INTRODUCTIONTHE UNDERGROUND SYSTEM OF ELECTRICAL DISTRIBUTION

OF POWER IN LARGE CITIES IN INCREASINGLY BEING ADOPTED, ALTHOUGH IT IS COSTLY SYSTEM OF DISTRIBUTION AS COMPARED TO OVERHEAD SYSTEM. IT ENSURES THE CONTINUITY OF SUPPLY APART FROM THE FOLLOWING ADAVNTAGES:IT ENSURES NON-INTERRUPTED CONTINUITY OF SUPPLYITS MAINTENANCE IS LESSIT HAS A LONG LIFEITS APPERANCE IS GOODIT ELEMINATES HAZARDS OF ELECTRO-CUTION DUE TO

BREAKAGE OF OVER HEAD CONDUCTORS.

UNDERGROUND CABLESAN UNDERGROUND CABLE CONSISTS OF ONE OR MORE

CONDUCTORS COVERED WITH SOME SUITABLE INSULATING MATERIAL AND SURROUNDED BY A PROTECTING COVER. THE CABLE IS LAID UNDERGROUND TO TRANSMIT ELECTRIC POWER.

THE UNDERGROUND CABLES ARE GENERALLY USED FOR DISTRIBUTION OF ELECTRIC POWER. UNDERGROUND CABLES ARE USED FOR THE TRANSMISSION AND DISTRIBUTION PURPOSES IN URBAN AREAS AND ALSO WHERE OVERHEAD SYSTEM BECOMES IMPRACTICABLE. THE UNDERGROUND CABLES MUST FULFILL THE FOLLOWING REQUIREMENTS GIVEN ON NEXT SLIDE:

THE CONDUCTOR USED IN CABLES SHOULD BE OF TINNED COPPER OR ALUMINIUM, HAVING HIGH CONDUCTIVITY. ONLY STRANDED CONDUCTORS ARE USED SO THAT THE CONDUCTOR MAY BECOME FLEXIBLE.

THE SIZE OF CONDUCTORS USED IN UNDERGROUND CABLES SHOULD BE SUCH THAT IT CAN CARRY THE REQUIRED LOAD WITHOUT OVER HEATING AND SHOULD NOT CAUSE A VOLTAGE DROP MORE THAN THE PERMISSIBLE LIMITS.

THE CABLE MUST HAVE PROPER THICKNESS OF INSULATION SO THAT IT CAN PROVIDE A HIGH DEGREE OF SAFETY AND RELIABILTY AT THE VOLTAGE FOR WHICH IT IS DESIGNED.

THE MATERIALS USED IN MANUFACTURING OF CABLES SHOULD SUCH THAT THERE IS COMPLETE CHEMICAL AND PHYSICAL STABILITY.

ALL THE CABLES MUST BE MECHANICALLY PROTECTED, SO THAT THEY CAN WITHSTAND THE ROUGH USE IN LAYING THEM.

CLASSIFICATION OF CABLESUNDERGROUND CABLES ARE USUALLY CLASSIFIED

ACCORDING TO THE VOLTAGE FOR WHICH, THESE ARE MANUFACTURED. ACCORDING TO THE VOLTAGE, THESE ARE CLASSIFIED AS:

A) LOW-TENSION (L.T.) CABLES......UPTO 1000V .

B) HIGH-TENSION (H.T.) CABLES.....UPTO 11000V.

C) SUPER-TENSION (S.T.) CABLES....FROM 22KV TO 33KV.

D) EXTRA-HIGH TENSION (E.H.T) CABLES.......FROM 33KV TO 66 KV.

E) EXTRA SUPER VOLTAGE CABLES.........BEYOND 132 KV.

SINGLE CORE V.I.R CABLE (BRAIDED)

TWIN CORE BELTED CABLE

3-CORE BELTED CABLE

4-CORE BELTED CABLE

3-CORE CABLE WITH STEEL WIRE ARMOUT

H & HSL TYPE CABLE

LOW-TENSION CABLES (L.T.)THESE CABLES ARE EMPLOYED FOR USE UP TO 1000V.

GENERALLY, THESE CABLES ARE USED FOR DISTRIBUTION PURPOSES AT ABOUT 400V. THESE CABLES MAY HAVE ONE OR MORE THAN ONE CORE DEPENDING UPON THE TYPE OF SERVICES. THESE CAN BE OF SINGLE CORE OR MULTICORE TYPE. FOR A 3-PHASE , 3-WIRE SERVICE , EITHER 3 SINGLE CORE CABLES OR THREE CORE CABLES CAN BE USED.

