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IS 1255 PDF

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Indian standard - Download as PDF File .pdf), Text File .txt) or read online. Code of practice for installation of power cables upto and including 33kV. IS - Download as PDF File .pdf), Text File .txt) or read online. IS: MAINTENANCE 61 MAINTENANCE OF CABLE INSTALLATION. Bureau of Indian standards, IS: (reaffirmed ), Code of practice for installation and maintenance of power cables upto and including 33 kv rating.


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IS Code of practice for installation and maintenance of power cables up to and including 33 kV rating. byBureau of Indian Standards. “Knowledge is such a treasure which cannot be stolen”. IS (): Code of practice for installation and maintenance of power cables up to and including. IS: – Indian Standard. CODE OF PRACTICE FOR INSTALLATION. AND MAINTENANCE OF POWER CABLES. UP TO AND INCLUDING 33 kV.

Uploaded by Public Resource on September 10, Search the history of over billion web pages on the Internet. Books by Language. IS Code of practice for installation and maintenance of power cables up to and including 33 kV rating Item Preview. EMBED for wordpress.

When pulling round a bend. The cable grip is made of wires woven in the form of a basket. The men at rollers should also apply graphite grease in the course of pulling. The gangman Mucadam should stand in a commanding position and make evenly timed calls. This enables the men positioned at each roller to pull the cable evenly. The cable is then slid off the bottom of the drum and laid immediately in the trench.

The cable loops must be distributed over a cable length of 4 to 6 m. The other half of the gang must hold on to the rope as closely as possible. It should be ensured that the cable is pulled off from the figure of eight only from the side in which it was first looped see Fig.

The order should not be given from the head or tail of the section. Where the distance is greater. The order should.

The cable should be lifted off over the drum sidewall towards the trench and in doing so further cable from the bottom of the drum should be pulled off. Another possibility for synchronizing orders is to use 2 to 3 hand held loudspeakers distributed along the section. NOTE — For heavy cables the Heave ho method of laying should be discouraged as far as possible since it tends to leave built-in stresses in the cable.

Long steel or plastics duct sections should be checked with a gauge and coated on the inside with lubricant for plastics and concrete ducts. They are not recommended for unarmoured cables. The duct should be cleaned before pulling the cable through it. They are particularly economical for laying several long lengths of cables in parallel. When aligning the cable. It is advisable to leave a crown of earth not less than 50 mm in the centre and tapering towards the sides of the trench to allow for settlement.

If cable has to be cut. If trench is partially filled with water. As a temporary measure. Each cable length should be aligned immediately after it is laid starting from one end. The requisite protective covering should then be provided. The position of the cable joint should be marked with markers. Walkie-talkie sets are necessary to permit the personnel at the drum.

Lead cap for paper cable and plastic cap for PVC cable should be used. Where necessary. The ends may be looped to provide extra lengths in case of extruded dielectric cables. To enable the cable to be laid on the inner or outer side of the trench. The quality of joint should be such that it does not add any resistance to the circuit. The joint should further be resistant to corrosion and other chemical effects. The materials and techniques employed should give adequate mechanical and electrical protection to the joints under all service conditions.

Where the surface is of special material. Such temporary reinstatement should then be left till such time that the soil thoroughly settles down. NOTE — Other accessories. In case of buried joint. The joint should not be near pipe end or at the bend. If the ground has been made up by tipping. For PILC cables it is always advisable to protect the factory plumbed cap by laying the end in solid bitumen until such time as the jointing is commenced.

There should be sufficient overlap of cables to allow for the removal of cable ends which may have been damaged see Fig. The two lengths of cable meeting at a joint are laid with an overlap of at least half the length of joint box when pulling in.

The floor of the joint pit should be well consolidated. This point is extremely important as otherwise it may result in a short piece of the cable having to be included.

This enables the jointer to adjust the position of his joint slightly to allow for any obstructions that may be encountered see Fig. The sides of the pit should be draped with small tarpaulin sheets to prevent loose earth from falling on the joint during the course of making. The samples of paper should be handled as little as possible to avoid contamination particularly by perspiration.

The belt insulation. Particular attention should be paid to the paper next to the sheath and to that next to the conductor. If the seals are found broken or the lead sheath punctured. The jointer should wear rubber boots or gloves or stand on a rubber mat.

Only a single strip of paper gripped by a pair of tweezers should be used for the test since if several thicknesses of a paper are immersed. The presence of moisture is indicated by the formation of bubbles when a piece of the paper is immersed in hot compound.

