DESCRIPTION ST/08B

CABLES AND OVERHEAD LINES DURATIONSHORT: 2 DAYS

CONTENT OVERHEAD LINES1. Different types of O.H. wires and where used.

2. Post , parts of post and paying of wires.

3. Jointing of overhead wires-different joints.

4. Testing of O.H. linesfaults, fault localisation and rectification

CABLES5. Different cables used in S&T and cable laying procedure6. Cable Jointing, Protection in jointing and connecting

Materials

7. Periodical tests on cables, fault localisation and rectification Page

8. Termination of S&T cables9. Colour code scheme in telecom cables identifying pairs

10. Study of jelly filled cable,

11. Normal joint, loading coil joint, capacitance joint and T-joint and joint practices

12. Cable used in exchange inside / outside and their periodicaltesting.

13. Soldering

2

OVER HEADINTRODUCTION

For Transmission of electrical energy from one point to the other distant point, to establishedcommunication, one of the modes is to use mettalic conductors. They are called “TransmissionLines”. Overhead lines are such types of transmission lines.

1) Different types of overhead wires and where used.Overhead lines can not be worked in those sections of Railways where the track iselectrified, due to inductive interference.Types of overhead wires used arei) Galvanised Iron wire i.e. GI wires.ii) Aluminium conductor steel Reinforced wire i.e. ACSR wires.

i) GI wires :- It is a iron wire protected from corrosion by depositing a thin film of zincover it. The different size of wires are used for diffent purpose.a) 42.5 Kg/Km) For local telephone 28.4 Kg/Km )b) 85.25 Kg/Km for Block circuitc) 170.5 Kg/Km for telegraph circuit.

ii) ACSR :- It is not a single solid wire. It consists of a six no’s alluminium wires(Strands)twisted together along a central Steel wire which forms the core of the conductor.Size Loop Resistance6/1/1.5 5.62 Ohms/Km6/1/1.96 3.28 Ohm/Km.6/1/2.11 3.00 Ohm/KmIt is used for trunk and control lines.

2) POST, Parts of post and paying of wiresRail Post:- The rails to be used may either MG(12 m) or BG (13 m) released rails, andmay be flat footed rail or bull headed.

PARTS OF POST: 1) Porcelain Insulator:- Supports the line wire on its neck groove and insulates it

from the stalk.2) Stalk:- Supports the insulator regidly and holds it at a fixed vertical distance

from the bracket.3) Bracket or cross arm:- Supports the stalk and also maintains the horizontal

distance between wires and pairs.4) U-back :- Fits the cross arm to the post rigidly.5) Distance piece :- Packs the gap between the head of the post and the cross – arm

at the right angle to the transmission wires.6) Ties or Tie Bars :- Maintain the vertical separation at cross – arms.7) Struts :- Keep the ties in correct position.8) Stay shackle :- Links the stay with the cross-arm.9) Stay clamp :- Links the stay with the body of the post.10) Straining screw :- Adjusts the tension of the stay.11) Stay wire :- Connects the straining screw with the stay rod.12) Stay rod :- Connects the stay wires with the mush-room anchor.13) Mush-room anchor :- Anchor bottom end of the stay in the ground.

3

14) Thimble:- Prevent the rubbing of stay wire at the point of attachment.15) Stay or Guy :- Keeps the post vertical.16) Base plate :- Prevents the sinking of the post.17) Lightning spike :- Provide a discharge path for lightning through the post to the

ground.18) Number plate :- Shows the number of the post.19) Concrete Base :- This gives extra strength specially in the water logged areas.20) Strut support :- The attachment of struts to the post.21) Paying of wires : Paying is the operation in which the bundles of GI wire

( 1 Coil = 1000 metres) or ACSR wire reeled on wooden drums( 5 to 20 Km).Special care is to be taken while paying of wires to avoid damage from rubbingagainst the bottoms of the wooden drum or any projection on the buttons etc.

3. Jointing of overhead wires – different jointsA. Galvanised Iron wires :

The joints made with GI wire are classified as i) Britannia joint and ii) Twist joint.

i) Britannia joint : This joint made with GI wires of 3.55 mm dia and above . Thetwo ends of the GI wires are kept overlapped and a GI binding wire is used tobind the two wire.

ii) Twist joint : The twist joint is adopted for thinner GI wires, less than 3.55 mmdia. In this the two ends of the two wires are twisted directly without using abinding wire.

B. Alluminium Conductor Steel Reinforce wire.

The joint made with ACSR are classified as i) Tensioned joint and ii) Non tensioned joint.

i) Tensioned joint : this joint will be under the tension of span wire and are to be located atleast 5 meters away from the insulator. Three types of tensional joints are a) Twist sleeve joint : The free ends at the ACSRT wires to be jointed are inserted from

opposite direction into a pair of oval shaped alluminium tubes, called sleeves, andthen twisted by sleeve twister.

b) Compression joint (Single sleeve ) : Only one aluminium sleeve of circular cross-section whose dia is slightly more than the wire is used.The two ends of the wires to be jointed are inserted from opposite directions so thattwo ends will be at centre of the sleeve and compressed tool at every 3 mm.

c) Compression joint (Separate sleeve) : A small steel sleeve is used for jointing theinner steel wires and a separate aluminium sleeve for overall joint of the ACSRconductor, first the steel wires are jointed.

iii) Non Tensioned joint : Non-tensioned joint are these which do not carry thetension of the wires. In these joints, mechanical strength of the joint is notimportant but electrical continuity is the main criterion.

4

4. Testing of OH lines faults, fault localisation and rectification

Testing of OH wires : O.H lines are periodically tested once in 3 months. Thefollowing tests are conducted.A) Insulation Test : It is to measure the insulation resistance between the twolimbs L1 and L2 (cross insulation ) and each limb to earth and thereby to assessof leakage. The testing is done with the help of Insulation megger at early morning whenleakage is maximum.

