conductors are formed from several
individual sector shapes (usually four for power cables). A thin
paper or other suitable insulation is applied over alternate
sectors. There is insufficient economic advantage to use this
construction below 1000 mm2 but Milliken design may also be used to
obtain increased conductor flexibility.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Neutral Conductor : In multiphase circuits the conductor used to
carry unbalanced current and in single phase systems the conductor
used for a return current path.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Oxygen Index : It is the minimum of oxygen in an oxygen
-nitrogen mixture in which the material will burn(air contains 21 %
oxygen) .
In the oxygen index test, temperature is approximately
maintained at room temperature.
In actual practice during fire the extent of burning may be
significantly influenced by the actual temperature involved. In
order to overcome this, the oxygen index is measures over a range
of temperature. From the results, a temperature index is obtained
by extrapolation. As the relationship is non-linear, extrapolation
results are not accurate.
A more valid method is maintaining the oxygen concentration at
21% and varying the temperature, the temperature index being
recorded as the minimum temperature at which a material will
support combustion following its burning.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Pay-off : The process of feeding a cable or wire from a bobbin,
reel, or other packages. Also a device used for paying out wire or
cable into a piece of equipment or machinery.
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CABLE TERMINOLOGY
Pitch Circle Diameter : The diameter of a circle passing through
the centre of the conductors in any layer of a multi-conductor
cable.
Proofed Tape : A tape applied to the insulation of rubber
insulated cables and composed of cotton cloth coated with the
rubber compound.
Polyvinyl Chloride (PVC) Sheathing: A sheathing of PVC compound
Used on an insulated cable or flexible cord to form an outer
protective covering so as to make the cable or cord reasonably
resistant to decay, mechanical abrasion, acids, alkalies and other
corrosive materials.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Resistance : The property of an electric circuit which
determines, for a given current, the rate at which electric energy
is converted into heat and has a value such that the current
squared, multiplied by the resistance gives the power
Rated Voltage : The voltage at which the cable is designed to
operate. In the case of ac system, the rated voltage means the
voltage between phases.
Reinforcement (Against Internal Pressure): A covering consisting
of metal tapes or strips or wires used to enable the cable to
withstand internal pressure.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Sealing, End (Sealing Box or Sealing Chamber) A box fitted to
one end of a single core cable to protect its insulation from air
or moisture at the point where connection is made with another
conductor.
Service Line : A line connecting the consumers installation to
the distributor. Service of an armoured or metal sheathed cable: A
layer or layers of material applied as a final
covering to the outside of a cable to protect it.
Sheath ( of a Cable) : A uniform and continuous covering used to
protect the insulation,
especially against moisture, or to protect an inner metallic
sheath or armour against corrosion.
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CABLE TERMINOLOGY
Stop Joint: A cable joint between two pressure cables in which
the fluid in each cable is
separated from that in the other cable by pressured resisting
barrier.
Straight through joint: A cable joint connecting two cable
together end to end.
Short Circuit Rating : It is required to determine cross
sectional areas of conductor and screen in respect of short circuit
current
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Tee-Joint: A cable where a branch connection is made to a main
cable.
Tough Rubber Sheathing: A sheathing used on an insulated cable
to form an outher protective
covering of tough rubber. It is composed of rubber mixed with
hardening substances and
suitably vulcanized to make it waterproof and reasonably
resistant to deca y, mechanical
abrasion, acides, alkalies and other corrosive materials.
Trifurcating Joint: A box connecting a three core cable to three
single core cables.
Temperature Index: Temperature index is the temperature at which
oxygen index becomes 21.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Waterproof Servicing: A layer or waterproof material applied to
the exterior of an armoured or lead sheathed cable.
Wire: Composed of a conducting material, uniform in diameter and
circular in cross section.
Useful Electrical Formulae for Determining Amperes, Horsepower,
Kilowatts and
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CABLE TERMINOLOGY
Kilovolts Amperes
To find Direct Current Alternative Current
Single Phase Three Phase
Amperes when Horsepower is known
HP x 746 E x Eff
HP x 746 E x Eff x PF
HP x 746 1.73 x E xEff x PF
Amperes when Kilowatts is known
KW x 1000 E
KW x 1000 E x PF
KW x 1000 1.73 x E x PF
Amperes when Kilovolt is known
KVA x 1000 E
KVA x 1000 E
KVA x 1000 1.73 x E
Kilowatts I x E 1000
I x E x PF 1000
I x E x 1.73 x PF 1000
Kilovolt Amperes I x E 1000
I x E 1000
I x E x 1.73 1000
Horsepower(output) I x E x Eff 746
I x E x Eff x PF 746
I x E x 1.73 x Eff x PF 746
Notes :
I = Amperes E = Phase to phase volts Eff = Efficiency expressed
as decimal (95% = 0.95) PF = Power factor expresses as decimal (85%
= 0.85) KW = Kilowatts KVA = Kilovolt Amperes HP = Horsepower
COMMON CONVERSION FACTORS
To Convert Into Multiply By Atm pressure Kilogram/sq.cm 1.0332
Centimeters Inches 0.3937
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CABLE TERMINOLOGY
Centimeters Millimeters 10 Horsepower (electric) Watts 746
Inches Centimeters 2.54 Inches Millimeters 25.4 Kilogram Force
Newton 9.80665 Kilogram/sq.cm Kilopascal 98.0665 Kilopascal
Kilogram/sq.cm 0.010197 Kilowatts Megawatts 0.001 Megawatts
Kilowatts 1000 Millimeters Centimeters 0.1 Millimeters Inches
0.03937 Newton force Kilogram 0.10197 Square Inches Square
Millimeters 645.16 Square Millimeters Square Inches 1.550 x 10 -3
Watts Horsepower (electric) 1.340 x 10 -3
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CABLE TERMINOLOGY
CABLES
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?
