UNDERGROUND 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.

Broad construction of any telecommunication cable

Core ::

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

Moisture Barrier :

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.

Protection ::

Telecom cables require Protection

from probable mechanical damages

from water and chemicals or soil conditions

from Induction due to Electrical lines

from diggings by different agencies and individuals

from damages while handling

The classification of underground cables with regard to design features are ::

Place where it is used - Underground / overhead / submarine
Insulation material used - Paper / polythene cables
The filling compound - Dry core / jelly filled cables
Mechanical protection - Armoured / unarmoured cables
Place of utilization - Primary / Distribution / Junction cable.
System for which used - Co-axial / PCM
Type of conductor - Copper cable / Optical fibre cable
Gauge of the conductor - 0.40 mm / 0.50 mm 0.63 mm / 0.90mm
Pressurization of core - Pressurized / unpressurised cables

DESIGN FEATURES ::

Before discussing the above classifications in a nutshell let us know what are the purposes of the above Design features in a underground cables.

PURPOSE OF INSULATION UNDERGROUND CABLES ::

(1) 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.

(2) The insulation is used as SHEATH to separate the insulated conductors from being corroded or eroded in soil.

(3) The insulation is being used for marking / identifying the pair or conductor in the unit and in the cable as a whole for that matter.

(4) The insulating material is used for preventing the grounding or earthing of the conductors.

(5) The insulating material is used for preventing the corrosion of armouring .

(6) Transmission characteristics of the cable

In the primitive stage paper insulation chosen as it has good di-electric properties and low specific inductive capacity which is about 1.5. Its physical properties also enable large proportion of air as dielectric. The ideal dielectric for telecom cable is air which has specific inductive capacity of 1.0. 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 (loss) and high dielectric strength. Mechanically it is tougher than paper and has abrasion resistant with ample tensile strength and elongation.

The insulating resistance measurement shall be measured with a DC voltage of magnitude not less than 500V 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 5000 Mega ohms / km at Room temperature.

( Cable length in Km x observed insulation in Mega ohms ).

Pairing and Overlay ::

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 adjacent pairs. The lay of various pairs shall be so chosen as to satisfy the capacitance unbalance requirements and cross-task requirement.

Unit Formation ::

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.

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. In PIJF cables non-hygroscopic and non-wicking polyester tape is used as core wrapping.

Filling compound ::

The cable should be filled with suitable water resistant compound which shall be compatible with the insulation, binders and tapes used in the cable. It shall be homogenous and uniformly mixed material containing an anti-oxidant. It shall not contain dirt, metallic particles or other foreign matter.

Paper insulated cables :: Dry air only

Polythene insulated cables :: Jelly compound.

SCREEN ::

An aluminum tape coated with polythene / copolymer on both sides shall be applied over the cable core with a minimum overlap of 6 mm for all sizes of cables. The nominal thickness of the aluminum tape shall be 0.2 mm and that of polythene / copolymer coating on each side 0.05 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 continuous throughout the length of cable.

SHEATH ::

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 5mm.

Paper insulated cables :: Lead sheath or Polythene sheath

Polythene insulated cables :: Polythene sheath only.

CONDUCTOR::

Each conductor is a solid round wire made of annealed high conductivity copper of diameter 0.32 mm, 0.40 mm, 0.50mm, 0.63mm and 0.90 mm.

ARMOURING ::

In the armoured cables bedding and armour are provided over the sheath to be followed by jacket. Then the cables are called Armoured cables.

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

BEDDING

two close helical lapping of polythene or polypropylene tape is applied over the sheath to provide sufficient mechanical protection during armouring. Each take is applied with a minimum 5% overlap. The second tape will cover the overlap of the first tape evenly.

Nominal thickness of the Galvanised steel Tape armouring

Diameter of cable over Polythene sheath	Thickness of Steel tape
Upto 40 mm	0.5 mm
Above 40 mm	0.8 mm

Armouring is the application of two layers of galvanized steel tape both applied helically in the same direction with a gap in the first tape of 25% +/- 10 % of the 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 15% of the nominal width of the tape on either side.

The standard armouring types are

Aerial tape armour

Jute protection

Burried tape armour

Modified tape armour

Steel armouring and polyjacketing

Corrugated steel armouring and jacketing.

Jacket ::

Most cables serve their lives with a basic sheath but after armouring the armouring is to be protected from getting rusty and corrosion and jacket is the protection which does the job. It should be reasonably circular, free from pinholes and other defects.

Nominal thickness of the Jacket

Diameter of cable over Polythene Jacket	Thickness of Polythene Jacket
Upto 46 mm	1.4 mm
Above 46 mm up to 64 mm	1.8 mm
Above 64 mm	2.2 mm

Identification and Length markings on a Cable

To enable proper identification of Telecom cables the following markings shall be embossed, engraved or printed on the polythene jacket 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 1 meter.

