external plant

8/11/2019 External Plant

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11UNDERGROUND CABLES

Objective of the lesson::This chapter provides the insight into the

basic requirements of any underground cable to survive the expected life

period and needs. The chapter also deals extensively with the different types of

cables in use in telecommunication networks and the parameters offered for

various utilities.

11.0Bro! constr"ction of n# teleco$$"niction cble

Core ::

All the insulated conductors compactly arranged in pairs, unitsand super units constitute core of the cable

%oist"re Brrier :

As the presence of moisture deteriorate the quality of

insulation of the telecom cables, moisture barrier protects entry

moisture into the core of the cable.

&rotection ::

Telecom cables require Protection

o from probable mechanical damages

o

from water and chemicals or soil conditions

o from nduction due to !lectrical lines

o from diggings by different agencies and individuals

o from damages while handling

11.0.1 'he clssifiction of "n!er(ro"n! cbles )ith re(r! to

!esi(n fet"res re ::


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Place where it is used " #nderground $ overhead $ submarine

nsulation material used " Paper $ polythene cables

The filling compound " %ry core $ &elly filled cables

'echanical protection " Armoured $ unarmoured cables

Place of utili(ation " Primary $ %istribution $ )unction cable.

*ystem for which used " +o"axial $ P+'

Type of conductor " +opper cable $ ptical fibre cable

-auge of the conductor " ./ mm $ .0 mm .12 mm $ .3mm

Pressuri(ation of core " Pressuri(ed $ unpressurised cables

11.1DES*GN +EA'URES ::

4efore discussing the above classifications in a nutshell let us know

what are the purposes of the above %esign features in a underground cables.

11.,&UR&OSE O+ *NSULA'*ON UNDERGROUND

CABLES ::

567 The insulation is used to separate the conductors bunched in a unit to

prevent short circuit between two conductors in a pair or between

conductor of one pair with the conductor any other pair in the unit or core

in the cable.

1st-ir B

Li$b r! -ir

A Li$b

,n! -ir

B Li$b 1st-ir A

li$b

nsulating

material

P8+ $PAP!9

Annele!

Co--er

Con!"ctors

*heath

*%

+ $ P8+

+

9

!

;illing

material %ry air $

)elly'!+ A?!9* ; -

*T9P*


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5>7 The insulation is used as *

from being corroded or eroded in soil.

527 The insulation is being used for marking $ identifying the pair or conductorin the unit and in the cable as a whole for that matter.

5/7 The insulating material is used for preventing the grounding or earthing of

the conductors.

507 The insulating material is used for preventing the corrosion of armouring .

517 Transmission characteristics of the cable

n the primitive stage paper insulation chosen as it has good di"electric

properties and low specific inductive capacity which is about 6.0. ts physical

properties also enable large proportion of air as dielectric. The ideal dielectric for

telecom cable is air which has specific inductive capacity of 6.. The thickness, width

and number of wraps per meter of insulating paper is selected to achieve the required

mutual capacitance of cables.

The Polythene insulation is easy to apply and have desired electrical

characteristic such as low dielectric constant, lo dissipation factor 5loss7 and

high dielectric strength. 'echanically it is tougher than paper and has abrasion

resistant with ample tensile strength and elongation.

The insulating resistance measurement shall be measured with a %+ voltage of

magnitude not less than 08 after steady electrification for one minute. The

insulation resistance values between each conductor in the cable and all the other

conductors connected together and to the screen and earth shall not be less than 0

'ega ohms $ km at 9oom temperature.

5 +able length in @m x observed insulation in 'ega ohms 7.


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11.,.1&irin( n! Overl# ::

Two insulated conductors shall be twisted together with uniform lay to form a

pair. The length of the lay of any pair shall be different from that of ad&acent pairs.

The lay of various pairs shall be so chosen as to satisfy the capacitance unbalance

requirements and cross"task requirement.

11.,.,Unit +or$tion ::

The pairs are then assembled into units with different length of unit twists

assigned to different units. These units are then assembled into a completed cable

core.

11.,.Core

wrappings are applied to the completed cable core to hold the units together

and provide high di"electric strength from core to shield and to protect the conductor

insulation from damage due to the heat of sheathing operation. n P); cables non"

hygroscopic and non"wicking polyester tape is used as core wrapping.