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
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CABLE TERMINOLOGY
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.
Back to Top
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.
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CABLE TERMINOLOGY
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.
Back to Top
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.
Back to Top
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
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CABLE TERMINOLOGY
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
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CABLE TERMINOLOGY
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.
Back to Main Faq's
CONDUCTORS
1. What are the different Conductor materials used as conductor?
2. What is the criterion for selecting particular type of
conductor? 3. What are the different types of 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.
Conductor selection depends on various parameters like current
carrying capacity , system voltage, voltage drop, flexibility,
shape, and economics.
The most commonly used metals are Copper and Aluminium.
Resistance (R) of a conductor is inversely proportional to the
area of cross section and is given by :
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R= x l / A
Where
= Resistivity of the conductor material. l= Length of conductor.
A = cross sectional area of conductor.
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Types of Conductor
Conductors are divided into different classes , the flexibility
of the conductor increases with the class number. These are as
follows:
A) Cables for fixed installations classes 1 and 2 B) The
flexibles classes 5 and 6
Solid Conductor (Class 1)
The conductor consists of single wire of plain or tinned
annealed Copper and its cross-section is circular. Solid Aluminium
conductor of sizes 1.5 mm2 , up to and including 16 mm2 is of
circular cross-section. Sizes 25 mm2 and above may be either
circular or shaped cross-section.
Stranded Circular Non-Compacted Conductors ( Class 2 )
The conductor consists of plain or tinned annealed Copper or
plain Aluminium. The number of wires in the conductor are not less
than the appropriate minimum number as specified in the
standards.
Stranded Compacted Circular Conductors and Shaped Conductors
(Class 2)
The conductor consists of plain or tinned annealed Copper or
plain Aluminium. The number of wires in the conductor are not less
than the appropriate minimum number as specified in the
standards.
Flexible Conductors(Classes 5 and 6 )
Conductor consists of plain or tinned annealed Copper. The
diameter of the wires in any conductor
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does not exceed the appropriate maximum value as given in the
standards.
INSULATION OF CABLES
1. What are different types of insulating material? 2. List the
important properties of cable insulation. 3. What are thermoplastic
and thermoset materials? 4. What is the function of Conductor /
Insulation Screen and material used?
Insulation is provided to electrically isolate the individual
cores from one another. The applied insulation must perform
adequately in the specified temperature range, and its dielectric
strength should be sufficient to sustain the electrical
stresses.
Cable insulation should have:
High Dielectric Strength Low dielectric constant Good Mechanical
properties Resistance to Ageing High temperature
withstandability
Insulating materials are classified as thermoplastic or
thermoset. Thermoplastic materials lose their form upon heating.
Thermoset materials maintain their form in spite of heating.
Various types of insulating materials used are : PVC, XLPE, Rubber,
Impregnated Paper, etc.
Polyvinyl Chloride (PVC)
Polyvinyl chloride (PVC or vinyl) is a thermoplastic. PVC
compound is the standard insulation for cables rated at 11000 volts
or less & sheathing of entire range of cables. PVC compound is
a mixture of PVC resin, plasticizer, fillers, stabilizers,
lubricant, pigment. The quantity and type of each ingredient
determines the properties. A broad range of electrical, physical
and chemical properties is possible.
PVC has good electrical properties. It is tough and resistant to
flame, moisture, and abrasion. Resistance to ozone, acids, alkalis,
alcohol, and most solvents is also adequate. PVC can be made
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resistant to oils and gasoline.
PVC has the disadvantage of having a high dielectric constant
and dissipation factor. Also plasticizer loss can cause hardening
and cracking.
Crosslinked Polyethylene
Crosslinked polyethylene (XLPE) is a thermoset. It is produced
by compounding PE (polyethylene) with a crosslinking agent, like
organic peroxide. The molecules of polyethylene are "crosslinked",
forming an interconnected network. The terms "cured" and
"vulcanised" are also used for "crosslinked".
Elastomer
Elastomeric material are used for insulation and for sheaths.
