The North American Electric Reliability Council (NERC) has a new standard on transmission vegetation management programs (TVMPs), in effect as of April 7, 2006, that is intended to improve the reliability of the electric transmission systems by:

  • Preventing outages from vegetation located on transmission rights-of-way (ROW)
  • Minimizing outages from vegetation adjacent to ROW
  • Maintaining clearances between transmission lines and vegetation on and along transmission ROW
  • Universal reporting of vegetation-related outages of the transmission systems to the respective regional reliability organizations (RROs) and NERC.

The program applies to transmission system owners, RROs, and all transmission lines operated at 200 kV and above and to any lower-voltage lines designated by the RRO as critical to the reliability of the electric system in the region.

Here's how the program works. The transmission owner (TO) must prepare and keep current a formal, documented TVMP. The program must include the TO's objectives, practices, approved procedures and work specifications, and must:

  • Define a schedule for and the type of ROW vegetation management inspections based on anticipated growth of vegetation and any other environmental operational factors that could impact the transmission lines.

  • Identify and document clearances between vegetation and overhead, ungrounded supply conductors, taking into consideration transmission line voltage; the effects of ambient temperature on conductor sag under maximum design loading; and the effects of wind velocities on conductor sway.

  • Create and implement an annual plan for vegetation management work to ensure system reliability.

  • Report to its RRO, or the RRO designee, sustained transmission line outages determined by the TO to have been caused by vegetation. The RRO will then report outage information quarterly to the NERC, as well as any actions taken by the RRO as a result of any reported outages.

For a draft of the entire standard, visit