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Managing power line hazards during harvest

It is fair to say that trees and power lines are not particularly compatible. Vegetation, including tree crops, that grow too close to power lines presents a hazard to power lines that can interrupt electricity supply (for example through trees touching or falling on lines). Conversely, providing a safe set back from the lines is seen as a significant cost borne by the land owner through a reduction in productive land area.

A discussion regarding the respective rights of lines owners and land/forest owners is one for another day. The scope of this discussion is focused on the planning and safe harvesting of mature tree crops within the power line hazard zone.

So why is this important?

  1. Logging of trees within close proximity of power lines presents a potentially lethal electrical hazard unless the power has been turned off.

  2. The scale and extent of the problem is sizable. If you look closely around the rural environment you will notice just how common it is for power lines to pass through or alongside forests and woodlots (or other wooded areas). PF Olsen has numerous clients, ranging from small woodlot holders to corporate forest owners that have power lines running through their production forest. To reinforce this point, PF Olsen is currently managing 12 operations in the Central North Island (either under way or in planning) that are affected by power lines.

  3. There is a legitimate expectation from both the forest owner (who wants to realise the full value of their tree crop) and power line owner (who desires the line hazard removed) for these trees to be harvested. Leaving a strip of edge trees along a power line corridor, in the too hard basket not only reduces the forest owner's return but leaves a potential wind-throw hazard.

Aerial photo showing powerline corridors through and adjoining forest plantations

Given the risk of a potentially fatal electrical shock from a line strike, providing the safest possible conditions should ideally involve eliminating the electrical hazard by shutting the power down during tree felling. However, lines companies/operators are often limited in their ability to turn off the power due to their own operating constraints. When the power cannot be easily turned off forest managers will typically resort to felling adjacent to the live lines under 'auto-reclose block (RCB)', often referred to as 'single shot relay', i.e. control the hazard

This is a valid approach under the various codes however an RCB is only designed to prevent automatic switches from re-livening a line should something strike it. The RCB will limit the number of electric shocks but not prevent the initial shock from happening.

So what can be done to plan the safe felling of the power-line corridor? First and foremost, it is important to understand the relevant codes of practice that relate to working around power (live) lines. These include:

  • The Approved Code of Practice for Safety and Health in Forest Operations (ACOP)
  • Approved Code of Practice for Safety and Health in Tree Work – Part Two: Maintenance or removal of trees around power lines
  • The Guide to Electrical Safety for Forest and Woodlot Felling and Logging Operations published by the Electrical Engineers' Association
  • NZECP 34:2001 – New Zealand Code of Practice for Electrical Safe Distances

The ACOP provides an overarching management framework and states that a felling plan shall be agreed between the line/network owner, the forest owner/manager and the contractor for trees within two tree lengths of the power lines.

The other codes referenced by the ACOP provide specific details regarding safe work around power lines and the minimum approach distances necessary to plan felling operations.

So what has PF Olsen been doing?

A number of projects recently completed by PF Olsen have highlighted the importance of early liaison with lines companies. We have found that face to face discussions with lines operators at the earliest possible stages in the planning process results in better understanding of the particular risks and issues for individual sites. Our initial discussions with lines companies are focused on whether or not a full line shut down can be secured. This is not always possible but where possible eliminating the live line hazard simplifies subsequent planning decisions.

In next month's Wood Matters we will outline case studies of managing difficult power line situations.