11.,./+illin( co$-o"n! ::

The cable should be filled with suitable water resistant compound which shall

be compatible with the insulation, binders and tapes used in the cable. t shall be

homogenous and uniformly mixed material containing an anti"oxidant. t shall not

contain dirt, metallic particles or other foreign matter.

Paper insulated cables :: %ry air only

Polythene insulated cables :: )elly compound.

11.,.SCREEN ::

An aluminum tape coated with polythene $ copolymer on both sides shall be

applied over the cable core with a minimum overlap of 1 mm for all si(es of cables.


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The nominal thickness of the aluminum tape shall be .> mm and that of polythene $

copolymer coating on each side .0 mm.

The aluminum tape shall be sealed at the overlap and bonded to the inner

surface of polythene sheath extruded over it. The tape shall be electrically continuousthroughout the length of cable.

11.,.S2EA'2 ::

A moisture resistant , gas tight sheath must be applied to all the paper

insulated cable other wise relative humidity conditions throughout will increase and

insulation resistance will decrease. The sheath also protects the cable form damage

during installation and service.

The sheath shall be reasonably circular and free from pinholes and other

defects. The variation between maximum and the minimum diameter at any cross

section shall not exceed 0mm.

Paper insulated cables :: ead sheath or Polythene sheath

Polythene insulated cables :: Polythene sheath only.

11.,.3CONDUC'OR::

!ach conductor is a solid round wire made of annealed high conductivity

copper of diameter .2> mm, ./ mm, .0mm, .12mm and .3 mm.

11.,.4AR%OUR*NG ::

n the armoured cables bedding and armour are provided over the sheath to be

followed by &acket. Then the cables are called Armoured cables.

f this arrangement is not done then we call them as un armoured cables.

4!%%=-two close helical lapping of polythene or polypropylene tape is applied over

the sheath to provide sufficient mechanical protection during armouring. !ach take is


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applied with a minimum 0 overlap. The second tape will cover the overlap of the

first tape evenly.

=ominal thickness of the -alvanised steel Tape armouring

%iameter of

cable over Polythene

sheath

Thickness of *teel tape

#pto / mm .0 mm

Above / mm .B mm

Armouring is the application of two layers of galvani(ed steel tape both

applied helically in the same direction with a gap in the first tape of >0 C$" 6 ofthe nominal width of the tape, the second tape evenly covering the gap of the first

tape. The overlap of the second over the first shall not be les than 60 of the nominal

width of the tape on either side.

The standard armouring types are

Aerial tape armour

)ute protection

4urried tape armour

'odified tape armour

*teel armouring and poly&acketing

+orrugated steel armouring and &acketing.

11.,.56c7et ::

'ost cables serve their lives with a basic sheath but after armouring the

armouring is to be protected from getting rusty and corrosion and &acket is the

protection which does the &ob. t should be reasonably circular, free from pinholes and

other defects.

=ominal thickness of the )acket

%iameter of cable over

Polythene )acketThickness of Polythene )acket

#pto /1 mm 6./ mm


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Above /1 mm up to 1/ mm 6.B mm

Above 1/ mm >.> mm

dentification and ength markings on a +able

To enable proper identification of Telecom cables the following markings shall

be embossed, engraved or printed on the polythene &acket in case of armoured cable

and on the sheath for unarmoured cables. These markings are at an interval of one

meter throughout the length and are distinct and visible to the naked eye from a

distance of about 6 meter.

Telephone handset emblem

=ame of the 'anufacturer

?ear of 'anufacture

+apacity of the cable in pairs

*i(e of the conductor

ength marking

Selin( of the En!s::

The cables will be sealed with thermo shrinkable end caps of adequate

thickness after completion of all tests in factory before dispatching to various stores

and workplaces directly.

&OL8'2ENE *NSULA'ED 6ELL8 +*LLED CABLES ::

These are popularly known as P); cables and consist of twisted pairs of

polyethylene insulated copper conductors.

Paper insulated cables 5*%+, P+#T, P+DT, P+D E P+D 7 are the

primitive underground cables that are used in the Telecommunications network. These

cables are available up to 6B pairs. ater on due to various factors like escalation in

the cost of ead and more incidence of faults due to paper insulation the Polythene

insulated )elly filled cables 5 Popularly known as P); 7 are used extensively now a

days in the telecommunication networks. The P); cables are available up to 21

pairs.

