For drawing a overhead line, different accessories like pole, conductor, etc. are required. A brief discussion is given here about varieties of accessories available in the market and their use in different jobs:- 1. Electric Poles or Supports 2. Overhead Line Conductors 3. Insulators 4. Cradle Guard 5. Lightning Arrestor 6. Horn-Gap Lightning Arrestor.
Accessory # 1. Electric Poles or Supports:
The long post by means of which conductors etc. of overhead lines remain supported above our heads is known as ‘electric pole’ or ‘electric post’. Different types of pole are in use for overhead lines of low and medium pressures. Of these wooden pole, steel tubular pole, rail pole, hamilton pole, concrete pole, lattice pole etc. are worth mentioning.
(1) Wooden Poles:
Formerly wooden poles were used only for temporary supply lines. But with the wide expansion of supply system of electricity, at present—be it a transmission line or be it a distribution line—in both cases large quantities of wooden poles are being used. The principal reason for such use is that sal wood suitable for making poles is available in sufficient quantity in different forest areas of our country and its cost is also considerably less.
ADVERTISEMENTS:
Indian Code of Practice No. IS: 876—1957 of Indian Standards has clearly stated what size, shape and class of wood are suitable for electric poles. Poles made of sal wood are usually 7.5 to 10.5 metres long.
The decision as to what length of pole is to be used at which site depends on whether the ground over which the line is to be drawn is plane or undulating, how many wires are to be drawn for the line, whether the wires will be arranged horizontally or vertically, etc. One-sixth of every pole, i.e. a part of 135 cm to 180 cm length from the lower end will remain burried in the ground.
Fig. 261 shows the shape in outline of a wooden pole. The pole, of necessity, should be as straight as practicable.
The wood used for making poles should be properly seasoned and before use it should be smeared with coal tar or creosote oil so that fungi or white ant may not damage the wood. At some places the surface area of the lower end which will remain buried in the ground is burnt to the roasting point for a depth of about 12.7 mm and then dipped into water. This helps the wood to resist easy wear and tear.
ADVERTISEMENTS:
As per Indian Electricity Rules No. 76(1)(a) the owner of every overhead line shall ensure that with wooden poles the minimum factor of safety is 3.5. The minimum factor of safety shall be based on such load as would cause failure of the support to perform its function assuming that the foundation and other components of the structure are intact.
(2) Steel Tubular Poles:
It is true that the use of wooden pole is economical, but a line on steel poles or concrete poles looks much more graceful than a line on wooden poles. Again, among all types of steel and concrete poles, the tubular pole looks most decent. That is why steel tubular poles are found to be used for street lighting even in large cities where underground cables are usually used for distribution lines. The cost is, however, much more for this type of pole.
ADVERTISEMENTS:
A tubular pole has three parts of different lengths and diameters. A sketch of this pole is given in fig. 262. A galvanised steel tube is drawn through moulds of different diameters so that desired lengths of desired diameters are obtained. Usually total length of a pole ranges from 9 metres to 12 metres. A cast iron base plate and a cap is used with each pole.
Generally the body of a pole is painted with aluminium paint so as to prevent rusting of the steel. This also enhances the gracefulness of the pole. According to Indian Electricity Rules, a metal pole of any variety must be at least 2.2 times as strong as the load which it is to bear on account of supporting the overhead lines.
(3) Rail Poles:
ADVERTISEMENTS:
In our county rail poles are used in large quantities for distribution lines in urban areas. Usually a pole is 9 metres long and weighs 29.8 kilograms per metre for low voltage lines. Poles are so erected that flange of the rail and its bull head or flat head remain across the line.
This adds strength to the support. 1.5 metres of the total length of each pole is grouted in the earth pit and the remaining portion lies above the ground level. The pole looks decent as well as lasts longer if the portion above the ground level is painted with aluminium paint. A rail pole is much costlier than a wooden pole, but less expensive than a steel tubular pole. A sketch of a rail pole is given in fig. 263.