Telephone handset emblem

Name of the Manufacturer

Year of Manufacture

Capacity of the cable in pairs

Size of the conductor

Length marking

Sealing of the Ends::

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.

POLYTHENE INSULATED JELLY FILLED CABLES ::

These are popularly known as PIJF cables and consist of twisted pairs of polyethylene insulated copper conductors.

Paper insulated cables (LSDC, PCUT, PCQT, PCQL & PCQL ) are the primitive underground cables that are used in the Telecommunications network. These cables are available up to 1800 pairs. Later on due to various factors like escalation in the cost of Lead and more incidence of faults due to paper insulation the Polythene insulated Jelly filled cables ( Popularly known as PIJF ) are used extensively now a days in the telecommunication networks. The PIJF cables are available up to 3600 pairs.

Some constructional features of Paper insulated cables are dealt in Job Aid – I, for academic interest and as still a few number of these cables are still serving some of the telecommunication networks.

The Pressurization of dry core paper insulated cables is now a avoidable feature as the replacement of paper-insulated cables with PIJF cables is nearing completion. As the PIJF cables are filled with Jelly as filling compound which takes care of prevention of entry of moisture / water into the core of the cable.

POLYTHENE INSULATED JELLY FILLED POLYTHENE SHEATHED UNDER GROUND CABLE (VIDE ITD S/WT -129 DT. 2.3.83 & ITD SPECIFICATION NO S/WT - 143 DT. 30.7.88 )

(a) Number of Pairs

The cables shall be in sizes 5, 10, 20, 50, 100, 200, 400, 800, 2000, 2400, 2800, 3200 and 3600 pairs.

Conductors

Each conductor shall be insulated with polyethylene of insulating grade. Different gauges of conductors 0.32mm, 0.40mm, 0.50mm, 0.63mm, and 0.90 mm are used in the cables.

Each conductor shall consist of a solid wire of annealed high conductivity copper smoothly drawn & circular in section, uniform in quality, resistance and free from all defects.

The average resistance of all the conductors in the cable shall not exceed the values shown in Table given below.

Nominal diameter of conductor	Resistance per km per conductor at 20 ⁰C	Tolerance on conductor Resistance	Minimum elongation of conductor	Min % conductor Resistance unbalance	Attenuation at 20 deg C
in mm	ohms/km	Ohms /km	percentage	Average	Db/km
0.32	223	-	15	1.5	-
0.40	135	+/- 4	15	1.5	12.0
0.50	86	+/- 3	15	1.0	8.25
0.63	58	+/- 2	18	1.0	6.30
0.90	28	+/- 1	18	1.0	4.40

The Percentage of Resistance Unbalance of any individual pair tested shall be calculated as follows::

Percentage of Resistance Unbalance =

Where R1& R2 are the resistances of individual conductors of pair under test and R1 > R2

The temperature correction for attenuation is:

Attenuation at 20 ⁰ C =

CORRECTION FACTOR FOR CONDUCTOR RESISTANCE ::

Temperature in deg C at which conductor Resistance is measured	10	20	30	40	50
Multiplier constant for conversion to 20 deg C	1.0419	1.0000	0.9622	0.9271	0.8945

Each conductor shall be insulated with solid medium density polythene of density 0.926 to 0.94 to a thickness. The insulation should be uniform, smooth and free from all defects. The insulation will have following color for identifying pairs /conductors under normal lighting conditions.

CODE FOR WIRE IDENTIFICATION

Primary colors	Secondary colors
For 1^st wire in a pair	For 2^nd wire in a pair and binder tape of unit in 50pr/100pr unit
White	Blue
Red	Orange
Black	Green
Yellow	Brown
	Slate / Gray

Unit

A number of twisted pairs laid up to form a group shall constitute the unit. The color scheme of pairs and wires in a unit shall be read as below.

CODE FOR TAPE OR BINDER FOR UNIT IDENTIFICATION

Unit number	1	2	3	4	5
Color of Binder	Blue	Orange	Green	Brown	Slate / Gray

2.1.6 CODE FOR CONDUCTOR INSULATION

Pair No	Color
	First Wire	Second Wire
1	White	B A lue
2	White	Orange
3	White	Green
4	White	Brown
5	White	Slate / gray
6	Red	Blue
7	Red	O B range
8	Red	Green
9	Red	Brown
10	Red	Slate / gray
11	Black	Blue
12	Black	Orange
13	Black	Green
14	Black	Brown
15	Black	S C late / gray
16	Yellow	Blue
17	Yellow	Orange
18	yellow	Green
19	Yellow	Brown
20	Yellow	Slate / gray
21	Natural	Blue
22	Natural	Orange

Note:

(a) In 5 pair cable, color code specified for pairs 1 to 5 above is used.