They are applied mainly where the product has to be particular
flexible. A wide range of elastomers is nowadays available to the
cable industry. This makes possible the manufacture of compounds
with specific properties, such as abrasion and oil resistance,
weather and heat resistance and flame resistance, combined with
good overall electrical and mechanical charateristics.
The classical elastomeric material, natural rubber, has declined
in significance in recent years. In its place, the synthetic
elastomers produced by the co-polymerisation of ethylene and
propylene, are constantly finding new areas of application in cable
engineering. This co-polymers, are generally known as EPR.
Rubber was the first insulant to be used in Electric cable
manufacture but gave way to other insulants like paper, PVC, XLPE
etc. Rubber is still considered the preferred insulation for
flexible cables and cables where very small bending diameter is
desired.
Paper
Paper tapes of specific thickness and suitable widths are lapped
around the conductor. The thickness of built up insulation depends
on the rated voltage of the cable. Paper insulated cores are dried
and impregnated using mass impregnating non-draining compound.
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CABLE TERMINOLOGY
Function of Conductor / Insulation Screen
The conductor shield is a layer of semi-conducting material.
Semi-conducting materials do not conduct electricity well enough to
be a conductor but will not hold back voltage.
It "smoothes" out the surface irregularities of the conductor.
The conductor shield makes the voltage on the inside of the
insulation the same. Industry specifications define the performance
of conductor shield.
Good insulation shields are extruded in tandem with the
insulation.
The insulation shield consists of two components. These
components are the extruded (auxiliary) shield and the metallic
(primary) shield.
The extruded shield consist of a semi-conducting layer similar
to the conductor shield. It makes the voltage on the outside of the
insulation the same.
The primary shield can consist of metal tape, drain wires or
concentric neutral (CN) wires. Grounding the primary shield makes
the voltage on the outside of the insulation ground. The Copper of
the shield is usually bare, but may be coated with lead or tin.
Some primary shields consists of drain wires and tape. Aluminium
and lead can also be used as the shield.
Concentric neutral wires serve a two-fold purpose. They function
as the metallic component of the insulation shield and as a
conductor for the neutral return current. Their cross sectional
area must be sized in order to function as the neutral
conductor.
Cables with paper Insulation The inner conducting layer consists
of several layers of semi-conducting paper. The outer conducting
layer consists of metallised paper.
Cables with PVC Insulation The inner conducting layer consists
of PVC compound having high carbon black content. For the outer
conducting layer, a cover of conducting tapes is preferred.
Cables with PE or XLPE Insulation Because of higher sensistivity
to partial discharge the reliable well adhesive gap and cavity free
bonding to conducting layers is of greatest significance. The inner
conducting layer consists of polymer compound made conductive by
adding carbon black. And the outer conducting layer is
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formed by semi-conducting compound along with semi-conducting
tapes.
PROTECTION OF CABLES
1. What is armour and why this is required in a cable? 2. What
are different materials used for armouring? 3. In single core
cable, why non-magnetic material is used? 4. What are different
armour types and functions of the same? 5. Which armour type is
generally used for different cable types? 6. What is an Outer
sheath? 7. What are the different materials used for sheathing? 8.
What are the functions of Metallic Sheaths used in cables? 9.
Comparison between Lead sheath and Aluminium sheath used in
cables
Armouring means metal in the form of wire, strip or tape applied
helically to a cable to provide either protection from mechanical
damage from external sources or additional mechanical strength
during installation and service. Armour also serves the purpose of
carrying earth fault currents.
The following are the general armouring materials that are used
:
Galvanised round steel wires Galvanised flat steel wires
(strips) Galvanised or ungalvanised steel tapes Non-magnetic
materials
Under normal circumstances, steel is used in much greater
quantity than any other material for armouring cables. However,
magnetic materials like ordinary steel would add considerably to
the electrical losses in case of single core power cables. Hence,
non-magnetic materials like Aluminium and some of its alloys are
used for single core cables.
Armouring is arranged over the core insulation (PVC) in case of
single core cables and over the inner sheath (common covering) in
case of multi-core cables. In case of paper insulated cables it
shall be arranged over bedding.
Round steel wire armouring will tend to press deeper into the
bending than Flat steel wire armouring because for flat wire the
force is more uniformly distributed as against the round wire
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which has only a line contact with the bedding.
Armouring is provided for offering mechanical protection to
cables after installation. When the cable is directly buried the
danger is mainly from crushing by sharp pike-axe used in
excavating. The double steel tape offers the best protection
against this.
In case of round steel wire armouring the chances of a pick-axe
piercing the inter-wire space are more than flat steel wire
armouring. This is because the tip of the pick-axe can easily slip
and be guided into the inter-wire space in case of round steel wire
armouring. The flat wire armouring apart from economic form of
wiring also provides economy in transport and ease in handling and
erection. It is much lighter in weight and smaller in diameter
which in turn allows a small bending radius.
