Incidental Line Clearance: What Arborists Need to Know About Working Around Electric Utilities

Electricity is the only product that is consumed the moment it is created, from the point of generation to end users, through a complex grid of towers, poles, wires and various types of hardware. The vast majority of the grid is above ground, where it often shares space with trees as well as the people who must maintain both the trees and this infrastructure.

The presence of utility infrastructure in and around the crowns of trees presents a real hazard to arborists. ANSI Z133 for Arboricultural Operation – Safety Requirements lays out rules for arborists with various levels of training for working in proximity to the electrical hazard. This article summarizes some of what incidental line-clearance arborists should know about electricity and the grid. The information presented here should be considered supplemental, and in no way should substitute for the training required to work in proximity to energized conductors.

Open land with electricity transmission lines
Photo 1: High-voltage transmission lines transmit large amounts of electricity over longer distances. Unless otherwise noted, all photos and graphics courtesy of the authors.

Electricity basics

Electricity is transmitted at various voltages. High-voltage transmission lines (generally above 70,000 volts, Photo 1) are used to move electricity over long distances, say, from a wind farm in a rural area to a city. There, voltages are stepped down to primary voltages (3,000 to 33,000 volts) to distribute electricity through neighborhoods. Finally, primary voltage is stepped down to secondary voltages appropriate for individual homes and businesses, generally 440 volts or less.

An electrical charge is essentially a temporary imbalance that is measured as voltage, and is resolved when the electricity is put to work or the charge goes to ground. This is why the hot side of the electrical system – basically the wires – must always be isolated from the surrounding environment. At household voltage, this isolation can consist of an electric cord with a plastic coating. With higher voltages, such coatings are impractical. At the top of a utility pole, bare primary wires are attached using ceramic insulators, which maintain separation between the hot wires and nearby objects. Higher voltages require greater separation and larger, more elaborate insulators (Photos 2 and 3).

Power lines with ceramic insulation.
Photo 2: The energized or “hot” portion of the electric grid is isolated from the rest of the system and the surrounding environment with ceramic insulators. Wires are copper or aluminum, and are not coated with insulation.
Grey clouds with high voltage wires
Photo 3: Higher voltage requires greater separation and larger insulators.

Electricity Seeks a path to ground

As electricity seeks a path to ground, it can be thought of as being under pressure. The higher the pressure, the higher the voltage. Using a water tank as an analogy (Figure 1), consider the pressure in the tank as comparable to voltage. The size of the pipe leading out of the tank is akin to amps (capacity of the conductor). The amount of water that goes through the pipe in a given time is like watts (amount of electricity used). And finally, the work performed, say, running a grist mill (or, in the case of electricity, lights glowing, blenders blending, etc.) is resistance, measured as ohms. Table 1 shows the mathematical relationship between these functions.

A drawing in blue depicting the flow of electricity
Figure 1: A water model can be analogous to the flow of electricity. Voltage can be thought of as pressure, amperage is the diameter of the pipe, wattage is flow rate and ohms (resistance) is work done.

To put things in perspective, a 1.5-volt AA battery is all that is needed to power an electric toothbrush. A gas-powered car runs its electrical system – the starter, lights, wipers, sound system, computers, etc. – on a 12-volt system. In North America, service to individual homes is usually delivered at 240 volts, with 120 volts used in most circuits. Mass-transit rail systems run on 600 to 750 volts. These are the voltage ranges we typically see operating on a daily basis, and these voltages can certainly be dangerous. The voltages in utility transmission and distribution systems are far greater.

Table 1: There is a mathematical relationship between volts, amps, watts and ohms.

Risk to arborists

Good conductors, such as copper or aluminum wire, offer very little resistance and are not damaged by electrical flow. Other materials, like the filament in an incandescent light bulb or the element in an electric stove, are designed to offer a certain level of resistance and give off light or heat. It should be clear that the human body is conductive, offers resistance and will heat up and be severely burned by the flow of electricity.

Primary-line voltages atop utility poles are often greater than household current by 100 times or more. This distribution system, running along city streets, rural roads and in people’s backyards, is where trees and wires most commonly come into contact and where arborists are most likely to encounter danger. Understanding this risk and knowing how to operate safely in proximity to these hazards is essential.