(4) Hamilton Poles:
ADVERTISEMENTS:
A hamilton pole is made up of four different parts joined together. The lowest part is made of cast iron which remains within an earth pit below ground level. The upper portion is made of galvanised steel. The three separate parts of this portion are so made that after joining them together, the entire pole narrows down gradually from the lower base to the top. An outline sketch of this pole is shown in fig. 264.
Now-a-days use of hamilton poles is rather scarce excepting, however, in those mofussil towns where the arrangements for supply of electricity have been in practice since many years. But still to-day, these poles are very much in use for telegraph and telephone lines.
(5) Concrete Poles:
Concrete poles are used for low and medium voltage distribution lines as well as for transmission lines carrying electricity at a pressure up to 11,000 volts. These poles should be made according to rules and regulations laid down for reinforced cement concrete work. At first the reinforcing frame is made up by arranging iron rods both length wise as well as crosswise and binding them with binding wires. Then a mixture of cement, sand and stone chips in the ratio 1:2:4 is prepared with proportionate amount of water. The pole is finally made by casting this mixture into the reinforcing frame of iron rods.
A concrete pole is highly suitable for marshy land or for a piece of land or road which remains under water during monsoon. When remains under water a wooden pole degenerates, a steel pole rusts, but a concrete pole remains intact. According to Indian Electricity Rules, a concrete pole must be at least 2.5 to 3.0 times as strong as the load on it due to overhead lines supported by it.
(6) Lattice Poles:
The use of lattice poles is rather rare in low and medium voltage lines. It is highly expensive, although it is much stronger and more durable when compared with other poles. Where many lines are drawn through the same pole or when along with a distribution line, a high voltage transmission line or a telegraph or telephone line is also supported on the same pole or where lamps like flood light are to be fixed at a great height (e.g. railway yard, pier for ships etc.), the pole should necessarily be very long.
To keep such a long pole straight, its fabrication should provide sufficient strength to it. In such cases usually lattice poles are used. According to Indian Electricity Rules, the strength of a lattice pole should be 1.5 times the load on it due to overhead lines supported by it. Fig. 266 shows the sketch of a particular type of lattice pole.
Accessory # 2. Overhead Line Conductors:
Bare conductors are mostly used in overhead lines. Use of insulated wire is limited. Usually C.T.S. or P.V.C. insulated wires are used for service connection to a house. Within the same boundary wall very often overhead insulated wires are drawn in order to take supply connections from one building to another. But when distribution line is drawn along roadside or in an open space, bare conductors are almost invariably used.
Generally copper or aluminium wires are used as overhead line conductors. However, for street lights even galvanised iron wires are drawn now-a-days in places where theft of line wires is expected any time. The line conductors should be very strong and durable. Indian Electricity Rule No. 74(1) states that the breaking strength of any overhead line conductor must not be less than 317.51 kilogrammes. Rule No. 74(2) states that where the voltage is low and the span is less than 15.24 metres (50 feet) and is on the owner’s or consumer’s premises, a conductor having an actual breaking strength of no less than 126.08 kilogrammes may be used.
The copper wire which is used for overhead lines is known as ‘Hard-drawn Bare Copper Conductor’ or in brief H.D.B.C. conductor. Now-a-days the use of this wire is out of practice because of very high cost of copper as well as the wire is stolen immediately after installation.
In some places it is found to be used in well protected areas inside a factory for giving supply connections to street lights and employees’ quarters. Information’s regarding size, current-carrying capacity, weight, breaking strength, resistance, etc. of different H.D.B.C. conductors are given in table no. 25.
The wire which is widely used for overhead lines in our country is aluminium conductor. Compared to copper the conductivity of aluminium is less, about 61 per cent. For this reason the cross-section of aluminium wire is somewhat greater than that of copper wire for the same current-carrying capacity. Besides, aluminium is a very soft metal. It has very low breaking strength. Hence, aluminium conductor consisting of single strand is never used for overhead lines.
Usually two kinds of aluminium wires are in use. For low or medium voltage lines, hard-drawn all aluminium stranded conductors are used. These conductors consist, of seven strands twisted together. As a result it does not snap easily. But in case of high and extra-high voltage lines as the span between two consecutive supports and the cross-section of the conductors are both comparatively large, the breaking strength of the conductors must necessarily be sufficiently high.
