Electricity is such an integral part of our daily lives that we take it for granted. We flip the switch on the wall, and the lights come on. We give no thought about where it came from or how. But to safely work around overhead power lines, it is essential that arborists understand the basics of our electric power system.
It is equally important for arborists who have no intention of working around conductors to appreciate the hazards associated with power lines. The Z133 and EHAP program revisions will make it clear that every arborist should have a fundamental understanding of and healthy respect for the electrical wires and hardware that line our streets, connect our homes and pass near or even through trees.
How electricity is generated
While solar panels are becoming more common, most electricity is still generated by converting the energy of motion to electrical energy. Electricity begins with spinning a turbine. The turbine blades spin from heat generated by fossil, biomass or nuclear fuels. The motion of wind and water also is harnessed to spin turbine blades. The objective of spinning turbine blades is to rotate magnets surrounded by coils of copper wire within a generator. The wire coils in the shifting magnetic field will induce an electric current.
Copper wire is used because of its high conductivity, defined as the relative ease with which electrons flow through a material. All matter – solids, liquids and gases – are composed of atoms. Atoms have a nucleus surrounded by an electromagnetically bound electron cloud. A conductive element such as copper easily loses its outer electrons. These electrons are continuously spun off and captured by another nucleus of the same matter – a copper atom to another copper atom – in random movements akin to balls bouncing off pegs and bumpers in a pinball machine.
A generator organizes the random flow of electrons across atoms to create electricity. The electrons flow across atoms in a wire in a uniform direction. This flow is not continuous in one direction. The rotating magnets cause it to switch back and forth 60 times every second to create an alternating current. The number of cycles a magnet completes in a second is measured in Hertz (Hz). The standard number of cycles per second used to generate alternating current in North America is 60 Hz.
This flow must follow a circular path, which is called a circuit. The flow must be continuous to function. The outlet in a house has two rectangular contact openings on the receptacle. One opening is to the live or hot wire, and the other is to the neutral. The flow is from the house breaker panel through the black live wire to the electrical device – a lamp, television or other electrical appliance – and back to the panel through the white neutral wire to complete the circuit. The third, usually a green or bare wire, is connected to a round opening that serves as a ground. The ground wire goes back to the breaker panel and to a grounding rod.
The three properties of electricity
There are three properties of electricity that are relevant to understanding the hazard: voltage, current and resistance. (Figure 1)
Voltage is the force or pressure that generates the flow of electrons. It is measured in volts (V), or kV for 1,000 volts. It is the potential difference between two points on the circuit. There is no flow if the two points are at the same potential.
Flowing water is often used as an analogy for the flow of electricity. If a bucket of water is suspended over an empty bucket, the water has the potential to flow, but will not until the spigot is turned on. The higher the water bucket is suspended above the empty bucket, the more force in the falling water droplets once the spigot is turned on, as the potential is greater. If the bucket of water is lowered and sleeved inside the empty bucket, they are at the same potential. Water will not flow even if the spigot is opened, since one is nested in the other. (Figure 2)
The second property of electricity is amperage, the current flow. This measures how many electrons are flowing past a point in one second. The flow is expressed as amperes, or amps (A), which equals 6.25 x 1018 electrons passing a single point in one second. Think of it as the number of water droplets dripping from the suspended bucket spigot. Many drops equal high current. Few drops equal low current. The number of electrons flowing and the force at which they are flowing, A times V, equals watts (W), a measure of electrical power. (Figure 3)
The third property of electricity is resistance. It is measured in ohms (R). One ohm is the resistance needed to stop one volt “pushing” one amp. The higher the resistance of a material, the lower the voltage or current that can pass through it. A porous screen set between the suspended water bucket and the empty bucket represents resistance to the flow of the water droplets.
The three properties of electricity represent the three points on Ohm’s triangle, also called Ohm’s law. An increase in one can cause an increase in another. An increase in voltage, with resistance remaining the same, increases current, A = V/R. So, if the voltage is increased, a corresponding increase in resistance is needed to keep the current the same.
Overview of the electrical system
We do not have a generator in every home and business, so there must be a way to move this electricity from the generator at the power plant to an outlet in the house. Electricity is generated between about 2 kV to 24 kV. It is moved from the generator through the utility system via aluminum alloy wires. Aluminum is used, as it is inexpensive and light. But it is not a perfect conductor, so there is resistance in the line. The longer the distance, the greater the resistance, resulting in the need for higher voltage to overcome that resistance.
Note, we are talking about force, not speed. The speed of the flow appears instantaneous, but it is not an electron moving from the generator to the light switch. It is a long chain of free electrons with one striking another over the distance, more like water droplets colliding with one another along a row.
If we transmitted the electricity at the voltage generated at the power plant – 2 kV to 24 kV – the voltage has the force to move only tens of miles, not hundreds of miles, against the resistance in the line. (Photo 1) This means power plants use transformers to step up the voltage so it can be moved hundreds of miles. Transmission lines are usually between 115 kV to 230 kV, but some are as high as 765 kV.
