Exposure to electric current is one of the many hazard sources during arboricultural operations. But electricity is one of the most unforgiving of these sources. A review of utility data shows that one in four contacts that resulted in an injury to an arborist working near overhead power lines was fatal. While contact with electrical current is the number-three fatal incident – behind falls and struck by a falling tree – the outcome of this contact is often death.
Electric shock is defined as the physiological reaction to the passage of electric current through the human body. There are several parts of this definition that arborists need to pay close attention to. First, it is the current, not the voltage, which causes the injury. Second, the current passes through the body, so internal injuries are a common outcome of contact. If the outcome of this contact is death, that is defined as electrocution – electric shock that resulted in death. No one survives electrocution.
Effects of current to the body
Current is what affects the body (Table 1). The human body can detect a current as low as 0.25 mA (milliamperes) or 0.00025 A (amperes). Contact with a current between 0.25 and 0.5 mA will cause a minor sensation, a slight tingling, as it flows through the body. Increasing the current to 5 mA makes the contact painful but the person can still – and usually does so quickly – break contact with the energized conductor.
A current of 10 to 15 mA causes muscles to contract. Muscles work in one direction. There are muscles in the fingers that contract to straighten them – the lumbrical muscles – and another that contract to close the fingers – the palmar interossei. The interosseous muscles are stronger than the lumbrical muscle, so an electric current will cause the fingers to close, often around the energized conductor being touched.
Effects of increasing current
As the current increases, muscles in the body can contract violently, causing forceful spasms. This often results in contact being quickly broken. Common injuries associated with these spasms are fractures from falls as the person loses their balance. Arborists have fallen from trees because of contact. There have even been instances of workers standing on the headache rack (the protective steel screen over the cab of an aerial-lift truck) reaching (with a pole pruner or their hand) to hold up a triplex to allow the boom to clear it and taking a fatal fall from the shock of touching the lines.
With current approaching 100 mA, ventricular fibrillation of the heart may occur. Ventricular fibrillation is a spasming of the heart’s ventricle walls, which interferes with the normal flow of blood. At 200 mA, the contraction of the heart is so severe it is forcibly clamped. If contact is instantaneously broken, as is often the case, the heart may resume normal beating. This does not mean there is no injury. At this high a current, severe burns are to be expected.
Direct versus indirect contact
Electricity flows to complete a circuit. If a worker is part of the fault pathway, then electricity can flow through them. There are two primary contact mechanisms: direct and indirect. Direct contact is when a worker touches the conductors. Indirect contact is when something the worker is touching then contacts the conductor. Arborists are injured through either mechanism, but the majority are indirect contact.
While fewer in number – about 20% of fatal incidents – direct contact is a risk to arborists. These incidents most often occur to aerial-device operators and climbers. A common direct-contact incident is an aerial-device operator touching an overhead power line with their back or neck. Usually these occur when the operators were not facing the power line, so they were not aware of the distance between themselves and the conductor. Sometimes they were not even aware of the presence of the overhead power lines.
Many of these incidents happen when the operator is in a lift that is not insulated and not designed to be used near electrical hazards. There are even labels on these units that warn against use with 10 feet of overhead power lines. But even an insulated aerial device provides only a measure of protection.
The dielectric properties of the device may have been compromised – the boom may have been gouged or scarred, the wax may have deteriorated or the interior of the boom might have accumulated debris. These are just some of the possibilities for loss of the dielectric properties. This is one reason for the requirement for annual dielectric testing of these units.
Contact with two conductors
Another potential direct-contact incident for aerial-device operators is contact with two energized conductors. The operator may be attempting to navigate the bucket through the space between the primary lines and the secondaries. The operator touches a primary through their neck or head and a hand to a secondary. This is direct contact and a phase-to-phase fault pathway.
Another direct-contact incident is a climber touching the overhead power line through their back or hand. Climbers have misjudged the distance between their hands as they reach up or did not see the line in the dense foliage and touched the conductor. More common is the climber attempting to squeeze themselves between the tree and the primary line that may be only feet away from the trunk. As they support themselves with their hands on the branches, their back touches the line.
The third though less common incident is to ground workers. These direct-contact incidents occur when a line is pulled off a pole either by a falling limb or tree. The ground worker is struck by the falling line or steps on the fallen line.