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Generally used armour
It is a general practice to use galvanised round steel wire
armouring for the cables having calculated diameter below armour
less than 13 mm. Where the calculated diameter below armour is
greater than 13 mm, the armour shall consist of either galvanised
round steel wires or galvanised steel strips.
The direction of lay of the armour is left hand. For double
wire/ strip armoured cables, this requirement is applied to the
inner layer of wires/ strips. The outer layer shall, except in
special cases, is applied in the reverse the direction.
The joints in armour wires/ strips are made by brazing or
welding & surface irregularities shall be removed. A joint in
any wire/ strip is at least 300 mm from the nearest joint in any
other wire/ strip in the completed cable.
The armouring by virtue of its being a metallic path parallel to
the lead sheath/ metallic sheath, shares the flow of line-to-line
earth fault currents. From experience it is found that as far as
earth fault ratings are concerned, flat strip armouring practically
as good as round wire armouring.
In case of cables such as mining cables or submersible cables
where the cable is subjected to lateral pulls during installation
and in service, single or double steel wire armouring is used.
Double steel tape armouring is commonly used for Paper Insulated
Lead sheathed Cables (PILC)
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while galvanised steel wires/ strips are used for armouring for
thermoplastic cables. The armour acts as a return path for such
cables and has a dual purpose.
For Aluminium sheathed cables, often no armouring is
required.
Unarmoured cables can be used more economically inside the
factory, ducts and protected places and due to this considerable
amount of money can be saved.
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Outer Sheath
Outer sheath /Jacket is usually an extruded plastic cover over
the laid-up and/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. Outer sheath provides protection to installed cores ,
and also designed to have characteristics like electrical
resistance, and suitability for environment in the vicinity. The
colour of outer sheath is normally black.
Types of PVC as per IS : 5831 are as follows:
Type ST1 General Purpose sheath intended for use in cables
operating at a maximum rated conductor temperature 70C
Type ST2 Heat resisting sheath intended for use in cables
operating at a maximum rated conductor temperature 90C
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Metallic Sheaths/Comparison between Lead sheath and Aluminium
sheath
Metallic Sheath is provided in cables to prevent the ingress of
moisture/water while maintaining the flexibility of the cable.
Water penetration is detrimental to insulation integrity. In the
presence of moisture and high electrical stresses, water tree
growth is initiated leading to premature failure of the cable.
Its other functions are:
Act as an electrical shield
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Carry fault current Provide mechanical protection
Metallic sheaths of lead or Aluminium are used as a moisture
barrier.
a) Lead Sheaths Lead is one of the oldest (metallic) sheathing
materials used on power cables. Lead is fed into a cylinder. A
hydraulic piston forces it around the cable. Lead is a very
effective moisture barrier. This contributes to the long-term
reliability of cable.
A disadvantage of lead is weight. Lead is prone to deform under
continuous load due to "creep". Lead sheaths are also susceptible
to fatigue failure from vibration and thermal cycling.
b) Aluminium Sheaths Aluminium is also used as sheathing. It is
lighter than lead and has good mechanical properties. Aluminium
sheaths may be extruded similar to lead. Aluminium sheaths can also
be applied as a thick metal tape around the core and welded. After
forming, an Aluminium sheath can be corrugated.
Superiority of Aluminium Sheath over Lead sheath
Aluminium Sheath is economical.. Superior mechanical properties.
(Al is stronger than Lead). It can dispense steel armouring. Al
sheath may work as neutral core. The smooth & soft Aluminium
sheath allows sufficiently small bending radii. Good electrical
conductivity ensures a good screening factor. Aluminium is not
susceptible to vibration & does not tend to re-crystallize even
at higher
ambient temperature.
Testing of Cables
1. Define Type tests, Acceptance tests, Routine tests and
Optional tests. 2. What are the typical tests performed as
Type/ Routine/ Acceptance / Optional tests ?
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CABLE TERMINOLOGY
3. What is the significance of following tests?
a. Annealing test b. Armour resistance test c. Bending test d.
Cold bend test e. Conductor resistance test f. Flammability test g.
High voltage test h. Insulation resistance i. Impulse Test j. Loss
of Mass Test k. Partial Discharge Test l. Tensile test m. Wrapping
Test (for Aluminium wires)
The tests performed on the cables are grouped as :
Type Tests : These are the tests required to be made before
supply on a general commercial basis a type of cable in order to
demonstrate satisfactory performance characteristics to meet the
intended application.
Acceptance Tests : These are the tests carried out on samples
taken from a lot for the purpose of acceptance of the lot.
Optional Tests : These are the special tests which are to be
carried out when required, by agreement between the purchaser &
the supplier.
Routine Tests : These are the tests made by the manufacturer on
all finished cable lengths to demonstrate the integrity of the
cable.
