Clearance Adherence: Avoiding Mid-span Violations
In the world of distribution engineering, utilities – the owners of distribution poles – are required to comply with National Electrical Safety Code (NESC) guidelines for clearance requirements, both at the pole as well as at the mid-span. Non-compliance with these guidelines can create safety hazards, the risk of liability, and expensive repair issues.
The most common NESC violation is a mid-span clearance violation. Ensuring that clearance requirements are met is a fundamental consideration when developing designs for distribution engineering. Horizontal clearance requirements are important, but vertical clearances are equally important to maintain compliance and public safety. Clearances at the pole are an obvious metric – either you meet the requirement or not. Mid-span clearances, however, can become a guessing game without proper analysis, especially when there are communication attachments to consider in the design.
Mid-span clearances are the most common consideration to be designed incorrectly – either knowingly or unknowingly.
With the increasing rollout of fiber/broadband across the country, utilities are seeing an uptick in third party applications to attach to distribution poles. States governed by the FCC have a 45-day shot clock to process the application, survey, and deliver the make ready required for the applicant to attach their devices. A 45-day time frame for large applications can be daunting, due to the engineering analysis required to determine what, if any, work needs to be done in order for the third party to attach without any violations.
Two different categories should be analyzed when evaluating mid-span clearances: mid-span clearance between attachments, and ground clearance from the lowest attachment. These considerations can, however, create competing constraints. The first consideration ensures that the third party attachments maintain proper clearances from each other as well as the electrical supply space at the mid-span. The second consideration ensures that the lowest attachment maintains required clearance to the ground. A solution that mitigates one of these constraints might cause a violation in the other. Additionally, in order to properly meet mid-span clearance requirements, multiple cases need to be analyzed. For example, the table below illustrates the weather conditions in NESC Clearance Zone 1 for a clearance analysis according to rule 230B.
|Figure||Design Consideration||NESC Rule||Secondary / Neutral||Communication Cable|
|A||Mid-span clearance between attachments||235C2b(1)c||
32 degrees F, plus 1/2" ice
|32 degrees F, no ice|
|B||Ground clearance from lowest attachment||232A1||n/a||32 degrees F, plus 1/2" ice|
The challenge associated with the above scenarios is that communication sag data is unavailable and unknown. Without this information, a design might be executed without all of the needed information, which will adversely impact clearance. If the wrong values are compared, this can result in an analysis that falsely shows no violations. The following diagrams show the two scenarios to watch out for.
Sixty degrees Fahrenheit was selected as an arbitrary ambient temperature as it doesn’t represent an extreme case in either hot or cold temperatures. In both cases, existing conditions were considered rather than worst case scenarios.
Why are NESC rules 235C2b(1)c and 232A1 not always included in a mid-span clearance evaluation?
- There is immense pressure to meet high volume and shot clock expectations. It’s easier and quicker to design based off of existing values rather than consider worst case scenarios outlined above.
- When considered with all of the information needed to create an informed design, mid-span clearance analysis may result in substantially more required pole replacements, which is a financial deterrent.
How can designs be executed properly?
- When trying to confirm clearances in Figure A above, use the line of sight method. If the communication cable is treated like a straight line between attachment points, that will ensure that clearances will be met. This is a very conservative method, and likely more than needed on long spans where one would expect a communication cable to sag.
- When trying to confirm ground clearance of an existing attachment, consider measured values as well as catenary curve behavior in the engineering analysis. Input values like measured sag as well as the ambient temperature into a suitable software to back your way into a sag table for that cable.
The ultimate goal of any engineering design is accuracy, safety, and compliance. In the case of mid-span clearances, with a little bit more information and attention to how worst case scenarios can affect the design, accuracy can be increased, safety can be enhanced, and compliance with NESC guidelines can be improved.
With more than 70 years of experience in the utility industry, Leidos has a proven history of helping its clients successfully assess their readiness to support regulations, requirements, and industry guidelines. Leidos helps utilities develop and implement solutions to their unique challenges by evaluating existing systems, assessing where improvements need to be made, and developing solutions that are uniquely tailored to meeting the challenges that each utility faces. Learn more about our power delivery services.