Tower Spec

  • Tower Spec

    Tower plotting

    The tower plotting may be performed by conventional methods using sag templates or with a computer plotting program where the template data are fed as input data to the computer.

    The sag template used shall be suitable for the respective sagging data and for the ruling span of the section where the plotting is performed.

    the equivalent span (ruling span) to be as near as possible to the basic span.

    The ratio of each span to the ruling span to be between 0.7 to 1.5.

    For all tower positions the specified maximum wind spans and the specified maximum and minimum weight spans (under min. temperature) shall be observed; the individual spans shall not exceed the maximum spans derived from the mid-span phase-to-phase distance.

    The tower plotting shall aim at lengths of successive spans in one section as close as possible. The maximum ratio of successive span lengths shall be 2.0.

    For suspension towers the minimum ratio of the weight span to wind span shall be such as to guarantee that maximum insulator set deflection angles will not be exceeded.

    The tower plotting shall take into consideration the minimum specified ground clearance as well as the minimum distances of the conductors to crossed obstacles such as power transmission and distribution lines, telecommunication lines, railway lines, trees etc. as specified.


    In general the towers shall be self supporting rectangular or square based galvanized steel lattice structures having:


    vertical phase configuration for the double circuit towers (see Annexes B1.7-2 and B1.7-4)


    and shall allow for the use of double bundle conductors.

    Tower types, design spans

    The table below indicates the design spans and line angles for the tower family. The Tenderer / Contractor is free to combine tower types or add types, e.g. heavy suspension towers, based upon his optimization criteria:




    Angle [°]

    Basic Span [m]

    Wind Span [m]

    Weight Span [m]

    Max. Span [m]




    0 .. 2







    0 .. 30






    2D3 spec

    0 .. 2







    31 .. 60







    61 .. 90







    0 .. 45







    Suspension towers

    The suspension tower shall be designed for the maximum height and maximum characteristic spans and shall be used with adequate body extensions.


    With reduced spans, the suspension tower can be used for a line angle of up to 2°.


    Heavy Suspension towers, if any, may also be used as angle suspension tower for line angles up to 5º, with corresponding reduced wind span.


    Tension towers

    According to the principles mentioned above, the following angle towers will be specified:


    · 30° angle tower

    · 60° angle tower

    · 90° angle tower and terminal.


    The Heavy Angle tower may also be designed as terminal tower with the incoming line direction normal to the crossarms and the slack span towards the substation at an angle of 0º - 45º.


    For angle-tension towers the transverse load capacity may be used either for increased wind spans or for line angles.

    Tower extensions

    The tower design shall include a suitable number of body extensions to allow increased tower height for crossing different obstacles as well as leg extensions for adapting the towers to sloping ground.

    For small ground dislevelment leg extensions may be used.

    As a minimum requirement the tower types shall have the following tower body and leg extensions:



    Tower Type

    Body Extension

    Leg Extensions


    -3,  0,  +3

    -2, -1, 0, +1, +2


    -3,  0,  +3

    -2, -1, 0, +1, +2


    -3,  0,  +3

    -2, -1, 0, +1, +2


    -3,  0,  +3

    -2, -1, 0, +1, +2


    -3,  0,  +3

    -2, -1, 0, +1, +2

    Design of towers

    As mentioned, the present specification encourages the use of existing tower designs. Hence the tower outline and dimensions shall follow the principles indicated in Annex B1.7-2 Normal Suspension Tower, 2DS Type – Outline view and Annex B1.7-4 Medium Angle-Tension Tower, 2D3 Type – Outline View.


    For tower designs and for the verification of the existing ones, the new regulations EN 50341 Part1using partial factors for loads (actions) and partial factors for material properties shall be used.


    The following points are to be given special consideration when designing new towers:


    the number of different tower types is to be kept as small as possible,

    low purchase, transport and assembly costs, maximum reliability and efficiency, long life and minimum maintenance, in the event of fault or damage it must be possible to replace individual components in the shortest possible time, tower body extensions shall be additional panels added to the base of the tower,


    Each tower type shall consist of a common portion (Basic Body) to which typical trunks for each body extension may be added. The common portion shall not require modification to accommodate the different body extensions. The legs shall be suitable for fitting to the common portion or to any of the body extensions, without modification to the legs.


    The towers can be erected by using:


    equal legs on flat places or on places possible to be leveled and where the soil permit the leveling and is approved by Engineer, or by using

    unequal legs. Stubs suitable for the foundation types and for legs of the tower types are part of the scope and templates for the alignment of the stubs shall be provided.  


    Towers shall be designed taking into consideration any combination of minimum and maximum leg extension heights used with the tower body or with tower body extensions.


    The reliability, security and safety of new tower designs are to be considered as per the empirical approach to the actions on the towers and corresponding partial factors in EN 50341. The partial factors on actions shall be considered in conjunction with the partial factors on material properties. The values for both - the partial factors on actions and the partial factors on material properties shall be considered as per Tender Schedules.

