EHAP Revision Series, Part 3: Switching Devices, Support and Other Utility Hardware

This is a continuation of the discussion of the electrical-system equipment found along distribution lines. One of the requirements for arborists working in proximity to an electrical hazard – within 10 feet of the lines – is familiarity with the equipment that shares the poles with the lines. The electrical-system equipment can be divided into three categories: 1) voltage-management devices, 2) protective devices and 3) support devices. There are also devices for switching to disconnect or close circuits.

Spool insulators
Spool insulators are used in lower-voltage situations such as this open secondary rack. All photos and diagrams courtesy of the author.

Part 2 of this series covered voltage-management devices and many of the protective devices. In this article, we will address the second part of Chapter 2, which is a continuation of electrical-hardware recognition.

  • Protective devices – insulators and surge arrestors.
  • Switching devices.
  • Support devices – poles, crossarms and guy wires.
  • Other hardware – streetlight and communication lines.

Protective devices: Insulators

Insulators inhibit the flow of electrical current. They support and isolate the ungrounded primary and secondary lines from the crossarm or pole. Insulators are made of fiberglass, glass, porcelain or a polymer. Porcelain insulators, fabricated from clay, are common on the distribution system. These are gradually being replaced by polymer insulators composed of silica and resin, due to their superior mechanical and dielectric properties.

The insulators are attached to cross-arms, poles or, occasionally, even a tree. The length and size of the insulator depends on the voltage. One means of estimating the operating voltage a line is designed to carry is to look at the type and size of the insulators. There are four types of insulators: spools, pin-type, line-post and suspension.

Insulator types

Pin-type insulator
Pin-type insulators are commonly used in lower-voltage distribution lines.

Spool insulators usually are porcelain but occasionally are made of glass. These are for low-voltage support, lines operating below 600 volts. Spools are used in open, secondary steel racks mounted on poles. The spools for the neutral also can be supported on house knobs, a steel housing attached to the pole with galvanized steel screws.

Pin-type insulators are used on distribution systems. They support lines with a voltage of 33 kV or lower, but are most common on lines operating between 2 to 15 kV. Pin-type insulators have a porcelain petticoat supported by a steel pin attached to the crossarm. The function of the petticoat is as a rain shield to prevent raindrops from forming a conductive path from the insulator to the crossarm.

Pin-type insulators have a tie-top, a small groove on the top that holds the power line. The line is held in the tie-top by tie wires. Pin-type insulators range in size from 6 inches in diameter and 5 inches tall for voltage 15 kV or less, to as large as 9 inches in diameter and 9 inches tall for 33 kV.

post-type insulator
A post-type insulator on a distribution crossarm. These are common with higher-voltage distribution lines.

Line-post or post-type insulators are used in distribution, subtransmission and transmission systems up to 345 kV. They often are used in distribution lines that operate at 15 kV or greater. Line-post insulators are a single unit of solid porcelain with sharp and narrow flanges on a mounting base. The line is attached to these with either ties or clamps.

Line-post insulators can be mounted vertically on a crossarm or horizontally on the pole. Distribution line-post insulators are 7 to 15 inches long, depending on the nominal voltage. Subtransmission line-post insulators may be 30 inches long, with some transmission-post insulators more than 50 inches long.

Suspension insulators are suspended from crossarms. These are typically used for operating voltages above 34 kV, and so are found on subtransmission and transmission systems. Suspension insulators are commonly manufactured from porcelain, though polymer insulators are becoming more common.

Suspension insulators are commonly constructed as 10-inch disk units attached in a string. The number of porcelain units in the string can provide an approximation of the line voltage, about one unit per 11 to 18 kV. So a line voltage of 34 kV has two to four units in the string, 69 kV has four to six units in a string and a 115 kV line will be supported by six to nine suspension units. A transmission line with an operating voltage of 765 kV may have 30 to 35 suspension units.


