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Draw in system of laying underground cable has drawback of
Higher cost
Reduced current carrying capacity of the cable
Requirements of much deeper and wider Trench
All of above
All of above
The 'Draw-in' system for underground cable installation involves placing cables inside stoneware or cast iron pipes or conduits buried in the ground. This method is costly due to the expensive ducting materials, limits current carrying capacity due to poor heat dissipation in the air-filled ducts, and necessitates deep, wide trenches to accommodate the conduit banks.
The 'Draw-in' system for underground cable installation involves placing cables inside stoneware or cast iron pipes or conduits buried in the ground. This method is costly due to the expensive ducting materials, limits current carrying capacity due to poor heat dissipation in the air-filled ducts, and necessitates deep, wide trenches to accommodate the conduit banks.
IcapacityтАЛтИЭRthermalтАЛ1тАЛтАЛ тАФ Ampacity is inversely proportional to the square root of thermal resistance.
╬Ф╬╕=PтЛЕRthтАЛ тАФ Temperature rise equation where P is power loss.
In this system, cables are drawn into pre-installed ducts. Since the cable is surrounded by air inside the duct, the thermal resistance increases significantly compared to directly buried cables (where soil contact provides better heat dissipation). This restricted heat transfer leads to a reduced ampacity (I=RthтАЛтЛЕr╬╕maxтАЛтИТ╬╕aтАЛтАЛтАЛ), where RthтАЛ is the total thermal resistance.
Draw-in systems allow for easy cable replacement without excavating the entire trench.
They are widely used in urban areas for street lighting or distribution networks.
The primary disadvantage is high initial capital cost and lower thermal rating compared to direct burial.
Cables are pulled through ducts using winches, which increases installation time.
Easy replacement/repair of faulty cables
Additional cables can be added later without excavation
Suitable for high-density traffic areas
Higher initial cost
Poor heat dissipation (lower current capacity)
Deeper and wider trenches required for duct banks
Congested city areas
Street lighting distribution
Industrial plant wiring
The 'Direct Burial' method is usually cheaper and offers better thermal conductivity but lacks flexibility for future modifications.
Option B is a consequence of the air gap inside the duct, which acts as a thermal insulator.
D is correct тАФ The draw-in system suffers from high initial costs, restricted thermal dissipation, and complex excavation requirements.
In competitive exams, remember that any cable laying method increasing thermal resistance (RthтАЛ) will always lower the permissible current carrying capacity of the cable.