Koi Koi Grace

Constructability refers to the “readiness to be built’’ and to the process of reviewing and ensuring that a particular transmission line project can be actually built as designed, given the numerous technical and non-technical parameters that control the construction phase. This “review’’ looks at the construction constraints, environmental barriers, political issues, risks and acceptance to local community.   a-) Construction considerations The primary construction-related issue is availability of access roads, both temporary and permanent. These roads are used to move crew, vehicles, materials and equipment in and out of the construction site. Their secondary purpose is to provide maintenance access to the structures for repair and to the ROW in general. Access needs also vary depending upon the type of structure (pole, frame or lattice tower), function (tangent, angle or deadend) and foundation (grillage, direct embedment, concrete shafts or guy anchors). These considerations in turn will dictate the type and number of vehicles and equipment that must reach the work site. Use of cranes and helicopters for heavy construction impose additional challenges. The work areas for conductor pulling and tensioning equipment must be integrated into access road plans. However, these activities are performed only during construction and therefore the access is temporary. Constructability assessment also involves evaluating the process of hauling poles to the site. Concrete poles are about four to five times heavier than steel structures are difficult to handle and move. Lattice steel towers, which are typically set on concrete shaft foundations, require the most concrete at each tower site. If unguyed poles are used at line angles, this mandates drilled shafts due to large base forces. Most utilities will have in-house procedures for inspection, assessment and maintenance and these are likely to dictate their choice of a structure type for a specific line. Steel or concrete structures require minimal maintenance while wood systems need frequent inspections and condition monitoring. Because of difficulty procuring ROW or easements, and obtaining permits for new lines, many utilities strive to improve their future options by selecting structure types for current projects that will permit easy upgrading or uprating initiatives.  b-) Environmental constraints Topographic characteristics of the terrain often provide the most difficult construction challenges. Areas with steep slopes, erodible soils, sensitive streams, wetlands, restricted lands and habitats impose constraints on the process. Construction activities in the neighborhood of federally-protected lands and wildlife refuges require a wide range of government permits and adherence to agency-imposed work schedules.    c-) Regulatory issues Any potential crossing of public (state and federal) lands invariably leads to regulatory requirements, consultations and approvals. Prior to construction, the project may require several permits at various levels. For example, road and highway departments require permits for transmission line construction where the line crosses a highway or expressway. Areas attached to sensitive natural resources and recreational usage demand additional regulatory compliances.   d-) Public acceptance Any project involving transmission line construction must take into account its impact on the residents of the area it covers and possible public opposition to the presence of large structures, noisy construction equipment as well as potential disruption to regular community activities. Most utilities conduct meetings to inform the public of their intended projects and seek their input and cooperation.            

Each structure must be securely embedded or anchored into the ground and facilitate safe transfer of structure loads to the ground strata below. To determine foundation requirements, the engineer must first evaluate the nature and condition of the soil in the vicinity of the structure. The choice of eventual foundation type will further depend on geotechnical characteristics of strata underneath, structure material, configuration, loads, constructability and economy. A majority of tangent poles are directly-embedded into ground; but systems defined by large lateral forces and moments require concrete drilled shafts whose design is a bit more labor-intensive. Design criteria for foundations depend on type of soil and loads imposed. Design loads are usually factored reactions obtained from structural analysis from computer programs. In lattice towers, the loads transmitted are primarily compression and uplift loads. For single pole structures, the loads transmitted are overturning moment, shear (lateral) and axial loads. Where moment foundations such as drilled shafts are required, it is important to specify allowable deflection or rotation criteria, both elastic and non-recoverable. Chapter 4 will contain more information on this issue. As with structures, foundations are also designed for given Strength Factors shown in Tables 2.15a and c. However, unlike structures, guidance in the area of foundation design is not well laid out by the codes and standards. Therefore it is common practice to use internal design criteria for foundations. These criteria vary from utility to utility. The structural design of all reinforced concrete systems is in general governed by the ACI 318 (2011). For drilled shafts with a diameter greater than 30 in (0.76 m), ACI- 336-3R (1998) is recommended. Computer programs such as CAISSONTM (2011), LPILETM (2015) or MFADTM (2015) are used to quickly size a drilled shaft or directly-embedded pole under moment loads. Drilled shafts under moment loads are primarily designed as laterally-loaded piles. Guyed systems require anchors to transmit guy wire loads to the ground. Anchor types range from classical (log) to grouted (rock) to helical (screw) with widely-varying holding strengths depending on site-specific soils. If soil data is available, helical anchors are evaluated using HeliCAPTM (2007). Geotechnical properties of ground strata are an integral part of the design criteria. In general, properties such as allowable bearing capacity, unit weight (dry, moist and submerged), friction angle, cohesion, subgrade modulus and others are required. Chapter 4 will discuss these issues in greater detail.  

