Routine Tree Maintenance Keeps Power Lines Clear

Preventing Outages

Trees and other vegetation cause about 20% of all electric service interruptions. Preventive pruning around aerial power lines is vital to providing reliable electric service.

Our vegetation management crews are trained in proper arboricultural pruning techniques, which meet the standards set by the American National Standards Institute (ANSI). These techniques take into account the species and location of the tree in relation to the overhead wires and the health of the tree.

Safety First!

Trimming trees near power lines is dangerous. ComEd’s contractors are specially trained to work in close proximity to electrical conductors. Please contact us when planning to do any tree trimming work near electrical conductors and we will advise you of options to work safely. Additional information is located on the Tree Trimming tab.

Right-of-Way Maintenance

ComEd maintains two distinct types of electric rights-of-way. Electric transmission rights-of-way carry power from power generation stations to substations; electric distribution rights-of-way carry power from substations to customers.

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Scheduled Maintenance

ComEd performs routine tree and vegetation maintenance on regular cycles. These cycles are four years in length. ComEd also performs some mid-cycle work where trees are especially fast growing, or there are other problems that may result in interruptions to electric service.

Electric utility pruning is often perceived differently from other types of pruning because the objectives are different. However, when pruning trees in urban and suburban environments, the tools and methods used are similar to those used for other pruning purposes. Likewise, utility arborists are expected to adhere to the same professional standards as other arborists.

Professional Standards

ComEd employs professional contractors to perform its line clearance work. The work is performed in accordance with American National Standards Institute (ANSI) Standard A300, Part 1: Tree, Shrub, and Other Woody Plant Maintenance – Standard Practices, Pruning.

The type of tree pruning used by ComEd is called directional pruning (or natural target pruning). This method is designed to encourage the tree to grow away from the electric lines and has been adopted as a national standard (ANSI A300, Part 1) and the International Society of Arboriculture as a Best Management Practice. Directional pruning allows ComEd to take characteristics of each tree into consideration when determining the extent of pruning needed. The pruning clearances to which we hold our contractors are based on the growth rate and mature size and shape of each tree, the location of the tree in relation to the power line, the type of utility facility, and a cycle length of 4 years.

ComEd has been recognized for 13 years as a Tree Line USA utility by the Arbor Day Foundation for our sustained commitment to proper tree care and maintenance while continuing to meet service objectives. The requirements for the award include training workers in quality tree-care practices, educating the public about planting trees for energy conservation and helping customers to plant appropriate trees near utility lines.

Pruning Technique

CORRECT: The diagram above shows how directional pruning guides the growth of the tree away from the wires.

INCORRECT: The diagram above shows how “topping” or “rounding over” of trees can cause a flush of fast growing sprouts that grow directly back into the wires. Find out further information about how to avoid tree and utility conflicts by visiting the International Society of Arboricultures’ Website. Additional information about reducing tree and utility conflicts is available on the Arbor Day Foundation Website.

Transmission Right-of-Ways

Powering lives begins with a journey that transports high-voltage electricity from power generation stations, to substations that “step down” the voltage to a level that can be distributed to your home. ComEd maintains more than 5,400 miles of high-voltage transmission lines that range from 69,000 volts to 765,000 volts.

Transmission rights-of-way (ROW) are typically identified by large steel poles or tower structures that are used to transport high-voltage electricity across large distances. ComEd owns or has express property rights to perform vegetation management in transmission ROWs in our service territory and uses a technique called integrated vegetation management to make sure that trees and other vegetation never cause an interruption to the system. View the EPA's Fact Sheet on Integrated Vegetation Management (pdf).

Federal Mandates

Federal standards mandate that utilities have a transmission vegetation management program to prevent widespread outages on the transmission system. The North American Electric Reliability Corporation ( strictly enforces these standards. Failure to meet a vegetation-related, zero-outage mandate can lead to fines of up to $1 million per day for a utility. The Federal Energy Regulatory Commission (FERC) has specific guidelines and requirements that electric utilities must meet. Visit the FERC Website for more information.

Acceptable Vegetation

ComEd does not allow tall-growing trees or vegetation on its electric transmission ROWs’ Wire Zones or Border Zones; and ComEd will not prune trees or vegetation to maintain their proper height. All such vegetation is removed.

Power outages on electric transmission lines can have far-reaching consequences, such as the loss of power to many thousands of customers, or even regional power blackouts affecting millions. In August 2003, more than 50 million people in the Midwest, Northeast and Ontario, Canada, were affected by a blackout that began when trees came in contact with transmission lines.

It is recommended that you contact ComEd before planting anything other than grass in an electric transmission right-of-way. ComEd encourages the establishment of native prairie grasses and flowers in a ROW's Wire and Border Zones.