The acceptable value is minimum 8 M Ohms/Km is in dry weather and 1.6 MOhms/Km in wet weather.If the value is sub-standard , the line is to be inspected to

i) Replace the partially broken or cracked insulator.ii) Cut the tree branches touching the wires.iii) Clean the insulator, if necessary.B) Conduction (Loop resistance ) test : It is to measure the loop resistance between

two lines L1 L2 and also L1 to Earth and L2 to Earth.The testing is done with the help of a Bridge Megger at Mid-day when resistance ismaximum.The acceptable value is maximum 5% may be tolerated in excess at the standardvalue. If the loop resistance is very high, checki) Wire jointsii) P.G Clampsiii) Pothead connectors.iv) Sag v) Broken strands of ACSR.vi) Fuses, lightning arrestors. Etc.

C) Transmission loss test : It is to measure the attenuation loss offered by the line atvarious spot.The testing is done with the help of a TMs (Transmission Measuring Set ). Oscillatorand a dB meter preferably at the early morning.The accepting value is maximum one dB for the total length may be tolerated.If the value is found to be excess check for i) Jointsii) Pot head insulatoriii) P.G Clamp Joints etc.

Faults & Faulty Localisation

Some common types of faults are :

i) Loop or twist : The two limbs of the line L1 and L2 may got twisted due toover sagging or shorted by stray wire etc.Symptoms:- The line will be salient but the station upto which loop is occurredcan be contracted No communication with the station is available beyond theloop from control office.ii) Earth fault : Both the lines or one of them get earthen partially heavyhum,i.e. inductive noise is available.Cracking noise is available due to intermittently earthed.

5

iii) Contact fault : One wire of one circuit touches the wire of another circuitlike section control wire touching block or telegraph line.

Symptoms : When the circuit comes in contact with Block lines, Block beats and feeblespeech will be heard on the circuit.iv) Break fault : Speech and ringing will be normal upto the point of disconnection andbeyond this point the station will be out off.Symptoms: The station upto which Break fault occurred ringing impulses will soundfrom control station.

Rectification of faults :As soon as a fault localised and the Block Section is identified, the JE Gr.II

(Tele) of that sub-section is to be informed immediately. The concerned like man is to beordered at once to proceed into the faulty section. He is to be given the followinginformation.i) Name of the circuit (Section control, Block cct etc.)ii) Block Section that is faulty.iii) Time of interruptioniv) Available trains for him to proceed to the nearest Block station if possible.

6

C A B L E

Introduction : Cable can be defined as, while a number of insulated electrical conductors bunched into acompact form by providing mechanical protection and electrical insulation is generally called cable.

Cable can be taken under ground, under water (Submarine cable) Overhead(Aerial orField Service) and drag the wall of building (Indoor cable). The underground type of cables can be furtherclassified into signal, communication and power cables as used by Railways.

The communication type of underground cables again comprises of cables used fortelephone purposes and for train control in R.E.Area.

1. DIFFERENT CABLES USED IN TELECOM CIRCUIT AND LAYING PROCEDURE: VARIOUS TYPES OF TELEPHONE UNDERGROUND CABLES :-

The underground telephone cables for telephone connections can be used in both R.E.and Non-R.E. area.

Types of U/G Telephone Cables:i) PCUT – Paper Core Unit Twin. ii)PCQT – Paper Core Quad Trunk.ii) PCQL – Paper Core Quad Local. iv) Multiple twin cables v)Poly – AL Cable – Polyethylene Aluminium Moisture Barrier. Vi) PJ - Petroleum Jelly FilledCable.Construction of PCUT cable :1. Conductor : A uniform solid annealed copper wire of nearly circular cross section.2. conductor Insulation : A chemically neutral paper tape is wrapped helically round

the annealed copper conductor. This paper tape encloses air around the wire,separates the conductor from the other cconductor carries the coloured marking ringsand counting of wires and pairs.

3. Pairs : Two insulated conductors as above are twisted about the common axis. Eachpair takes one telephone circuit.

4. Layers : The twisted pairs are arranged in concentric layers.5. Units : A 50 pair or 100 pair cable will have all the pair as one unit and this single

unit itself forms the core of the cable. Each layer in the unit is tied with a cottonthread called ‘Interlayer Binders’. Each unit is tied with a colour cotton thread binder.In each layer the first unit will have Orange binder & last unit will have green binder.This binder helps in the identification of various UNITS in each layer of the givencable (Paper type is also used instead of cotton thread). In that case the paper typecarries the number of the unit. Then the entire cone is wrapped with insulating paper.

Paper wrapping - Minimum two layers of paper is wrapped, Brown paper with redrings indicates that the sheath is lead alloy but not pure lead.Sheath - A metallic sheath of lead - Antimony alloy is extended over the core. Thesheath provides.i) Mechanical protection to the cores, ii) Screening against induced voltage iii) Moistureproof and water tight arrangement to the core, iv) A barrier for the insects and v) alsoreduce the impact due to lightning on the cable.Impregnated paper - Minimum two layers of paper impregnated with an insulatingcompound are applied over the sheath .Impregnated Jute: Two layers of impregnated jute are applied over the impregnatedpaper. (The above impregnated paper and jute form into BEDDING).Armouring - Two layers of galvanised mild steel tape are applied . The second layercovers the gaps left by the first layer.Serving – Two layers of impregnated jute are applied over the steel tape are applied overthe steel tape armour. Then the cable is white washed so that it does not stick to the othercoils when the cable is reeled over wooden drums.