The utility industry now uses tubular steel poles as well as steel lattice towers, spun prestressed concrete poles, wood poles and composite poles. Simple suspension (tangent) and small-angle single pole structures can be quickly analyzed using spreadsheets. The advent of powerful digital computers and software such as PLS-POLETM, TOWERTM and PLS-CADDTM, now enables accurate modeling and analysis of not only individual structures of a transmission line but also the entire transmission line in a single modeling session. POLE and TOWER also have provisions for input of foundation capacities to facilitate fuller representation of the structure. Plan and Profile drawings can now be digitally processed, printed and saved in various formats and sizes.   1-) PLS-POLE PLS-POLETM is a powerful structural analysis and design program for transmission structures. The program is capable of handling wood, laminated wood, steel, concrete and composite structures and performs design checks of structures under specified loads. It can also calculate maximum allowable wind and weight spans to aid in structure spotting. Virtually any transmission, substation or communications structure can be modeled, including single poles, H-Frames and A-Frames. These models can be rapidly built from components such as pole shafts, davit arms, cross arms, guy wires, X- and V-braces and all types of insulator configurations. Pole shaft databases for standard classes of steel poles from various suppliers are built into the program and cover both galvanized and weathering steel poles. The component databases that are used with PLS-POLE include various steel shapes (round and multi-sided pole shafts), wood, laminated wood, concrete and composite poles of various classes, guy wires and insulators etc. Custom elements can be added by the user as needed. PLS-POLE is capable of performing both linear and nonlinear analyses. Nonlinear analysis allows 2nd order or P-Delta effects, helps to detect instabilities and to perform accurate buckling checks. The program can perform steel pole design checks based on ASCE 48-11 or other specified codes. The current version of the program has built-in checks for various international codes. For poles used as communication structures carrying antennae, the program also has a design check option using ANSI TIA-222 (2006) standards.   Structures assembled in PLS-POLE can be exported to PLS-CADD and installed in the line’s 3-dimensional model. Figures 2.6a, 6b and 6c show renderings of typical structure models developed with PLS-POLE.  2-) TOWER TOWER is a powerful structural analysis and design program from Power Line Systems (PLS) for lattice steel transmission towers based on a 3-dimensional finite element analysis scheme for axially-loaded trusses. Both self-supported and guyed towers can be modeled. The program performs design checks of structures under user specified loads. For a given lattice tower structure, it can also calculate maximum allowable wind and weight spans and interaction diagrams between different ratios of allowable wind and weight spans. The program is capable of performing both linear and non-linear analysis and facilitates detailed input (angle members, bolts, bolt configurations, member slenderness ratios, connection eccentricity and restraint etc.) It calculates the forces in the members and components and compares them against calculated capacities for the selected code or standard. Overstressed members easily identified graphically and in the TOWER output reports. TOWER is capable of performing design checks based on various world standards: ASCE Standard 10-15, ANSI/TIA-222, Canadian Standard CSA S37, ECCS, CENELEC, AS 3995, BS 8100 and others. The component databases that are used with TOWER include various steel angles (equal and unequal legs, single and double angles), tower bolts of various specifications and insulators etc. Unique shapes (flat bars, channels, T-sections for example) can be created where necessary. Structures assembled in TOWERTM can be exported to PLS-CADDTM and installed in the line’s 3-dimensional model. Figure 2.7 shows rendering of a double-circuit deadend tower developed with TOWERTM.     3-) PLS CADD PLS-CADD is a line modeling, analysis and design program for transmission lines and structures. This program in integrated with its sister modules PLS-POLE and TOWER. A typical 3-dimensional view of a line segment showing single poles with braced post insulators is shown in Figure 2.8. A Plan and Profile drawing generated using the software is shown in Figure 2.9. 3-a) Load and strength criteria The fundamental design philosophy behind current transmission structure design is that all structures shall be designed and detailed in such a way so as to sustain imposed factored design loads without excessive deformations and stresses. The objective is to design a structure with resistance exceeding the maximum anticipated load during its lifetime and to produce a structure with an acceptable level of safety and reliability. Mathematically, The strength factor ‘∅’ limits the resistance ‘R’ and accounts for the variability of the resistance property. The load factor LF accounts for the uncertainty of the given load and/or simplifying assumptions made in the analysis. This concept is similar to the LRFD (Load and Resistance Factor Design) used in general structural design where the erstwhile deterministic approach is replaced with a more robust probabilistic approach, matching strengths with loads of various types and incorporating statistical variations associated with different load categories and strength of components. Strength and Load factors suggested by RUS and NESC are shown in Tables 2.15a, 2.15b and 2.15c. All values refer to new, Grade B Construction which is the highest grade associated with safety and reliability per NESC. Both NESC and RUS also provide guidelines for strength factors for replaced or rehabilitated structures. The strength factors of RUS are somewhat conservative and are often adopted by utilities in rural areas and whose lines and structures are designed and built according to RUS/USDA guidelines.   Grades of construction NESC defines three grades of construction, namely, Grades B, C and N. Grade B is the highest or the type of construction with stringent strength standards while N is the lowest grade. Grade B standards are typically used by US utilities for transmission lines.   3-b) Structural Design Criteria Overhead transmission lines are continuous structural systems with supporting elements (poles, towers, frames) and cable elements (conductors and shield wires). To ensure safe and reliable service, these are typically designed by considering the various load conditions listed in Table 2.16. 3-c) Weather Cases A comprehensive analysis and design of a transmission line involves various weather cases related to both the performance of the line itself (conductors and insulators) as well as the supporting structures (poles, frames and towers). A collection of these weather cases is called “Criteria File’’ which is a critical input to line design software PLS-CADD. Tables 2.17a and 2.17b show the full list of weather cases that are typically considered in transmission design in the United States. Several parameters such as wind pressures and ice thickness magnitudes vary from utility to utility and the numbers provided in the table are for illustration purpose only. The case labeled Warm Islands is applicable only for islands located from 0 to 25 degrees latitude, north or south. This zone covers Hawaii, Guam, Puerto Rico, Virgin Islands and American Samoa. The ‘k’ factor is explained below.  3-d) Load Cases Table 2.18 gives a typical list of load cases needed for transmission structure analysis and design. These loads are applied on the structures, supported wires, hardware, insulators and other equipment. The philosophy behind each of the load cases is explained below. The reader must note the additional NESC Factor ‘k’ applied for the NESC district load cases only. This is an arbitrary factor added to the resultant of the vertical and horizontal loads as shown in the figure at the bottom of the Table 2.18.     NESC district loadings These 4 cases – also called District Loadings – are included to meet the requirements of Rule 250B of NESC and include the mandated climactic and load factors. The ‘k’ factor is an additional load item applied to the resultant of the conductor vertical and wind load components while determining the sag and tension of the wire. Only the structural design of angle and deadend structures and tangent structures located on   the line angle are impacted by the ‘k’ factor because the tension of the wire is affected by the factor. The designations of Heavy, Medium, Light and Warm Islands are based on ice and wind expected in those areas. The user may refer to NESC for geographic boundaries of these 4 zones. For a given transmission line, only one of the four zones is applicable unless the line crosses more than one zone. Extreme wind The purpose of this NESC load is to ensure that the structure is capable of withstanding high winds that may occur within the geographic territory. As mentioned earlier, the wind force used is based on the fastest 3-second gust at 33 ft (10 m) above ground. Per NESC, this load is currently applied to structures over 60 ft (18 m) in height. However, RUS Bulletin 200 recommends all structures be checked for Extreme Wind regardless of height. Maps showing design wind speeds are provided by NESC as well as RUS Bulletin 200. NESC also requires all structures (irrespective of height) to be checked for extreme wind applied in any direction on the structure without conductors.  