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 to coincide with the revision of the ANSI Z133 Standard. We will have one or more articles for each of the program’s six chapters. There may be some variation in this series in terminology or content from the actual EHAP revision. Articles planned for the series include:

Part 1, Chapter 1: Electricity and the Utility Industry (TCI Magazine, November 2023)

Part 2, Chapter 2, Part 1: Electrical Hardware Recognition: Voltage Management and Protective Devices (TCI Magazine, December 2023)

Part 3, Chapter 2, Part 2: Electrical Hardware Recognition: Switching Devices, Support and Other Utility Hardware

Part 4, Chapter 3: Recognizing Electrical Hazards

Part 5, Chapter 4, Part 1: Work Practices Near Utility Conductors: Different Categories of Tree Workers Relative to Electrical Hazards, Conducting a Jobsite Hazard Assessment and Job Briefing

Part 6, Chapter 4, Part 2: Work Practices Near Utility Conductors: Work Practices Near an Electrical Hazard

Part 7, Chapter 5: Emergency Response and Aerial Rescue

Part 8, Chapter 6: Safety Standards

Protective devices: Arrestors

Surge arrestors (sometimes spelled arresters) are devices to prevent extremely high-voltage surges from damaging equipment. They provide a path for the overvoltage to ground. Severe overvoltage may occur from lightning (lightning arrestors) or a primary or secondary line contacting a higher-voltage line (surge arrestors). The most frequent source of voltage surges is lightening.

Lightning arrestor
Lightning arrestors will be found on poles with transformers attached.

Lightning arrestors are used to protect equipment such as transformers, reclosers, cutouts and switches, among other devices. During a high-voltage surge from lightning, the arrestor serves as a conductor, carrying the voltage to earth through a cable attached to a ground rod. Lightning arrestors are constructed of polymers. They often are posts about 10 inches tall with a series of narrow flanges.

Switching devices

Metal poles
Metal poles are becoming more common with subtransmission and transmission lines. This picture shows a distribution underbuild on the crossarm of the same pole.

The function of distribution air-break switches is to open and close a three-phase circuit. They have horns that form an arc as they open. As the opening increases, the air gap eventually breaks the arc. The switches can be operated by a handle connected to a rod from the switches. The handle is padlocked to reduce the chance of vandalism. They also can be operated remotely by mechanized equipment connected to the rod. Arborists should never assume open means deenergized.

Disconnect switches are air-break switches not equipped with a load-break device. Disconnect switches cannot be opened until the circuit is interrupted by other means. Switching while the current is flowing will result in an arc that can cause a short and dangerous arc flashing.

Support devices: Pole types

Poles support overhead power lines and communication lines at a height and spacing to provide adequate clearance. The poles must have sufficient strength to support ice-coated lines or those subjected to high winds. Poles are made of wood, reinforced concrete or metal (aluminum or steel).

The distribution poles in many regions of the United States are made of wood. These poles are usually constructed of Douglas-fir, ponderosa pine, southern pine (a collective name for loblolly, longleaf, shortleaf and slash pine) or western redcedar. Some regions may utilize local wood such as lodgepole, red pine or even eucalyptus (most common is rose gum). Most poles come from tree farms growing trees specifically for this purpose.

Wood poles are given a preservative treatment to slow deterioration, as the service life is expected to be 30 to 40 years or longer. Ponderosa pine and southern pines have the preservative applied under pressure due to the wood-sap flow. Douglas-fir and eucalyptus may be pressure treated. Western redcedar rarely is pressure treated, or treatment is limited to the lower 6 feet. The most common treatments for poles are creosote, pentachlorophenol (penta) and chromated copper arsenates (CCA). Copper naphthenate and ammoniacal copper fatty acid also are used as preservatives. Penta is being phased out.

Each pole has a birthmark (or brand), an identifier set at about eye level.
Figure 1: Each pole has a birthmark (or brand), an identifier set at about eye level.


Each pole has a birthmark (or brand), an identifier set at about eye level. (Figure 1) There is a two-letter code that identifies the species (Table 1), usually followed by a one- or two-letter code for the preservative (Table 2).

Numbers identify the treatment pressure (in pounds of retention per cubic foot) and the pole class and length. A number 1 to 50 denotes a class-1 pole, 50 feet tall. The class refers to the load-bearing capacity. A class-1 pole has a horizontal load-bearing capacity of 4,500 pounds, while a class 5 would only be 1,900 pounds. This 50-foot pole will have 7 feet buried, a butt dimension of 15.5 inches and a top diameter of 6.5 inches.