7

PCQT CABLE : R.E.TELECOM. CABLE.In R.E area main and derivation cables are used for train control communication

purposes. The main cable runs from one end of the section to the other and along thetrack. The derivation cable is used en-route to the various utilisation points like waystations, Signal cabins switching posts, feeding posts, etc., The main cable carriesaluminium sheath and it is a long distance cable. The derivation cable carries lead sheathand it is a short length of cable. The size of the derivation cable will also be generallysmall. But, the other features are same for both. Construction of R.E. Cable (PCQT): Conductor : Each conductor is of Plain annealed high conductivity copper with circularcross section. Conductor Insulation : It consists of paper insulated and Polyethylene (PET) insulatedconductors. For paper-insulated conductors, first the copper conductor is given a helicalwhipping of paper string and then an insulating paper lap is applied over the paper string.The insulating paper carries the coloured rings for identification purpose. This is called‘Air spaced Dry Core’. This improves insulation. Polyethylene insulated conductors carry soiled colours with Nylon coating. The NylonCoating prevents the PET from softening due to heat at the time of jointing andmanufacturing.QUADS : Four conductors insulated with paper are twisted together uniformly around apaper string (axis) to from a ‘Star Quad’. The PET Quad consists of Polyethylene string at the centre (axis). Each quad is providedwith helical cotton whipping around it is keep the four conductors of quad intact and tofacilitate the identification and counting of layers and quads.Core : It is formed of all the quads arranged in concentric layers. Each layer is helicalwrapped with a insulating paper. Then its centered core is given at least four overallhelical wrappings of insulating papers. A metalised paper tape is applied to drain awaythe heat to protect the PET from heat at the time of plumbing.Sheath : The case furnished as above is extended with the sheath 99.55% aluminium isused for main cable sheaths for better screening ability. All sheath is first given a primarwash and then an anticorrosive compound is applied over it . A part from screening,moisture (and insect) barrier for the core.Bedding : Over the sheath, ‘PVC’ jacket and impregnated paper are applied for main Al-sheathed cables. For derivation cables anti-corrosive paper and layers of jute are providedas bedding. Since lead is less prone to corrosion.Armouring : Galvanised steel type is applied helically over the bedding in two layers,the second layer covering the gaps left by the first layer. A layer of anti corrosivecompound is also applied on the steel tape armoring.Jacking : Black PVC overall jacket doped with 2% carbon(to prevent ultra violetradiation hazards) is applied over steel tape armour. The PVC can also be doped withsuitable or rodent repellant difficult. The PJ cable has also get the Poly-Al moisturebarrier, polyethylene sheath and jacket. But the colour code is different.

8

Laying of Telecom. CableLife and performance of a telecommunication cable depend to large extent on the

care taken in installation of the cable and it’s jointing.

Testing of cable before LayingBefore the cable is laid, visual inspection of cable shall be made and it shall be

tested for insulation and continuity of the cores.Bending and armouring shall also be inspected to see that there has been no

damage during transit or in storage.Before the cable is unwinding from the drum, its insulation must be measured

after removing the end seals. No cable having insulation lower than that specificationshall be laid.

Paying Out Cables:For paying out cables, drums shall be mounted on cable wheels. The drum on the

wheel shall be brought to one end of the trench and the end of the cable freed and laid inthe trench. The cable wheel shall then be drawn along the track.

A party of labourers shall follow the drum and guide the cable from the read intothe trench carefully so that the cable is not damaged or bent unduly.

In case where the wheels are not available, the drum shall be mounted on an axleat one end of the trench and cable laid out and carried by labourers.

In no case, shall the drum be reeled off on to the road for laying the cable and thecable dragged on the ground for laying purposes.

It should be ensured that no kink is formed while laying out the cable.In no case shall the cable drum be laid flat on the ground and cable unrolled from

the drum by twisting the cable. This procedure may cause permanent damage to thecable.

LAYING OF CABLESCables shall be laid generally as per standard instructions. Special precautions

shall be taken in the station, yards etc. where a number of other utilities may be existingand may be detailed in joint circular issued by the Civil Engineering, Signalling andElectrical department (Where applicable) of the Railway.

The width of the cable trench shall normally be 0.5 meters. The bottom of thecable trench shall be leveled and get rid of any sharp materials. In the bottom of thetrench previously leveled. In the soft ground, the cable shall be laid at the bottom of thetrench previously leveled. In the rocky ground the cable shall be laid on a layer on a layerof sand or shifted earth of 0.05 m. thickness previously deposited at the bottom of thetrench. To both the above cases, the cable shall be covered with a layer of sand or shiftedearth of 0.10-meter thickness.

The cable laid parallel to the track and between home signals of a station shall belaid at the depth of 1 meter. Cable laid across the track shall be 1 meter below the railflanges. Cable laid beyond the home signals in a yard, automatic signal area or IBH orLevel Crossing Gates must be at a depth of 1.2 meters. All station yards, which are theftprone, the cable shall be laid at cable crossing.

When a cable has to cross the track, it should be ensured that :i) The cable crosses the track at right angles,ii) The cable does not cross the track under 10 meters from the centre of the

nearest track.iii) Within the station limits, the trenches shall preferably be dug at a

distance of not less than 3 meters from the centre of the track, width ofthe trench being inside the 3 meters distance.

9

iv) At the end of the main cable of the extra loop length of 6 to 8 metersshall be kept.

It is desirable that the excavation of the trenches is not done in long lengths does notremain uncovered overnight. It is preferable that trenches are dug, cables laid andrefilling done on the same day.

The inspecting official, who supervises the excavation works, shall have theshorting materials ready in hand so that in banks where ashes or loose materials areencountered, shoring can be adopted.

Back filling of the trenches should be done properly, rammed and consolidated.During excavation, the earth of the trenches shall not thrown on the ballast. The

earth shall be thrown by the side of the trenches away from the track.In places where cables are to laid close to the track, though preliminary after

digging upto 0.5 m. may be done, excavation to the full depth should be done only justbefore laying the cables in the presence of an official from Engineering department.

The work shall be personally supervised at site personally be an official of theSignal and Telecommunication Department below the rank of a Telecom. Inspector.

2. CABLE JOINTING, PROTECTION IN JOINTING AND CONNECTING MATERIAL.TELEPHONE CABLE JOINTING

Jointing procedure for the lead sheathed PCUT Telephone cable:

The standard drum length of cable manufactured by M/S Hindustan Cable Limited is of500 mts. Or 1000 mts., depending on the size of cable . So, when more than above length isrequired, two drum lengths of cables are to be jointed, this type of joint is ‘Straight ThroughJoint’. But in telephone cables, it is commonly called as “Cable Joint”.