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Type tests :
a) Tests on conductor
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Annealing test (for conductor) Tensile test (for Aluminium)
Wrapping test (for Aluminium) Conductor resistance test
b) Test for armouring wires/ strips
c) Test for thickness of insulation and sheath
d) Physical tests for insulation and outer :
Tensile Strength and elongation at break Ageing in air oven
Shrinkage test Hot deformation Loss of mass in air oven Heat Shock
Test Thermal Stability
e) Insulation Resistance Test
f) High Voltage Test (water immersion test )
g) High voltage test at room temperature
h) Flammability Test
Acceptance tests
Various test performed under this group are:
a) Annealing test (for Copper)
b) Tensile test (for Aluminium)
c) Wrapping test (for Aluminium)
d) Conductor resistance test
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e) Test for thickness of insulation and sheath.
f) Tensile strength and elongation at break of insulation and
sheath.
g) Insulation resistance test .
h) High voltage test at room temperature.
Routine tests
Various tests performed under this group are:
a) Conductor resistance test
b) High voltage test at room temp.
c) Armour resistance test (for mining cables )
Optional tests :
Various tests performed under this group are:
a) Cold bend test
b) Cold impact test
c) Armour resistance test (for other than mining cables)
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Significance of Tests (Refer IS : 10810)
Annealing test : As the conductor of a cable is subjected to
twisting & bending, it is necessary that it should be flexible
enough to take any desired bend without breaking. This test takes
into consideration the extent of work hardening which may be caused
during the stranding & laying up the process.
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Armour resistance test : In case of short circuit faults the
armour has to carry the short circuit current. The resistivity of
the armour must be maintained within the designed value so that
there is no excess temperature rise above the designed value &
also there is no excess voltage rise , to the unsafe level.
Bending test : All electrical cables are subjected to bending
operation during handling & installation. The minimum radii are
specified in the relevant specifications. In order to ensure that
the cables withstand, without damage or showing any cracking of the
sheaths, the recommended bending radii during installation &
handling, it is necessary to conduct a bending test which is more
rigorous than what is likely to be subjected to in actual
practice.
Cold bend test : Sometimes the cable have to be laid in the area
of low temperature climatic conditions or frost. Due to constant
exposure of the cable to the low temperature or frost conditions,
the insulation or outer sheath of the cable will become hardened
& stiff. However, this is a long term effect. Such hardening or
stiffness causes cracking of the insulation or outer sheath &
may lead to the failure of the cable. In order to ascertain the
suitability or withstandability of the insulating or sheathing
materials at low temperature or frost conditions (when usage of the
cable so warrants), this cold bend test is carried out.
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Conductor resistance test : Accurate control of resistance is
necessary to meet system design parameters. Resistance is
influenced by conductor dimensions & construction, processing,
conditions, temperature, resistivity (composition/ impurity &
temper of the material). It is expressed in terms of ohms per
kilometre corrected to 20 0C.
Flammability test : For certain location it may be important to
use a cable which retards flame in case of a fire & is
self-extinguishing when the source of the fire is removed. This
test is therefore carried out on finished cables to verify this
property.
High voltage test : The insulation material in a cable is used
to isolate the conductors from one another and from ground, as well
as provide the necessary mechanical strength. The fundamental
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requirement of the insulation in an electric cable is that it
withstands the voltage imposed on it in service. It is necessary
that evaluation of the condition of the insulation be made by
imposing a higher voltage stress for a short duration.
Insulation resistance : The test is performed to determine
insulation resistance, calculate volume resistivity and insulation
resistance constant of the dielectric material of electric cable by
direct current method. The dielectric material in a cable is used
to insulate the conductors from one another and from ground as well
as to provide mechanical support for the components. For this
purpose it is desirable to have the insulation resistance as high
as possible consistent with acceptable mechanical, chemical and
heat resisting properties. During the manufacturing process
non-uniformities may develop in the dielectric either in the form
of conductive impurities in the material or in the form of
mechanical imperfections in the dielectric affecting the quality of
the cable. This test helps in detecting these imperfections.
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Impulse Test : This test involves impulse testing ( simulated
lightning impulse ) of electric cables. Insulating material used in
high voltage cable may be subjected to transient voltages resulting
from nearby lightning strokes. The ability of the insulating
material to withstand these transient voltages is important in
establishing the reliability of the cable insulation
Loss of Mass Test: This test is performed to determine the
thermal effect on the mass of thermoplastic insulation and sheath.
Themoplastic insulation and sheath exposed to heat are subjected to
many types of physical and chemical changes. The severity of
exposure , in both time and temperature, determines the extent and
type of change that takes place. Extended periods of exposure of
insulation and sheath to elevated temperature cause degradation
with progressive changes. These changes are assessed by measuring
the loss of mass on subjecting the material to accelerating
ageing.
Partial Discharge Test: This test involves detection and
measurement of partial discharge occurring in screened electric
cables. Partial discharges remain unnoticed in the normal high
voltage tests and could be harmful to the life of the insulants.
