Distribution line Equipment

The distribution lines can be either overhead or underground. These are usually overhead, though for higher load densities in cities or metropolitan areas, these are underground. The choice between overhead and underground depends upon a number of widely differing factors such as the importance of service continuity, improvement in appearance of the area,feasibility in congested areas, comparative annual maintenance cost, capital cost and useful life of the system.

Overhead Lines

An overhead power line is intended for transmission of electric power by a bare or covered overhead conductor supported by insulators, generally mounted on cross-arms near the top of poles. The overhead line may be 66, 33, 11 kV or LT line. The basic equipment used for the line remains the same. The main equipment required for an overhead line is as follows:

b) Close-up

• supports,

• cross-arms,

• insulators,

• earthing knob,

• earthing coil,

• strain hardware set,

• conductors,

• line accessories,

• guard wires, and

• LT line spacers.

We describe each one of these, in brief.

a) Overhead
Lines Mounted
on Cross-arm
Poles;
b) Close-up

•Supports

A support is a column of wood, concrete, steel or some other material supporting overhead conductors by means of arms or brackets. The supports used for overhead line construction vary in design and the purpose they have to perform. The different types of supports for overhead lines are: wood poles, concrete poles, steel poles and lattice type towers.

Wood poles: Chemically treated wood poles are used for distribution lines. The advantage of using wood poles is that they are low in cost.However, they are susceptible to decay. The specifications for wood poles are covered by IS:876 and IS:5978. According to this standard,the timber suitable for poles has been classified into three groupsdepending upon its strength. For example, IS 6056 for jointed wood poles for overhead lines specifies that sal, deodar, chir, kail, wood be used. Jointed wood poles with wire bound lap joint are considerably less expensive and found to be very suitable for LT and HT lines in rural areas.

Concrete poles: Concrete poles are more expensive than wood poles but cheaper than steel tubular poles. Concrete poles are of three types:

− Pre-cast cement concrete poles (PCC) made of cement concrete;

− Reinforced cement concrete poles (RCC);

− Pre-stressed cement concrete poles (PSCC).

The low maintenance, competitive price and aesthetic appearance of PCC poles makes them superior to steel or wood for use in electric lines. Ease and speed of installation means faster project completion and lower installation cost. RCC poles have an extremely long life and need little maintenance but they are bulky in size and comparatively heavy. They have shattering tendency when hit by a vehicle. PSCC poles take care of these shortcomings to some extent. However, the handling, transportation and erection of these poles is more difficult because of their heavy weight.

Steel poles: The steel poles are of the following types:

− Steel tubular poles whose specifications are covered by IS:2713-1967. Due to their light weight, high strength to weight
ratio and long life, they possess distinct advantages over other types of poles. The use of a pole cap at the top, concrete muff
in the ground and regular coating of paint prolongs their life.

− Old and second hand rails and Rolled Steel (RS) joists are frequently used as supports for overhead lines. The portion embedded in the ground should be protected by concrete muff and the remaining portion by regular paint unless galvanised steel is used.

Lattice type supports: These are fabricated from narrow base steel structures. They are light in weight and economical and can be assembled at site if bolted construction is used. Normally both welded and bolted types are used.

•Cross-arms

The shape and length of the cross-arms depend upon the desired configuration of conductors. The following types of cross-arms and
brackets are used:

−V cross-arms for tangent locations with clamps;

− double channel cross-arms for tension or cut point locations where double poles are used; and

− top clamps.

Cross-arms of hand wood (sisso, sal), or creosoted soft wood (chir) or fibre glass are mostly used. Steel cross-arms are stronger and last much longer. MS angle iron and channel iron sections are generally used for this purpose. Smaller sections are used for communication circuits.

•Insulators

You have learnt that an electrical insulator resists the flow of electricity.Application of a voltage difference across a good insulator results in negligible electrical current. Insulators made of glazed porcelain, tough glass and polymers are used for supporting the conductors. Porcelain insulators prevent the electrical current from energizing the power pole.

The principal types of insulator are described below:

− Pin insulators are manufactured for voltages up to 33 kV and are cheaper than the other types. IS:1445 and 731 cover detailed specifications for these. The pins for the insulators are fixed in the holes provided in the cross-arms and pole top brackets. The insulators are mounted over the pins and tightened. The cost of pin insulators increases very rapidly as the working voltage is increased.For high voltages these insulators are uneconomical. Moreover, replacements are expensive.