Cable Jointing Pit :- At the site of cable jointing, a pit is to be dug up, the dimentions ofwhich can be 1.2m x 0.8m x 0.8m, and a tent is to be taken to prevent entry of water into the pitin the event of rain etc.Preparation of Lead Sleeve : This is a cylindrical pipe of Lead which is placed at the joint ofinter connection of the two ends of the cable as an extra piece. This serves as a metal cover overthe jointed open core. As per P&T standard size of Lead sleeve for commonly used cable size aregiven below :-

Lead Sleeve (Length x internal dia) in mm.No. of pairs in the cable6 ½ Ibs / mile 10 Ibs / mile 20 Ibs / mile

10 152 x 19.1 152 x 19.1 305 x 25.420 305 x 25.4 305 x 25.4 305 x 38.150 305 x 38.1 305 x 38.1 381 x 50.8

100 381 x 50.8 381 x 50.8 457 x 63.5

A lead sleeve of length as shown above, is prepared out of lead steel if ready-made one isnot available. For the correct size of the lead steel is cut and it is made into a cylindrical tube. Themeeting edges are thoroughly cleaned with thick knife and it is neatly wiped with plumbing metalusing a little tallow (flux). Then it is inserted into an iron rod (or crow bar) and it is neatly dressedwith wooden mallet. The final shape should be inform in cross-section and free from bends. Thedia meter of this Lead Sleeves will always be larger than the diameter of the joint. A vent hole ismade at the centre of the lead sleeve by making a V-Cut.

10

Preparation of Cable: The two cables to be jointed are made straight and over lapped. Thenleaving an overlapped. Then leaving an overlap of one and half times the length of the lead sleeveto be used for jointing, the extra length of cable is cut.

Preparation of ends of Cable :-1) On one end of the cable, a binding is done with a thin G.I wire at a length equal to one and

half times the length of Lead Sleeve to be used.2) Then the jute serving is removed upto the point of binding by cutting it with a knife and

hammer.3) Now, a second G.I binding is provided over the steel tape armouring (50 mm) away from the

first binding.4) The double steel tape armouring is cut by a Hacksaw and removed one by one by cutting

pliers. (Care should be taken not to damage the inner Lead sheath).5) Then the two layers of jute bedding and paper are removed by hand after slightly warning

with a kerosene blow lamp and cutting with a knife (This flame should not be burn the coreinsulation ).

6) Lead sheath is cleaned with cotton waste moistened with a little kerosene.7) After that all the tools used so far and the hands are cleaned thoroughly.8) Now the lead sheath of the cable is cut by a hacksaw leaving 3” (75 mm) from the second

binding core should be taken not to damage the core while cutting the sheath .In the same way the end of the other cable is also prepared.

Insertion of Lead Sleeve :- The prepared Lead Sleeve is now inserted into one end of the cablebefore the conductor jointing is commenced.

Jointing of Conductor : A twin knot is made over the core insulation paper ¼” away from themouth of the lead sheath.Then similar Twin knots are made for every layer. They keep the pairs in correct position whilejointing the individual conductors.Now the two ends are kept face-to-face leaving some space between them. This space is called‘Jointing Space’.This jointing space for commonly used cables is as given below : (as per P & T standard0

Length of Lead Sleeve Jointing Space152 mm 100mm305 mm 230 mm381 mm 305 mm457 mm 381 mm

Before commencing the jointing of copper conductors of the cable is to be tested to know thecondition of the cable.Jointing Operation : the jointing operation is started with zero layer with marker pair. Papersleeve is inserted is one of the conductors before jointing.First both the conductors are twisted twice alongwith paper insulation. Then the paper insulationis carefully removed with a side cutter and again the bare copper wire is evenly twisted for alength of about 1” (25 mm) and rest is trimmed off with a pair of nippers. Then the twistedportion is bent on the side opposite to the paper sleeve and the paper sleeve is drawn over so thatthe base portion of the twist is completely covered. The paper sleeve for the next pair is insulatedon the other side to reduce the bulging at the centre.Soldering The tip is soldered for conductor gauges more than 6 ½ Ibs/mile. The tip is alsosoldered when conductors of different gauges are jointed.Lacing :The complete joint is laced with twin to reduce bulging and to hold all the pairs in a grip.

11

Heating : The above jointed portion is heated by charcoal flame to remove the inner moisture(Do not use naked flame).Wrapping of paper :Paper is wrapped over the joint to protect the conductors from heat whichwill be applied during plumbing.Plumbing : Now the lead sleeve which was inserted on one side of the cable drum over the jointand it is dressed with a wooden mallet on the ends to taper the either ends to grip the sheath of thecable (Keep the vent hole on the top).Both the ends of cable sheath and the sleeve ends are cleaned with a shave hook and tallow isapplied to prevent any oxidation. The plumbing metal is melted in an iron – ladle and poured onone side of the dressed sleeve and the same is neatly wiped all over the end by a wiping pad. Amirror can be use to see the bottom portion of the joint. Now, tallow is applied which cools thejoint. Similarly, the other end also wiped. Lastly the vent hole enables the inner gasses to escapeat the time of plumbing.

Straight through joint Box :- The above is kept in a straight through joint box for protection andis fitted properly. Then bitumen compound is filled inside the box. If the G.I box is not available,a brick chamber is to be made and the joint is secured in that and bitumen is filled completely tomake it moisture proof.Earthing : An earth is used to connect armour for electrical continuity.

Note :-1) In one end of the cable, the pairs appear in clockwise direction. This end is called ‘A’ end

and a red band is provided at this end.2) At every straight through joint, the A-end should meet the E-end to avoid cross-connection of

pairs. So, at the time of cable drum length faces the E-end of subsequent drum length.