Extruded dielectric tends to deteriorate very fast due to discharge
in very small voids and cavities. It is therefore necessary that
such voids should be avoided in the extrusion process. Still
certain minute voids are unavoidable
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CABLE TERMINOLOGY
and these remain in the insulation. The magnitude of the
discharge in such voids are measured so as to determine whether
these are within permissible limits
Tensile test : This test is performed on conductor material to
determine the strength of the material when subjected to tensile
stress. Since cable conductors to be pulled from one end along
trenches are subjected to considerable force during pulling as well
as manufacture, it is necessary to ensure that the conductor
material has adequate tensile strength.
Wrapping Test (for Aluminium wires) : This test is performed to
determine the ductility of aluminium wire used as conductor for
electric cables. This test brings out the property of the material,
which makes it suitable for winding and twisting. Cable conductors
during the process of manufacture as well as during installation
are subjected to torsion due to axial twist and might break, if
material is not sufficiently ductile. The properties checked by
wrapping test ensure the suitability of material as cable
conductor.
Back
Cable Accessories
1. What are the different types of cable accessories? 2. What
are the objectives to be achieved by Termination Kits? 3. What are
the objectives to be achieved by joints? 4. List down the various
types of cable accessories. 5. Explain Heat Shrink technique
Types of cable accessories
i. Terminations :
Termination kits serve as a connection to electric apparatus or
machines or switchgear and are further categorized as Indoor and
Outdoor Termination kits.
6. Explain Cast Resin technique
Indoor Termination kits (IDT) : IDTs are used for the indoor
application i.e. for termination at Panels,
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Transformers etc.
Outdoor Termination kits (ODT) : ODTs are used for terminating
at switchyards, outdoor substations, on poles etc. where these are
directly exposed to atmospheric conditions.
ii. Joints :
Cable joints connect lengths of cables in long routes or at
points of repair. These facilitate transfer of power over long
distances as many a times it is not possible to supply long cable
lengths due to various reasons such as manufacturing &
transportation constraints, site conditions etc.
There are two types of joints :
1. Straight Through Joint : These joints are required when two
cables having the same type of insulation are to be joined ( eg
:XLPE to XLPE insulated cable).
2. Transition Joint : When two cables having different types of
insulation are required to be joined, transition joints are
required ( eg : XLPE insulated cable with PILC insulated cable etc
).
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Objectives to be achieved by Termination Kits
The following objectives must be fulfilled :
connection of the conductor stress grading at screen cut point
of both radial & longitudinal stresses for medium and high
voltage cables Avoid ingress of water and to protect against
ambient influences protection of core insulation (e.g. against U.V.
radiation) insulation from earthed parts
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Objectives to be achieved by joints
The following objectives must be fulfilled : connection of the
conductors v insulation of the conductors
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reinstatement of the elements of the cable for Medium & high
Voltage Cables protection against all ambient conditions of the
ground establishment of branch points for service cables in low
voltage cables.
List of various types of cable accessories.
The various types of cable accessories are :
Heat Shrink type Pre-moulded /Pre-fabricated type (Factory
Moulded) Cold Shrink type v Push-on type Cast Resin (
poly-eurethene / Epoxy compound filling ) type Cast Iron ( bitumen
compound filling ) type
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Heat Shrink technique
The heat shrink technique is a versatile technology which can be
used for a variety of applications vis-a-vis cable
applications.
Thermoplastic based materials are used for manufacture of heat
shrink components. After cross-linking process the material is
moulded into shape, expanded in hot condition itself and suddenly
cooled.(eg: expanded insulating tube, break-out etc). The component
remains in the expanded form. This pre-stretched form when heated
recovers to its original dimensions. This is termed as the Shape
Memory Phenomenon. In this process the tube "shrinks" on to the
subjacent parts.
As a heat source, a hot air blower or soft flame may be used.
Shrink on products are easy to install, simple, quick and
independent of ambient temperature. They can also be used, because
of their flexibility, as additional protection against mechanical
stress. These accessories are best suited for application in 1.1 kV
to 33kV range.
Cast Resin technique
This is a time tested method of jointing & terminating of
cables. In this technology, basic insulation is provided by lapping
of insulating tapes. Stress grading, in case of MV & HV cables,
is provided by means of stress-cone which is built-up by lapping of
semi-conducting tapes. Cast-resin compound when set, gives
protection against mechanical damage & moisture ingress. These
accessories are
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successfully in service in 1.1 kV to 33kV range.
Cable Characteristic
1. What is current carrying capacity of the cable? 2. What
factors affect cable current carrying capacity? 3. What is Short
circuit rating of the cable? 4. Calculation of Short Circuit Rating
5. What are different losses in Power cables? 6. What are the
factors affecting cable selection?
Current carrying capacity of the cable
The current carrying capacity of a cable is determined by the
thermal resistance of insulation, outersheath and surroundings.