Pin Type Insulator

Disc Type
Insulator 11 kV

− Disc insulators are made of glazed porcelain or tough glass. They are used as insulators on high voltage lines for suspension and dead ending.The line conductor is suspended below the point of support by means of the insulator or a string of insulators. A disc insulator consists of a single disc-shaped piece of porcelain, grooved on the under-surface to increase the surface leakage path between the metal cap at the top and the metal pin underneath. The cap is recessed so as to take the pin of another unit, and in this way a string of any required number of units can be built up. The cap is secured to the insulator by means of cement. Disc insulators are “ball and socket” or “tongue and clevis” type. A suspension clamp is used to support the  conductor, if suspension configuration of the line is chosen.

− Shackle insulators are used for distribution lines dead ending and supporting conductors laid in vertical formation. IS:1445-1977 covers shackle insulators for voltages below 1000 V. The two standard sizes listed in this specification are 90 mm dia x 75 mm height and 115 mm dia x 100 mm height. A shackle insulator is supported by either two straps and two MS bolts or one U clamp or D strap and two MS bolts as per IS:7935.

− Stay insulator/Guy strain insulators of egg type porcelain are used for insulating stay wire, guard wires, etc. wherever it is not

Insulator 11 kV
proposed to earth them. As per IS: 5300, two strength sizes (ultimate tensile strength) are used: 44 kN and 88 kN, respectively, for LT and HT lines.

•Stays/Guys and staying arrangement: Guys of stranded steel wire are used on all terminal, angle and other such poles where the conductors have a tendency to pull the pole away from its true vertical position. The guys are fastened to the poles near the load centre point with the help of pole clamps. The other end of the guy/stay is secured to a stay rod embedded in the ground. The stay rod should be located as far away as possible.

Earthing Knob

The earthing knob is used for supporting the neutral-cum-earth wire used for earthing of metal parts of supporting structures of low-tension lines,i.e., 400/230 V lines. The knob is generally made of cast iron 52x42 mm and its electrical resistance is not to exceed 200 mega ohms. Moreover,the breaking strength at the neck of the knob is not to be less than 11,500 kg when force is applied.

•Earthing Coil

Two types of earthing arrangements are used. One is with GI pipe and the other is with GI wire. In case of GI pipe earthing, 40 mm dia and 2500 mm long pipe is used for earthing of supports and fittings. GI wire is used for earthing of lines. Generally 8 SWG wire with 115 turns, 50 mm dia and 1500 mm length is used.

•Strain Hardware Set

The conductor is strung between sections through a strain hardware set. It is fixed with the last disc of the string of disc insulator. It is made from malleable iron or aluminium alloy. Alloy hardware is preferable as the losses are less.

•Conductors

Conductor represents 30 − 50% of the installed cost of the line. All aluminium conductors (AAC), all aluminium alloy conductors (AAAC) and aluminium conductor steel reinforced (ACSR) are generally used.Technical specifications of conductors are covered in IS: 398. These conductors are of standard construction and the ultimate tensile strength of the whole conductor is based on the total strand strength.

•Line Accessories

This is the associated equipment required for fastening the conductors to supports and taking off the power or supply points such as joints material,clamps and compounds. For lines up to 33 kV, the following fittings are used:

♦ Conductor dead-end fittings
−LT conductor dead-end grips,

− guy grips dead-end,

− service grips,

− full tension splices,

− distribution ties,

− side ties,

− spool ties,

− tee connectors,

− lashing rods, and

− line guards.

Preformed fittings made of aluminium alloy are used as it saves cost,labour and time. It also eliminates chances of error of judgment. No tools are required. These fittings are fast and simple to apply and assure uniformity of application every time.

♦ Joints should conform to IE Rule 75. For conductors up to 50mm 2 , crimped joints are made with simple hand crimping tools
and for higher sizes, compression type or hydraulic type crimping tools are used. Joints are of the following types:

− uni-joints/ compression joints,

− twisting joints,

− two part compression joints, and

− dead-end joints.

♦ Insulator ties secure the conductor to the insulator. In general, the tie wire should be the same kind of wire as the line wire, i.e., for tying aluminium conductors on insulators, aluminium wire should be used. The tie should be made of soft annealed wire so that it is not brittle and does not injure the line conductor.