3. PERIODICAL TESTS ON CABLES, FAULTS, FAULT-LOCALISATION AND

RECTIFICATIONThe instruction deals with the periodical Tests of cable :SCHEDULE OF TESTS: - PERIODICITY

1. Transmission loss test (Repeater to Repeater) Monthly2. Insulation test (Repeater to cable Hut)

i) Between conductors of untapped quadsii) Between conductors and earth Monthly

3. Loop resistance of untapped quad (Repeater to cable Hut) Monthly4. Measurement of Signalling voltage at input and output of repeater. Monthly5. Input and Output level of Repeater station . Monthly6. Measurement of Cross talk (RPTR to RPTR) Quarterly

i) Near Endii) Far End

7. Measurement of noise on cable Yearly

PROCUDURE OF TESTINGa. Transmission loss test (Repeater to Repeater)

12

Feed a tone of 0 dB at 800 Hz to the cable pair under measurement. Measure theattenuation of the measuring end by keeping the switch of the TMS Kit in ‘Trans’(Loss)position. The transmission loss should be 0.25 dB/Km (for loaded RE Cable).

b. Insulation resistance :The insulation resistance should be measured by a 100V Megger only betweenconductors of pairs of untapped quads and between each limb and earth. The insulationresistance should not be less than 625 Meg Ohms/Km of each conductor is at 16 degreecentigrade.During the month from July to October the insulation resistance should be measureddaily on a test quad. This will give a pre warning to the maintenance staff if the insulationof the cable goes down below the determined value so that immediate action may betaken before the complete failure of cable.

c. Loop resistance :The loop resistance measurement should be carried out on untapped quads betweenRepeater and cable Hut by a meter of an approved type or by a wheat-stone Bridge.

d. Signalling Voltage:The signalling voltage should be measured on load by feeding permanent voltage fromTest Room . The input and output voltage of all repeater stations should be measured by ameter of approved type. The signalling voltage should be measured on all quads havingphantom derivation.

e. Input and Output Levels:The performance of the cable circuit can be checked by conducting this through test.Feed a tone of 5 dBm at 800 Hz on Pair 1 from Test room and measure at the input of thefirst Repeater station by disconnecting the plug (U links). The reading should bemeasured by TMS Kit on Trans position. This will give loss of cable of betweenCT/Room and 1st RPTR connect the plug and measure the level at output of repeaterdisconnecting between the second & first reading gives the gain of the trans amplifier ofthe circuit under test. Put through the plugs and measure the level at the input of the ofthe second repeater in the same manner as described earlier. The difference in reading of3rd. and 2nd gives the loss in cable between first in the second Repeater stations. Likewise,reading should be taken on all Repeater stations.For receive pair (Pr.2), the last repeater station will feed a tone and the other repeaterstations will measure as describe.The above test is to be conducted by keeping the switch of the TMS Kit in Loss position& all reading should be taken by isolating the plugs.

f. Near End & Far End Cross Talk:The measurement of cross talk shall be carried out by a cross talk meter. Far end crosstalk level should be better than –65 dB & Near talk level should be better than –61 dB.i) Near and Far end cross talk should be measured by feeding a tone of 0 dB at

800Hz. On Pr.1 (Disturbing pair) of quad and measuring the talk on Pr.2(Disturbed pair)at the same station. While conducting test the cable at the otherend should be kept isolated.

ii) To measure far end cross talk a signal of 0dB 800 Hz shall be feed on Pr.1 andresulting cross talk should be measured on Pr.2 at the far end. While conductingfar end talk, the far end should be terminated by a characteristic impedance.

Noise on Cable:Interference to speech transmission, is, the psophometric voltage in communication cableis to be measured by Psophometer. This is to be done by connecting a 600 Ohmsresistance across the telephone circuit. The Psophometer voltage should not exceed2 mV.

13

FAULT LOCALISATION ON COMMUNICATION CABLES :-

Fault occurring in cables which are in use on Lower distribution voltages will beconsidered. The common faults which are likely to occur in such cables are:

i. Group Fault : The insulation of the cable may break down causing a flow of currentfrom the core of the cable to the lead sheath or to the earth. This is called “Group Fault”.

ii. Short Circuit Fault : If the insulation between the conductors is faulty, a current flows between them . This is called a ‘Short circuit Fault’. iii. Open circuit Fault : An open circuit fault occurs when a conductor gets between or when a joint pulls out.

Method used for localisation ground and short circuit faults differ from these used forlocalisation open circuit faults.METHIOD USED FOR LOCALISING GROUND AND SHORT CIRCUITFAULTS :In case of multicore cable it is advisable first of all, to measure insulation resistance ofeach core to earth and also between cores. This enables to sort out the core that is earthedin case of Ground fault, and so sort out the cores that are shorted in case of a Short circuitfault Loop tests are used for location of Ground and Short circuit faults. These tests canonly be used if a sound cable runs along with the faulty cable of cables.The Loop tests work on the principle of Whetstone Bridge. The advantage of these testsis that the resistance of fault is connected in the battery circuit and therefore does notaffect the result. However if the fault resistance is high, the sensitivity is adverselyaffected. Murray and Varley Loop tests are generally carried out for localising Groundand Short circuit faults.

LOCALISATION OF OPEN CIRCUIT FAULTS IN CABLES :The Localisation of an Open circuit in a cable may be done by a capacitance test. Theprinciple of this method is based upon the fact that the capacitance of a cable isproportional to its length. In case the total capacitance measurement is necessary. Thecapacitance measurements may be done by any of the Known methods by using Ianty’sbridge or using a Ballistic galvanometer.

4. TERMINATION OF CABLE

Pot heading of cable: It is the operation of connecting the terminal end of the undergroundtelephone cable to PVC switchboard cables. The PVC switchboard cables are then directlyterminated on equipment. First, the conductor of the PVC cable. Then a separate Lead sleeve isplumbed (after dressing) to the lead sheath of the pot sleeve . the inner space of the pot sleeve iscompletely filled with wax to seal against moisture entry. A circular lid of lead is then solderedon the top end of the sleeve. The PVC cables are taken through the holes in the top lid . This isindoor type of joint and hence it should not be exposed to outside weather. These pot heads arefixed vertically on walls or kept inside the cable trench on the floor.

