Covered Conductor System
Covered Conductor System: Overhead distribution systems can be categorized into two broad heads based on voltage rating and insulation system. Distribution system based on voltage rating is broadly either low voltage (up to 1.1 KV) or medium voltage (up to 36KV), Transmission and sub-transmission of power is done at a higher voltages. Voltages between 33kV and about 230kV, called high voltage are used for sub-transmission & transmission of power. Voltage level above 230kv up to about 800kv is called EHV or Extra high Voltage and voltage level above 800kV is called UHV or Ultra High Voltage.
We now also have HVDS systems where lines right up to consumer’s small single phase transformers are run at medium voltage. In this system LV mains are conspicuous by their absence. Categories based on insulation type are either bare or covered.
In case of bare lines most of the insulation is provided by air and the insulators are used to support the dead end conductor on poles and pylons with the help of tension strings and support hardware. Insulators also prevent leakage of current between conductor & supports. Spacing between phases and vertical ground clearance are critical safety requirements. Insulated spacers are also sometimes used to maintain inter-phase spacing and to prevent accident contact between them.
In case of LV applications, fully insulated ABC system is increasingly becoming the most acceptable option with a perfect fit in the equation of cost/quality binomial. Main features / advantages of low voltage ABC system are being separately outlined here.
Low Voltage: Aerial Bunched Cable (ABC) system which is an insulated system was introduced in Low voltage (LV) to provide certain advantages like reduction of leakage current, line outage, jungle fire etc. Since conductors are bundled together, bundled together, phase to earth or phase to phase clearances are not required and thus compacting the line to reduce ROW substantially. This system also reduced maintainance of line and consequently operating cost.
FEATURES OF LV ABC SYSTEM
- The phase conductors are fully insulated. The messenger may be either bare or insulated depending on type of system being followed.
- More than one conductor is part of the bundle. Commonly theya re four comprising of three phases and one neutral messenger. Very often 5th conductor can be used for street lighting is also a part of the bundle.
- They are self-supporting and suspended between supports (generally existing poles)
- They can be installed through forest areas without cutting of aisles prescribed for overhead lines.
- ABC Bundle and accessories being fully insulated protect lineman actively as the design is touch proof.
- They are twisted together to form a bundle as there is no requirement of phase to phase clearance. Individual cores are separated only at dead ends, supports, joints and tap-offs.
- Prescribed clearance required for bare conductors under NEC/ NESC/VDE/EDFcodes not applicable here and the same may be reduced without limitation as long as insulation doesn't not become vulnerable to mechanical damage.