THESE LOW TENSION CABLES HAVE NO SPECIAL CONSTRUCTION BECAUSE THE STRESSES DEVELOPED IN THE CABLE FOR LOW VOLTAGE (UPTO 6600V) ARE SMALL AND THE THERMAL CONDUCTIVITY IS ALSO OF NOT MUCH IMPORTANCE.

THESE LOW TENSION CABLES HAVE NO SPECIAL CONSTRUCTION BECAUSE THE STRESSES DEVELOPED IN THE CABLE FOR LOW VOLTAGE (UPTO 6600V) ARE SMALL AND THE THERMAL CONDUCTIVITY IS ALSO OF NOT MUCH IMPORTANCE. THE CONDUCTORS ARE INSULATED WITH RUBBER OR POLYVINYL CHLORIDE AND A LEAD SHEATH COVER OVER THE INSULATION TO RESTRICT THE MOISTURE TO ENTER THE CORE OF THE CABLE. THE LEAD SHEATH IS COVERED WITH A COMPOUNDED FIBROUS MATERIAL . SINGLE CORE CABLES ARE USUALLY NOT PROVIDED WITH AMOURING IN ORDER TO AVOID EXCESSIVE LOSS IN THE ARMOUR.

HIGH-TENSION CABLES (H.T.)HIGH TENSION CABLES ARE USED UPTO 11KV. A3-CORE BELTED TYPE

CABLE GENRALLY EMPLOYED AT 11KV. IT CONSIST OF THE FOLLOWING MAIN PART:

1. CORE OR CONDUCTOR: A THREE CORE CABLES HAS THREE CONDUCTORS.

2. INSULATION: EACH CORE OR CONDUCTOR IS PROVIDED WITH A SUITABLE THICKNESS OF INSULATION, THE THICKNESS OF LAYER DEPENDING UPON THE VOLTAGE TO BE WITHSTOOD BY THE CABLE. THE INSULATING MATERIAL IS USED TO INSULATE THE CONDUCTORS ARE PAPER, VARNISHED CAMBRIC AND VOLCANISED BITUMEN, BUT IMPREGNATED PAPER IS MOSTLY USED.

3. METALLIC SHEATH: IN ORDER TO PROTECT THE CABLE FROM MOISTURE, GASES OR OTHER DAMAGING LIQUIDS (ACIDS, ALKALIES ETC.) IN THE SOIL AND ATMOSPHERE, A METALLIC SHEATH OF LEAD OR ALUMINIUM IS PROVIDED OVER INSULATION AS IN NEXT FIGURE.

PARTS OF CABLE

X SECTION OF CABLE

4.BEDDING: IN ORDER TO PROTECT THE METALLIC SHEATH FROM CORROSION AND MECHANICAL INJURY DUE TO ARMOURING, A LAYER OF BEDDING IS PROVIDED OVER THE LEAD SHEATH. THE BEDDING CONSISTS OF FIBROUS MATERIAL LIKE JUTE OR HESSIAN TAPE.

5. ARMOURING : OVER THE BEDDING,ARMOURING IS PROVIDED TO PROTECT THE CABLE FROM MECHANICAL INJURY WHILE LAYING IT AND DURING THE COURSE OF HANDLING. ARMOURING IS USUALLY CONSISTS OF ONE OR TWO LAYERS OF GALVANISED STEEL WIRES OR STEEL STRIPS.

6.SERVING: IN ORDER TO PROTECT ARMOURING FROM ATMOSPHERIC CONDITIONS, A LAYER OF FIBROUS MATERIAL(LIKE JUTE) SIMILAR TO BEDDING IS PROVIDED OVER THE ARMOURING. THIS IS KNOWN AS SERVING.

LAYING OF UNDERGROUND CABLES

BEFORE LAYING CABLE UNDER THE GROUND, ITS ROUTE SHOULD BE SURVEYED & SELECTED. THE POSITION OF WATER MAINS OR DRAINS ETC. SHOULD BE ASCERTAINED. CABLES WHICH ARE TO BE BURIED UNDER GROUND MUST POSSESS FOLLOWING PROPERTIES:

A) MOISTURE OF SOIL SHOULD NOT ENTER THE CORE OF CABLE.

B) IT MUST POSSESS HIGH INSULATION RESISTANCE.

C) IT SHOULD NOT BE COSTLY.

D)IT SHOULD BE SUFFICIENTLY FLEXIBLE.

E) IT SHOULD NOT BE BULKY.