14

Termination of Cable :- The under ground cables terminated in termination box. There are twotypes of boxes:

i) Cable termination box (C.T box) ii) Protected cable termination box (P.C.T.box), C.T. box is used for indoor

terminations. The conductors of the pairs are terminated and soldered on the tagsprovided on the back side of box. The PVC cable is connected to the terminalprovided on the front side of the box. The back portion is completely filled withbitumen compound to avoid entry of moisture. The C.T. boxes are mountedvertically on walls. A gland is plumbed to the cable sheath at the entry of the box.An earth clamp is also provided as shown in fig.6. P.C.T box is used when over-head lines are connected to underground cable. This

Is meant for outdoor use. Hence it is protected from weather conditions.Protective devices like fuses etc.,are also provided.

5. COLOUR CODE SCHEME IN TELECOM. CABLES IDENTIFYINGPAIRS AND QUADS

COLOUR CODE (FIG. 2) OF PCUT CABLE IDENTIFYING PAIRS :Colour Coding Scheme :

i) PCUT Cable : Here to remember marker and layer colour of ODD and EVEN layerthe following may be followed :-

Blue => Layer colourOrange => Marker colour

B O R G Red => Layer colour (Blue) (Orange) (Red) (Green) Green=>Marker Colour

ODD Layer EVEN Layer

ii) Polythene insulated cable : The following may be followed to remember colourcode of switch board, jelly filled etc. cables for long duration.

B O G B S this indicate colour code for Main wire

Blue Orange Green Brown Slate

Mate wire for PJ filled cable : W R B Y (White) (Red) (Black) (Yellow)

Mate wire for SB (Switch Board) Cable : W Y B R P (white) (Yellow) (Black) (Red) ( Pink)

15

iii) PCQT cable : In case (i) i.e. in PCQT cable it has been observed that BORG is followed for layers (ODD and EVEN) but here in case ii) i.e. in PCQT cable colourcode of string (thread) is the striking criteria. In that case, the following order may befollowed for durable remembrance.

Coloured Ring for Coloured Thread forSlNo.

Layer1st Quad(Marker)

LastQuad(Ref.)

Others 1st Quad(Marker)

LastQuad(Ref.)

Others

1. EVEN RED BLUE Bl. & Ralternate

W.O(WhiteOrange)

W.O W(White)

2. ODD RED BLUE -DO- BK,O(Black,Orange)

BK, O BK

Pictorial Description Of PCQT cable :-

i) Distribution of layers :-

Layer 0 (May be treated as EVEN Layer) Layer 1 ODD layer.

Layer 2 EVEN Layer Layer 3 ODD Layer.

ii) Distribution of marker and reference quads :

SlNo.

ITEM FIGURES DEPICTING COLOUR CODE

1. First quad of layer2(EVEN)(MARKER)

16

SlNo.

ITEM FIGURES DEPICTING COLOUR CODE

2. Last Quad of layer3(ODD)(REFERENCE)

3. Fourth Quad of layerzero(EVEN)(OTHERS)

In case of 3rd . quad in layer zero the only change will be Blue will be replaced by red colour ring.

In this cable the paper insulation on the conductor carries the coloured rings atequal lengths throughout the cable as shown in fig. 2 : 1. The pairs twisted (in fig. 2.3)are arranged into the concentric layers.i) Pairs in ODD layers carry Blue rings. But first and second pair in each ODD

layer carry ORANGE rings. Out of the two ORANGE ring pairs, single anddouble pair is the First one and three and four ring pair is the second one (fig.3).

ii) Pairs in EVEN layers carry RED rings. But first and second pairs in each evenlayer carry GREEN rings. Out of the two GREEN ring pairs, single and doublering pair is first one and three and four ring pair is the second one. First and second pairs in any layer is called ‘Marker 1’ and ‘Marker 2’respectively. Zero layer is the even layer. The above system of identification andcounting of pairswith the help of colour is called ‘colour code’ or ‘ColourScheme’

COLOUR CODE SCHEME IN R.E (PCQT) TELECOM. CABLESIDENTIFYING QUADS AND SPECIFICATION :

The four wires A,B,C,D in a quad carry Blue or Red coloured rings in groups asshown in fig. 7.The diagonally opposite wires in a quad form a pair, each concentrically arrangedquad in the core carry overall external cotton thread (whipping) in a helical

17

manner. The colour of this cotton whipping is white – For here and even layersexcept the first and last quads in each of the above layers. Black – For ODDlayers. For the first and last quads in all ODD and even layers an extra orangethread is added to the black or white whipping.

Identification of first and last quads in each layer can be done as per the tableshown below :

Zero layer and Even layer Odd layerQuadColour ofRings

Colour of cottonwhipping

Colour ofRings.

Colour ofcottonwhipping

First quad Red White Red BlackLast quad Blue Orange Black Orange

Different parameters of R.E Cables : R - ResistanceC - CapacitanceL - InductanceG - Leakance

6. STUDY OF JELLY FILLED CABLE

Jelly filled cable is an underground cable having polythene as insulation on conductorsand the interspaces between the conductors is fully filled with petroleum jelly. Petroleum jellyprevents entry of moisture and water inside the core area in the event of any damages to the cable.The cable is circular in shape through out its length and free from any physical defect.

Jelly filled cable is wound on strong wooden drum. The length of the cable on any drumis of 500 mtrs. + 10% .

Uses : In Railway Polyethylene sheathed Jelly filled cable with poly-Al moisture barrier is used

for providing telephone connection to the subscribers and jelly filled quad cable is used forspecial purposes in Railway electrified area .

Fault localization is easily possible in Jelly Filled cable as water does not spread into it.

The different sizes of cables to be used for various telecommunication circuits are as followingtable :

Sr No. Size of conductor Circuit1 2.81 Kg/Km (10 Ib/mile) Subscriber connections no exceeding 5 Km2 5.65 Kg/Km (20 Ib/mile) Subscriber’s connections and different lines 5

to 8 km long.3 11.26 Kg/Km (40 Ib/mile) Tie lines on more than 8 Km in the same

local area.