Indirect contact
Indirect contact is the most common type of contact, comprising about 80% of electrical-contact incidents to arborists. The most common incident is the climber touching a metal pole saw to the overhead power line. The fault pathway is from the saw touching the line to the hands holding the saw to the legs on the branch to the ground. This contact is a phase-to-ground fault pathway.
Another indirect contact is through a branch that is being cut and then deflects into the line as the cut opens. Indirect contact also occurs when the climber or aerial-lift operator is still holding the just-cut branch and it contacts the energized conductor. Climbers and aerial-device operators must be aware of the distance between the branch and the lines and must maintain control of the branch while cutting. The fall path of the cut branch also must be clear of the lines.
Step and touch potential
When current flows down an aerial device in contact with a line and to the ground the device is sitting on, a voltage gradient will occur based on the resistance of the soil. The farther from the ground contact, the lower the voltage. If a ground worker is standing near the outriggers, there will be a difference in voltage potential between the feet. The foot closer to the outrigger will be at a higher voltage than the foot more distant. Current will flow as there is a difference in potential.
If a ground worker is standing near the truck when it becomes energized, the first action to take is sliding the feet together, so they are touching one another. Voltage was discussed in the first article in the series and was illustrated by a bucket of water far above another – high voltage potential – to the one bucket sleeved into the other – no difference in potential. Putting the feet together is akin to placing the bucket in a bucket. Once the feet are together, the ground worker shuffles away from the truck, never separating the two feet. The next question is how far to shuffle. The safest distance is to shuffle at least 30 feet before beginning to take steps.
If the worker is touching the truck when it makes contact, there will be a difference in voltage from the hand touching the truck to the feet on the ground. This is touch potential. More ground workers suffer from electrical shock through touch potential than step potential. Touch potential also can involve workers feeding brush into a chipper hitched to the aerial device in contact with the line.
Electrical injuries
Injuries occur from the conversion of electric energy into heat energy as the current penetrates the skin and passes through the tissues of the body. Electrical shock is often categorized as low voltage – less than 1,000 volts – or high voltage – 1,000 volts or greater. Power, or watts, has been considered a better classification of electrical injury, but voltage is still the most-used threshold.
Low-voltage contact is more likely to induce ventricular fibrillation. High-voltage contact is most likely to result in severe burns rather than V-fibrillation.
High-voltage contacts cause partial-thickness and full-thickness burns that penetrate through the skin into deeper tissue. Other organ injuries include kidney damage. This is not a direct injury, but muscle destruction from the current can lead to myoglobin release, which may cause renal failure. Indirect injuries are from secondary trauma sources, such as the fall from a tree or burns, from the flash of an electric arc or flames from cloths burning.
Job-hazard analysis
One of the simplest ways to reduce electrical hazards is to avoid them. Arborists are used to inspecting the tree before working on it. But we also need to look for electrical hazards. Too often following an electrical incident, the crew mentions they never even knew there was a line in or near the tree until their climber touched it. Every job-hazard analysis must include a check for overhead power lines and electrical equipment. The next article in this series will explore job-hazard analysis and the job briefing in more detail.
Minimum approach distances
One means to manage exposure to electric current is using minimum approach distances (MAD). This is the minimum distance a worker and conductive objects they are holding can be from a line. Air is an excellent insulator. This is the electric component that formulates MAD distance. The higher the voltage, the greater the air gap required between the line and the worker to maintain a safe work environment.
The dielectric strength of air is a function of air density – temperature and pressure – and humidity. Anyone who has stood outside on a hot, humid day as thunderstorms are building almost feels the air crackling. But the factor that makes the most measurable difference in MAD, and the only one that can be placed in a standardized table, is altitude. According to Paschen’s Law, the dielectric properties of air decrease with increasing altitude. At higher altitudes, above 5,000 feet, air is not as good an insulator as at 1,000 or 2,000 feet.
The air gap is not the only component to MAD. There is also an ergonomic component. This accounts for inadvertent movement by the workers or conductive object they are holding. The minimum distances set in the MAD tables assume a reasonable movement by the worker. Hence, MAD is not just the distance between the worker and the line. It includes the possible arm reach and any conductive tool (or cut branch) they may be holding. At voltages lower than 72.5kV, the electrical component is less than the ergonomic (only 6 inches for 22 kV, the rest is the ergonomic!).
What are the different MADs for arborists?