When rubber mats are used in wet holes. The cut ends should be separated and tested to ascertain if either or both are live or dead. It is advisable to step the positions of the cuts. A small piece of rubber insertion should then be placed between the core to be cut and the remaining cores and cut through the selected core. Rubber insertion between this core and the rest should not be removed till this exposed core is completely taped. This process should be repeated for other cores taking care that only one core is handled at a time.

A rubber insulation should be inserted between this core and the rest and prepare the core for tee-jointing.

Irrespective of being live or dead. The same procedure should be followed with the remaining cores. The paper insulation of the main cable core should be cut for a suitable length and the jointing work completed.

The amount of armouring and length of lead sheath. The most convenient core to be teed should be selected and separated from others by means of a wooden wedge as far as possible. The cores of the cable to be tee-jointed should be spread and suitable tapping positions selected on the main cable. For 22 kV compound and 33 kV grade. RCC or fibre glass protection boxes are used see Fig. Since 22 kV and 33 kV cast iron boxes and moulds for HT joints in cast resin system are not yet standardized. Cast iron boxes with bitumen based filling compounds can be used with PVC cables with certain precautions.

For voltages above this neither moulds nor casting resins are standardized. Core identification should be properly studied for this purpose. Hence cable supplier may be consulted for his advise on selection.

Tee joints on HT cables up to and including 11 kV may be done only in exceptional cases. Moulds for such joints should conform to IS: Cold setting casting resins are used for further protection against water and corrosion. A typical HV dividing box is shown in Fig.

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Comprehensive jointing instructions should be obtained from the cable manufacturer and should be followed see Fig. Termination procedures of 22 kV and 33 kV grades vary widely with the design of the boxes and type of cables used.

Preformed stress-relieving cloths or tubes are also used for stress relieving mechanism in place of self-amalgamating tapes. Four standard methods which are most commonly used are: The conductors should then be preheated by basting with solder. Aluminium conductor thus prepared may be soldered on to copper cable lugs. Welded conductor joints have lesser resistance and equal or better mechanical strength than the conductor itself.

No flux is used in any of the operation. The ferrule should then be closed firmly and basted with solder. The excess solder should then be wiped off quickly and aluminium solder flux should be applied to the conductor by a stiff brush on all sides of conductor.

For These fumes contain small quantities of fluorine and it is. It is also recommended that proper ventilation be maintained at the place of jointing. If necessary the flux should be applied again and the conductor basted with solder till a bright shining appearance is obtained.

The excess solder should be wiped off and the joint allowed to cool. If necessary the strands can be stepped. Emphasis should. Then all the strands are tinned by rubbing a special friction solder stick over the heated strands. This method is not recommended for jointing conductors in XLPE cables. This is known as metallizing. Tools and accessories should meet the requirement of relevant Indian Standards where available.

The methods normally used are indent compression. In this method the end of the stranded conductor are first welded to the cable lug. After cooling welded connections are filed smoothened and cleaned.

The voltage rating of the insulation resistance tester for cables of different voltage grades should be chosen from the following table: It should be noted here that insulation resistance test gives only approximate insulation resistance and the test is meant to reveal gross insulation fault s. After satisfactory results are obtained cable jointing and termination work should commence. This is particularly more pronounced in the case of PVC cables.

Attention is drawn in this case to the presence of standing voltages along armour or lead sheath and to the considerable increase in such voltages when cables carry fault currents. A fairly low insulation resistance reading compared to the values obtained at factory testing should not be a cause of worry since the insulation resistance varies greatly with parameters such as length and temperature. These voltages must be taken into account when considering safety and outer sheath insulation requirement.

In non-screened cables. With screened construction the insulation resistance of each core is measured against all the other cores and the metal screen connected to earth. With properly installed and jointed cables. For this purpose conductors at other end are looped together with connecting bond of at least same effective electrical cross-section as conductor. The contact resistance is kept to a minimum by proper clamped or bolted connections. This measurement may be carried in case of cables above 11 kV.

In any case. Capacitance bridge is used for this purpose. The recommended values of test voltages are given in Table 6. In case of screened cable it is measured between conductor and screen.

