The National Electrical Safety Code (NESC) governs safety for overhead lines in all states except California. The committee consists of seven subcommittees that have a targeted focus:
SC 1 Definitions
SC 2 Grounding
SC 3 Substations
SC 4 Overhead Lines - Clearances
SC 5 Overhead Lines - Strength & Loading
SC 7 Underground Lines
SC 8 Work Rules
The subcommittees met in Piscataway, New Jersey, IEEE Headquarters, during the weeks of August through October to place final votes on changes for the 2017 edition of the code. A draft will be submitted for letter ballot in January and the final document will be published in August 2016 becoming effective January 2017. Some of the more significant changes related to Strength & Loading are highlighted below.
Over the past 10 years, manufacturers have worked with industry personnel to develop new insulator ratings. A great deal of testing was conducted and the outcomes are significant for certain insulator types:
Line Post Ratings
Previous: Rating was an average value with no insulator testing lower 85% of the average
New: Minimum rating for all insulators
Transmission Suspension Ratings
Previous: 1.2 standard deviations
New: 3.0 standard deviations
To coincide with the new insulator rating practices, the allowable percentages of the strength rating will be adjusted in Table 277-1.
Wire tensions for open supply conductors and overhead shield wires are addressed in Rule 261.H.1. This rule will be rewritten in the 2017 edition and most notably adds the requirement to consider Aeolian vibration. It goes on to define methods for mitigation if analysis indicates a need and sets tension limits when limiting tension is the only method applied to mitigate the issue.
Grades of Construction
There was a concerted effort to improve the readability of the code during this change cycle. One result is a much more intuitive format for Table 242-1, Grades of Construction for conductors and cables.
Changes Not Accepted but Likely to Resurface in the Next Code Cycle
Several Strength & Loading change proposals submitted for the 2017 edition were not accepted in the final voting; largely because analysis or justification was inadequate. However, there are activities within and outside of the NESC committee that are purposed to focus on how to keep the code relevant and useful in the future.
Major Storm Resiliency
The existing electric utility reliability indices measure the frequency, duration and expanse of outages but exclude major storms. Since Superstorm Sandy, the Federal government has created a new metric intended to measure how infrastructure of all types resist a major storm and then recover from that storm. The term resiliency is intended to capture both aspects.
The National Institute of Standards & Technology (NIST) has published a Disaster Resilience Framework Document that addresses infrastructure like water, sewer, transportation, electricity and telecom along with others. The recommendation relating to the energy infrastructure states:
"The baseline design criteria in the NESC and RUS should be increased to provide consistent and unified guidance to all entities designing above these minimums. This increase will ensure all hazards are addressed for the same return period of event."
It is not clear if the Department of Energy or some other agency is intent on increasing loading and strength requirements but the NESC committee is working to establish relationships and communication that will maintain involvement with any such activity.
One of the references in the NIST document pertains to a 60-foot exclusion that exists in the NESC. Criteria for extreme wind events are only applied to electric structures that extend more than 60 feet above ground. The report points out that building, billboards and streetlights are designed to the extreme wind criteria.
However, a strong sentiment exists that wholesale application of extreme wind conditions would not be technically correct nor an efficient method to increase storm resilience for overhead lines. Subcommittee 5 has considered modifying the 60-foot exclusion for several code cycles and has not reached agreement on a suitable solution. The issue will be revisited and analyzed in the next code cycle.
Distribution overhead lines have traditionally been designed using a linear analysis meaning deflection of the structure and the resulting change in applied load is not considered. In most cases, the deflection of the structure is likely to increase the resulting applied load. The impact of deflection increases as pole length increases and heavy equipment attachments are added.
There was a change proposal for the 2017 code to introduce different strength or load factors to account for using non-linear analysis. However, the impacts of the modified proposal turned out to be inconsistent and in conflict with the original intent. Nonetheless, this issue is also likely to be revisited in the next code cycle.
The k-factor was introduced to the code in 1941 when the District Loads were lowered to represent weather conditions that were more typical. The reduced loads also decreased sags and accordingly tensions. Clearances in the NESC had been developed using the previously higher load criteria so they were no longer in "calibration" with the reduced loads.
Clearance requirements in the pre-1941 edition had been based on sags, not the tension of the wire. Therefore a k-factor was introduced to approximate the same resultant loading on the wire as the pre-1941 loadings which in turn created very similar sags as were previously determined.
As subcommittee 5 has considered changing load criteria for strength and loading requirements, the clearance subcommittee chose to adopt the existing District Load map into the clearance section. In this way the loads for computing clearances would remain the same if the District Load map changed in the Strength & Loading sections.
Further, many feel that the original purpose of the k-factor was to calibrate the clearances and the additional tension was not intended for structural loading purposes. A change proposal for the 2017 code eliminated the k-factor in Table 251-1 for these reasons. However, it had not been coordinated with the clearances subcommittee to analyze all potential impacts. This change proposal will likely appear again in the next code cycle.
As you can see, there will be a lot of activity focused on continuous improvement of the NESC. Public input, comments and participation are encouraged. To learn more about the IEEE, NESC and possibilities to participate, visit the NESC Zone of the IEEE website at: http://standards.ieee.org/about/nesc/erp/form.html.