(b) In 10 pairs cables and 10 pairs units of 50 pair cables, color code specified for pairs 1 to 10 is used.

(d) The number of the pairs with respect to the color scheme is only for the purpose of identification of pairs, the actual numerical sequence of the pairs varies as the size increase.

The different colors of the binder shall be readily distinguishable under normal lighting conditions.

(e) Stranding

A 50 pair cable consists of 5 number of 10 pair units

A 100 pairs cable consists of 10 number of 20 pair units.

These units shall be stranded into a compact and symmetrical cable. the sequence of the units in the cable shall be same throughout the length of the cable.

An open lapping of 0.02 mm miler tape of any other suitable material of appropriate thickness shall be applied for each unit.

The tapes shall be so colored and have lay not exceeding 200 mm. This tape is not necessary on the 5 pairs, 10 pairs and 20 pairs cables.

In case of 5, 10, 20 and 50 pair cables, one spare pair will be stranded as the last pair. The color of the spare pair shall be in accordance with pair No. 21 of above table .

In the case of 100 pair cable, 2 spare pairs shall be provided. the color of the pair shall be as specified for pair No. 21 & 22 of above table. The spare pairs in the case of 50 pairs and 100 pairs cables shall be provided within the cable core, but shall not be within any unit.

The 200pair and 400 pair cables ( cables above 100 pr and unto 400 pairs) will be formed by super units of 50 pairs. and the units stranded in the form of layers

The cable over 400 pairs is formed be the super units of 100 pairs and the units stranded in the form of layers.

Identification of 50 pair super units in cables of 200 pair and 400 pair & 100 pair super units in cables of over 400 pairs

Position of the unit in the layer	Color binder
First (Marker)	Red
Intermediate	White
Last (Reference)	Black

Note:: The numbering of the units will be clock wise as running end.

Different configurations of PIJF cables availability is given in table.

Cable size	No of Units / Super units			Nominal length in a drum
In pairs	Centre layer	Middle Layer	Outer Layer	0.50mm	0.40mm
5	1x5 (unit)			1000	-
10	1x10 (unit)			1000	-
20	1x20(unit)			1000	-
50	5x10 (SU)			1000	-
100	5x20(SU)			500	-
200	4x50(SU)			400	-
400	2x50(SU)	6x50(SU)		400	-
800	2x100 (SU)	6x100 (SU)		200	200
1200	3x100(SU)	9x100(SU)		200	200
1600	1x100(SU)	5x100(SU)	10x100(SU)	-	200
2000	1x100(SU)	6x100(SU)	13x100(SU)	-	200
2400	2x100(SU)	8x100(SU)	14x100(SU)	-	200

(f) Armouring

When required the cable sheath shall be armoured. For armoured cable a close helical lapping of waterproof cotton tape shall be applied over the inner-sheath. The cable shall than be armoured with two applications of galvanized steel tape each applied helically with a gap of 25% + 10% of the width of the tape, the second tape covering the gap of the first.

(g) Overall Polythene Jacket

A tightly fitting jacket of polythene shall be applied on the armoured cable. The minimum thickness of the jacket shall be 1.2 mm.

USES OF DIFFERENT TYPES OF CABLES

Gauges 0.32 mm and 0.44 mm for primary cable.

Gauges 0.4 mm and 0.5 mm for secondary cable.

Gauge 0.5 mm and 0.63 mm for distribution cable.

Gauges 0.63 mm and 0.9 mm for distribution cable.

Higher gauges of cable for distribution side having longer lengths.

Unarmoured PCUT cable to be laid in duct and to be pressurized.

Armoured jelly filled cable may be laid direct in the ground and and Unarmoured jelly filled in Ducts not to be pressurized.

DISADVANTAGES OF PAPER INSULATED CABLES WITH POLYTHENE INSULATED CABLES

Numbering of pairs is in coded form. Require more skill. Color markings also fade with due course of time.

Jointing of cables require skill and perfection is required while plumbing as even a slight pinhole will cause entry of moisture / water and damage all the pairs.

Extra care is required for handling like coiling, uncoiling to avoid damage.

Water / moisture entry will affect the complete cable at once instantaneously.

Termination in cabinet / pillars / DPs and at MDF is very expensive and time consuming & increases number of joints.

ADVANTAGES OF POLYTHENE INSULATED JELLY CABLES.

Counting of pairs is easy and human mistakes are avoided.

Jointing is easy and require no chamber or additional place.

Failure of joints is less.

Entry of moisture / water is prevented by Jelly in the core.

Cables can be directly terminated on MDF / Cabinet / Pillar and DPs, thus avoiding additional joints decreasing the cost and time.

Handling of cable is easy not delicate like paper insulated cables.

Life of cable is more.

NOTE :: The systems used in our telecommunication underground network other than PIJF cables are discussed in brief at the end of this chapter.