Test voltage in these cases should be determined by the Engineer-in-charge of the work. The leakage current shall also be measured and recorded for future reference. This test should be carried only when essential. Details of construction. The frequency of inspection should be During the high voltage test. All repairs and new joints in connection with repairs should be made in the same manner as joints on new cables see Preventive maintenance in the form of regular inspection of all digging operations by other utilities or persons.

The derating factors due to grouping of several cables. Cable inspectors should patrol the various sections of the city and where it is found that cables are exposed. Important heavily loaded lines will require more frequent attention than less important lines.

In some cases where the insulation has not been damaged severely. In the case of medium voltage distribution cables emanating from distribution pillars. Distributor loads should be checked at intervals not exceeding three months. This eliminates the possibility of the cable sinking due to subsidence of the backfilling as it gradually consolidates.

Particular attention should be given to joints. The faulty length of cable having been isolated. On completion of the work. If a second test made after the joint has been opened indicates the same position as the first. If this is not done. If none of the cores is likely to be burnt through or broken.

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While excavation is proceeding. The exact nature of the fault can be ascertained by taking the following tests which may be made with an insulation testing set and a Wheatstone bridge or a resistance tester of the bridge type.

If possible. If the equipment is available to carry out the induction test see It is a good practice to choose a test which can be applied with the fault conditions as found.

In such a case. But for large currents required for burning down the faults. After initial tests are completed. This process consists of a judicious application of voltage. In the case of power cables. Having obtained the location with one method. When the amount of moisture is very small. Large ac test sets or in case of higher voltages. If an unbroken loop can be found in the cable. The resonance fault burning instrument. The battery terminal is connected to a long piece of wire at the other end of which a knife edge is provided to make contact with the CTS or VIR cable placed above ground as described before.

A reference to Fig. Its dimensions and weight in relation to output are very small. The point where the balance is obtained is thus the exact point where the fault exists. In this test the ratio arms P and Q are fixed. In the ground test shown in Fig.

The connections are shown in Fig. When the balance is obtained with the throw-over switch in the battery circuit on contact c. Q and S being known.

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At balance. S2 P In Fig. In its simplest form. A galvanometer is also joined across the open ends of the loop and a battery or a dc hand generator supplies the current for the test see Fig. Balance is obtained by adjustment of the slide or resistance and the fault position is given by the formula: The distance to the fault in equivalent length having been found out.

The equivalent length of each of the other sizes is given by: NOTE — When a loop test is taken on a cable made up of length of different cross-sections and conductor material. The cable will usually consist mainly of one size and this may be taken as standard. This is. The only essential instruments consist of an accumulator. There are many different circuit arrangements and the accuracy is not as high as that of the Murray loop test see Fig.

The measurements give the voltage drop up to the fault and by comparing the voltage measurements made from each end. For the purpose of locating breaks. V1 and V2 and then reverse the battery and take a set of readings V3 and V4. To avoid false readings.

The true readings are given by: The usual method of measuring the capacity is to charge the cable under test to a certain voltage for about 15 seconds and then discharge it through a moving-coil galvanometer.

Most faults of this type can be located with the aid of a compensated capacitance bridge which embodies a resistance shunted across the variable capacity arm for balancing out the resistance to earth at the break see Fig. A similar test is next made on a standard condenser of known capacity.

It is not very essential to have conductor data for computing results. This test can be used for all types of faults. High ohmic earth faults. The test is carried out with an ac bridge in much the same way as the ac capacitance test.

An audio-frequency current is passed through the cable loop thus formed and a search coil with head-phones and an amplifier tuned to this signal is used to trace The standard ac bridge is usually arranged to enable both the capacitance and inductance tests to be made see Fig.

From a scrutiny of the image on the screen. The impulses are reflected and the reflection from the faults insulation failure or conductor break can be differentiated from those obtained from cable joints. The intensity of the noise is maximum above the exact location of fault.

The capacitors are charged from high voltage dc test set and the spark gap is set to flash over at suitable voltage. A bank of capacitors is connected to the faulty core through a spark gap. Pulse echo test which now supersedes original radar test. It is desirable to set the spark gap to flash-over at as high a voltage as permissible to obtain good results. A typical bell mouth prepared from lead sheath fitted on to cas: Cables can be laid on bridges duly suspended from catenary wire at regular intervals.

When the cables are laid under railway tracks the cables should be laid in reinforced spun concrete or cast iron or steel pipes at such depths as may be specified by the railway authorities but not less than 1 m measured from the bottom of sleepers to the top of the pipe.