Extreme ice This load case considers the possibility of extreme ice storm or a storm that develops icing conditions. Usually this case is defined by accumulation of radial ice of 1 in (25.4 mm) thickness on conductors; but often utilities located in icing regions adopt a higher value of 1.5 in (38.1 mm) or more.   Extreme ice with concurrent wind The intent of this NESC load case is to design a structure for extreme ice accompanied by wind. Per NESC, this load is currently applied to structures over 60 feet (18 m) in height. Since ice can stay on conductors for 4 to 5 days and may see subsequent wind, a 40 mph (64 kmph) wind at 4 psf (190 Pa) wind is shown in Table 2.18; this is used to satisfy ASCE Manual 74 requirement that wind supplementing ice must be equal to about 40% of extreme wind case. Maps showing uniform ice thickness with concurrent wind speeds are provided in NESC as well as RUS Bulletin 200. These maps show uniform ice thickness typically ranging from ¼ inch (6.35 mm) to 1 inch (25.4 mm) and concurrent wind speeds ranging from 30 mph (48.3 kmph) to 60 mph (96.5 kmph), equivalent to a wind pressure of 2.3 psf to 9.2 psf (110 Pa to 440 Pa). In all NESC load cases in which wind is included, the horizontal wind pressure is applied at right angles to the direction of the line, except where wind is applied in all directions without wires. Also, NESC does not consider ice on structures and wind-exposed surface areas.   Construction This load case is to ensure the structural integrity of not only the main structure but also the arm or steel vang supporting the insulator/stringing block and arm strength during wire tensioning. This is because one of the worst loadings that a given arm or conductor attachment point will endure is during the stringing of the conductors. A small wind is also considered in this case. This load is applied as additional vertical and horizontal loads to the phase that induces the highest structural stresses with all conductors installed and stringing in the last conductor. A typical example will be tensioning at the structure tensioner down slope (1:1).   Broken wires The idea behind including this case is to ensure that in the event any phase wire or overhead ground wire fails, the failure does not cause any additional damage to the structure or lead to a cascading type of line failure. Another motivation is related to the cost and availability of replacement structures and the long lead times for fabrication. Designing a structure for potential broken wire cases and the slight cost associated with it is well worth considering that removal of a line from service, even temporarily, is avoided. This is critical for lattice transmission towers carrying HV and EHV circuits. Broken wire loads are generally applied at selected wire location (conductor or shield wire) that induces the greatest stress in the structure. More than one load case may be needed if the highest stress location is not readily apparent. ASCE Manual 74 provides more information on design longitudinal loading on structures, historically called everyday wire tension or broken wire load.   Failure containment This load case is designed to reduce potential for catastrophic failures. Severe climactic conditions often produce a cascading type of failure where one structure fails and collapses completely. This subjects adjoining structures with severe unbalanced loads – much more than what they were designed for – upon which they collapse in a dominolike sequence. Deadends and heavy angles, and in some cases tangent structures too, are subjected to such a load criteria. Containment loading is characterized by the absence of ALL wires on one side of the structure. The conditions that define this load case are usually set by the utility based on their judgment. ASCE Manual 74 also provides guidelines for longitudinal loading on structures including a procedure based on Residual Static Load (RSL), which is a final effective static tension in a wire after all the dynamic effects of a wire breakage have subsided.       Uplift Uplift is defined as negative vertical span. Figure 2.10 depicts the situation where uplift occurs in a transmission line. On steeply inclined spans in hilly terrains, when the cold sag curve shows the low point to be above the lower support structure, the conductors in the uphill span exert upward forces on the lower structure. The magnitude of this force at each attachment point is related to the weight of the loaded conductor from the lower support to the low point of sag. This uplift force is more pronounced at colder sub-zero temperatures. Uplift must be avoided for suspension, pin-type and post insulators. A quick check for uplift can be made using a sag template (see Chapter 5).   Deflection In addition to the above loading cases, designers also often check situations such as Deflection Loading where tangent poles and frames are designed to limit the pole top deflection to a specified level, usually to 1% to 2% pole height above ground for a given loading. This ensures adequate stiffness of the structure to limit flexural deformations and thereby help keep insulators plumb and clearances intact. The climactic conditions for this load case usually include the average annual ambient temperature for the structure geographical location.    

1-) Structure spotting Prior to any design, the engineer must establish the alignment of the transmission line taking into consideration the various criteria. These criteria include property boundary issues, soil conditions, road and other clearances, regulatory issues, environmental concerns, costs, impact on public property, aesthetics and construction access. Potential structure locations are identified along the chosen alignment using structure spotting process. Structure spotting is the design process which determines the height, location and type of consecutive structures on the Plan and Profile (P & P) sheets. The potential points are identified based on terrain, expected access issues and land use. However, structure’s height and strength requirements are not considered as primary criteria. Survey coordinates of the points will be input into computer programs such as PLSCADDTM (2012) to generate the terrain showing graphical view of the transmission line. Structures with known strength ratings (i.e.) maximum allowable span for a given configuration and height are used alternatively to ‘spot’ structures to make best use of their height and strength. Among the key factors that impact structure spotting are vertical and horizontal clearances, structure capacity, insulator swing, conductor separation, galloping and uplift. Both manual and computerized spotting processes are available. In the case of manual spotting, structure strength is expressed in terms of maximum allowable Weight and Wind Spans. The weight span (also called vertical span) is the horizontal distance between lowest points on the sag curve of two adjacent spans. The wind span (sometimes called horizontal span) is the horizontal distance between the geometrical midpoints of the adjacent spans. Figure 2.5 shows the definitions of wind and weight spans. 2-) Rulling Spans The concept of Ruling Span (RS) is used in the design and construction of a transmission line to provide a uniform span length representative of the various lengths of spans within a tension section (i.e.) between deadends. A ruling span is an assumed design span which approximates the mechanical performance of given tension section. This span allows sags and tensions to be defined for structure spotting and conductor stringing. Mathematically, the Ruling Span is defined as: Example 2.3 A tension section between two deadends of a 161 kV transmission line contains the following spans: 920 ft (280.4 m), 1060 ft (323.1 m), 1010 ft (307.8 m) and 870 ft (265.2 m). Determine the Ruling Span. Solution: RS = {[9203 + 10603 + 10103 + 8703]/[920 + 1060 + 1010 + 870]}0.5 = 973.55 ft (296.7 m)   3-) Sags and Tensions Sags and tensions of wires are interdependent. Sags depend on the span length, tension, loading condition, type of wire (conductor, overhead ground wire or optical ground wire) and tensions in turn depend on history of stressing and wire temperature relative to a given weather condition. Long-term stress behavior includes creep which in turn affects tension. Aeolian vibrations of wires induce dynamic stresses. Extreme operating temperatures soften the conductor and induce higher sags. Therefore, the determination of conductor and ground wire sags and corresponding tensions – as a function of temperature and loading – is of fundamental importance in transmission line design. Wire stringing charts showing these sags and tensions are critical to field technicians during installation process. Wire sags and tensions are generally determined using well known computer program SAG10TM from Southwire (2014). Alternatively, wire sags and tensions can also be calculated within PLS-CADD. The maximum anticipated sag of a conductor – usually at its highest operating temperature or heavy ice loads also helps determine the required height of a pole or structure at a location to satisfy mandated ground clearance.   