*ome constructional features of Paper insulated cables are dealt in )ob Aid F ,

for academic interest and as still a few number of these cables are still serving some

of the telecommunication networks.


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The Pressuri(ation of dry core paper insulated cables is now a avoidable

feature as the replacement of paper"insulated cables with P); cables is nearing

completion. As the P); cables are filled with )elly as filling compound which takes

care of prevention of entry of moisture $ water into the core of the cable.


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1,Cbinet9 &illr9 D&

1,.0.1+orecst of tele-hone !e$n!

The first step in local network planning is forecasting of telephone demand for

0, 6, 60 and > years for the area under study. This process has been explained in

detail in chapter G"A. The forecast values are obtained for smaller areas say of the

order of 6 sq. m. called as survey unit.

1,.0., N"$ber n! Loction of tele-hone e;chn(es

4ased on the forecast figures, the optimum number of exchanges required are

determined. Then ideal location of each exchange is worked out on the basis ofdefinite area demarcation for each exchange and exchange and available sites. The

complete procedure for this exercise is given in subsequent chapter.

1,.0. Cble ro"tin( sche$e.

After the location of each exchange is finali(ed and the area to be served by

this exchange demarcated, the forecast figures for each block at the end of the

economic planning periods 5i.e. > years7 are as curtained. -enerally two stage

flexibility scheme is being adopted in our department. The determination of the cable

routing as well as the cable si(e involves the following main steps:"

a. 'arking of general layout of an exchange area.

b. ocation of %istribution Points and %emarcation of %P area.

c. ocation of Pillars and %emarcation of pillar area.

d. ocation of cabinets and %emarcation of cabinet area.

e. %etermination of economical and feasible cable route.

'arking of general layout of an exchange area.

1,.0..1*. Bsic Lo! n!


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!ach stage of cable reduction helps to reduce only the portion of the pairs

required to meet the variable load but that required to serve the basic load remains

unaltered. therefore, the effective flexibility at a cable reduction point is in no way

reduced by not terminating the pairs corresponding to the basic load. The pairs which

are not terminated are said to be through " connected. The arrangement with through "connected pairs is generally refered to as a partial flexibility generally implies that the

through"connected pairs correspond at the most to the basic load.


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%istribution +able

1,.0..Generl l#o"t of n e;chn(e re :=

The general layout of an exchange area will, therefore be as follows :"

The exchange area will be divided into two distinct (ones " the inner (one and

the outer (one.

The inner (one will be made up of %P areas which will be independent units

with no flexibility between then. 4ut some times in large systems where demand is

high inner (one is also fed from a pillar called LM pillar directly connected to

exchange.

The outer (one is primarily made up of a number of pillar areas. The pillar

areas may be either independent units or may be controlled from the cabinet. !ach

pillar area comprises of %P areas with the pillar providing the flexibility between the

%ps. The cabinet areas are always independent units with no flexibility between them.A pillar will be provided in each pillar area and a cabinet will be provided in

each cabinet area. The cabinet will be located in the area nearest the exchange, i.e.

the one through which the cable feed to other pillars controlled by the cabinet passes,

so that back " feeding is avoided in the secondary network. This particular pillar area

will therefore, have both the pillar and the cabinet.

1,.0../Sh-e of res serve! b# cbles :=

The most economical shape for an area served by cables is a long and narrow

rectangle with the main cable route passing across it through the mid points of the

longer sides of the rectangle as shown in fig. /.6 4y keeping the shape long and

narrow instead of round or square, the length of the cables in the network is made a

minimum. The shape of the exchange areas in mullet " exchange systems, cabinet

divisions and %P areas are like wise to be made long and narrow.

The longer side of the cabinet area should be parallel to the shorter side of the

exchange area and similarly the longer side of the %P area should be parallel to the

shorter side of the cabinet areas.


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'ain

+i( : 11.1

1,.0.. Loction of D& n! De$rction of D& re:=

i. Service Loo-.

The subscriber loop consists of connection from %P to subscriber premises.

'ay consist of open wire lines or drop wires or buried served wires. pen wire lines

are generally metallic iron wires 5bare or nsulated7 erected on insulators and

terminated on pot heads at subscriber windows as well as on the %Ps. ;rom the pot

head terminationMs the connections are made through P8+ wires.