For such conditions. The catenary wire should be of galvanized steel of adequate strength and of stranded convt ructiou. If the duct is on the same side of drawing pit.

On bridges. This type of installation takes care of expansion of bridge and vibrat ion problems. SUIl- Some road bridges are provided with built-in cable-ducts and on such bridges the cables may be installed in these ducts. The catenary wires in suitable spans. On most of the rail-cum-road bridges. It is advisable that cables laid in bridges are provided with shields. In the case of single-core cables the cast iron or steel pipes should be large enough to contain all the three single core cables forming the circuit in the same pipe.

While designing a cable layout on a bridge. I t is necessary to obtain formal approval of the appropriate authorities PWD.

Pulling is carried out with manual labour by means of winches or other mechanical means when the cable to be pulJed is fairly long.

Stockings are slipped over the cable surface while the pulling eyes are connected to the conductors. The duct" having bell mouth ends should be laid at all man-hole positions in the cable unless otherwise protected to avoid sharp edges. Petroleum jelly or graphite powder or a combination of both is effective for this purpose and through lubrication will reduce the pulling tension by about 40 percent. A strong rope or flexible steel wire rope is attached to the end of the last cane rod and the whole thing is pulled from the other end of the duct.

Considerable economy can be achieved using standard factory made racks. In this way the pulling rope or wire may be passed through the duct. Racks may be ladder or perforated type and may be either fabricated at the site or pre-fabricated. Cane rods of 2 or 3 m length each fitted with brass screwed caps with screwed portion on one side and fitting socket the other side are passed into the duct one by one. The necessary. Each tray should preferably contain only one layer of cables.

It is advisable that cable suspended from a catenary wire is provided with sun shield to protect the same from direct heating by sun's rays. The catenary wires should be of galvanized steel of adequate strength and of stranded construction. Where more than one cable has to be supported either separate 'J' hooks may be used or a strip with multi-tier projections in the form of 'J' can be provided.

Another method of mounting cables along walls is with the provision of 'J' hooks which are fixed on the wall for supporting the cable. Multi-core cables need not be clamped. When there is a change in direction of the wall.

Stacking cables one above other in 2 or 3 layers on one rack or tray reduces their current carrying capacity to a very great extent. The suspenders are hung from catenary wire by means of GJ dropper wires.. Distance between these supports depends on size and weight of the cable.

In certain section it may be necessary to install a cable across an area. More than one tier trays are permissible if the cables present cannot be accommodated in a single tray. They are generally fixed to the wall or supported by free standing columns or structures enabling easy installation or replacement of cables.

The two ends of the catenary wires should be connected to rigid supports by means of GJ turnbuckle. Consequently there is an increase in the temperature of the cables installed in the tunnel and accordingly a proper derating has to be applied to the current carrying capacity of the cables installed in the tunnel. The increase in air temperature is given by the following formula: When calculatin.

If neither of these is available. The cable will unwind and become loose if the drum is rolled in the opposite direction Fig. The cable drum should then be rolled over the ramp by means of ropes and winches. Cables can be routed inside the building along with structural elements or with trenches under floor ducts or tunnels. A ramp or crane may be used for unloading cable drums. The route of proposed cable should be such that intersection with other cables will be minimum The route should not subject these cables to any vibrations.

Additionally a sand bed at the foot of the ramp may be made to brake the rolling of cable drum. Due consideration should be given for adequate ventilation. It IS necessary to rewind a cable on to another drum. During storage. The cable should. It is desirable for the drums to stand on battens placed directly under the flanges. To avoid damage to the protective covering and the insulation the cable must not be pulled across hard and sharp objects and must not be bent in an inadmissible wa y.

C on the flat'. The cable laying must be carried out swiftly. When pulling. In doing so. Warming of cables may be achieved by storing the cables for adequately longer period not Jess than 24 hours in a heated building or in a tent with hot air heaters. The drum is jacked up with a drum axle to such a height that the plank needed for braking cannot jam. The kinks nooses are particularly dangerous and should be avoided at all costs. A simple plank can serve as drum brake.

To facilitate laying. When the cables are arranged in a duct or on a rack in this way. No pull should be exerted on the end of the cable. Single-core cables should not be installed individually in protective steel ducts. Instruction to back fill the trench should not be given until the entire length is protected by cable cover and checked. Identification tags should also be attached at every entry point into the buildings and at the cable end termination.

