These loading zones are designated as Heavy (Zone 1), Medium (Zone 2), and Light (Zone 3). Kentucky is in the medium loading zone. Utilities that construct overhead facili- ties in Kentucky are required to build those facilities to meet the minimum requirements of medium loading construction as specified in the NESC. Indiana and some portions of Ohio are located in the heavy loading zone. In Kentucky, current NESC medium loading and Class B construction standards apply to all structures at the time of design. Some utili- ties construct to a heavier loading standard in order to improve reliability and durability of their facilities and structures.
According to data provided by several utili- ties, the cost of construction to meet the heavy standards ranges from 10 percent to 30 percent higher than the cost of construct- ing comparable facilities to a medium loading standard.
The NESC serves as the basis for the mini- mum mechanical strength and electrical clearance requirements for the design of new distribution line construction and the minimum strength limitations for in-service distribution lines. For design purposes the NESC re- quires poles, crossarms, pins, insulators and conductor fastenings to bear their own weight plus the weight that they support, including all conductors and cables, as well as loading due to radial accumulation of ice.
Figure 15: NESC loading zones
Example: Heavy Loading (Zone 1): ½ inch of ice, 40 mph horizontal wind, and 0° F. Example: Medium Loading (Zone 2): ¼ inch of ice, 40 mph horizontal wind, and + 15° F.
Ice is assumed to weigh 57 pounds per cubic foot. In addition to the radial ice, for heavy and medium loading, the poles, crossarms, pins, insulators and conductor fastenings must also have sufficient strength to support the force exerted by a 4 pound-per-square- foot wind (approximately 40 miles per hour) blowing perpendicular to the conductors and cables. In determining the required structural component strengths, the appropriate
strength and load factors are applied to the calculated loadings according to the type of structural component and the required grade of construction.
Utilities have reported that during major storms the mechanical loading experienced from tree contact is the primary cause of the majority of outages and damage to distribu- tion and transmission lines. That analysis was
not altered by the Hurricane Ike wind storm of 2008 and the 2009 ice storm. In both those events, utilities reported that tree contact with lines is the major cause of customer outages and the major contributor to damage to the distribution and transmission lines, regardless of wind and ice loadings on structures.
To illustrate how application of loading criteria affects structure design, consider the follow- ing example, which uses two common distri- bution line pole/conductor combinations: a single-phase tangent pole (35 ft/class 5) with #2 Aluminum Conductor Steel Reinforced (ACSR) phase and neutral conductors at maximum allowed tension (60% of rated breaking strength), and a three-phase tan- gent pole (35 ft/class 5) with #1/0 ACSR phase and neutral conductors also at maxi- mum allowed tension.
Figure 16: Single-phase and three-phase construction
The term “single-phase” refers to a type of construction on a distribution circuit. A single-phase con- struction consists of two conductors: a primary phase conductor and a neutral conductor. “Multiphase” or “Three-phase” construction consists of 2 or 3 primary phase conductors and a neutral conductor. Some three-phase systems may or may not have a neutral wire.
Using the medium ice and wind loading re- quirements, the maximum allowable horizon- tal span (based on pole strength) would be 1,162 ft. for the single-phase example, and 470 ft. for the three-phase example. For the corresponding examples using heavy loading requirements, the results would be 720 ft. for the single-phase example, and 302 ft. for the three-phase example. Thus, the pole spacing requirements and resulting span lengths are roughly one-third more stringent for the heavy loading zone than for the medium zone.
As shown in Figure 17 below, below, the NESC imposes a further requirement for ex- treme ice with concurrent wind pressure load- ing for structures exceeding 60 feet in height. This requirement varies with location. For most of Kentucky, it requires that structures be built to withstand 0.75 inch radial ice thick- ness loading with 2.3 lb/sq.ft. horizontal wind pressure for the majority of the state. A 0.5 inch radial ice thickness standard is in place for the extreme eastern counties, while a 1.0 inch radial ice thickness is assumed for the extreme western counties. This requirement applies primarily to transmission structures.
Figure 17: NESC Figure 250-3(b) - extreme ice loading
Example: Medium Loading (Zone 2) for structures above sixty feet could be shown as 3/4 inch of ice, 30 mph horizontal wind, and + 15° F.