For this reason use of all aluminium conductors is not permitted in these lines. The conductors used for these lines consist of a steel core with seven or more strands of aluminium wire placed over it. The steel core may have one steel wire or more. Such a conductor is called ‘Aluminium Conductor Steel Re-inforced’ or A.C.S.R. conductor in abbreviate form.
Necessary information in regard to size, weight, breaking strength, resistance, etc. of all aluminium conductor or A.A.C. Conductor are given in table no. 26.
Accessory # 3. Insulators:
To draw an overhead line the accessory that is urgently necessary after poles and wires is the insulator. Insulator holds the wire on the pole so that no current leaks into the earth from the conductor. As bare conductors are usually used for overhead lines, insulators should be made perfectly satisfied and of good material.
Besides, the other qualities required of an insulator are:
(a) Insulator should be durable and strong enough to withstand the pull of the wire and does not break easily.
(b) Insulator must be able to withstand the electric pressure of the line for which it is used.
(c) The surface of the insulator should be very smooth and highly glossy, otherwise dust, dirt etc. deposited on its surface may lead to short-circuit between conductor and bracket.
Insulators are usually made of glass or glazed porcelain. However, use of glass insulator is rare in our country. Insulators are set on to brackets or clamps fixed to the poles and the line conductors are drawn on them.
A brief discussion on different types of insulators generally used in low or medium voltage lines is given below:
i. Cross Arms:
Cross-arms are usually used for drawing overhead lines in horizontal configuration. A cross-arm is made of angle iron or channel iron. The size of the angle iron is 5 cm x 5 cm x 6.4 mm (2″ x 2″ x 1/4’”) and that of channel iron is 7.6 cm x 3.8 cm x 0.69 gm per metre (3″ x 1.5″ x 5 lb per foot). The length of the cross-arm depends on line voltage and on the number of wires to be drawn side by side horizontally. However, a length less than 45 cm is not used for any cross-arm.
Cross-arm remains firmly clamped to the pole. There are two holes at the two ends of the arm for fixing the pins of the pin insulators or the straps of the shackle insulators with the help of nuts, bolts and washers. Cross-arm should necessarily be strong enough to withstand stormy wind pressure, weights of line conductors, insulators etc. The body of the cross-arm is painted with aluminium paint or any other suitable paint so that it does not rust quickly. In some places galvanised angle iron or channel iron is used to serve this purpose.
ii. D-lron Clamp:
Cross-arm is not required for fixing insulators to the poles where vertical configuration of the line conductors is adopted. In that case shackle insulator is fixed to a clamp which is shaped like D with the help of nuts and bolts. As this clamp is made of iron or steel, it is called D-Iron Clamp.
D-Iron clamp is made of galvanised flat iron. Size of the flat is 4 cm x 6 mm (1 ½” x ¼ “). With this clamp only shackle insulator can be used. D-Iron clamp is fixed to the pole with the help of another clamp.
iii. Stay-Set:
The terminal pole or the tangent pole (the pole where the overhead line changes direction) is subjected to such a pull that it tends to become inclined towards the line instead of standing erect. In this case, therefore, a suitable stay is to be fixed up to the pole in order to keep it standing erect (vertically) against the tension of the line. If the arrangement of stay is not properly done, the line pole becomes inclined within a few days and it may even fall on the ground leading to interruption of supply of electricity.
In order to fix up a stay with a pole, different accessories are required. These accessories taken together is known as stay-set.
The accessories in a stay-set are briefly described below:
i. Stay Wire:
Generally hard-drawn and stranded galvanised iron wire is used as a stay wire. There are seven strands in this wire. Size of each strand is 10 S.W.G. or 8 S.W.G. (7/10 S.W.G or 7/8 S.W.G.). However, the total capacity of stay wire to withstand tension must not be less than 70 kilogrammes per square millimetre of sectional area.