Heat losses in a current carrying conductor are given by I2R. The
heat generated causes increase in the conductor temperature. This
heat is dissipated to the atmosphere through the cable materials
and soil. As shown in the following fig. cable materials and soil
represent a series circuit of thermal resistors. The thermal
resistance controls heat dissipation from the conductor. The more
the dissipation of heat the more is the current carrying
capacity.
Some of the factors affecting the underground cable current
carrying capacity and hence the selection of cable are :
Depth of installation: the deeper the installed cable, lower is
the current carrying capacity. Thermal Resistivity of Soil : the
higher the thermal resistivity, the lower is the ampacity Ambient
temperature of soil: the higher the ambient temperature, the lower
is the ampacity Method of Installation: if installed in a duct ,
the lower is the ampacity
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Short circuit rating of the cable
The conductor size necessary for an installation is often
governed by its ability to carry short-circuit rather than
sustained current. During a short circuit there is a sudden inrush
of current for a few cycles followed by a steadier flow for a short
period until the protection operates, normally between 0.2 to 3
seconds.
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CABLE TERMINOLOGY
At the commencement of the short circuit the cable may be
operating at its maximum permissible continuous temperature and
increase in temperature caused by short circuit is a main factor in
deriving acceptable ratings. However, the current may be 20 or more
times greater than the sustained current
As the time involved is short and cooling follows rapidly, the
cable insulation can withstand much higher temperature than are
allowed for sustained operation. The difference between maximum
conductor temperatures for sustained rating , and the temperature
during short circuit, provides a maximum temperature which can be
used in short-circuit rating calculations.
It may be assumed that the whole of the energy input appears as
heat which is absorbed by the conductors
Following equation is obtained on the basis of equating heat
input (I2 RT) to the heat absorbed (product of mass, specific heat
and temperature rise ) :
I2 = K2 S2
Here the conductor refers to the current carrying component. The
constants are given in the
loge { (1 + )/ (0+ ) }
T
Where I = short circuit current (r.m.s over duration ) (A)
T= duration of short circuit (second)
K = constant for the material of the conductor
S= area of conductor (mm2 )
1 = final temperature (0C)
0 = initial temperature (0C)
= reciprocal of the temperature coefficient of resistance of the
conductor ( per degree Celsius)
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CABLE TERMINOLOGY
following table :
Material K Qc 20 Copper 226 234.5 3.45 X 10 -3 17.241 X 10
-6
Aluminium 148 228 2.5 X 10 -3 28.264 X 10 -6
Lead 42 230 1.45 X 10 -3 214.000X 10 -6
Steel 78 202 3.8 X 10 -3 138.000 X 10 -6
in which
K2 = Qc ( +20 }
20
where Qc = volumetric specific heat of the conductor at 20 0 C
(J/ 0 C mm3)
20 = Resistivity of conductor metal at 20 0 C (mm)
For XLPE cable,
Conductor Temperature rise (0C)
Short-Circuit Current (kA)
Copper 90-250 0.143 ST -1/2
Aluminium 90-250 0.094 ST -1/2
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Example illustrating the calculation of the Short Circuit
Rating.
Refer Tropothen-X catalogue
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CABLE TERMINOLOGY
Table No. 1
Tropothen-X Armoured Cable. Type- A2XCEFY-3.8/6.6 kV (E)
No. Of cores & Cross sectional area of Conductor
Short circuit Rating for 1 second
No. x sq. mm kA
3 x 25 rm/v 0.094 x 25 x (1) -1/2 = 2.35 3 x 35 rm/v 0.094 x 35
x (1) -1/2 = 3.29 3 x 50 rm/v 0.094 x 50 x (1) -1/2 = 4.70 3 x 70
rm/v 0.094 x 70 x (1) -1/2 = 6.58
Similarly, we can calculate short circuit ratings for other
cable sizes.
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Different losses in Power cables
The losses in the Power Cables are :
Conductor losses : Atoms of the conductor offer specific
resistance to the flow of the electrons (current) which causes loss
of power in the form of heat. Heat generated is directly
proportional to the square of current & conductor
resistance.
Heat generated = I2 x R
where I = current carried by the conductor
R = Resistance of the conductor
This power loss is also called Voltage Drop across the
conductor.
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CABLE TERMINOLOGY
Dielectric Loss : Electric Insulation surrounding a conductor
creates a capacitor when the conductor is electrically energised.
Thus all insulated conductors are capacitors in which insulation
material acts as a dielectric. There is a loss of power which is
proportional to the dielectric power
factor (tan ).
The dielectric loss in watts per kilometre per phase is given
by
D = 2 f CU02 tan 10-6 (watt/km per phase)
where f = frequency
C = Capacitance
Uo = Voltage applied over conductor
tan = Dielectric power factor
The power factor of the dielectric of cables should be of low
value.
Theory of Dielectric Losses :
When a voltage is applied to a cable with a perfect dielectric,
a charging current IC flows which is in leading quadrature with the
voltage .In such a perfect dielectric there would be no component
of the current in phase with U. However in actual practice the
dielectric is imperfect and there is a small current IR which is in
phase with U. This current causes losses IRU in the dielectric in
the dielectric which generates heat.