Example 2.4 The maximum sag of a conductor at a particular level span in a 230 kV transmission line is calculated as 13 ft (4 m). Determine the nominal height above ground of a pole required for that location. If 11% of the total pole is embedded into ground, what is the total pole length needed? Assume a ground clearance buffer of 2.5 ft (0.76 m). Solution: This problem serves to illustrate the use of clearance tables discussed in Section 2.2.3. From Table 2.7 – Minimum phase-to-phase separation for a 230 kV line = 9.1 ft (2.77 m) Minimum separation between OHGW and phase wire = 5.9 ft (1.80 m) From Table 2.6a-1 – Minimum Ground Clearance required = 22.4 ft (6.8 m) With buffer: Design Ground Clearance = 22.4 + 2.5 = 24.9 ft or 25 ft (7.62 m) Height of pole required = 5.9 + (2)(9.1) + 13 + 25 = 62.1 ft (18.93 m). Total Length of the Pole needed = 62.1/(1.00 – 0.11) = 69.8 or 70 ft (21.34 m) Use 70 ft (21.34 m) Additionally, the following Sag-Tension relationships will be useful in many situations:   The requirement of using iced upper and un-iced lower conductors is related to the issue of differential ice accumulation. Ice on overhead ground wires is unlikely to melt whereas ice on phase wires is prone for melting given the heat generated by passing of current. This unbalanced ice is also a condition to check separation between ground wires and energized phase wires. Equations 2.4 and 2.6 can be combined to determine maximum allowable horizontal spans for tangent structures based on horizontal separation of conductors. The example below illustrates the process for an H-frame.   Example 2.5 The following data apply to a 69 kV line with a ruling span of 800 ft (243.8 m). The suspension insulators each carry 4 bells for a string length of 2.5 ft (0.76 m). Experience Factor is taken as 1.00. Determine the maximum horizontal span limited by conductor horizontal separation. H = Horizontal separation of phase wires = 10.5 ft (3.2 m) Ruling Span Sag = 20.4 ft (6.22 m) at 60◦F final φ = Swing angle under 6 psf (290 Pa) wind = 59.4 deg. (sin φ = 0.861)   Solution: Substituting these values in Equation 2.4: 10.5 = (0.025)(1.05)(69) + (1)(Sf) + (2.5)(0.861) or Sf = 42.72 ft (13.02 m) Using this value in Equation 2.6: (Lmax/800)2 = (42.72/20.4) = 2.09 or Lmax = 1157 ft (352.9 m)     Example 2.6 Given a 138 kV structure with a phase separation at pole of 9.64 ft (2.94 m), a ruling span of 800 ft (243.8 m) with the following additional data: Sag of lower conductor at without ice = 18.7 ft (5.7 m) Sag of upper conductor at with ice = 22.4 ft (6.83 m) Required separation at mid-span = 5.7 ft (1.74 m) Determine maximum span limited by conductor vertical separation. Solution: From Equation 2.8: Lmcs = (800)(sqrt[(5.7–9.64)/(18.7–22.4)]) = 825.5 ft (251.6 m)    3-a) Galloping The issue of galloping was discussed in Section 2.2.10 where wire tension control is noted as one of the methods of controlling galloping vibrations. Usually, for long spans in excess of 600 ft (183 m), the optimum wire tension is often governed by galloping movement defined by double loop ellipses. This tension is determined in a trial-anderror procedure by adjusting the tension so that the ellipses do not touch each other. At angles and deadends, a factored value of this wire tension is used to design the structure itself; therefore, it is important to evaluate the impact of galloping while designing transmission lines with large spans. 3-b) Tension Limits Throughout the life of a transmission line, conductors are subject to a variety of mechanical and climactic loading situations including wind, ice, snow and temperature variations. NESC and RUS Bulletin 200 therefore specify tension limits for conductors and shield wires which refer to state of loading, temperature and climactic parameters. Table 2.14 shows these combined limits. These limits generally refer to ACSR conductors. For other types of conductors, manufacturer’s guidelines must be followed. Initial Unloaded Tension refers to the conductor as it is strung initially before any ice or wind is applied. Final Unloaded Tension refers to the state of the wire after it has experienced ice and wind loads, long term creep and permanent inelastic deformation. Standard Loaded situation refers to the conductor state when it is loaded with simultaneous ice and wind per NESC loading districts as defined in Section 2.1.2. Extreme Wind Tension is the tension when wind is acting on the conductor as defined in Section 2.1.1. No ice is allowed on the wire for this condition. Extreme Ice Tension is the tension when the conductor is loaded with specified amount of radial ice as defined in Section 2.1.3. No wind is allowed on the wire for this condition. This load case is outside of NESC requirements. Extreme Ice with Concurrent Wind refers to the situation where extreme ice on the wire is accompanied by a moderate amount of wind as defined in Section 2.1.3. Tension in the wires is also a criterion to determine whether vibration dampers are needed or not (see Section 5.1.1.12 for details). 4-) Insulator Insulators are an integral component of the mechanical system defining a transmission structure and are employed in several basic configurations: post or suspension, angle and deadend. All insulators specified for a structure must consider both mechanical strength as well as electrical characteristics. Post insulators are used where insulator swing is not permitted and are usually subject to cantilever forces. Suspension (and angle) insulators are subject to vertical loads due to wire weight and tensile loads due to line angles and are rated by their tensile strength. Deadend insulators carry direct wire tensions and are selected on the basis of tensile strength. NESC recommends the following allowable percentages of strength ratings for line post insulators: Cantilever 40% (Ceramic and Toughened glass) & 50% (Non Ceremic) Tension 50% Compression 50% For suspension type insulators, NESC recommends allowable strength rating of 50% of combined mechanical and electrical strength in case of ceramic and toughened glass insulators. For non-ceramic insulators such as polymer insulators, it is 50% of the specified mechanical load. This recommendation applies only for the case of combined ice and wind district loading (Rule 250B) with load factors of unity. These percentages are applied to the insulator manufacturer ratings which are different for ceramic and non-ceramic insulators. For post and braced post insulators, manufacturers also provide interaction strength diagrams showing combined effects of simultaneous application of vertical, transverse and longitudinal loads. For porcelain or glass suspension insulators, the ultimate strength rating is usually denoted as “Combined Mechanical and Electrical’’ whereas for polymer insulators the terms “Specified Mechanical/Cantilever/Tensile Load’’ are used, as appropriate.    5-) Hardware All components defining a transmission structure also contain various connecting hardware and must meet or match the strength ratings of the main connected part. For example, the anchor shackles and/or yoke plates used to connect deadend insulator strings to the structure must withstand the loads imposed via the insulator string. This also applies to all other associated accessories such as conductor and ground wire splices, suspension and deadend clamps and other hardware.  6-)  Guy Wires and Anchors The design of guyed structures involves both guy wires as well as anchors that transfer wire load into the ground. Guy wires are generally galvanized steel (often the same stranding and rating as the overhead ground wire) or aluminum-clad steel. Strength ratings apart, corrosion resistance is also often considered while selecting a guy wire. A wide variety of anchoring systems are available to choose from: log anchors, plate anchors, rock anchors, helical screw anchors etc., each with a specific set of usage and design criteria. The nature of soil/rock at the location plays an important role in anchor design. As with insulators, limits are placed on allowable loads on guy wires and anchors. Typical allowable percentages of strength ratings for guys and anchors are: Guy Wire 90% (NESC) Anchor 100% (NESC) Guy Wire and Anchor 65% (RUS) In situations where the guy wire may be in close proximity to energized conductors, a guy insulator is often used on the guy wire to provide additional protection. For visual safety, the bottom portion of all guy wires are typically enclosed in colored PVC markers, usually yellow, to render them visible during night time.          