The drop wire consists of insulated cadmium copper or copper coated steel

wire insulated by P8+ coating, these wires are directly terminated in the %P as well

as to the subscriber window.

n +ase of buried service wire, the connection from %P to subscriber premises

is through buried service wire and thus overhead line is entirely eliminated.

n our department we are using open wire lines or drop wires only. The buried

service wires are not being used.

ii. Distrib"tion &oints :=

%P area is the smallest territory in the cable system. As there is an element of

uncertainty in forecasting. The actual incidence of telephone demand may vary from

the forecast by different deters. There is always a greater percentage of error in

forecasting a smaller area than a larger one. The %P area is the victim of such

variations. The only way to made good uncertainty of the forecast is to over"provide

cable pairs to the %Ps to the !xtent of maximum variations. t is desired to allow 2

extra provision over and above !conomic Planning Period demand forecast.

The number of distribution points is a compromise between two opposing

factors :"

. The larger the number of distribution points, the smaller will be the

length of the aerial lines from the distribution point facilitating rapid

installation of telephone connections.

b. *maller the number of distribution points, the lower is the total cost of

the cable terminals and all the spare distribution cable pairs terminated

there in.


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+onsidering above, it is desirable that the number of distribution points should

be large and the capacity as small as possible.

There are four si(es of %Ps which are provided depending upon the

requirements :"

,o &ir D-s : These %Ps are generally used in case of overhead lines feedinglong areas and lowly populated areas.

10 &ir D-s := These %Ps are generally suitable for drop wire type

subs loops where the telephone density is high.

&ir D-s:= *uch %Ps are normally provided where number of

telephones in a building or contiguous building is small.

, &ir D-s := These are normally provided in subscribers premises

where requirement may not be exceeded more than two

telephones.

*ince the length of subscribers loop is directly proportional to the si(e of %P

and accordingly fault liability, it is preferable to provide smaller si(e of %ps. n our

department at present mainly 6 Pair and > %Ps are being used.

ii. Loction of !istrib"tion -oint :=

To meet the > years requirements, %ps will have to be provided after

consulting the telephone requirement details in each street of a particular block. some

of the %Ps will have to be opened immediately to meet first three years requirements .

The %Ps should be located at one end of %P area in the direction looking towards the

cabinet. t might even be situated opposite the area it is expected to serve. n large

buildings, which require over 6 pairs. All the pairs that are taken into the building,

must be terminated at a convenient point preferably on the ground floor, before they

branch off for internal distribution frame if the number of pairs required is large.

iii. De$rction of D& re :=

%epending upon the si(e of %P decided for each location, the area to be served

by each %P should also be demarcated as far as possible on the basis of demand

forecast say first 2 to 0 years requirements. *ome times practical difficulties may be

encountered for exact demarcation of %P area for external %Ps. n such a case, it must

be ensured that kriss crossing of the connections is avoided .

Thus in this step the %Ps for each block are marked on the basis of > years

forecast. Then on the basis of the existing development stage of the area as well as

future time schedules, the %Ps which are to be opened immediately are identified. ;or

example, if the area is not developed at all, then there is no need to open any %P at


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this stage. n case the area is scantily developed then the planner has to decide

whether all %Ps marked in that area should be opened right now or some should be

opened . As far as possible locations should not be altered. n this way a scientific

exercise will have to be done to decide the number of %Ps to be opened immediately

and the number %Ps to be opened later on.

1,.0..Loction of -illrs n! !e$rction of -illr re.

i. &illr re:=

The provision of flexibility between %Ps by means of cross connection

requires the cable pairs from the exchange and %P sides to be terminated at the cable

terminal point 5+T 4ox7 suitably mounted on frames inside steel casing or shell.

uter (one of each exchange has to be divided into portions which are already

developed and built up and portions where most of the development has yet to take

place and no cable feed exists at present.

The developed portions are again sub"divided into a group of smaller survey

blocks of territory such that in course of time, the distribution to each block can be

conveniently made independent to feed the ad&oining blocks. *uch groups of these

blocks will be the pillar areas. There will be a number of points in the existing cable

network where a pillar can be introduced with an advantage. ;or instance the points

where a number of distribution cables meet are generally suitable for locating the

pillars. t will be convenient to first fix these points and mark out the areas to be

served be more pillars as well as the pillars already installed.

