General

Towers shall be erected by piecemeal method on the foundations not less than 14 days after concreting and after such time that the concrete has acquired its full strength. The towers shall be erected in the best workman like manner and its members shall not be strained.

Treatment of Joints

Care shall be taken to see that the jointing surfaces are clean and free from dirt or grit and if necessary Zinc rich point shall be applied.

Assembly

The method followed for the erection of towers shall ensure the points mentioned below-

• Straining of the members shall not be permitted for bringing them into the position. If however, it is, necessary to match hole positions at joints to facilitate this tommy bars not more than 450 mm long  be used.
• Before starting erection of an upper sections, the lower section shall be completely braced and all bolts provided in accordance with approved drawings.
• All plan diagonals relevant to a section of tower shall be placed in position before assembly of upper section in taken up.

Checking as per Drawing

The tower erection shall be done in strict accordance of the approved drawings. If the stubs of super structures after the erection are found to differ from approved drawing or to be put off alignment, the tower shall be dismantled and re-erected.

Vertically

The towers must be truly vertical after erection and no straining will be permitted to bring them in alignment. Tolerance allowed for vertically shall be one degree that is 1 in 360 height.

Tightening and Punching of Bolts & Nuts

All nuts shall be tightened properly using correct size spanners. Before tightening, it will be seen that filler washers and plates are placed in relevant gaps between members and bolts of proper size and length are used with flat washers. The tightening shall progressively be carried out form the top downwards, care being taken that all bolts at every level are tightened simultaneously.

The threads of bolts projecting outside the nuts shall punched by chamfering the threads at three positions on the diameters to ensure that the nut are not loosened in course of time. If during tightening a nut is found to be slipping or running over the bolt threads, the bolt together with the nut shall be replaced.

Replacement

If any replacement are to be effected after stringing and tensioning or during maintenance, leg members and main bracings shall not be taken out without reducing the tension of the tower with proper towing or releasing the conductor. If the replacement of cross arms be comes necessary after stringing, the conductor shall be suitably tied to the tower at tension point or transferred to suitable roller  pulleys at suspending  points.

Insulator Hoisting

In 132 KV and 220 KV transmission lines, suspension insulator strings are used on all suspension 'A' type towers (upto a deviation of 2 degrees) and tension insulator strings are used on 'B' and 'C' type Tension towers. 

In 400 KV lines V-type insulator strings are used on all suspension A-O and A-5 towers (up to a deviation of 5 degrees). Double tension insulator strings are used at all B-30, C-60 and DE type Tension towers.

The number of insulator units to be used on various types of strings are as follows :

Table 4-I

Note :

• *1. Double suspension insulator strings are used in 132 KV & 220 KV lines on Suspension towers of River and Power line Crossings only.
• **2. Double Tension insulator strings are used in 132 KV & 220 KV lines on Tension towers of Railway, River and Power Line Crossings only.

The following precautions shall be taken for hosting of insulator strings:-

• The insulator strings shall be assembled on the ground before hosting.
• The strings shall be fixed on the towers just prior to the stringing of conductors.
• It shall be ensured that all current carrying parts are smooth and without dirt, grit, cuts, abrasions, projections etc.
• Damaged insulators and strings shall not be used in the assemblies. Insulators which have cracks or chip or those having glazing defects exceeding half a centimeter Sq. shall not be used.
• Before hosting all insulators shall be deemed in a manner that will not spoil, injure or scratch the glaze of the porcelain, but in no case shall any oil be used for this purpose.
• Security clips shall be placed properly in position for the insulator assemblies before hosting.
• For 132 KV and 22 KV lines in suspension strings the arcing horns are used on line side only and in tension strings the arcing horn are used both on line and tower sides.
• Arcing horns and Anti-corona rings shall be fitted as per recommended practice of the manufacturer/supplier.