The ANSI Z133 Safety Requirements for Arboricultural Operations has a complete section devoted to electrical hazards. The Electrical Hazard section was easy to understand prior to the 2017 edition; an arborist was either qualified to work within 10 feet of overhead power lines or they were not. There were two categories; the unqualified arborist who could not work near power lines and the qualified line-clearance arborist who could. The 2017 edition of the Z133 added a third category, the incidental line-
clearance arborist.
The incidental line-clearance arborist category was for arborists who had the training and skills to work within 10 feet of overhead power lines but who were performing the work for the property owner, not on behalf of the owner or operator of the electric supply lines. The lines, whether the primaries running over the edge of the yard or the service lines running from the pole transformer to the mast on the house, were incidental to the arboricultural operations – the lines just happened to be there.
The proposed 2024 Z133 Standard expanded this to four categories, and the names changed to numbers: Electrical Level 1, 2, 3 and 4 Arborists. Electrical Level 1 Arborist replaces the unqualified arborist. The Electrical Level 1 Arborist has only a fundamental education in electrical hazard safety awareness. They do not have the skills to work safely near power lines nor an understanding of the function of the electrical-system equipment.
Electrical Level 1
Electrical Level 1 Arborists shall not work in trees that are within 10 feet of any electric supply lines, communication lines or equipment. If any overhead line – even a communication line – comes within 10 feet of the tree, the Electrical Level 1 Arborist cannot access the tree canopy (Table 2).
Note that this restriction is for an arborist working in an elevated position, whether on a ladder, climbing or in an aerial device, not from the ground. If a service line is elevated 6 feet from the top of a crabapple tree, the Electrical Level 1 Arborist cannot be in the tree. They can stand on the ground, however, and prune with a pole saw if the saw or the branch being cut is not within 10 feet of the service line.
Electrical Level 2
The Electrical Level 2 Arborist may access tree canopies that have an electrical hazard involving lines with a nominal voltage of 750 volts or less. This includes secondary lines, service drops and communication lines. These arborists have the training and skills to safely work within this hazard. This new category was added at the request of tree/landscape companies that do not work near primaries, but near service drops that hang in or near the trees they are pruning or removing. They also may be hired by a homeowner to prune branches away from the service lines – a task few utilities will perform.
Electrical Level 2 Arborists may not directly or indirectly contact any line with a nominal voltage of 300 or less. This means they cannot deflect a service drop while they fell a nearby tree by throwing a rope over it. This is indirect contact, and holding rope draped over a service line kills an arborist every year or two.
They also must maintain a minimum approach distance of 1 foot, 6 inches distance from any electric supply line between 300 and 750 volts. This means no part of their person, including the potential reach of their arms, any conductive tool or the branch they are pruning or holding, can violate this distance. If the line has a nominal voltage greater than 750 volts, the 10-foot rule applies (Table 3). The Electrical Level 2 Arborist cannot work within 10 feet of the primary lines.
Electrical Level 3
The Electrical Level 3 Arborist category replaces the incidental line-clearance arborist. These are arborists who have the training and skills to work within 10 feet of electric-supply lines of 35,000 volts (35 kV) or less. These also are arborists who are not working on behalf of the utility or owners of the electric supply lines.
A change from the 2017 incidental line-clearance arborist standard is the range of voltages in which they can work (Table 4). The incidental line-clearance arborists could work within the appropriate MAD up to 72.5 kV, which is sub-transmission voltages. The new Electrical Level 3 Arborists can work up to 35 kV, the upper end of distribution voltages. Working at any higher voltage requires an abatement plan with the approval of the utility. The utility may decide the work must be done by an Electrical Level 4 Arborist.
Electrical Level 4
The final category, the Electrical Level 4 Arborist, replaces the qualified line-clearance arborist. This category is restricted to workers with a high degree of training and skills to work within a wide range of voltage. They are working on behalf of the utility or operator of the
electric-supply line. There is no change to the MAD chart for line-clearance arborists.
Who determines the electrical level of a worker? It is the employer’s responsibility to determine whether a worker has the training and skills to move up to an Electrical Level 2 or 3 Arborist. The training should be a combination of field and classroom and be documented. But the employer makes the decision, not the employee. So, if an employee changes employment, they need to be re-evaluated by the new employer.
Conclusion
How to safely work near overhead power lines will be discussed in the next article in this series.
John Ball, Ph.D., BCMA, CTSP, A-NREMT (Advanced-National Registry of Emergency Medical Technicians), is a professor of forestry at South Dakota State University.