This losses in the dielectric are proportional to the cosine of
the angle between the resultant current It and applied voltage
U.
Now It = (IR2 + IC
2)1/2
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CABLE TERMINOLOGY
IRU = ItU cos
and
cos = IR / (IR2 + IC
2)1/2
As is close to 900, cos equates approximately to tan (90 - ),
i.e. equates (approximately) to tan, and the dielectric power
factor of a cable is frequently referred to as tan, where is known
as the dielectric loss angle (DLA).
The dielectric loss in watts per kilometre per phase is given
by
D = 2 fCU02 tan 10-6 (watt/km per phase)
The power factor of the cable insulation is dependent on
frequency, temperature and applied voltage. It is of a very low
order and consequently for cables of up to 50 kV operating voltage
the dielectric losses are small in comparison with conductor
losses. However, for cables for operation above this level the
losses rise rapidly with voltage and this must be taken into
consideration when calculating the current carrying capacity of the
cables.
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The Factors affecting cable selection
Selection of a Cable for a given system depends upon :
System voltage Rated current (future needs/ factor of safety)
Ambient temperature Cable installation (laying method, no of cables
in parallel, air/ under ground) Permissible voltage drop Initial
size/ Type selection Economics with higher size (2 step) cables
Comparison of initial cost + Running cost
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CABLE TERMINOLOGY
Final selection & specification freezing
To illustrate the selection procedure a following typical LT
distribution circuit is considered.
Incoming cable
Rating factor for 45 0 C=0.9
Rating factor for 1m depth = 0.97
i.e. Rated current for cable selection of 300A ckt = 300/(0.9 X
0.97)
= 343 A
The minimum cross-section area for 343 A = 500 mm2
The initial selected cable is 3.5 X 500 / 240
Feeder cable
I) Rating factor for 450C =0.9
(Tropodur cables catalogue Table 12 : Rating factor for
variation in Ambient Air Temperature)
Rating factor for cable laid in open rack = 0.96
(Tropodur cables catalogue Table 14 : Rating factor for
multicore cables laid on open racks in air)
Therefore,
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CABLE TERMINOLOGY
Current for cable selection of 200A ckt under specified service
conditions
=200/(0.9 X 0.96) = 231A
Corresponding minimum cable size for 231 A (Tropodur cables
catalogue: Table 2) 3.5 x 185 / 95.
Similarly,
Current for cable selection of 100A ckt under specified service
conditions
=100/(0.9 X 0.96) = 116A
Selected minimum cable size 3.5 X 70 / 35
Now having selected the minimum cable cross-sections for given
service conditions, the same may be verified for voltage drop not
exceeding the limits
Maximum permissible voltage drop limit is 9 % of voltage
applied.
Therefore, permissible voltage drop for 240 V=21.6 volts
Voltage drop is = R x I x l
Where R = Approx. A.C. resistance at operating temperature 700C
in / km.
I = circuit current in Amperes.
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CABLE TERMINOLOGY
l = cable length in km.
500 mm2 (400 m) = 0.0767 x 300 A x 0.4
=9.2 volts
185 mm2 (300 m)= 0.198 x 200 A x 0.3
= 11.94 volts
70 mm2 (500 m) =0.532 x 100A x 0.5
=26.6 volts (not permissible)
70 mm2 (200 m) =0.532 x 100 x 0.2
=10.64 volts
Voltage drop for the selected cables is within permissible
limits except for 70 mm2 , 500 m cable length.
Therefore selecting next higher size (i. e. 3.5 x 95/50 sm ) for
this cable length we get voltage drop for
95 mm2 (500 m ) = 0.385 x 100 A x 0.5
= 19.25 volts which is within permissible limits.
So, the selected cable size for this cable length is 3.5 x 95/50
sm.
Modified Cable Selection is
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CABLE TERMINOLOGY
CableLength (m)
CircuitCurrent(Amp) Selected Cable
400 300 3.5 X 500 / 240 sm
300 200 3.5 x 185 / 95 sm
500 100 3.5 x 95 / 50 sm
200 100 3.5 X 70 / 35 sm
Economics
The above calculations enable to select the minimum conductor
cross section area. However economics will govern the final optimum
size which will offer the minimum capital + running cost. For this
purpose, the minimum size along with two or three higher sizes are
considered, and annual costs are worked out by calculating the cost
of power loss across the cable and interest/depreciation of the
cable cost. The cross-section is one with minimum Total cost.
Besides this, for large distribution system other factors such as
standardisation of cable sizes, future expansion, standardisation
of accessories etc., should also be considered for final selection
of cable size.
Braiding: The plaited protective covering of a cable.Barrier
Joint: A cable joint between two mass-impregnated cables in which
the impregnating compound in each cable is separated from that in
the other.Service Line : A line connecting the consumers
installation to the distributor.