The areas of each block has to be such that the !conomic Planning Period

forecast of telephone development may be catered by a particular si(e of pillar

planned to be installed in the area. n fast developing network, !conomic Planning

Period and telephone development is very difficult to assess. n such networks, the

ultimate capacity of the exchanges planned have to be kept into view. n the cases of

new developing areas the requirement arising due to shifting of the telephones to

these areas which get priority over all types of the new connections have to be taken

into consideration. The location of pillars should be so planned that it serves the

number of blocks, some of which can be taken out from this pillar in case the

telephone development reaches to a point which can not be met by the pillar installed

to meet the anticipated demand in the area.

!ach pillar area is distinct which comprises of one or more complete blocks

serving the %Ps located within its area but not the %Ps located in any other pillar area


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to avoid criss crossing of distribution cables. !ach pillar must have a separate

distribution network originating from the pillar and there should be no need of teeing

in any of the section.

The total terminating capacity of pillar may vary from / pairs to > pairs.They are available in the si(es of /, B, 6, 61 and > pairs. The

terminations on distribution side are more than the terminations on the secondary side.

ii. Loction of &illr.

The pillar should be located taking into account the following considerations :"

a. The function of the pillar is to provide flexibility. #nless there are 0 to

1 %Ps in a block, a separate pillar for the block may not be &ustified .

nstead the %Ps can be served from the pillar in an ad&oining or near

block. n such cases, the entire block must be included in ad&oining

pillar area.

b. Although the telephone load in a block may not &ustify a pillar, it is

necessary to provide a pillar irrespective of telephone load in the

blocks where cabinets are installed. This will avoid back feeding of

distribution cables from other pillar areas to meet the demand of the

particular block.

c. The Pillar areas should be so demarcated that no ma&or road crossings

are encountered while laying distribution cables to feed the %Ps of the

area.

d. Primary and *econdary cables being ducted are safer as compared to

the distribution cables which are a weakest element in the system.

Therefore, the distribution cables length from pillar to the last %P

should be minimum possible be keeping the pillar nearest to the

anticipated load centre of the area to achieve better reliability,

minimum fault liability and economy in material and labour.

iii. L# o"t of Distrib"tion cbles :=

The distribution cables are generally provided taking into account economic

planning period requirements. After plotting the %Ps, cable of suitable si(e to feed a

group of %Ps in a particular direction survey blocks are planned.

The economic planning period for the secondary cable is 6 years. Thus after

working out the pillar area and pillar locations on the basis of > years forecast, the

developed stage of the area as well as future plans of the area for the next 0 years have


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ii. Loction of Cbinets :=

The cabinet should be located taking into account the following

considerations:

a. A cabinet should serve 2 to 1 pillars. n no case, the number of

pillars should be less than 2.b. The best point for location of cabinets is on the primary cable route

after it enters the cabinet area.

c. t should not be at the &unction of two main roads.

d. There should be sufficient space near the cabinets to accommodate

number of &oints which may be made in the near future. consider

construction of manhole for all &oints located at the cabinet.

e. There must be space to install the PT8s near by. 5Pressure Test 8alve

in case of Pressurised Paper insulated cables7.

f. The cabinet should not be target for the vehicular traffic.

g. t should be away from the verb stone i.e. ad&acent and parallel to the

wall.

;urther a cabinet should be so chosen that the primary cable length is greater

than secondary cable length. *imilarly pillar should be so chosen that the secondary

cable length is always greater then the distribution cable length except for the first

pillar.


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1+AUL'S LOCAL*SA'*ON

b&ective of the esson :: The chapter deals with the necessary information

regarding the types of faults and the causes of faults occur in underground cables. t

also deals with the detection, locali(ation and rectification of the faults

1.0CABLE +AUL'S = DE'EC'*ON LOCAL*SA'*ON

n analysis it has been found that about G of the external faults are noted

as under cable faults so the efficiency of the maintenance of external plants largely

depends on the proper planning, execution, constructional practices and maintenance

of cable network i.e. the primary, secondary and distribution cables, cabinets, pillars,

%Ps etc. 'oreover one cable fault normally effects a large number of circuits even

upto 21 circuits. *o proper care and vigilance on the cable network is of utmost

importance.

1.1 '2E GENERAL '8&ES O+ CABLE +AUL'S:=

5A7 Erth f"lt :: Khen the insulation between the earth

and the conductor in the cable becomes very low.