Most jurisdictional electric utilities facilities are constructed specifically to meet medium load- ing standards as required by the NESC. How- ever, Kentucky Utilities Co. (KU), Louisville Gas & Electric Co. (LG&E), Duke Energy Kentucky (Duke Kentucky), and Kentucky Power have developed their own construction standards that meet or exceed the NESC. The rural electric cooperatives build their fa- cilities to meet or exceed the NESC and RUS construction standards. However, there are design requirements applicable to these con- struction standards that actually produce fa- cilities which meet many, but not all, aspects of the heavy loading standards, depending on the circumstances under which they are con- structed. For example, poles, spacing of poles, crossarms, guying, and other material used during the construction could meet the heavy loading requirements. However, be- cause distance-to-ground (sag) specifications vary, utilities may not be able to meet that aspect of heavy loading standards while meeting it in all other respects.
The six jurisdictional utilities with transmission facilities – Big Rivers Electric Corp. (BREC), Duke Kentucky, East Kentucky Power Coop- erative (EKPC), Kentucky Power, KU, and LG&E – all have a mix of transmission lines meeting either the medium or heavy loading standards. In recent years, most of these utili- ties have decided to build their transmission lines above 69 kilovolts to meet the heavy loading requirements. Nonetheless, severe wind loading and mechanical loading from trees contacting conductors caused conduc- tor or structure failures during the 2008 wind storm.
Kenergy Corp., a distribution cooperative, util- izes the heavy loading standard for all feed- ers and major three-phase extensions. The medium loading standard is utilized for single phase construction and individual extensions. Kenergy observed no difference between the damage done to facilities constructed under the medium loading standard, as opposed to facilities constructed to meet the heavy load-
ing standard, in either the Hurricane Ike wind storm or the 2009 ice storm. It appears that within Kenergy’s service area, both these storms were so severe that the loading condi- tions exceeded those anticipated even under the most stringent construction standards, leading to massive structural failure through- out the system.
In the areas hardest hit by the ice storm – western and central Kentucky – the LG&E and KU transmission systems were built prior to 2003 and were designed to meet the NESC medium loading standard. These transmission systems suffered several struc- ture failures due to extreme weight from ex- cessive ice loading. Since the 2003 ice storm, KU and LG&E have revised their transmis- sion system construction standards for new construction and line upgrades and are meet- ing or exceeding the heavy loading require- ments of the NESC.
Due to its experience in the 2003 ice storm, Fleming-Mason Energy now constructs all new facilities to meet the NESC’s heavy load- ing requirements, and upgrades to heavy loading requirements when replacing older facilities. During the 2009 ice storm, most damage to the Fleming-Mason Energy sys- tem was due to ice loading on older conduc- tors. Fleming-Mason Energy is continuing to re-conductor the system to eliminate aged copper and aluminum conductors. The poles that were broken on the system were mostly older poles that failed due to longer span lengths. The three-phase circuits built for both heavy and medium loading performed well due to shorter span lengths.
In assessing whether the use of more strin- gent loading standards to govern electric sys- tem construction in Kentucky would have miti- gated the damage from the two storms, the PSC reviewed the extent of damage in Indi- ana and Ohio. Both states experienced sig- nificant accumulations of ice during the 2009 storm, and the 2008 wind storm affected most of Ohio. Even though utilities in those states construct their facilities to the heavy loading
standard, both states suffered significant out- ages during the ice storm. Outages in Ohio as a result of the wind storm were more ex- tensive than in Kentucky.
The PSC finds that most utility facilities constructed to both the medium and heavy standards simply could not with- stand the physical stresses placed upon them by both the weather conditions and the attendant loadings from falling trees and limbs during the wind storm and ice storm. The PSC does not believe that Ken- tucky should be placed into the heavy loading zone in the NESC.
However, construction to heavy loading standards, rather than the medium loading standard required in Kentucky, appears to have improved system durability in some instances.
Therefore, the PSC recommends that juris- dictional utilities should consider upgrad- ing to heavy loading standards in some circumstances. For example, it may be beneficial to shorten span lengths when building lines in treed areas, thus improv- ing the ability of those lines to sustain the weight of fallen vegetation.