18

4 28.19 Kg/Km (100 Ib/mile) Control cables and long distance trunk circuit5 Quaded cable, 0.mm copper

conductor.Telecommunication’s control circuits in REarea

Construction of Jelly Filled cable :

A B C D E F

LEGENDS:

A Polythene outer jacketB Galvanised steel tapeC Polythene TapeD Polythene Inner sheathE Poly-Al-Laminated TapeF Polythene Insulated Copper Conductor

7. Normal Joint, Loading coil joint, Capacitor joint and T joint and Joint PracticeJoints in R.E. Cable:

Types Spacingi) Straight through (Normal Joint) (SJ) 457.5 mts.

19

ii) Loading Coil Joint (LCJ) 1830 mts.iii) Derivation joint (Transformer Joint) (DJ) Tapping points.iv) Straight through cum derivation joints (SJ/DJ) 457.5 mts.v) Capacitor Joint (CJ) 915 mts.

Purpose of the above different joints :-

i) Straight through joint is made to interconnect two successive lengths of cable.ii) Loading coil joint is made to interpose extra inductance in the form of loading coils in

series with the conductors of VF pair to keep the attenuation low in the VF range andmake the circuit to work as a Low Pass Filter.

iii) Derivation Joint – is made to connect the derivation cable with the main cable throughtransformers wherever tapping of circuits are required.

iv) Straight through cum Derivation Joint : In case of a derivation joint falling very near to anormal joint, both the joints are clubbed into single joint.

v) Capacitor Joint – is made to connect capacitors in parallel externally to the VF quads forthe purpose of reducing noise and cross talk in the V.F circuits.

Then with a smooth file (without damaging the core ) and removed to twin. Knot is puton the core 25 mm. Away from the bell mouth of the aluminium sheath.

Insertion of Lead Sheath : A Lead Sleeve with top vent hole to escape hot air and moisture isprepared and slipped into one end of the cable. For Al sheathed cables only Lead Sleeves areused, not Al Sleeve.

Layer Separation : The various layer are separated and quad No. Rings are inserted on bothsides.

Testing : The cable is tested with a megger.

Jointing of Quads: The quad conductors are jointed starting from zero layer – Marker quad onboth sides as per quad No. rings.

Jonting of PET Quads : The PET conductors are butt jointed by using tinned split copper sleeve.

Bandaging : This splitted joint is laced with cotton twin to keep it intact and compact.

Dressing and Plumbing of Lead Sleeve :- Now the Lead Sleeve is slides. Now the lead sleeve isslided over the joint with vent hole on top and both ends are dressed taper with a wooden malletends are dressed taper with a wooden mallet to grip the Al sheath. After that both end of thesleeve are sealed with plumber’s metal heated by a white petrol blowlamp.Bonding wire is connected from armour to armour via Al sheath and Lead Sleeve to maintaincontinuity for screening.

Anti-corrosive protection :- Anti-corrosive tape and paint is applied on it.

20

Straight through joint Box :- The entire portion of jointing placed into a straight through jointbox and filled with bitumen compound.Precaution :-1. The pit should protect from entry of water.2. The joint should be kept horizontal.3. ‘A’ end of cable should meet the ‘E’ end.4. A heap of mud should be placed over the burried joint to flow water away.5. Bitumen compound should be cooled before pouring the CI box also with nuts and bolts

of CI box to protect from resting.

Loading Coil Joint :In a transmission line the attenuation (dB loss) increases with increase of frequency and

decrease of diameter of the conductor. So in underground cables the attenuation is much higherthan that of overhead lines. In case of transmission lines the condition for minimum attenuation isgiven by the formula :

L G = C R Or, L = C R

G

Where R (Resistance ), C (Capacitance) L (Inductance) and G ( Leakance) are the primaryconstants of the transmission are the primary of the transmission line.But in R.E. communication cables with .9 mm, diameter copper conductors by construction thevalue of L is very less and C is more due to closer spacing of cable conductors, R is also moredue to thinner conductors and G is very small (in micro Ohm) due to the better insulation. So, tokeep down the attenuation in R.E. communication cable, either we must increase the L and / ordecrease C in order to satisfy less free line condition Viz. LG = CR.C (Capacitance ) of the cable pair can be decreased only when the pair separation is increased.This increased the size and cost of the cable. So it is found convenient to increase the L(inductance) by artificial means, which is termed as LOADING of cables.Different methods are adopted for loading of cables. One method is to wrap each conductor of thecable with a wire or tape of soft magnetic material. This is called Continuous Loading and it iscostly, so it is adopted for submarine cables only.In R.E. cables loading is done by connecting Lumped Impedance (called Loading coil) of 88 mH.To VF pairs only at regular intervals of 1830 mts. This method is called ‘Lumped Loading’.

Requirement of Loading Coils:They must have 1) Low resistance 2) High insulation, 3) Low capacitance, 4) Smaller size withhigher inductance, 5) Shielding to avoid interference to other coils, 6) Equal electrical effects oneach by of the circuit (Electrical balance to avoid cross talk and 7) Reasonable cost.

Construction of Loading Coil :The wire is wound on a core of toroidal shape in two exactly equal parts. Each part is inserted inone conductor of a pair in series so that exactly half the inductance is added 15 each conductor ofthe pair. In the latest construction ferrite rings are used as cores. The two parts of the coil shouldhave balanced electrical properties to avoid Cross talk.Connection of a Loading Coil :-

The loading coils are connected as shown: -A A

B B

21

Loading Coil Loading CoilTo balance the parts of the coil electrically the actual winding is done as shown :-Note : 1. Only the V.F pairs are loaded in R.E. Communication cable . The Phantom and PETpairs are not loaded.

2. The local telephone cables are not loaded.3. The coils are sealed and only the loads will be available outside the box.

DERIVATION JOINTS.For tapping the required circuits, from the main cable, a derivation cable is not directly connectedto the main cable but it is connected through transformers as shown in fig. So, it is also called asTransformer Joint. The transformers are used for impedance matching.