CO-AXIAL CABLE

The coaxial cable consists of a inner solid cylindrical conductor placed along the axis of an outer hollow cylindrical conductor. A coaxial cable may consist of two or more cores layed up with suitable lay with proper insulation along with quads laid in the interstices between them all enclosed in a lead sheath. The cable is recognized with

Number of cores i.e. either 2 core or 4 core

Size of the inner diameter of tube - r i.e. 0.375 type ( large tube – 0.375” ) or 0.174 type (small tube – 0.174”).

The interstice Quads or pairs having diameter of 0.9mm

TWO CORE 375 coaxial cable

OPTICAL FIBRE CABLE ::

Optical fiber is the latest underground cable that is being used extensively in all the networks including long distance trunks, junction circuits and even the local subscriber loops to enhance the data transmission.

The OF cables are detailed extensively in the other modules of the basic course. How ever for academic interest the advantages of OF cables over copper cables are discussed here under

Optical Fibers are non conductive, hence does not require ground and surge suppression

Optical Fibers are immune to electromagnetic interference

Un authorized tapping is not feasible.

Easily upgradeable to higher bandwidth.

Low loss ( 5db per km to < 0.25 db per km on a typical fiber)

Long and unrepeated links , hence inexpensive

Small light, and hence cost is less and easy for installation.

It does not attract lightning, It does not carry electricity hence not hazardous

The Application of the Fiber Optical cable in communications are ::

Is the common carrier nation wide networks

Inter connecting all Trunk automatic exchanges

Inter connecting all the Exchanges.

Under sea cable

Control systems

Customer premises communication networks.

SDH systems

8 MB MUX for 120 channels

34 MB for 480 channels

140 MB for 1920 channels

PAIR GAIN SYSTEMS ::

The pair gain systems are introduced in to the local network to provide temporary relief to the Technically not feasible (TNF) areas. The PGS is a switching system which provides more subscriber lines using a single cable pair. The numerical value of pairs gained is therefore defined as

Pair gained = No of customers connected – No of network pairs connected .

The types of PGS are divided into basic groups depending on how pair gain is achieved

Concentrator

Grade of service requirement will limit the degree of concentration practically possible.

Multiplexer

Uses FDM or TDM technique to assign a dedicated frequency or time slot resulting in non-concentration. Better pair gain is possible.

Concentrating Multiplexer

It is a combination of multiplexer with concentrator. The different types of PGS are given in the table.

PGS	Description	Customer pairs to network pairs	Break even distance in Km
1+1 FM	Analog customer carrier	2 / 1	0.02
4D PGS-1	4 channel Digital PGS phase I	4 / 1	0.04
4DPGS –2	4 channel digital PGS phase II	4 / 1	0.03
SLC	Small line concentrator	14 / 5	0.08
LCU	Line concentrator Unit	96 / 16	5.2
RCM	Remote customer multiplex	30 / 2	2.0

Benefits of PGS units :

Primary Benefits : Flexibility to provide rapid service as less effort is needed to install it than conventional cable .

Secondary Benefits : To improve customer satisfaction with quick service and network benefits by restricting the cable pair re-arrangements to necessary cases only.

CONSTRUCTION OF solid polythene insulated fully filled under ground pcm telecom cables used for 2mbps digital system.

The cables are available in sizes 10+2, 20+4, 48+6, 96+8 and the nominal conductor diameter of 0.63 mm. The core shall be formed in units o 5 pair in the case of 10+2oair cable, units of 10 pairs in case of 20+4 cables and units of 12 pairs in case of 48+6 and 96+8 pairs.

The conductor material and insulating material are same as PIJF cable construction.

The color code for conductor insulation is :

Pair No

Color

1^st wire

2^nd wire

Color code for Binder for Unit identification :

Unit No.	1	2	3	4	5	6	7	8
Color of Binder	Blue	Orange	Green	Brown	Grey	White	Red	Black

Cable Laying Up :: As shown in the diagram

Size of cable	No of Units	Size of unit	No of Extra pairs
10+2 pairs	2	5 pairs	2
20+4 pairs	2	10 pairs	4
48+6 pairs	4	12 pairs	6
96+8 pairs	8	12 pairs	8

Color code for conductor insulation for Extra pairs ::

Pair No	1	2	3	4	5	6	7	8
1^st Wire COLOUR	BK	BK	BK	BK	Y	Y	Y	Y
2ndWire COLOUR	B	O	G	BN	B	O	G	BN

Core separator Tape (Screen)

A poly-al core separator tape screen is in continuous length to physically separate the core into two compartments. The thickness of the tape shall be suitable to meet the cross-talk requirements. In addition a Poly-al tape shield if provided with 0.2 mm thickness of al tape and 0.05 mm coating of polythene

PCM CABLE LAY

sai@janu

Thursday, 15 September 2011

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