5b7 Lo) ins"ltion f"lt.:: Khen the insulation between conductors

in the cable or between the pairs or between pair and earth falls below a prescribed

limit 5normally .0 meg ohm7 This may be due to entry of moisture or due to failure

of wire insulation.

5c7 Disconnection +"lt:: Khen the +onductor is cut then the fault

is called break fault or is called


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equipment is foreign potential or contact of the conductor with other circuit having

potential.

'2E ABO


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21/29

>. Availability of up"dated cable records vi(., cable diagram, cable

plans, %P cards and cable route and &oints indicators etc.

2. Availability of proper type of testing instruments in god working

condition and skilled staff.

GENERALL8 '2E +OLLO@*NG ARE '2E %ED*A '2ROUG2@2*C2 CABLE +AUL'S ARE DE'EC'ED

6. +omplaints from the subscriber, testing of the circuits from test desk or

'+.

>. 9outine testing from test"desk or '+.

2. =on availability of pair at the time of putting through a new

connection.

/. Alarm from the control panel of the pressuri(ation system.

ocation of a fault in working cable caused due to the digging of

any agency can be easily identified $ detected by cable route patrol or

through informationMs from local people. These types of faults and

breakdowns are immediately brought to notice or identified by the

complaints given by the effected subscribers.

;ailure of weak and imperfect &oint due to entry of moisture is a

common phenomenon in this country. Presently usage of polythene

insulated &elly filled cable has made this type of &oint failure a rare

phenomenon and detection an easy affair. f there is any damage in the

sheath, the entry of the moisture will be avoided due to the presence of

&elly.


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Con!itions :=

6. +able used is &elly filled

>. +able diagram is very much up"to"date

The above diagram is a part of the cable diagram of a pillar. ;or

example the telephones of %P nos. 6 and > are reported to be faulty. =ow

consulting the pillar diagram it can immediately be referred that &oint " >

is faulty. =o necessity of locali(ation testing is required. =ow by

consulting the cable plan and with the help of route and &oint indicator

the faulty &oint " > can be physically locali(ed.

;or paper insulation cable, this type of locali(ation is not possible

because fault may arise any where in the body of the cable between &oint

" 6 and &oint " > due the entry of moisture through sheath puncture.

tion of +"lt b# instr"$ents

The following instruments are generally in use for the

locali(ation of cable fault.

%"lti$eter or A.


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5c7 =umbering of the pairs, if there is reversal at any

intermediate point.

BR*DGE %EGGAR : This instrument is extensively used in all the

cable maintenance sections. This works on Khetstone 4ridge principleand is used generally for the following purposes ::

5a7 Accurate measurement of loop and wire

resistances

5b7 'easurement of cross insulation between the

wires of the same pair or between the pairs of a cable.

5c7 !arth insulation between a wire or pair and earth.

5d7 ocali(ation off dead"earth fault 5when resistance

between the faulty wire and earth is very low7.

5e7 8ery rough locali(ation of L%isc.M fault 5principle

of condenser discharge7.

EAR'2 +AUL' LOCAL*SA'*ON B8 BR*DGE %EAGER

'2E +OLLO@*NG COND*'*ONS %US' BE

SA'*S+*ED :=

6. The fault must be dead F earth

>. ne good wire must be available between the testing station and

the point from where test is given at the looping end. the length and

gauge of the good wire is not required to be the same as that the faulty

wire. The good wire may have a separate route than the faulty wire.

2. =o presence of foreign battery.

/. #seful for locali(ing four faults

5a7 +ondition of the fault 5whether dead earth or not7

5b7 !arth insulation of the good wire

5c7 9esistance of the faulty wire and good wire

looped at the distant end

5d7 8alley loop test. Fused for locali(ing earth or contact

faults.


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5 constant arm radio 4ridge 7

5e7 'urray test " used for locali(ing earth or contact faults

on short lines.

5variable arm radio bridge7

The procedure for connections, testing and formula for

manipulating the test"data to find out the distance of the fault from the

testing end are available with each instrument.

&ULSE EC2O 'ES'ER OR EC2O %E'ER

This is the modern method and reliable method for measurement ofcables of any type.

This instrument provides reasonably an accurate distance

measurement.