Non-magnetic clamps may not be essential in this case and it suffices to bind the cable with steel. A rope is attached to the cable stocking pulling eye. Of these" only the first two are possible in cities and towns where there is a risk of damage to other properties existing underground. However, the final choice would depend upon the type of ground to be excavated.

It is usual, therefore, to lay pipes or stoneware conduits and to allow several spare way, for future developments. If a large number of pipes are laid across the road, a manhole should be built on either side to terminate the pipes. To avoid accidental damage to other installations in the path of the tunnel during the tunnelling operation, the tunnel should be well illuminated" preferably by powerful torch lights when men are at work. Trailer should move very slowly and the cable drum should be rotated by hand and braked if necessary in order to prevent excessive tensile stresses or kinking of the cables see Fig.

The drum is jacked high enough to fit in braking plank. Weak shaft should not be used; otherwise drum would revolve unevenly. The drum should never be kept flat on its side on the ground and the cable taken away from the same.

This invariably leads to kinking and bird-caging. At Jeast three solid plates for guiding the cable around the bend should be used maintaining minimum bending radius. The drum is slowly raised equally from both ends by using both the jacks.

The cable drum is mounted and is kept at proper position in such a way that the cable is paid out from the top of the drum Cable rollers are placed at 3 to 4 metre intervals in the cleaned and bedded trench. Where the trench is deep, it is desirable to provide 2 or 3 long rollers at different heights so as to form a ramp for the cable from the cable drum into the cable trench. A cable grip is provided at the end of the cable and rope connected to the cable grip is passed down the trench to pull the cable.

The cable grip is made of wires woven in the form of a basket. Men may also directly grip the cable, positioning themselves near the cable rollers and pull after a sufficient length of about 50 m has been pulled. The gangman Mucadam should stand in a commanding position. This enables the men positioned at each roller to pull the cable evenly, simultaneously and without jerks. The men at rollers should also apply graphite grease in the course of pulling, as required.

When pulling round a bend, corner rollers should be used so as to minimize abrasion. NOTE - For heavy cables the Heave ho method of laying should be discouraged as far as possible since it tends to leave built-in stresses in the cable.

The other half of the gang must hold on to the rope as closely as possible. When the order 'Heave ho' is given, they grip the cable and at the word 'ho' they tug it vigorously. The order should not be given from the head or tail of the section, since sound takes a second to travel a length of only m. If the gang reacts even only i second too late to the 'ho' order, the cable will not budge. The order should, therefore, be given from the centre of the section.

Where the distance is greater, 2 or 3 deputy foremen should be stationed along the section who then follow the orders given by the foremen waving the flag.

Raising the flag means 'Heave' and lowering it means 'ho', Another possibility for synchronizing orders is to use 2 to 3 hand held loudspeakers distributed along the section. The drum should be rotated further and vertical loops 3 to 4 m long should be pulled off in the direction 'B'. The cable should be lifted off over the drum sidewaJl towards the trench and in doing so further cable from the bottom of the drum should be pulled off.

The cable loops must be distributed over a cable length of 4 to 6 m, The cable is then slid off the bottom of the drum and laid immediately in the trench.

It should be ensured that the cable is pulled off from the figure of eight only from the side in which it was first looped see Fig. The duct should be cleaned before pulling the cable through it. They are particularly economical for laying several long lengths of cables in parallel. Long steel or plastics duct sections should be checked with a gauge and coated on the inside with lubricant for plastics and concrete ducts.

They are not recommended for unarmoured cables. If cable has to be cut. The ends may be looped to provide extra lengths in case of extruded dielectric cables. It is advisable to leave a crown of earth not less than SO mm in the centre and tapering towards the sides of the trench to allow or settlement.

If trench is partially filled with water. Walkie-talkie sets are necessary to permit the personnel at the drum. To enable the cable to be laid on the inner or outer side of the trench. As a temporary measure. The requisite protective covering should then be provided. The position of the cable joint should be marked with markers. When aligning the cable. Where necessary. Each cable length should be aligned immediately after it is laid starting from one end. This is generally used to connect a cable to switchgear terminal in switchboards and distribution pillars.

This is normally used for jointing a service cable to the main distribution cable in city distribution network lee The joint should further be resistant to corrosion and other chemical effects.

Such temporary reinstatement should then be left tilJ such time that the soil thoroughly settles down. Where the surface is of special material. This joint is used to connect two The materials and techniques employed should give adequate mechanical and electrical protection to the joints under all service conditions.