3-) Shielding When lightning strikes a transmission line, it may hit either the overhead ground wire or a phase conductor. If a conductor is hit, there will certainly be a flashover of the insulation. To prevent and minimize such an occurrence, the overhead ground wire is used to intercept the lightning strike, “shielding’’ the conductors. To optimize this shielding, the shielding angle (see Figure 2.4) must be 30◦ or less. If a location is known to have an unusually high exposure to lightning strikes, and structure heights are over 90 ft (27.4 m), even smaller shielding angles should be used. Table 2.10 shows typical shielding angles relative to structure height as well as voltage. Note that for situations where the height and voltage give two different shielding angles, the smaller value shall be adopted.   For voltages exceeding 230 kV, the shielding angle often controls the required vertical and horizontal separation between the ground wire and conductor. For areas with a history of lightning strikes, the clearance between the overhead ground wire (OHGW) and the conductor shall be determined with specific reference to the Isokeraunic levels. 3.a) Lightning Performance and Grounding Design criteria should include the line’s target Isokeraunic Level of the area. This level is usually given in terms of number of thunderstorms in the area per year. An overhead ground wire (OHGW) should be provided at all places where the Isokeraunic Level is over 20 per RUS Bulletin 200. This wire should be grounded at every structure by way of the structure ground wire. For H-Frame structures with two (2) ground wires, the OHGW must each be connected to the structure ground wire and to one another. This will be beneficial in the event that if one structure ground wire breaks, both overhead ground wires will still remain grounded. RUS Bulletin 200 recommends that lightning outages of 1 to 4 per 100 miles per year is acceptable for lines in the 161 to 230 kV range. All circuits must be adequately grounded to facilitate protection against lightning strikes. RUS recommends that individual transmission structures should have a footing resistance of less than 25 ohms, measured in dry soil conditions, especially within ½ mile (0.8 km) of a substation in high Isokeraunic areas. Outside of this distance, a resistance of less than 40 ohms is allowed by some utilities at voltages upto 230 kV. Ground rods or counterpoise wires may be used to obtain a footing resistance below the maximum allowable value. A map of Isokeraunic levels at various locations in the US is given in RUS Bulletin 200.   3.b ) Insulation Requirements Insulators are needed to provide both a mechanical means to hold the line conductors as well as electrical isolation required to withstand impulse voltage events such as lightning impulses and switching impulses. Insulators are basically a physical means of providing an “air gap’’ between an energized conductor and the grounded (earthed) portion of the structure. Toughened glass, porcelain and non-ceramic (polymer) insulators are commonly used on overhead lines. Recommended insulation levels are generally in terms of number of “bells’’ (porcelain) or “sheds’’ (polymer) in an insulator string. The number of bells or sheds also determines the so-called “leakage distance’’ or “dry-arcing distance’’ – the distance measured along the insulating surface from the ground point to energized point. Tables 2.11, 2.12 and 2.13 provide guidelines for suggested minimum insulation levels in suspension and horizontal post insulators (See also Appendix 11 for more insulator data). Additional insulation may be warranted when required due to higher altitudes, contamination and high soil resistance. Other insulation considerations include critical impulse flashover, switching surge factor flashover and strength requirements. For deadends, where the insulator string is in line with the conductor, two extra bells should be provided relative to the tangent string. It must be noted that for voltages up to 230 kV, the most severe stress on insulators is generally due to lightning strikes; therefore, the most important characteristic is the impulse flashover. RUS Bulletin 200 recommendations for insulation levels include both positive and negative flashover values.  3.c ) Conductor Operating Temperature Clearances from energized conductors to ground are generally evaluated at the wire’s specified Maximum Operating Temperature (MOT). ACSR (Aluminum Conductor   Steel Reinforced) wires function well even at an elevated temperature of 100◦C (212◦F) without any significant loss of strength. High performance wires such as ACSS (Aluminum Conductors Steel Supported) can sustain temperatures up to 200◦C (392◦F). ACCR (Aluminum Conductor Composite Reinforced) conductors often used in long span crossings can withstand temperatures over 200◦C (392◦F). Nominal wire clearances to ground are usually given onP&P (Plan and Profile) drawings in terms of MOT. This in turn will help determine the required structure heights for that location. Conductor operating temperature also influences line ratings (see Ampacity subsection below). 3.d) Corona and Field Effect Corona is the ionization of the air that occurs at the surface of the conductor and hardware due to high electric field strength at the surface of the metal. Field effects are the secondary voltages and currents that may be induced in nearby objects. Corona is a function of the voltage of the line, conductor diameter and the condition of the conductor and may also result in radio and television interference, light and ozone production. 3.e ) EMF and Noise Operation of power lines produce electric and magnetic fields commonly referred to as EMF. Noise on transmission lines is also due to the effect of corona. The EMF produced by alternating current in the USA has a frequency of 60 Hz. Electric field strength is directly proportional to the line’s voltage; the higher the voltage, the stronger the electric field. But this field is also inversely proportional to the distance from the conductors. That is, the electric field strength decreases as the distance from the conductor increases.  3.f ) Galloping Galloping is a phenomenon where transmission conductors vibrate with large amplitudes and usually occurs when steady, moderate wind blows over a conductor covered with ice. Ice build-up makes the conductor slightly out-of-round irregular in shape leading to aerodynamic lift and conductor movement. Such movement of conductors results in: (a) contact between phase conductors or between phase conductors and ground wires resulting in electrical short circuits (b) conductor failure at support point due to violent dynamic stress caused by galloping During galloping, conductors oscillate elliptically at frequencies of 1 Hz or less. Shorter spans, usually less than 600 ft (183 m), are anticipated to gallop in a single loop configuration; longer spans are expected to gallop in double loops. Overlapping of these ellipses means possible conductor contact which must be avoided. Therefore, adequate clearance must be maintained between phase wires and between phase and ground wires, to prevent loop contact. Another way to address galloping problems is to adjust the wire tensions to an optimum level so that sag and lateral movement are minimal. Shorter line spans or advanced conductors such as T2 (Twisted Pair) may also help in reducing galloping effects. Anti-galloping devices are also used on existing lines to mitigate galloping issues.  3.g ) Ampacity   Ampacity of a conductor is the maximum current in amperes the wire can carry at its maximum design temperature. This rating is a measure of the conductor’s electrical performance and thermal capability. The maximum conductor design temperature for sags and clearances is also the line’s maximum operating temperature (MOT) for Ampacity. Although the MOT is the primary criteria governing these ratings, wind, ambient temperature and sun (solar heating) conditions are also considered in the calculation of Ampacity. IEEE Standard 738 (2013) provides excellent guidance for determining Ampacity Ratings for conductors. For example, according to RUS Bulletin 200, the Ampacity ratings for Drake ACSR conductor (795 kcmil 26/7), which is very popular in North America, are: 972 Amps (summer) and 1257 Amps (winter), both calculated at an MOT of 212◦F (100◦C) and with a wind speed of 2 fps (0.61 meters/sec) and at ambient temperatures of 104◦F (summer) and 32◦F (winter).