This instrument works on the pulse reflection principle. The

pulses 5sin> form 7 which are created by a generator and which are

suitable for the location or places of error are transmitted through cable,

the part of the pulse power is reflected at the fault according to

magnitude of power. This is observed on the +9T screen. t makes use of

characteristic impedance of the conductor pair whose deviation from the

nominal characteristic impedance determines the intensity of reflection

or the reflection co"efficient

The pulse echo test method provides quantitative as well as

qualitative information on any transmission cable " impedance loss, 9ise

time, electrical length and discontinuities in a single measurement.

Pre"requisite :" while using this instrument for locali(ation of

fault, the gauge of the conductor and type of insulation of the cable

under test must be known.

This instrument can detect and locali(e various types of faults inopen wire and under -round cable provided the impedance irregularity

on the pair at the fault"point is large enough to cause a distinctly visible


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reflection of the screen. The type of the faults those can be locali(ed by

this instrument are:"

5a7 pen circuit$disc 5b7 *hort

circuits5b7


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.1 +UNC'*ON O+ CON'ROLS AND *ND*CA'ORS ::

+9T intensity control :: +ontrols intensity or brightness on the screen.

6. +9T focus control :: ;or focus or sharpness of

scilloscope display.

>. +9T screen :: 8iewing transmitted pulse and

reflection from fault.

2.


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4attery charge ow 4attery level :: To indicate the sufficient level of battery

when !% requirement of recharging when !% is on.

%istance readout :: ;ive !% type digits to display distance in

meter.

Power on $ off :: To switch power on $ off for the instrument.

6/. 9ange selector :: To select ranges of distances expected to be

fault for providing progressively wide pulses required for".2km, 2 km, "6km

ranges respectively.

60. Alternate trace selection :: Khen pressed +9T shows complete trace of the

cable under test and allows the reflection to be shifted towards the transmitted pulse

as controlled by Trace shift control. Khen not pressed only one trace is displayed.

61. *hift $ measure selector :: n depressed 5*!T7 position, the trance shift

control can be used to pinpoint a reflection. Khen pressed 59!A%7 allows digital

readout of the distance to the selected pulse reflection.

6G. 9eference pair selector :: Khen pressed allows reference pair connected

to > to be shown on alternate trace for comparison to faulty pair connected to 6.

6B. Trace shift :: A 6 turn control allows alignment of reflected

pulse with the leading edge of the transmitted pulse to obtain the distance

measurement.

63. !xternal %+ power :: )ack for external 6> to 60 8 %+ "ve ground

source.

>. +hassis ground :: )ack for chassis ground for user safety.

>6. *un shied :: To operate the test set in bright sunlight.

., &ROCEDURE 'O 'ES' ::


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a. +heck 4attery level before taking to the place of testing.

4attery charge !% should be off on the panel.

b. There should be no voltage on the pair to be tested.

c. *et the 8$> value for the cable using velocity selector switch 5

one half the velocity of propagation value for the cable under

test.

d. Ad&ust the vertical gain sensitivity control to obtain pulseheight of two divisions height.

e. Ad&ust the hori(ontal position control so that the foot of pulse

is aligned with vertical line.

f. +onnect the meter to the pair to be tested at 6 and reference

pair at >

g. Ad&ust the hori(ontal expansion control to show the reflected

pulse on the right side of the screen.

h. %epress the %epress the alternate trace selector switch. Ad&ust

the trace shift control, so that the foot of the transmitted pulse

is observed as below.

i. %epress the *!T $ 9!A% switch. 9ead the distance in

display. *ubtract the length of the test leads to obtain the

actual distance.

. O&ERA'*NG CONS*DERA'*ONS ::

6. A good cable with a matched termination displays a flat

trace with no reflection.

T

'

R


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>. f the measured fault location is near a splice, or if near a

construction site where recent digging has been done, it is

quite probable the fault is located there.

2. Kater existing in a splice case or in a &elly filled cable

may produce a very small reflection due to the confined

area of water. Kater reflections can be seen on the 'odel

223 long before actual deterioration of the cableinsulation occurs as the water changes the velocity factor

of the cable, making the measured distance to the wet

section sub&ect to error.

A cable pair may have more than one fault, so the first

fault can easily mask a more remote fault. Then locate the closest

fault then locate and repair the more distant faults.

/. *ND*CA'*ON O+ '8&*CAL +AUL'S :: Khen the fault is with in few

meters from the testing point it is difficult to get an accurate reading. This can be

overcome by connecting a dummy 0 to 6 meter length of cable between meter and

pair to be tested. The final actual measurement is got by substracting the length from

reading shown in the meter

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