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The quality of joint should be such that it does not add any resistance to the circuit. The basic types of cable joints are described under The core insulation should be wiped dry and all parts. The basic types of cable jointing accessories available in the country. It comprises: Other accessories. The joint should not be near pipe end or at the bend. This enables the jointer to adjust the position of his joint slightly to allow for any obstructions that may be encountered see Fig.

For PILe cables it is always advisable to protect the factory plumbed cap by laying the end in solid bitumen until. If the ground has been made up by tipping. The floor of the joint pit should be well consolidated. This point is extremely important as otherwise it may result in a short piece of the cable having to be included. The two lengths of cable meeting at a joint are laid with an overlap of at least half the length of joint box when pulling in. There should be sufficient overlap of cables to allow for the removal of cable ends which may have been damaged see Fig.

Whenever practicable. During the preliminary st. The sides of the pit should be draped with small tarpaulin sheets to prevent loose earth from falling on the joint during the course of making. When jomtmg cable In water-logged ground or under monsoon conditions. Insulation Resistance. The presence of moisture is indicated by the formation of bubbles when a piece of the paper is immersed in hot compound.

The test lamp is one of the apparatus used to determine whether the cable is 'alive' or 'dead'. The jointer should wear rubber boots or gloves or stand on a rubber mat. If the seals are found broken or the lead "heath punctured. Particular attention should be paid to the paper next to the sheath and to that next to the conductor. Before jointing is commenced. The samples of paper should be handled as little as possible to avoid contamination particularly by perspiration.

When rubber mats are used in wet holes. As far as possible a tent should be used where jointing work is being carried out in the open. Only a single strip of paper gripped by a pair of tweezers should be used for the test since if several thicknesses of a paper are immersed.

A small piece of rubber insertion should then be placed between the core to be cut and the remaining cores and cut through the selected core. This precaution win prevent a 'short' to earth if a tool slips between the live conductor and the sheath or armour. The most convenient core to be teed should be selected and separated from others by means of a wooden wedge as far as possible.

It is advisable to step the positions of the cuts. Rubber insertion between this core and the rest should not be removed till this exposed core is completely taped. The cores of the cable to be tee-jointed should be spread and suitable tapping positions selected on the main cable.

This process should be repeated for other cores taking care that only one core is handled at a time. The amount of arrnouring and length of lead sheath. The same procedure should be followed with the remaining cores. The cut ends should be separated and tested to ascertain if either or both are live or dead. A rubber insulation should be inserted between this core and the rest and prepare the core for tee-jointing.

Irrespective of being live or dead. The paper insulation of the main cable core should be cut for a suitable length and the jointing work completed.

Paper tape 16 CI box Cord 3. Soldering sleeves Sealing cord s. Asbestos cord Lead inner sleeves RCC or fibre glass protection boxes are used see Fig.. Earthing conductor FIG. Thread Bitumen compOund 8 Impregnated paper For 22 kV compound and 33 kV grade. Tinned copper wire 2 nun dia 8. Tinned Iron wire 1 mm dia 7. Protection cage 8. Cord binding 9. Cord binding 6. PVC insulated core.

PVC tape covering 2. Outer covering 3. Rubber tape wrapping 5. Inner Cf clamp 5. Recess filled with compound 7. Bituminous compound CI filling plate Filling cap wiped over Lead sleeve filled with bituminous compound Impregnated cotton tape 11 kV only Armour DSTA shown. Paper separator positioning tape Weak back jointing ferrule Cast iron box Antimonial lead wire soldered to lead sheath for screened cable only Cast iron boxes with bitumen based filling compounds can be used with PVC cables with certain precautions.

For voltages above this neither moulds nor casting resins are standardized. Hence cable supplier may be consulted for his advise on selection. Lead sleeve Impregnated Cotton Positioning Tape for separator Impregnated cotton tape Armour clamp 1. Jute pack ing salvaged from cable Lead tape packing salvaged from cable Recess filled with bituminous compound Cast iron box filled with bituminous.

Cast iron filling plate 5. Antimonial lead wire Cold setting casting resins are used for further protection against water and corrosion. IS I Moulds Tee joints on HT cables up to and including 11 kV may be done only in exceptional cases.

A typical Hv dividing box is shown in Fig. Termination procedures of 22 kV and 33 kV grade" vary widely with the design of the boxes and type of cables used. Comprehensive jointing instructions should be obtained from the cable manufacturer and should be followed see Fig.