2-) Electrical Design Overhead transmission lines are designed to satisfy a wide range of local and regulatory requirements. While the demands of these requirements vary from location to location and are often line-specific, the underlying principle is one of legality, safety and reliability.   2.a) Regulatory Codes These relate to the local and national standards pertaining to both electrical operations as well as other land use considerations. For example, in areas close to runways in airports, U.S. Federal Aviation Administration (FAA) has laws limiting the height of a transmission structure to 200 feet (61 m). Protected lands such as wetlands do not permit or severely limit construction activities. In some areas, operating high-voltage transmission lines can be hazardous to raptors (eagles, hawks, falcons and other protected species of birds) and in such areas avian protection guidelines are specified. Other land use restrictions include wild life habitats and environmentally-sensitive locations. From a design perspective, regulations refer to the NESC to which all designs adhere to. The other code is the RUS Bulletin 200 which is applicable to all rural highvoltage lines funded by the government. Most states in the USA have adopted NESC as governing code without any changes; but some states require supplemental standards via their administrative codes. In California and Hawaii, for example, electrical design is usually governed by GO-95 (2016) and GO-6 (1969), respectively. Supporting these codes are the various material and structural codes, design manuals and design guides   (steel, concrete, lattice, foundations etc.). Many individual utilities in the US have their own in-house design standards which meet or exceed the NESC standards. Other international codes, standards and manuals will be briefly discussed in Section 2.7. Another special situation is when transmission lines of one utility cross those of another or when transmission lines cross distribution circuits. Wire clearances in these cases must satisfy the applicable regulatory code. Lastly, overhead lines near highway or railroad crossings and underground pipelines in the vicinity of transmission lines are regulated by various state or local laws with special permitting and clearance requirements.  2.b) Right Of Way The transmission line must be designed with adequate right-of-way (ROW) width to provide legal access to line repair and maintenance crews, vegetation management and to facilitate adequate distance from objects nearby. The actual width of line alignment depends on the voltage, number of circuits, type of structure adopted for the line and clearance from various objects required. Figure 2.1 shows the concept of ROW and how it relates to insulator swing and clearance to objects. Right of Way needs (referring to H-Frame type structures and various voltages) are shown in Table 2.5. RUS Bulletin 200 provides a formula for calculating the required ROW width W for a typical pole structure given the required distance from conductors to a nearby object or such as building and other installation under moderate wind conditions. Referring to Figure 2.2:   Sf = Conductor final sag at 60◦F at 6 psf (290 Pa) wind C = Required horizontal clearance between conductors and nearby objects ∅ = conductor/insulator swing angle under 6 psf (290 Pa) wind δ = pole deflection under 6 psf (290 Pa) wind In some cases the structure deflection is very small and negligible, especially for braced H-Frames. In such a case, the equation simplifies to: W = A + 2(Li + Sf )sin ∅ + 2C It must be noted that weather conditions other than moderate 6 psf wind are also often considered. The required horizontal clearance C for objects such as buildings and other installations is provided by NESC. Without wind (i.e.) conductor at rest, the required C values are higher; and therefore, one has to check the ROW for conductor at rest condition also. Note that some utilities allow a buffer of 2 ft (0.61 m) to be added to the required horizontal clearance. Additionally, ROW width is also required to satisfy the horizontal and radial clearances to   Example 2.1 A 69 kV line is planned on an available strip W = 75 ft (22.9 m) wide by an electric utility. The following data is given: Horizontal distance between outermost insulators = A = 9 ft (2.74 m) Suspension Insulator with 4 bells, Length = Li = 2 ft-6 in (0.76 m) Conductor Sag at 60◦F with 6 psf wind = Sf = 5 ft (1.52 m) The swing angle of the insulator under moderate 6 psf wind at 60◦F =∅= 38◦ The required C per internal standards = 10 ft (3.05 m) away. Neglect pole deflections. Determine if the strip provides adequate ROW for the line based on Equation 2.3.   Solution: See Figure 2.2. From Eqn. 2.3, we have the required ROW width as W = 9 + (2)(2.5 + 5)(sin 38◦) + (2)(10) = 38.23 ft (11.7 m) The 75 ft strip is adequate for ROW. 2.c) Clearances Design clearances are one of the primary parameters of a transmission line design process. Clearances are primarily provided to safeguard public for activities reasonably anticipated in the vicinity of a transmission line. In general, the typical clearances required for a transmission line design are: (a) Vertical Clearances of energized conductors above ground and other surfaces (b) Clearance between Wires – Underbuild (c) Clearance to Nearby Objects (d) Clearance to Structure Surface (e) Clearances between Wires – Phase and Ground Wires Figure 2.3 shows the typical situations in which clearances are identified. The NESC and RUS Bulletin 200 provide tables and formulas for minimum recommended clearances in each of the above categories. One of the parameters influencing clearance calculations is the Maximum Phaseto-Ground Operating Voltage (MOVPG) which is a function of Maximum Phase-toPhase Operating Voltage (MOVPP). In general:     Clearances for voltages above 230 kV are more complex to evaluate considering the influence of several extra variables such as switching surge factors and electrostatic effects. For more information on electrostatic effects, the reader is referred to RUS Bulletin 62-4 (1976). Several clearance tables are provided below. In these tables, the voltages are AC and nominal phase-to-phase unless specified otherwise. Altitude correction is not applied to these clearances. Additional buffers are mandatory over the minimum provided in these tables.   