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CONDUCTOR
:-
The most acceptable metals for conductor are copper
and Aluminium due to their higher conductivity and ductility.
As copper has got higher affinity for sulphur, in the
atmosphere where sulphur fumes are present. In these
conditions tinned copper should be used.
Aluminium oxide film, which is always present on Aluminium
conductor surface, acts as a barrier and it protects
the Aluminium conductor from corrosion in fumes laden
atmosphere. |
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CONDUCTOR
CONSTRUCTION :-
The most economical construction is solid conductor
is made of one single wire. As the area of conductor
increases, solid conductor becomes stiffer and hence
difficult to handle. In this case stranded construction
is adopted. Here the conductor is made of number of
strands. The strands are arranged in spiral layers in
1+6+12+18+... formations. This construction provides
more flexibility. Where crimping of lugs are required,
the conductor has to be stranded construction only.
To economies in insulating material, weight and overall
diameter, shaped conductors are employed in bigger sized
cables. Here the stranded conductor is shaped in to
a segment of a circle so that when all the cores are
laid, they form a complete circle.
I.S. 1554 permits solid conductor construction up to
10 sq.mm. in Aluminium and up to 6 sq.mm in copper.
It permits the use of shaped conductors for sizes from
16 sq.mm onwards. |
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COLOUR OF
CORES :-
The following colour code code is used for identification
:- |
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Single core
|
Red, Black, Yellow or Blue, White,
Grey. |
| Two cores |
Red and Black. |
| Three cores |
Red, yellow and Blue. |
| Three & half |
Red, Yellow, Blue and Reduced neutral Black. |
| Four cores |
Red, Yellow, Blue and Black. |
| Five cores |
Red, Yellow, Blue, Black and Grey |
| Six cores |
Two adjacent cores. Blue and Yellow (counting
and direction Core) and remaining Grey in each layer.
ORBy printing numbers on each core. |
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LAYING UP
:-
The cores are laid up with suitable lay. The final layer
always has a right hand lay i.e. if you look along the
cable, the cores move to your right hand. |
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INNERSHEATH
:-
Inner sheath is provided over the laid up cores. It is
provided to give circular shape to the cable and it provides
bedding for the armouring. |
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| I.S. 1554 permits following two methods
applying the innersheath of any thermoplastic material
i.e. PVC, polyethylene etc. |
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a) EXTRUDED INNERSHEATH
:-
Here the innersheath is provided by extrusion of thermoplastic
over the laid up cores. This type of innersheath is
generally provided in cables having round cores i.e.
in control cables and power cables up to 10 sq.mm. Size.
This type of innersheath is also acts
as a water barrier & between cores and Outersheath.
In case of puncture in the outersheath the water cannot
reach to the core and hence we recommend that cables
for outdoor underground uses should have extruded inner
sheat
b) TAPPED INNERSHEATH :-
Here wrapping a thermoplastic tape over the laid up
cores provides the innersheath. It is generally employed
in cables having sector shaped cored i.e. multicore
cables of 16 sq.mm. And above. This method saves a process
and hence manufacturers always provide this type of
innersheath useless the purchase specifications ask
for extruded innersheath. |
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ARMOURING
:-
In case of armoured cables, generally galvanized steel
wire /strip armouring is provided over the innersheath
in multi-core cables and over the insulation in single
core cables. It provide the mechanical protection to
inside cores and it carries earth return current in
case of short circuit of core with armour.
As per I.S 1554 (Part 1) 1988 round wire armouring is
provided in cable where calculated diameter under armour
is up to 13mm.Above this the armouring is either with
round wire or strip of size 4mm x 0.80mm. As per strip
construction is economical, the manufacturers always
provide steel strip armouring unless wire armouring
is specially specified.
In long run of cables and in case of mine, round wire
armoring is must, as strip construction provides higher
resistance to earth fault current and sometimes this
current may not be sufficient to operate the circuit
breaker in case of earth fault.
In mines, the resistance of armour in no case should
exceed the resistance of the main core by more than
33% for safety reasons. To achieve this sometimes-tinned
hard drawn copper wires are required to be used along
with galvanized steel wires. Sometimes two layers of
steel wires are provided to give extra protection.
In case of single core armoured cables for use in AC
circuits, the material for armouring has to be non magnetic,
as in this case the return current is not passing through
the same cable and hence it will not cancel the magnetic
lines produced by the current. These magnetic lines
which are oscillating in case of AC current will give
rise to eddy current in magnetic armouring and hence
armouring will become hot, and this may lead to the
failure of the cable. Generally hard drawn aluminium
wire /strip are used for armouring in this case. |
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OUTERSHEATH : -
The PVC covering on armouring in case of armoured
cables and over the innersheath in unarmoured cables
is called outersheath I.S. 1554 specifies nominal
and minimum thickness of outersheath for unarmoured
cables and only minimum thickness of outersheath for
armoured cables.
It permits the following types of outersheath PVC
compounds.
-
Outersheath with type ST1 PVC
compound as per IS-5831, which is suitable for 70°
C continuous operation.
-
Outersheath with type ST1 PVC
compound as per IS-5831, which is suitable For 85°
C continuous operation.
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PVC has got fire retardant
properties due to its halogen content. The fire in the
gets extinguished immediately on removal of the fire
source. In the modern power, Chemical, fertilizer and
cement plant many PVC cables are bunched in the cable
shaft or on cable trays. In case of fire in these cables
the fire becomes self-sustaining. Moreover due to the
burning of PVC a dense corrosive smoke is emitted which
makes fire fighting very difficult, due to poor visibility
and toxic nature of the smoke. HCL content of the smoke,
not only damages other costly equipment lying near by,
but also penetrates the RCC and corrodes the steel reinforcement.
Due to this there is an extensive damage to the property.
To overcome these deficiencies FRLS
i.e. Fire Retardant Low Smoke PVC was developed. If
required, we can provide Fire Retardant Low Smoke (FRLS)
PVC inner sheath and/or outer sheath. This PVC compound,
apart from meeting the requirements of Type ST2 as per
IS-5831, has got better fire retardant properties and
it emits lower smoke and acid fumes When it catches
fire (For more information please refer our catalogue
on FRLS cables). |
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CABLE CODE
:-
The following codes are used for designating the cables
as per IS-1554. |
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CONSTITUENT
|
CODE LETTER |
| COPPER CONDUCTOR |
-- |
| ALUMINIUM CONDUCTOR |
A |
| PVC INSULATION |
Y |
| STEEL ROUND WIRE ARMOUR |
W |
| STEEL STRIP ARMOUR |
F |
| STEEL DOUBLE ROUND WIRE
ARMOUR |
WW |
| STEEL DOUBLE STRIP ARMOUR |
FF |
| PVC OUTER SHEATH |
Y |
|
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| YWY means copper conductor, PVC insulated,
round wire armoured and PVC sheathed cable. AYFY means
Aluminium conductor, PVC insulated, steel strip armoured
and PVC sheathed cable. |
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