    Spacing and clearances 


    Clearances and spacing of conductors and live insulator set parts shall be in accordance with EN 50341-1:2001or equivalent and the following requirements, whichever result more stringent. The figures denote minimum clearances; the conductor being at maximum working temperature in still air or when deflected. Tower clearance diagrams for insulator strings and jumpers shall be submitted.


    The positioning of the conductors and of the earth wires on the tower shall be determined considering:


    a) the clearances between the conductors and between conductors and earth wires in midspan

    b) the clearances between live and earthed parts of the line on the tower construction

    c) the earth wire's shade protection angle


    Clearances within the tower geometry

    The length of cross arms and their vertical distances shall observe the minimum phase to earth clearance, the length of the insulator sets and shall take into account the maximum deflection of the conductors due to wind.


    Vertical spacing between ground wire and top conductor cross arm shall be so derived that the specified shielding angle shall not be exceeded.


    For all towers the clearance of the conductor, electrical control fittings, jumper loops and all live metal to the tower steelworks shall not be less than the values given in the Tender Schedules. These values refer to two hypotheses: first - the suspension insulator set and jumper loop vertical or very slightly inclined and second - the assumed maximum swing of insulator sets and jumper loops.


    For angle towers carrying deviation angles up to 60º, cross arms shall generally be so proportioned that live metal clearances are maintained under all conditions without the use of jumper suspension insulator sets.


    Earth wire's shade protection angle

    A shade protection angle of the earth wires of 0 degree to the vertical of the phase conductors shall be considered. In addition, at everyday temperature the sag of the earth wires shall not be more than 95% of the conductor sag.


    The dimensions of the crossarms of the angle-tension towers shall be such to ensure that horizontal spacing between conductors in a plan normal to the conductors are not less than that at normal suspension towers. The earth wire support positions must also ensure the corresponding spacing between earth wires as well as the assumed shielding angle.


    For the D6 and D9 type towers having a line deviation angle of 60 or 90 deg., rectangular crossarms may be used so that live metal clearances are maintained with or without the use of jumper suspension insulator strings.


    The crossarms of suspension towers shall be designed to allow the attachment of double insulator strings directly to the structure.


    The crossarms of tension towers shall be designed to allow the attachment of double insulator strings directly to the structure and an attachment for maintenance purpose.


    The minimum vertical clearances to ground and within line crossings over different obstacles are specified in the Technical Schedules.


    The maximum and minimum conductor sags shall be calculated under still air condition, for the conductor maximum and minimum temperatures, as indicated in the Technical Schedules.


    The Contractor shall indicate in his bid the total creep which he will consider after ten years operation and shall base his bid on the assumption that this creep will be compensated by stringing the conductor correspondingly at initial sags.


    Midspan clearances

    The minimum mid-span phase to phase and phase to ground wire clearance shall be checked according to EN 50341-3-4:2001, clause 5.4.3,


    a = k x sqrt (f+l) + S [m]


    where: l= length of suspension insulator set [m]

    f= maximum final conductor sag [m]

    S= minimum electrical clearance, defined for 132 kV nominal voltage [m], equal to:

    S = 1.05 m, in case of phase-to-phase and

    S = 0.90 m, in case of phase-to-earth

    k = coefficient function of the type of conductor and the relative position of phases



    for AAAC 400:

    k= 0.85 for phases in vertical or quasi vertical disposition,

    k= 0.65 for phases in quasi horizontal disposition, and

    k= 0.70 for oblique phase disposition.


    Allowance shall be made for increasing the length and varying the arrangement of the cross arms at terminal towers and gantries to enable a rearrangement and/or transposition of the conductors.


    For the geometry of angle-tension towers, the following requirement shall be considered:


    · vertical phase to phase distance of angle towers as per the formula shown above,

    · horizontal phase to phase distance shall be maintained close to the value for suspension towers. It shall therefore be determined for the average value of the range of line angles for which the angle tower is to be used (for instance, for an angle tower for line angles of (30° - 60º) the average would be 45º). Different cross arm lengths for the inside and outside of the line angle may be considered. For the heavy angle tower square cross arms may be considered for the outside of the angle.


    Clearance to grounded tower members is the minimum clearance between conductors or between the live parts of the insulator strings and the grounded members of the tower.


    For a suspension tower:

    from still air to 10° swing of insulator from vertical:  1.40 m

    from 10° to 50° swing of insulator from vertical:  0.50 m


    For tension towers:

    jumper loop from still air to 10° swing from vertical:  1.40 m

    jumper loop from 10° to 40° swing from vertical:  0.50 m

    minimum plan clearance from arc horn tip to

    grounded members of the tower:  1.40 m


    Clearance to ground and obstacles

    The minimum clearances which have to be observed under worst condition of maximum sag from the phase conductors to ground and to crossed obstacles are listed in the minimum requirements data sheets. They have to be considered during tower spotting:


    The Tenderer shall indicate in his offer the total creep which he will consider after 10 years and shall base his offer on the assumption that this creep will be compensated by appropriately increasing the initial stringing tension.


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