♦ Taps and jumpers are made by various accessories, which are not subjected to mechanical tension. Tapping should be taken off
only at a point of line support.

•Guard Wires

Guard wires are to be used at all points where a line crosses a street, road or railway line, other power lines, telecommunication lines,canals, rivers, along the road and public places. As per IE Rule 88, guard wires of galvanized steel of minimum 4 mm dia having breaking strength not less than 635 kg should be used.

•LT Line Spacers

Very often clashing of LT conductors in the mid span takes place due to sag, wind and longer spans. This results in faults and interruptions.Spacers are provided to overcome this problem.

Underground Power Cables

Due to the fast growth in load densities in major towns and cities, 33 kV, 11 kV and LT underground cables are being used to meet the ever growing demand of electric power. The underground cable system has attained considerable importance in distribution networks. This is because in towns and cities, almost all roads are already occupied by LT, HT overhead lines, telephone lines, street lights, advertising boards, etc., on either side of the roads. Further, high-rise buildings make it difficult to go for overhead systems for sub-transmission or distribution. Moreover, the overhead system with bare conductors is prone to frequent breakdowns causing interruption in power supply. Uninterrupted power supply can be maintained by employing underground cable ring system. The underground cabling system is particularly important for metropolitan cities, city centres, airports and defence services.

Underground distribution costs are between 2 to 10 times that of the overhead system. Yet, it is preferred due to elimination of outages caused by abnormal weather conditions such as snow, rain, storms, lightning, fires, stress, accidents, etc. Moreover, this system is environment-friendly and has a long life. In addition, improved cable technology has reduced the maintenance cost
of the underground system compared to the overhead system. We summarise the main reasons for underground cable systems in Box 

Box : Reasons for Having Underground Cables

The right of way for erecting overhead systems is no longer available;

It is possible to extend the supply from source to load centres on any route profile;

Fairly uninterrupted and reliable power supply can be maintained;and

Aesthetic beauty of the town/city as a whole can be ensured.

We now describe, in brief, the selection criteria, sizing, jointing and terminating of underground cables.

•Selection of Cable

The following factors influence the selection of cable:

− load;

− system voltage;

− type of insulation;

− short circuit rating;

− mode of installation; and

− economy and safety.

For the same conductor size, the maximum continuous current carrying capacity depends on the depth of laying, ground temperature,
silicon oil resistivity, ambient temperature, proximity of other cables,type of ducts used. Paper Insulated Lead Covered, PVC and XLPE
cables are being used. Depending upon the voltage at which the power is transmitted or distributed, the cables are designed as
follows:


The sizing of cables depends on the following factors:

− current carrying capacity,

− short circuit current,

− voltage drop, and

− losses.

•Jointing and Terminations

Cables are laid in lengths supplied over reels. Cable extensions are made through joints and terminated at the ends to connect them to the system for use. Since the cable consists of many items right from the conductors to the outer sheath, all joints are to be made as straight through joints so that each joint has the same features/characteristics of the original cable.

Straight jointing is ensured by providing:

− core continuity;

− stress controlling screens;

− insulation;

− continuity of earth potential parts of the cable by clamping and running the earth lead;− mechanical protection by installing the brass or aluminium covers; and

− finishing over the mechanical protective cover.

Cable ends are terminated by providing:

− stress control screens;

− the earthing clamp lead, etc.;

− insulation;

− lugs; and

− rain sheds.

While making joints and terminations, it is essential to know the size and type of the cable in order to select appropriate kits for joints and terminations. The kits contain the accessories required along with instruction sheets for step-by-step procedure for making joints and terminations. The cable and end terminations should be prepared as per the dimensional drawing and procedure given in the instruction sheet.

Types of Joints and Terminations

The joint is considered to be the weakest link in the system but the overall reliability of a distribution system depends on it. Therefore, jointing accessories and techniques have an important and critical role despite their comparative low value in the overall investment.

The following types of joints and terminations are used:

− cast iron moulded,

− epoxy resin type,

− heat shrinkable,

− cold shrinkable, and

− ‘push on’ type.

The heat shrinkable, cold shrinkable and ‘push on’ type joints and terminations do not need any setting time and can be taken into service immediately.