Grey clouds with high voltage wires
Photo 3: Higher voltage requires greater separation and larger insulators.

Know your hardware

Working safely around electrical infrastructure requires basic knowledge of electrical hardware and its function. It should be noted that electric utilities are increasingly adding technology and hardware to improve the quality of service and “harden” the grid against service interruptions. Describing the functions of these various types of hardware and how to identify them is beyond the scope of this article. However, it should be understood that some of this hardware retains a charge and can be dangerous, even when the power is out or if it is knocked to the ground in a storm.

In general, higher voltages are placed higher on the utility poles – for example, primary lines at the top – with secondary and service lines (which carry power to individual customers) lower down on the poles. Transformers step down voltage from primary to secondary levels. In most systems, there is a neutral wire between the primary and secondary lines, which completes the circuit. Communication lines (telephone, cable, etc.) are usually mounted lowest. Z133 requires that all of these items be regarded as energized with potentially lethal voltages.

However, some systems, especially in urban environments, are more complex. (Photo 4) In this case, multiple sets of primary lines are mounted on tall poles. Transformers step voltage down to secondary levels for use in street and traffic lighting and for individual homes and businesses. At the very bottom are the communication lines. There may be multiple agencies – in this case, the local electric utility, a municipality and cable and internet-service providers – who are responsible for the hardware at various levels on the utility pole. With so many lines and service providers on the same pole, identifying the nominal voltage of each becomes problematic.

What does the Z say?

Employer and employee responsibility

When it comes to electrical safety, ANSI Z133 lays out responsibilities for everyone, and makes it clear that the amount of training provided should reflect the amount of risk based on the hazards present. And those hazards are plentiful! Employers should be keenly aware of requirements laid out by OSHA and Z133, and should design their training programs accordingly.

Some important highlights (for more detail, consult ANSI Z133) include:

  • The human body is conductive and provides electricity with a path to ground.
  • Contact with an energized object, including electrical hardware, a tree branch, tool or equipment, may lead to shock, injury or death.
  • Trees and tree parts are conductive.
  • When live lines are grounded, injury or death may occur from ground shock or step potential.
  • Employees without adequate training may not approach within 10 feet of an energized conductor (these approach distances increase above 50,000 volts).
  • Minimum approach distance (MAD) includes not just the employee’s body, but their tools, clothing and equipment as well.
  • If MAD cannot be maintained, work must stop until the system operator/utility de-energizes, grounds and tests the lines.

Incidental line clearance

Incidental line clearance includes work in proximity to electrical hazards when the arborist is not employed by or working under contract to a utility-system operator. This typically includes municipal and commercial arborists working where power lines are present, but the objectives do not include clearing vegetation for the power company.

In addition to the list above, incidental line-clearance arborists must be able to recognize exposed energized parts, the nominal voltage to which they are exposed and the MAD that must be maintained for that voltage. However, while it is possible to recognize what is energized, the facilities are not labeled with the voltage. It may be true that much of the system runs on a certain voltage, but many utilities have circuits with different voltages or different primary voltage on the same pole or, as noted above, share their poles with other utility providers. (Photo 4) The only way to be certain about voltage in a given location is to contact the utility-system operator.

trees in utility lines
Photo 4: In urban environments, there may be multiple levels of primary and secondary lines with different voltages mounted on the same pole, sometimes with separate system operators.

Understanding and respecting the risk

Perhaps even more important is understanding and respecting the risk and the reasons for having MAD in the first place (see sidebar). When an arborist is working in a tree with power lines running through it, there is plenty that can go wrong. The wind might pick up. The rope could shift. The arborist might lose their footing and swing closer to the line. The arborist’s lanyard or saw might unexpectedly move toward the line. Any of these could result in violating MAD and instant catastrophe. To prevent this, margins of safety and situational awareness must be built into the operation.

Arborists should consider why they need to be incidental at all, and where the majority of their work is to be performed. If the incidental work consists of working only downstream from the transformer at secondary voltages (<300v), then the incidental arborist can “avoid contact” for those lines and maintain a full 10 feet from primary voltages.

