Utilities Address Common Problem: Mid-Span Vertical Clearance Issues

Following the NERC alert of 2010, many utilities were left with the gargantuan task of addressing excessive slack in transmission line spans that do not meet the National Electric Safety Code requirements for ground clearance. Surveys were completed in the years prior to the NERC Alert, and measures were taken to “Make Safe” and fence off mid-span clearance issues while planning and construction put their heads together to address hundreds if not thousands of line spans that were in violation. The common solution: The Nip & Tuck.

The Nip: The first step removes a section of conductor in order to shorten the length of conductor in a given span. This increases the tension in the span which improves the conductor-to-ground clearance in the given span.

The Tuck: The second step slides a length of conductor from one span to another. This shifts the amount of slack from one span to another. Improves clearance in one span at the sacrifice of another span.

A good example includes a 765kV Transmission line span approximately 1250 feet in length between two 130 foot tall box lattice towers that violates the NESC code by just over 6" or half a foot. Simply removing 3 feet of conductor from the belly of the span via the Nip & Tuck method resolves the issue.

What is Mid-Span Vertical Clearance?

NESC requirements state the following clearance standards for available clear area under a drop cable at mid-span, where the cable’s swag is deepest and it hangs the lowest:

• 15.5 feet over roads, streets, alleys, parking lots, fields and other areas subject to traffic that may be more than 8 feet tall—primarily trucks.
• 11.5 or 15.5 feet over residential driveways, depending on the height of its attachment to a building.
• 23.5 feet over railroad tracks
• 9.5 feet over sidewalks and other pedestrian-only traffic areas.

Why Fix the Mid-Span Vertical Clearance Problems?

Mid-span clearance issues pose a significant danger to system reliability, but even more importantly, to the public.  If not remedied, these conditions in violation of the NESC or flirting with violating it may create unnecessary costly repairs, along with foreseen and predictable costly liability issues should human life be lost or injured as a result of a low-hanging transmission lines.