A stay wire has two parts. One end of the upper part is clamped with the pole and its other end is bound up with the guy-insulator. According to Indian Code of Practice the angular distance between pole and stay wire should be 45°. However, it must never be less than 30°. Also, according to Indian Electricity Rules a guy insulator should remain at a height of at least 3.046 metres above the ground level.
One end of the lower part of the stay wire is bound with the guy insulator and its other end is bound up with the thimble just above the bow which is fixed with the stay rod. Often, for the protection of stay set, lower part of the stay wire, part of the stay rod above the ground level, bow and thimble are kept inside a galvanised iron pipe of length 1.5 metres to 3.0 metres (5 ft. to 10 ft.). Fig. 273 shows how to bind a stay wire with a thimble.
ii. Stay-Rod with Anchor-Plate:
A galvanised iron rod is used as a stay rod. Generally this rod is 19 mm (¾ ”) in diameter. The galvanised iron plate used at the lower end of the rod is known as anchor plate. Its size is 30 cm x 30 cm x 6.4 mm (12″ x 12″ x ¼”). This plate and a length of 1.67 metres (5 ½ ft.) of the stay rod are set inside of an earth pit and grouted with a casting of cement concrete. The ratio of cement, sand and stone chips should be 1:3:6.
The angular distance between pole and the stay rod may be maintained at 45° if:
(i) The stay rod is properly inclined and set Inside the earth pit, and
(ii) The distance of the earth pit from the foot of the pole is exactly equal to the height above the ground level where the stay wire is clamped to the pole.
If, however, sufficient space is not available at this distance for the earth pit, a little less distance will do. A length of 46 cm (18″) of the stay rod remains above the ground level. As an anchor plate is attached to the lower end of the rod, it cannot come out of earth pit easily due to an external pull. Besides, the use of anchor plate becomes further necessary for proper earth connection of the stay set.
iii. Stay-Bow and Thimble:
The upper end of the stay rod is threaded. In this threaded portion the bow can be lowered down or raised up by turning it. This has to be arranged so that the tension in the stay wire can be adjusted as and when required. A separate grooved part made of galvanised iron is provided as a cap at the head of the bow. This is known as thimble. The stay wire is taken round the thimble, and each strand of the wire is separately wound for binding the stay wire.
iv. Stay Insulator:
Stay insulator or guy insulator has already been discussed before. It is made of porcelain and it remains at least 3 metres (10 feet) above the ground level. The stay wire remains clamped to the pole near the top and thus remains very near the line conductors. It may, therefore, for some reason or other, come in contact with the live conductor and get charged. But the lower portion of the stay set near the ground does not get charged as it is electrically separate from the upper part by the guy insulator. Thus, the safety of man or animal somehow coming in contact with the stay wire is assured.
Fig. 274 shows a complete stay set (including its different accessories) attached to a pole.
It may not be possible to attach a stay set with a pole as per rules and regulations on account of intervening road, building etc.
Under the circumstance any one of the following alternative arrangements for keeping the pole vertically erect may conveniently be adopted:
(i) Strut:
An angular pole or an end pole may be kept properly erect by a strut-pole in place of a stay-set if sufficient and suitable space is available in front of the pole just opposite to the space where stay-set is supposed to be installed. A strut may be a pole similar to line pole or it may be made of angle iron or channel iron.
It is placed inclined against the line pole in a direction in which there is tension on the line pole due to line conductors. Fig. 275 shows a line pole kept erect with the help of a strut. A strut must be capable of withstanding the pressure exerted by the tension on the pole due to line conductors. It remains fixed with the pole either by a clamp or by a head-piece.
(ii) Flying-Set:
If the space at the side of a pole where stay-set is to be installed is intervened by a road or a pond, a separate pole may be-planted on the other side of the road or pond arid the main pole is then affixed to it by a stay wire. This arrangement to keep the pole erect is known as flying stay. Fig. 276 shows such an arrangement.
At the time of drawing stay wire over the road, there must be at least 5.49 metres (18 feet) clearance from the ground according to Indian Electricity Rules. The separate auxiliary pole that becomes necessary for fixing the flying stay wire is known as stub-pole. One end of the flying stay is fixed with the main pole and its other end is fixed with the stub-pole by means of clamps.