A cautionary note: The term “avoid contact” could be interpreted to imply that even millimeters of distance are OK, as long as contact is avoided. Remember that secondary voltage is powering every electric appliance in the houses served by that line. Stoves, air conditioners, dryers, sound systems – everything. Secondary lines are mounted lower on the pole, below transformers, and are the most likely to be encountered when doing routine arboricultural work. Some secondary lines and services are coated with insulation; however, many are bare, “open wire.” (Photo 5) Secondary voltage can kill and should be treated with respect.

Blue sky with telephone pole and power lines
Photo 5: Secondary lines are attached below the transformer. Some are coated with insulation, but, as shown here, many are bare wires.

Utility line clearance

When the arborist is working on behalf of the electric utility, employers are explicitly required to verify that every arborist is qualified, and that they have received specific training. This exceeds requirements for incidental line-clearance arborists.

In addition, different rules apply per OSHA’s 29 CFR 1910.269. As noted earlier in this article, MAD distances are slightly less for utility line clearance. However, some rules are more stringent; for example, a second qualified line-clearance arborist must be on site at all times. Considering the risk, this does not seem unreasonable; indeed, it has been a requirement through several revisions of Z133. It does beg the question, why would this requirement not apply to incidental line-clearance arborists, who in general have less experience?

Interestingly, a fully qualified utility line-clearance arborist with years of experience becomes, by definition, incidental, and must maintain incidental MAD as soon as they are given a non-utility line-clearance work assignment. Incidental line clearance is not about the experience of the worker, but rather the nature of the job assignment.


This article only touches on the details provided in the 2017 ANSI Z133, which devotes six pages to Section 4, Electrical Hazards. The standard leaves no doubt that there is plenty to be concerned about when working around utility facilities. And clearly there are some inconsistencies and ongoing concerns about incidental line clearance. There is a revision of Z133 due this year, and, as always, we can expect changes and clarifications, including in the Electrical Hazards section.

Electricity is an ever-present danger, silent, swift and often hidden in the crowns of trees. Every arborist, whether nonqualified, incidental or utility line clearance, must take the electrical hazard seriously.

Comparison of MAD for Incidental and Line-Clearance Arborists

Considering that utility line-clearance arborists spend most of their time working in proximity to energized conductors and receive more training than incidental line-clearance arborists, the differences between the corresponding MAD is surprisingly small.

MAD for incidental is derived from OSHA General Industry, whereas MAD for utility line clearance is from clause 1910.269, which applies only to those directly employed in electric-
power generation, transmission and distribution.

A comparison of the MAD distances for incidental line-clearance arborists and utility line clearance are shown in Table 2. Note that MAD distances for typical distribution primary voltages differ by just six to nine inches. This is far less than the mandatory 10 feet for non-qualified personnel.

Furthermore, arborists must estimate the distance they and their gear are from conductors – they obviously cannot violate MAD to check with a measuring device. At transmission voltages (above 72,500 volts), where incidental work would be exceedingly rare, MAD is much greater for incidental line-clearance arborists.

All arborists working in proximity to energized conductors, whether non-qualified, incidental or line clearance, must be continuously aware of the location of energized parts of the system, and must take MAD very seriously.

A comparison table of data
Table 2: A comparison of MAD distances for incidental line clearance and utility line-clearance arborists at typical distribution and secondary voltages. At transmission voltages, MAD is increased significantly for incidental work.

Additional information

ANSI Z133, Safety Requirements for Arboricultural Operations, International Society of Arboriculture, Atlanta, GA.

Utility Arboriculture, The Utility Specialist Certification Study Guide, International Society of Arboriculture, Atlanta, GA.

Geoffrey Kempter is a technical-services manager with Asplundh Tree Expert LLC, a 48-year TCIA member company based in Willow Grove, Pennsylvania. He was the chair of the 2017 ASC A300 Pruning Revision Subgroup. He has served as the Asplundh representative on the ASC A300 Committee since 1996, and was the recipient of TCIA’s 2018 Pat Felix Volunteer of the Year Award.

Stephen Hilbert also is a manager of technical services for Asplundh Tree Expert LLC, and has more than 20 years’ experience as an arborist.  He serves on various industry boards and is the voting member for Asplundh on the ANSI A300 committee.

1 Comment

  1. I’m so glad Husqvarna developed and launched this year a MAD SAW meeting OSHA 1910.269 Standards.

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