Vertical clearance of energized conductors to ground and other surfaces The most important of all clearances is the vertical clearance of an energized conductor from the ground which governs structure heights. For voltages exceeding 98 kV AC to ground, Rule 232D of NESC also provides an alternative procedure for calculating vertical clearances of energized conductors above ground and other surfaces. The clearance is defined basically as a sum of a reference height and an electrical component accounting for maximum switching surge factor, non-standard atmospheric conditions and a margin of safety. This procedure also uses the Maximum Crest-to-Ground Operating Voltage (MOVCG) which is defined as: MOVCG = Sqrt(2) ∗ MOVPG (2.3d)   Tables 2.6a-1 and 2.6a-2 show the typical ground and other vertical clearances recommended by NESC and adopted in the US using Rule 232B and Rule 232C. For comparison, Table 2.6a-3 shows typical ground and other vertical clearances for EHV Lines computed per the alternative procedure (Rule 232D) using switching surge factors shown in the table. Utilities themselves also have their own in-house clearance standards, including buffers (adders), based on experience and consideration of various design and construction uncertainties. These standards meet and often exceed NESC values. Clearance between wires – under-build   Dual-use structures (Figure 2.3b) often contain a distribution under-build or communication wires below the transmission circuits. The clearances between wires of the transmission circuit and those of the under-build are very important in determining the relative heights of the structures. Table 2.6b-1 shows these clearances. This table refers to clearances both at the structure as well as in mid-span and is a modified form of clearances from RUS Bulletin 200. The vertical clearances at structure apply regardless of horizontal separation between transmission and under-build conductors. Minimum recommended vertical clearances within span apply to one of the following conditions which yields the least separation between the upper and lower conductors:  regardless of horizontal separation between transmission and under-build conductors. Minimum recommended vertical clearances within span apply to one of the following conditions which yields the least separation between the upper and lower conductors: a. Upper conductor final sag at 32◦F (0◦C) with no wind and with radial ice thickness as applicable for the particular loading district; b. Upper conductor final sag at 167◦F (75◦C) c. Upper conductor final sag at maximum design temperature with no wind (usually 212◦F or 100◦C for ACSR conductors) The sag of the under-build conductor to be used is the final sag, measured at the same ambient temperature as the upper conductor without electrical and ice loading.   Clearance between wires on different supporting structures   Transmission lines often cross another, usually a higher voltage (upper) line crossing a lower voltage line. The clearances between wires of the upper level circuit and those of the lower level are very important in determining the relative heights of the structures for the upper line. Table 2.6b-2 shows these clearances. This table refers to the situation where higher voltage wires cross lower voltage wires and where the   wires are on different supporting structures. Lower voltage wires crossing over higher voltage wires, though theoretically possible, is not operationally recommended. Note that Table 2.6b-2 is a modified form of RUS clearances from Bulletin 200.   Clearance to nearby objects   Horizontal clearances to various objects are given in Tables 2.6c-1 (modified RUS) and 2.6c-2. The tables provide clearances required for wires at rest and when wires are displaced under 6 psf (290 Pa) wind at 60 deg. F temperature. The horizontal clearance   covers what is commonly known as conductor displacement due to wind. However, the clearances do not cover the blow-out of the conductor (determined with a computer program such as PIS-CADDTM). Vertical clearances to various objects are given in Tables 2.6d-1 (modified RUS) and 2.6d-2. Buffers of 2.0 ft (0.61 m) to 5.0 ft (1.52 m) can be added to various clearances to provide additional safety. These adders are not absolute but are based on the judgement of the engineer. Some utilities in the US are known to use clearance buffers ranging up to 10.0 ft (3.05 m) on EHV lines. Codes in various other countries more or less adopt similar clearances, quantitatively and qualitatively. Operating voltages often differ widely between the North American continent, Europe and Asia; so, any attempt to compare and contrast clearances can only be nominal at an informational level. Clearances between wires – phase and ground wires   The separation between energized conductors of the same circuit and between conductors and ground wires should be adequate to prevent swinging contact or flashovers.   NESC provides guidelines to determine the minimum required phase separation, both at the structure as well as mid-span. Tables 2.7 (modified RUS) and 2.8 show commonly used phase separation on structures. Clearances for circuits 345 kV and above included in Table 2.8 are based on data from actual lines built at those voltages. These values serve only as a guideline; the actual required clearances are a function of several variables including type of structure adopted, span lengths, voltage, terrain and possibility of Aeolian vibration and galloping. Longer spans are often associated with galloping and require larger separation between phase wires and between phase wires and overhead ground wires. Insulator swing Suspension insulator strings are usually free to swing about their points of attachment to the structure. It is, therefore, necessary to ensure that when the insulator strings do swing, minimum clearances are maintained to structure surfaces and guy wires. The amount of swing is a function of conductor tension, wind velocity, insulator weight, line angle etc. The RUS Bulletin 200 provides guidelines for determining minimum swing clearances (i.e.) separation from the structure, both in terms of distance as well   as maximum allowed swing angles. These clearances are generally evaluated at the three weather conditions shown below and are derived from the NESC for conditions (a) and (b). (a) No Wind (b) Light or Moderate Wind (6 psf minimum) (c) High Wind Swing clearance is aimed towards minimizing the possibility of a structure flashover during switching operations. Additionally, it helps protect electrical maintenance workers by providing safety clearances while working on the transmission structures. Table 2.9a shows safe clearances suggested by RUS for voltages up to 230 kV; Table 2.9b gives safe clearances suggested for EHV voltages of 345 and 500 kV. The values also indicate safe distance of energized conductors from guy wires (air gap). The RUS Bulletin 200 also specifies the required horizontal separation between various wires on a transmission structure. This separation is mandated to prevent swinging contacts or flashovers between phases of the same or different circuit.   Elevation effect   The various clearances discussed so far are applicable for line locations at altitudes of 3300 ft or less. For locations at a higher altitude, NESC recommends altitude corrections to be added to the given clearances. These corrections include adding a specified amount of clearance for each 1000 ft in excess of 3300 ft. Example 2.2 For the same data/line of the previous example E2.1, determine horizontal separation required between conductors. Assume Fc = 1.20, Sf = 4.0 ft (1.22 m). Solution: From Eqn. 2.4, we have: H = (0.025)(1.05)(69) + (1.20)(sqrt(4.0)) + (2.50)(sin 38◦) = 5.75 ft (1.75 m) Note: This Sf is defined differently than in Example 2.1.    