Preformed stress-relieving cloths or tubes are also used for stress relieving mechanism in place of self-amalgamating tapes. Four standard methods which are movt commonly used are: This method is not recommended for jointing conductors in XLPE cables. If necessary the strands can be stepped. Then all the strands are tinned by rubbing a special friction solder stick over the heated strands.

No flux is used in any of the operation. For smaller In this method each strand of the conductor is carefully cleared and scraped with scraper tongs to remove oxide film.

These fumes contain small quantities of fluorine and it is. This is known as metallizing. It is. The excess solder should be wiped off and the joint allowed to cool. Welded conductor joints have lesser resistance and equal or better mechanical strength than the conductor itself. Aluminium conductor thus prepared may be soldered on to copper cable lugs.

The ferrule should then be closed firmly and basted with solder. The excess solder should then be wiped off quickly and aluminium solder flux should be applied to the conductor by a stiff brush on all sides of conductor. The conductors should then be preheated by basting with solder. If necessary the flux should be applied again and the conductor basted with solder till a bright shining appearance is obtained.

Emphasis should. Welding method gives the best possible results. Tools and accessories should meet the requirement of relevant Indian Standards where available. In this method the end of the stranded conductor are first welded to the cable lug. After cooling welded connections are filed smoothened and cleaned. The methods normally used are indent compression. In this method conductor and lug ferrules are pressed together firmly by means of tools and dies to form a joint.

All new cables should be tested for insulation resistance before jointing. These voltages must be taken into account when convidering safety and outer sheath insulation requirement. After satisfactory results are obtained cable jointing and termination work should commence.

ND EL. This is particularly more pronounced in the case of PVC cables.

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It should be noted here that insulation resistance test gives only approximate insulation resistance and the test is meant to reveal gross insulation fault s.

Attention is drawn in this case to the presence of standing voltages along armour or lead sheath and to the considerable increase in such voltages when cables carry fault currents. A fairly low insulation resistance reading compared to the values obtained at factory testing should not be a cause of worry since the insulation resistance varies greatly with parameters such as length and temperature. The voltage rating of the insulation resistance tester for cables of different voltage grades should be chosen from the following table: In non-screened cables.

For this purpose conductors at other end are looped together with connecting bond of at least same effective electrical cross-section as conductor. The test of completed installation may be measured and entered into record book for comparison purposes du ring service life of cable installation and during fault location.

With properly installed and jointed cables. The contact resi-tance is kept to a minimum by proper clamped or bolted connections. With screened construction the insulation resistance of each core is measured against all the other cores and the metal screen connected to earth. Insulation resistance is measured by a suitable bridge. Test voltage in these cases should be determined by the Engineer-in-charge of the work. In any case. This measurement may be carried in case of cables above J I k V: Capacitance bridge is used for this purpose.

In case of screened cable it is measured between conductor and screen. The leakage current shall also be measured and recorded for future reference.

The recommended values of test voltages are given in Table 6. Details of construction. The maintenance of cable installation includes inspection. During the high voltage test. This test should be carried only when essential. In some cases where the insulation has not been damaged severely. The derating factors due to grouping of several cables. Distributor loads should be checked at intervals not exceeding three months. All repairs and new joints in connection with repairs should be made in the same manner as joints on new cables see Preventive maintenance in the form of regular inspection of all digging operations by other utilities or persons.

Cable inspectors should patrol the various sections of the city and where it is found that cables are exposed. Important heavily loaded lines will require more frequent attention than less important lines. Repairs of cables generally involve replacement of a section of the defective cable by a length of new cable and insertion of two straight joints.

The frequency of inspection should be determined by each electric supply undertaking from its own experience. In the case of medium voltage distribution cables emanating from distribution pillars. On completion of the work. The faulty length of cable having been isolated. This eliminates the possibility of the cable sinking due to subsidence of the backfilling a" it gradually consolidates.

Indian standard 1255

Particular attention should be given to joints. In case of cable termination filled with liquid insulating compound. If a second test made after the joint has been opened indicates the same position as the rirst.

If this is not done. If possible. While excavation is proceeding. The exact nature of the fault can be ascertain. If the equipment is available to carry out the induction test see If none of the cores is likely to be burnt through or broken.

Large ac test sets or in case of higher voltages. After initial tests are completed. Having obtained the location with one method. In such a case.