F) IT SHOULD BE ABLE TO WITHSTAND HEAT PRODUCED DUE TO FLOW OF CURRENT.

G) IT SHOULD NOT BE CAPABLE OF BEING DAMAGED WHILE LAYING IN THE GROUND.

THERE ARE THREE MAIN METHODS OF LAYING UNDERGROUND CABLES:

1) DIRECT LAYING

2) DRAW-IN-SYSTEM

3) SOLID-SYSTEM

LAYING OF UNDERGROUND CABLES

DIRECT LAYINGIN DIRECT LAYING METHOD, THE CABLES WITH STEEL TAPE OR WIRE ARMOURING ARE LAID DIRECTLY AS THEY AFFORD EXCELLENT PROTECTION FROM MECHANICAL INJURY. THIS METHOD OF THE LAYING UNDERGROUND CABLES IS SIMPLE AND CHEAP AND IN MUCH USE. IN THIS METHOD OF LAYING, A TRENCH ABOUT 1.5 M DEEP AND 45CM WIDE IS DUG THROUGH OUT THE ROUTE OF THE CABLE. THE TRENCH IS COVERED WITH A LAYER OF FINE SAND AND THE CABLE IS LAID OVER THIS SAND BED. THE PURPOSE OF SAND IS TO PREVENT THE ENTRY OF MOISTURE

FROM THE GROUND AND THUS PROTECTS THE CABLE FROM DECAY. AFTER THE LAYING OF THE CABLE IN THE TRENCH, IT IS COVERED WITH ANOTHER LAYER OF SAND OF ABOUT 10 CM THICKNESS, CARE BEING TAKEN THAT NO SHARP STONE SHOULD COME IN DIRECT CONTACT WITH THE CABLE. THEN A SINGLE LAYER OF BRICKS OR TILES IS LAID TO PROTECT THE CABLE FROM PROTECTED BY WOODEN PLANKS OR CONCRETE SLAB. WHEN MORE THAN ONE CABLE IS TO BE LAID IN THE SAME TRENCH, HORIZONTAL OR VERTICAL INTERAXIAL SPACING AT LEAST 30CM IS PROVIDED IN ORDER TO REDUCE THE EFFECT OF MUTUAL HEATING AND ALSO TO ENSURE THAT FAULT OCCURING IN ANY ONE CABLE DOES NOT DAMAGE THE ADJACENT CABLE. THE CABLES WHICH ARE TO BE LAID BY THIS WAY MUST HAVE SERVING OF BITUMINISED PAPER AND HESSIAN TAPE, SO AS TO PROVIDE PROTECTION AGAINST CORROSION AND ELECTROLYSIS.

DRAW-IN SYSTEMIN CONGETSED PLACES, WHERE EXCAVATION IS EXPENSIVE AND INCONVENIENT, THE DRAW-IN SYSTEM IS OFTEN USED. IN THIS METHOD, A LINE OF CONDUITS OR DUCTS ARE OF THE GLAZED STONEWARE CEMENT OR CONCRETE. AFTER LAYING CONDUITS OR DUCTS, THE CABLES ARE PUT INTO THE POSITION FROM MAN-HOLES OR BRICK PITS SPACED AT REGULAR INTERVALS. FIGURE SHOWS SECTION THROUGH FOUR WAY UNDERGROUND DUCT LINE. THREE OF THE DUCTS CARRY TRANSMISSION CABLES AND THE FOURTH DUCT CARRIES RELY PROTECTION CONNECTION, PILOT WIRES.

SOLID SYSTEMIN THIS METHOD OF LAYING OF THE CABLES, TRENCHES ALONG THE ROUTE ARE MADE AS IN DIRECT LAYING SYSTEM. TROUGHS OF CHINA CLAY, EARTHEN WIRE, TREATED WOOD, CAST IRON, STONEWARE OR ASPHALT ARE PLACED IN A ROW AND THE CABLE IS LAID OUT IN THESE TROUGHS. AFTER THE CABLE IS LAID IN POSITIONS, THE TROUGHING IS FILLED WITH A BITUMINIOUS OR ASPHALT COMPOUND AND COVERED OVER. CABLES LAID IN THIS MANNER ARE USUALLY PLAIN LEAD COVERED, AS THE TROUGHING AFFORDS GOOD MECHANICAL PROTECTION.

COMPARISON BETWEEN UNDERGROUND & OVERHEAD SYSTEM

COMPARISON BETWEEN UNDERGROUND & OVERHEAD SYSTEM CONT..