The process of designing an overhead transmission line involves various phases or sub-processes. From a Project Management (PM) perspective, there are five (5) main sequential phases as shown in Figure 1.9. This blog is concerned with Detailed Engineering Design, which includes structures and foundations. The basic contents of each of the sub-processes are shown in Figures 1.10, 1.11 and 1.12. The other sub-processes will be discussed in the companion volume. For small projects, a PM-based process is not necessary; however, for large, time- and cost-constrained projects, where several entities and stakeholders interact, a formal PM-based approach is warranted. In such situations, project managers coordinate the design with engineers and serve to optimize costs, schedules and finally quality assurance and control. A brief description of each component of Detailed Engineering Design block is given below. Structure Locations: Before any design takes place, the engineer must finalize the alignment or route of the transmission line taking into consideration the various constraining criteria. These criteria include Right of Way (property boundary) and landowner issues, soil conditions, road and other clearances, cost and regulatory issues and construction access. Potential structure locations are identified and structures are “spotted’’ or placed at these points. Survey coordinates of the points when input to small projects, a PM-based process is not necessary; however, for large, time- and cost-constrained projects, where several entities and stakeholders interact, a formal PM-based approach is warranted. In such situations, project managers coordinate the design with engineers and serve to optimize costs, schedules and finally quality assurance and control. A brief description of each component of Detailed Engineering Design block is given below. Structure Locations: Before any design takes place, the engineer must finalize the alignment or route of the transmission line taking into consideration the various constraining criteria. These criteria include Right of Way (property boundary) and landowner issues, soil conditions, road and other clearances, cost and regulatory issues and construction access. Potential structure locations are identified and structures are “spotted’’ or placed at these points. Survey coordinates of the points when input to PLS-CADDTM will generate a 3-Dimensional Terrain to facilitate graphical assessment of the line. Wire Sags and Tensions: This step involves determining the design tensions of the conductors and ground wires selected. Selection of optimum wire tensions is the most important activity of a transmission line design; the chosen tensions impact structure loads at line angles, sags under various weather situations, which in turn affect clearances, vibrations and forces in guy wires and anchors. Appropriate conductor tensions are also important at locations of line crossings where one line crosses another directly above or below. The sag of the wires and the mandated clearances will govern the eventual heights of the structures. Structure Design: Transmission structural design basically involves ensuring safety and integrity of the system when subjected to various wire and climactic loads. This process must adhere to and satisfy various code and industry regulations defining the performance of the structure. Structural materials may differ but the underlying design concepts are the same. Foundation Design: Each structure must be securely embedded or anchored into the ground and facilitate safe transfer of structure loads to the ground strata below. To determine foundation requirements, the engineer must first evaluate the nature and condition of the soil in the vicinity of the structure as well as the variation of soil with depth and along the alignment. The choice of eventual foundation type will further depend on structure material and configuration and loads. Design Drawings: The framing drawings show the full geometry of the structure, insulator and wire attachment points and heights. Individual hardware units and their sub-components are shown in the assembly drawings. Finally, the Plan and Profile (P & P) drawings constitute the view of the entire line – plan and elevation – and showing all significant features defining the terrain.  

Historically, the term “transmission structures’’ usually implied iron or steel latticed towers. The early “pylons’’ dating back to 1829 were iron structures; the basic shape of later pylons was mostly inspired by the famous Eiffel Tower. Figure 1.1 shows some of the early shapes and forms of transmission structures. Single wood poles directly-embedded into the ground formed the bulk of the transmission structures family for a greater part of the 20th century. France in the late 1900s and later Belgium in 1924 began producing concrete poles. The first steel tubular transmission pole in USA was erected in 1958. Wood H-frames and lattice steel towers became popular later on, dictated mostly by height, availability, strength and urban convenience. Prestressed concrete poles are also used in various places. The world record for the largest transmission structure is now held by China’s 500 kV Yangtze River Crossing double-circuit tower, 1152 ft (351 m) tall, supporting a maximum span of 7667 ft (2337 m) and weighing 8.4 million lbs (3.81 million kgs). Little historical information is available on how foundations were designed for transmission structures in the early days. It is conceivable that some rule of thumb and field tests were used while determining how much a pole needs to be embedded into the ground. One of the earliest discussions on soil behavior in wood H-frames can be traced to 1943 (Hughes Brothers, 1943). Figure 1.2 illustrates the earth pressures below the ground on the legs of an H-frame.  

   Count Alessandro Volta (1745−1827), the Italian physicist and inventor of the battery, was the first person to suggest the idea of a transmission line by writing in 1777 “… the igniting spark could be transported from Como to Milan with barbed wire supported by wooden poles planted here and there …’’ The structures in use those days for telegraph poles were wooden poles with zinc iron barbed wire supported by porcelain insulators fixed to the pole with screws and bolt hooks (TERNA, 2013). The first industrial transmission line ran somewhere between Tivoli and Rome in 1882. The line carried a 5.1 kV single-phase circuit supported by metal fixtures made of double beams, concrete bases and insulators mounted on bolt hooks with wires made of copper. On September 16, 1882, Miesbach in Germany became the starting point for the first long distance transmission of electric power in the world. A 2.4-kilovolt direct current (DC) power transmission line transferred electricity from Miesbach over a distance of 31 miles (50 km) to Munich. However, the first long distance transmission of electrical energy occurred in 1884 during the Turin Expo. A 3 kV single phase current was sent over a 26-mile (42 km) line from Como to Lanzo, Italy. The supports were wooden poles and bell insulators with bronze wires were used. The construction of the first three-phase 12 kV alternating current (AC) overhead transmission line took place in 1891 between Lauffen and Frankfurt, about 112 miles (180 km), coinciding with the International Electricity Exhibition in Frankfurt. Back in Italy, the Tivoli-Rome line was followed in 1898 with a 20-mile (32 km) line between Paderno and Milan: the first 3-phase circuit with metal pylons and delta-type multiple bell insulators with copper wires. In the United States, the first power transmission line operated at 4 kV. It went into operation in June 1889 between Willamette Falls and downtown Portland in Oregon, running about 13 miles (21 km). In 1912, the first 110 kV overhead transmission line was constructed between Croton and Grand Rapids, Michigan. The year 1913 saw the construction of the biggest and longest high-voltage line – the 150 kV Big Creek Line in California – which spanned 250 miles (402 km). The following years witnessed technical advances and rapid developments everywhere. The first 220 kV lines were constructed in Germany and Italy in 1928; by 1936, a 287 kV line was built between Hoover Dam in Nevada and Los Angeles, California. Sweden built the world’s first 380 kV line from Harsprannget to Stockholm, running 596 miles (959 km), in 1953. At the same time, American Electric Power (AEP) constructed the first 345 kV transmission line. In most cases, the average design spans between structures ranged from 1000 to 1500 ft (305 to 457 m); almost all lines used aluminum-steel conductors, bell insulators and latticed steel towers. Hydro-Quebec in 1965 built Canada’s first 735 kV overhead line; soon, Russia and USA built overhead lines at 765 kV – then the largest voltage in the world. A 1200 kV line was commissioned in the Soviet Union (now Russia) in 1982.