Other information gleaned from the birthmark may include the manufacturer, the year it was manufactured and the owner. Poles also have a number to identify the pole on a utility map. This identifier can be helpful if the need to contact the utility/operator of the lines arises, so they know the location.

Concrete poles

Concrete distribution pole.
Example of a concrete distribution pole.

Concrete poles are used for distribution and subtransmission lines in some areas of the country. They are used in areas where wood poles decay too quickly. They also are used in hurricane-prone areas, as concrete poles can withstand stronger winds and do not splinter. The major disadvantages of concrete are the cost, difficulty in climbing and installing new equipment. Steel structures are used for subtransmission and transmission lines.


Crossarms are used to support insulators on power poles. They may be used as a single arm or as a double arm at terminals or corners where additional line strain occurs. Usually, the arms are of equal length on each side of the pole, but there also are alley arms or side arms that cantilever out from one side of the pole.

Armless construction also is used to support lines. Single-phase distribution poles often have a single pin-type insulator installed on a vertical brace near the top of the pole, with a spool insulator attached to the side for the neutral. Fiberglass brackets also may be used to support pin-type insulators from the poles. Line-post insulators may be installed horizontally on poles.

Guy wires

A light-colored guy protector is placed around the base of the wire to improve visibility.
A light-colored guy protector is placed around the base of the wire to improve visibility.

Distribution poles are usually supported by guy wires. Guy wires are 3/16- to 1½-inch-diameter-strand steel wire coated with aluminum or zinc to protect from corrosion. They are anchored by a variety of devices, from a double helix to a rock anchor, depending on soil conditions. The National Electrical Safety Code requires that ungrounded guys be insulated when attached to poles with supply lines carrying more than 300 volts. A porcelain or fiberglass guy insulator is placed in these guy wires, usually above midline.

A light-colored, usually yellow, guy protector at least 8 feet long is placed at the ground end to make any guy wire more visible to pedestrian and vehicle traffic.

Guy wires are part of the electric delivery system. While they are grounded, if there is a fault, they can become energized with potentially fatal voltages. There have been fatal incidents when a ground worker grabbed guy wires detached from their anchor and swung them into a conductor.

Other hardware

Streetlights are powered by low voltage (120/240 v) for many lights, such as LED lamps. The older, high-pressure sodium lamps may require higher voltage. Streetlight lines should be treated with the same respect as any lines in the electrical-supply system. They may not be touched, either directly or indirectly.

Communication lines

An alley arm.
An alley arm.

Below the electrical supply lines – the primary, secondaries and neutral – there is an open space and then the communication lines. The open space, called a neutral or safety space, provides about 40 inches or more of space to provide a measure of safety for telecom workers. It is not a space in which to push through an aerial-lift bucket. This is a dangerous maneuver that has resulted in electrocution of arborists who try to duck down in a bucket to navigate through this space.

The communication lines are the lowest lines on the pole. The telephone line is usually the lowest line, with the cable line just above it. Telephone lines are usually thicker than cable lines. They also have a dark, rectangular junction box next to the pole.

The thinner, sometimes lighter-colored cable lines have distinct D-shaped expansion loops. They also have inline equipment such as amplifiers that can be identified by their small rectangular shape, light color and heatsink slots. There will be two cables entering on each side of the amplifier.

Fiber-optic lines also may be found in the communication zone. If they are attached, they are usually the lowest on the pole.

Communication lines are very low voltage, often less than 12v, to propagate the signal. Communication lines are directly attached to the pole without an insulator. Despite this low voltage, they still represent a hazard source for arborists. They can carry potentially fatal voltage under fault conditions. Do not touch them directly or indirectly.

The utility does not own or operate these lines. They are the responsibility of the communication company that installed them. Arborists need to contact the owner of the communication lines with any questions, not the utility.


This completes the discussion of the electrical-system equipment. All arborists should have a fundamental knowledge of the electrical-supply system. Arborists who are working within an electrical hazard, i.e., closer than 10 feet to lines, must have a deeper understanding of all the equipment and devices found along the distribution system. They must be able to identify the various pieces of electrical-system equipment and know their function in the system.

The next part in this series will cover recognizing electrical hazards.

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.

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