In certain places turn buckle is also used along with the stay wire so that the tension in the wire may be adjusted. A stay set has to be used with the stub-pole to keep it erect against the tension in the flying stay.
(iii) Bow-Stay:
Where shortage of space may not permit the use of even a strut or a flying stay, another kind of stay, known as bow stay, maybe used in such cases. If the tension due to line conductors is not too strong, a bow stay can keep the pole erect. Bow stay is fixed with the line pole in the direction opposite to that to which the pole has a tendency to incline on account of the resultant pull due to conductors.
At the top of a pole, just below the bracket or clamp for the line conductors, a bow is fixed by means of a clamp. About 90 cm (3 feet) below the clamp a brace made of iron angle or iron channel remains fixed with the pole. The brace is fixed in such a manner that its one end remains attached to the pole while its other end remains about 75 cm (2.5 feet) away from the pole.
Somewhere a hole may be found at this other end of the brace or elsewhere at the same end a pulley of diameter 5 cm (2 inches) is found to be in use. One end of the stay wire is at first bound together with the free end of the bow and the other end of the wire is then passed through the hole in the brace or round the groove of the pulley in case where pulley is used, and finally bound to a clamp at the bottom of the pole if no space is available near the bottom. This is shown in fig. 277(a).
If, however, such space is available, an earth pit is made at a distance of 75 cm away from the pole bottom. A stay rod with an anchor plate is grouted with cement concrete in this earth pit. The lower end of the stay wire is then bound to the upper end of the stay rod. This arrangement has been shown in fig. 277(b).
On account of existence of a bow at the upper end of the stay wire, the pole can be maintained in erect position by adjusting the tension of the wire. The pole and the stay wire together look like a bow, and hence the name ‘bow stay’.
Accessory # 4. Cradle Guard:
The use of only a safety device is not considered sufficient to keep away from hazards wherever the overhead line is drawn across a road or railway line or a domestic yard. At such places Cradle Guard is used below the line so that, on snapping accidentally, a live wire may not at all fall on the ground. Fig. 283 shows how cradle guard is used for medium and low voltage lines.
At first two poles are to be erected on two sides of the road or railway line or domestic yard, and one or more cross-arms suitable for drawing the line conductors are to be affixed to both the poles. Below the lowest cross-arm a bracket, somewhat longer, is fixed for drawing the cradle guard. This bracket which is parallel to the cross-arm may be made of either galvanised iron channel of size 75 mm x 40 mm (3″ x 1 ½”) or galvanised iron angle of size 50 mm x 50 mm (2″ x 2″).
From the two ends of the bracket two pieces of galvanised iron wires are drawn parallel to each other along the overhead lines. The size of these wires should be at least No. 8 S.W.G. Next, in order to complete the cradle guard, lacing wires are bound to them across the line at intervals of 30 to 45 cm (1′ to 1 ½’). No. 10 S.W.G. galvanised iron wire is sufficient size for lacing.
Both ends of a cradle guard should be properly earthed. In order that a pole may not bend due to tension of the guard wires, a special type of stay-set is used with each of the two poles. Such a stay looks like ‘Y’, and as such it is called Wye-Stay. Fig. 283 shows an Y-stay fixed to a pole carrying cradle guard.
Sometimes earth wire and neutral wire are used as cradle guard wires in low or medium voltage lines. This arrangement avoids use of two separate galvanised iron wires. One egg-type insulator is to be used with that end of each lacing wire which is bound to the neutral line so that neutral wire remains insulated from earth wire.
Where earth wire is drawn at the topmost height, two neutral wires are drawn horizontally (i.e. parallel to each other) for the purpose of cradle guard. These are called Split Neutral. The lacings are then bound to two neutrals in a cross-wire manner.