TX

Quad Main Cable

RX

CAPACITOR JOINT (CABLE BALANCING).During the process of cable manufacture, care is taken to keep the various capacitance

unbalances within close limits. But due to the handling of cable in the process of paying out andlaying the various capacitance unbalances are created. This capacitance, unbalance causes, crosstalk and noise in V.F circuits carried in the cable. In RE communication cable, the capacitanceunbalance exists between:a) Conductors of a quad and sheath (Earthed). This is called of a ‘Earth Coupling’.b) Conductors of a quad and between adjacent quads. This is called ‘Cross talk coupling’.

Cross talk couplings cause cross talk and Earth couplings produce noise in VF circuits. Henceto limit the cross talk and noise in R.E V.F circuit, the two couplings are measured in the fieldafter laying the cable by an instrument called CUM set (Capacitance Unbalance Measuringset)and the measured values are recorded in a tabular form. With the help of these tabulatedreadings, the corrective measures are adopted.4. By transposing the two conductors of a pair where necessary. This is called ‘poling’and

‘Poling’ is done to reduce Earth coupling.5. By connecting capacitors externally in shunt. This is done for reducing the cross talk

coupling.The above operation, by which the capacitance unbalances are brought within the permissiblelimits, is termed as Balancing.Permissible limits of capacitance unbalance for full and half loading sections are 40 PF and20 PF respectively.

CUM Set :-Capacitance Unbalance Measuring set is the instrument by which the various capacitance

unbalances of the cable circuits are measured.The unit basically consists of an oscillator circuit and a calibrated capacitor with the

associated controls to simulate the various circuit configurations of e and k.

22

The principle of working of this meter is, feeding an audio tone in one side circuit andmonitoring the level of the tone on the affected side circuit. This monitoring is done by a headphone (or a meter deflection) . The level of the tone is brought to minimum by introducing knownvalue of capacitance. Thus the amount of unbalance can be directly read on a calibrated variablecapacitor. The conductor across which the external balancing charts or from the Campbellequation.

4. CABLE USED IN TELEPHONE EXCHANGE INSIDE / OUTSIDE AND THEIR PERIODICAL TESTING.

INDOOR WIRING CABLES :i) Cable VIR (Vulcanised India Rubber) braided and compounded single core

copper conductor 250/440 V for indoor wiring of telecommunication circuits.ii) Cable PVC (Poly Vinyle Chloride) insulated black PVC Sheathed flat twin core

copper conductors, 250/440V iii) Switch Board Cable: - It is an indoor type of cable used for extending the

connection to various locations in the administrative building or forinterconnection of various stages of equipment in Telephone Exchange, TrunkBoards, etc. These cables are run along the walls on wooden batten fixed withlink clips. These cables are used previously for wiring manual switchboards,hence the name. This cable consists of high-grade annealed tinned copperconductors with PVC insulation and cable is PVC sheathed.The core is wrapped with a moisture proof tape (Cellulose). The copperconductors are tinned to avoid oxidation during soldering cables with conductordiameters of 0.5 mm and 0.6 mm are available Rip cord is a nylon thread. It isused to cut open the sheath.

iv) FS cable : Field service cable is used for outdoor applications for providingcommunication on temporary basis in floods, cyclone, etc. They can be substitutefor over head lines also. The specification of Field Service cable, Single coretwin twisted, size : 7/0.0148(0.375 mm) 250 V /440V grade PVC insulated andPVC sheathed.

9. SOLDERING :It is a process of jointing two metal pieces permanently like metal plates, wires

etc.SOLDERING PROCESS :

Metal pieces are first cleaned with help of sand paper or knives and then heatedat the place to be jointed by the ‘Bit’ of the soldering Iron. When the place is sufficientlyheated, a little flux is applied so that the moisture or any oil and grease is removed bychemical action. A little solder is then applied on the place, which melts away and flows

23

through the entire heated surface. Now the soldering Iron is removed from the jointingplace and the joint is allowed to cool down which will make a strong permanent joint.RESIN CORED SOLDER.

It is a wire type solders flexible and soft whose inner core is hollow which isfilled with resin powder. The resin power functions as ‘Flux’.FLUX: To remove oxidise layer, any oil, grease etc. Flux is made of rasin, Borax, HCl,ZnCl2, salamnonial.

DO’S & DON’TSDo’s

Overhead

1. Periodical Testing should be done within the predetermined time for better maintenanceof Overhead alignment.

2. Lightning spike must be provided in every post.3. Stay wire must be provided in all posts and should be tightened properly.4. During power line (LT & HT) crossing standard spacing must be followed and guard

must be provided overhead wires.5. Specified gap must be maintained for Railway & Road crossing and in congested

localities.6. Care must be taken for any type of overhead jointing so that high resistance may not

formed. Grease must be applied during jointing.

Cable

1. Keep the proper distance at the time of cable laying alongwith Railway track.2. Soil depth of cable must be followed as per extend rule.3. Laying cable should not be exposed without filling of soil and ends of the cable to be

covered with proper Nalco taping.4. Cable sheath and Armour should be earthed with an interval of one Kilometer.5. Distance of cable hut should be maintained properly for termination and testing purpose.6. Insulation of cable should be maintained properly .7. A lead sleeve with top vent hole to escape hot air and moisture.

Don’tsOverhead

1. More than tolerated sag cannot be allowed.2. Corrosive and edged wires don’t use for O/H transmission.3. No strands of ACSR wire should not be kept. That should be immediately treated.4. The insulator should not be dirty5. Joints of ACSR wires should not bend in the process of twisting.

24

6. Bulging at the centre of the joint not to be formed.

Cable

1. No induced voltage may arise to dangerous values. Precautions to be taken for avoidingelectric shock.

2. Don’t mount the drum without cable wheels.3. Don’t expose the cable without filling of soil during laying.4. width of the cable trench should not be kept below 0.5 meters.5. Don’t keep cable trench at a distance below 3 mtrs. from the centre of the track within

station limits.

***************************