Transmission lines are elevated high off the ground and are suspended from the towers by a long string of insulators. The insulators provide the resistance to keep the voltage from flowing from the lines down the towers. (Photo 2)
There also will be a wire running from the top of one tower to the next tower. This is a static line. Its function is as a ground for lightning strikes on the transmission lines. This protects the transmission lines from an overcurrent.
Transmission lines end at a transmission substation, where the voltage is stepped down to subtransmission voltage.
Subtransmission lines radiate from these substations, carrying electricity to distribution substations in communities. Subtransmission voltage is between 23 kV and 115 kV, but many are designed for 69 kV. These lines also are supported high off the ground on either shorter strings of insulators or large post-type insulators. Subtransmission lines can carry voltage for 50 miles or more before the resistance in the wire significantly drops the voltage. (Photo 3)
Eventually, the subtransmission lines reach a community and its distribution substations. The distribution network is where interactions among trees, lines and arborists typically play out. Only a small percentage of our electrical incidents occur on subtransmission or transmission lines.
Primary distribution voltage is typically between 2.4 kV and 23 kV, but can be as high as 34.5 kV. This lower voltage means it can only be maintained for 10 to 50 miles before a significant voltage drop due to the line resistance. The distribution line may be supported by post-type or pin-type insulators.
Wye and delta systems
Two systems are commonly used to move electricity through the distribution lines, wye and delta. The wye is by far the most common.
A wye system has phase wires that carry the current, usually in three primary wires, with a common neutral wire. This is typically seen as three wires supported by insulators on a crossarm, with the common neutral attached on a smaller insulator to the pole beneath the crossarm. (Photo 4)
The primary phase wires are ungrounded. They must be for electricity to flow from the substation to the outlet in the house. The neutral is grounded. It is the return path for the flow of electricity. Along a span – a length of poles – there will be poles with a ground wire running from the ground up the pole. A wire will connect the neutral to a ground wire.
Delta systems do not have a common neutral. There are only phase wires, usually two or three, on the poles. It is difficult to look up at a pole and determine whether it is a two–phase delta – two wires supported by an insulator on a crossarm – or a single-phase wye with the phase wire and neutral supported on the same crossarm. The best way to know is to familiarize yourself with the local system. Another way is, if one of the wires on the crossarm has a wire that connects it to a ground wire on the pole, it is a wye system. Deltas do not have grounds.
Wye systems dominate the utility grid. The wye system will radiate out from the distribution substations typically as three phase – three primary wires with a common neutral beneath. Since residences and most businesses do not require three-phase power, eventually the three phases will be tapped off to a single-phase feeder. There will be a wire connecting one of the three phase lines on the crossarm to a lower crossarm, usually set perpendicular to the upper one, and then to a single-phase line. There will also be a neutral.
Primary distribution lines are connected to transformers that decrease the voltage for the end user. (Photo 5) This “secondary distribution” or “service” voltage is 120/240 volts for residential buildings, though it can be higher for industrial uses. This voltage has enough force to move the current for several hundred feet before a significant drop in line voltage. A single-phase line in the country, where homes may be far apart, will have a transformer on the pole for every house. The closer housing found in communities can have a pole-mounted transformer serving multiple homes.
The two 120-volt lines and a common neutral coming off the transformer can be carried by an open secondary, when the three aforementioned wires are strung between poles. These secondaries also can go from the transformer to the mast on the house as a service drop. This is usually as a triplex, where coated “hot” wires are wound around a common bare-metal wire. The 120-volt wires and the neutral run to the house, the breaker panel and then the outlets.
The importance of current
Voltage has been prominent in the description of the electric power system. Voltage is gradually reduced as electricity flows from the transmission line at the power plant to the service drop that enters the house. Everyone is familiar with warning signs that say, “Danger – High Voltage.” Arborists have much respect for high voltage, but seem to treat lower voltage, such as secondary voltage, too casually. But current is what kills.
A common saying is, “Volts get your attention; amps are what kill you.” It is not just the force of the electrons moving through the human body, it is the number of electrons.
We’ll relate more about electrical-system hardware in Part 2 of this series.
EHAP Revision Series Review
The goal or purpose of this eight-part series is to inform readers about changes to TCIA’s Electrical Hazards Awareness Program (EHAP), being made in an ongoing revision of the program to coincide with the revision of the ANSI Z133 Standard. We plan to have one or more articles for each of the program’s six chapters. Articles planned for the series include:
- Chapter 1: Electricity and the Utility Industry
- Chapter 2, Part 1: Electrical Hardware Recognition: Voltage Management and Protective Devices
- Chapter 2, Part 2: Electrical Hardware Recognition: Other Switching Devices, Support and Other Utility Hardware
- Chapter 3: Recognizing Electrical Hazards
- Chapter 4, Part 1: Work Practices Near Utility Conductors: Different Categories of Tree Workers Relative to Electrical Hazards, Conducting a Job-Site Hazard Assessment and Job Briefing
- Chapter 4, Part 2: Work Practices Near Utility Conductors: Work Practices Near an Electrical Hazard
- Chapter 5: Emergency Response and Aerial Rescue
- Chapter 6: Safety Standards
Chapter 1 is a review of how electricity works and the wires/hardware used in its distribution by utilities.
- How electricity is generated.
- The three properties of electricity.
- Overview of the electrical system.
- The importance of current.
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.