Accessory # 5. Lightning Arrestor:
Overhead line conductors remain open to the atmosphere under the sky. During the season of rain and storm, lightning may strike the line at any time. With lightning stroke the electrical pressure of the line increases many times. In order to protect the equipment, machinery and apparatus in factories and houses supplied by the line as well as to protect the line itself from the damaging effect of this excessive voltage, a device known as Lightning Arrestor is used with the line.
According to Indian Electricity Rule No. 92, the owner of every overhead line which is so exposed as to be liable to injury from lightning shall adopt efficient means for diverting to earth any electrical surges due to lightning.
This rule further states that, the earthing lead for any lightning arrestor shall not pass through any iron or steel pipe, but shall be taken as directly as possible from the lightning arrestor to a separate earth electrode subject to the avoidance of bends wherever practicable.
In overhead lines where earth wire is drawn all along its length at the topmost height, the earth wire is very much helpful in the matter of protection of the line from the lightning strokes. The lightning at first strikes the earth wire on account of its being at the topmost height.
If the earth wire is in good electrical contact with the mass of earth, the transient excessively high voltage instantly goes to earth. For this reason special attention is to be given for maintaining good earth connection of the earth wire. It is the usual practice to dig an earth pit near the terminal pole and every fifth pole along the line and to install an earth electrode at each point so that earth wire is connected to these electrodes for efficient earth connection.
But in places where service connection is given to a house or workshop, or where transformer and other equipment’s remain connected with the line, lightning arrestors are to be used for the protection of the electrical installation. Various types of lightning arrestors are available from the market. Among these the one that is mostly used in low and medium voltage lines is named “horn-gap lightning arrestor”.
Accessory # 6. Horn-Gap Lightning Arrestor:
Among all different types of lightning arrestors used in overhead lines, horn-gap arrestor is manufactured in easiest method and its cost is also comparatively much less. It is called horn-gap arrestor, as this arrestor is made of two conductors in the form of two horns of an animal like cow or buffalo keeping some gap between them.
One end of one conductor is connected to live wire and one end of another conductor is connected to an earth electrode. As many arrestors are to be used as there are live wires in the line. Fig. 284 shows a horn-gap arrestor. In this arrestor two brass or copper strips in the shape of two horns are fixed on a pin insulator. One strip remains fixed at the neck of the insulator with the help of a machine screw.
One terminal of a choking coil is connected with a terminal screw attached to the strip by the side of the machine screw. The other terminal of the choking coil is connected to live line drawn towards the main switch of the house or workshop or transformer.
The other strip of the arrestor is fixed upon the pin of the insulator. There is an oblong hole in this strip. The pin of the insulator is passed through this hole and is properly fixed with the cross-arm or bracket by means of washers and nut below the strip. As the hole is oblong, the strip can be moved to and fro but not around the pin. This facilitates adjustment of the gap between the two strips as and when required. Usually this gap is 3.2 mm (⅛ ”) after fixing the strip upon the pin over the cross-arm, it is properly connected to earth.
Under normal condition the frequency of current flowing through the line is not very high. In our country supply frequency is usually 50 hertz. This offers very little opposition to the flow of current due to impedance of the choking coil, and it can easily flow through the domestic or industrial load circuits. On the other hand the current cannot flow towards earth owing to gap maintained between two strips of the arrestor.
But when the lightning strikes the line, both the frequency as well as the pressure of the supply line become very high. Due to rise in frequency the impedance of the choking coil becomes so high that practically no current flows through it. On the other hand an electric arc is temporarily produced in the small air-gap between two strips of the arrestor due to very high pressure of the line. It is through this arc that the transient high pressure and current of the line discharge to earth.
The electric arc having been formed in the air-gap, the surrounding air becomes heated, the hot air begins to move upwards and in the process pushes the arc upwards also. As a result the length of the arc increases gradually till the arc becomes too long to be maintained by the normal line voltage.
Thus the arc is extinguished and further discharge of current to earth through it is stopped. Although the arc is temporary, the whole of the transient high voltage in the line due to lightning stroke is completely discharged to earth.
Fig. 285 shows how a horn-gap lightning arrestor with an air-cored choking coil remains connected between supply line and consumer’s service connection.
