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CRITERIA
FOR SELECTION OF POWER CABLES
The
following factors should be taken into account while
selection, the correct size and type of cables.
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SYSTEM
VOLTAGE : What
is the system voltage and the type of system
? Single phase, Three phase, earthed or unearthed
or dc ?
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CURRENT
CARRYING CAPACITY :
The current rating is the main and basic criterion.
Tables Nos. 1-22 give the carrying capacity
of various type and size of cables, under different
condition of laying . These should be considered
before the correct size and type of cables is
decided upon. Suitable derating/uprating factor
should be used depending on the conditions of
laying as under :
2.1 : Depth of laying.
2.2 : Ambient temperature of ground or Air.
2.3 : Soil resistivity.
2.4 : Whether one or more groups of cables are
laid together.
2.5 : Any heating source near cable run.
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MODE
OF INSTALLATION : The
mode of installation determines the type of
cable to be used. Electricity regulators require
the use of armoured cables for underground applications.
In general, armoured cables are recommeded where
there is any chance of mechanical damage. If
subsequent mechanical demage after laying cables
is not likely, cheaper, unarmoured cables can
be used.
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PERMISSIBLE
VOLTAGE DROP :
For longer length of cable run it is necessary
to check that with the cable size selected the
voltage drop does not exceed the prescribed
regulations limit. A higher size cable may have
to be used if the voltage drop limits are not
to be exceeded.
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LOAD
CHARACTERISTICS :
One should take into account the characteristics
of load. It is essential to ensure that
the cable selected is capable of handing
temporary overloads. "AVOCAB"
cables permit a conductor temperatures
of up to 130°C under temporary overload
conditions. ( It possible, the complete
load cycle may be furnished ).
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SHORT
CIRCUIT RATING : Short
circuit rating depend on the expected
level and the expected duration of the
short circuit . In certain cases, a
larger size of cable than be cable required
for normal full load current may be
needed. The cables with high KVA capacity
are expected to carry short circuit
current of high magnitude "AVOCAB"
cables permit a short circuit temperature
of 160°C.
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ECONOMIC
CONSIDERATIONS : The
most economical construction and size
of cable persistent with current carrying
capacity and laying conditions has to
be selected. A detailed study of 3 or
4 approximate sizes is made. Actual
running costs are worked out taking
into consideration |2R loss and interest,
depreciation of the total cable cost.
The size which gives minimum running
costs is to be preferred.
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