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Home / Technical Articles / 6 beautiful animals that cause interruptions and hazards to electric utilities

Problems & mitigation techniques

Animals are one the largest causes of customer interruptions for nearly every electric utility worldwide, right after human and other factors. Problems and mitigation techniques are as varied as the animals involved. Identifying and addressing physical root causes is often the most cost effective way to address reliability problems.

6 beautiful animals that may cause interruptions to electric utilities
6 beautiful animals that may cause interruptions to electric utilities

This technical article describes reliability concerns and common reliability improvement strategies for the following classes of animals:

  1. Squirrels,
  2. Mice, rats and gophers,
  3. Birds,
  4. Snakes,
  5. Fire ants, and
  6. Large animals (cattle, horses, bison, and bears).

1. Squirrels

Squirrels are extremely cute animals. Everybody love them, but unfortunately they are a reliability concern for all overhead distribution systems near wooded areas. Squirrels will not typically climb utility poles, but will leap onto them from nearby trees and cause faults by bridging grounded equipment with phase conductors.

Of the more than 365 species throughout the world, reliability concerns lie primarily with gray squirrels and red squirrels (see Table 1).

The gray squirrel, inhabiting most of the Northern Hemisphere, is the most common type of tree squirrel. Typical gray squirrels are fifteen inches long, weigh about one pound, and have bushy tails about as long as the combined head and body length. They have a typical diet of nuts, seeds, and fruit.

North American red squirrels (also known as pine squirrels and chickarees) are smaller then the gray (eight to ten inches in typical length), have fur that ranges from red to black, and have bushy tails about the same length as their body. Some subspecies can have red or black ear tufts and most have a white belly.

The red squirrel is found in coniferous forests, where it feeds on the seeds and cones of the pines, fir, and spruce trees.

Table 1 – Common types of squirrels and their typical maximum length

Squirrel TypeLocationLength
Eastern GrayEastern US, southern Canada20”
Western GreyWest Coast of US22”
FoxEastern US29”
North American RedAlaska, Canada, North US and West US12”

Squirrels are extremely agile, persistent and committed to travel routes that may include utility poles. Because of this, attempts to keep squirrels off of overhead utility equipment are usually futile and reliability improvement efforts should focus on equipment protection.

The most common technique is to install plastic animal guards on bushings and insulators to prevent squirrels from simultaneously touching the tank and phase conductors (Figure 1).

The figure to the left shows a squirrel causing a fault by bridging the phase voltage to the grounded transformer tank
Figure 1 – The figure to the left shows a squirrel causing a fault by bridging the phase voltage to the grounded transformer tank

The middle figure shows the bushing fitted with a plastic animal guard to prevent the squirrel from causing a fault. The right figure shows the bushing fitted with an electrostatic guard that deters an approaching squirrel with a harmless but noticeable electric shock.

Squirrel proofing electric transformers

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2. Mice, Rats and Gophers

Mice, rats, and gophers are rodents that cause faults by gnawing through the insulation of underground cable. Rats and mice are the most common cause of animal related outages on underground equipment, and gophers are third (snakes are the second most common cause).

Besides chewing through insulation, mice and rats also create reliability problems by attracting snakes. To prevent these rodents from gaining access to underground equipment, all equipment cabinets and duct openings should be tightly sealed.

Some utilities have successfully used ultrasonic devices to ward off mice and rats.

An example of delivered ultrasonic animal protection device with HV equipment
Figure 2 – An example of delivered ultrasonic animal protection device with HV equipment (photo credit: INMR)

When gophers dig tunnels, they chew their way through any obstacle in their path, including electric cables and plastic conduit. Gophers are most common in the Midwestern US where they are often the most common cause of underground equipment failure.

Effective gopher control is difficult to achieve, with methods including traps, poisoned bait, poison gas, and ultrasonic techniques.

What happens to a rat that hides in a substation

The poor rat ended up with a huge corona. Substation crew found him lying with his tail toggling in the wind.

Some of the modern solutions against rodents:

  1. Woven fibre glass tapes: are a dielectric solution that offer a very good protection because glass sticks to their mouth and irritates them. They have shown to be very effective when it comes to the prevention of small rodent attacks.
  2. Steel tape: reliable protection but can produce electromagnetic interferences
  3. Polyamide 12 outer sheath: used to protect the fibre optic cable from termites, however rodents can still gnaw through.
  4. Polypropylene outer sheath: Effective in protecting fibre optic cables due to its hard surface, not as hard as PA 12 Outer sheath and rodents can still gnaw though.
  5. Chemical repellents: not that effective because the rodents do not ingest what they bite at.

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3. Birds

Birds are the most common cause of animal faults on transmission systems, subtransmission systems and air insulated substations. Different types of birds cause different types of problems, but can generally be classified as nesting birds, roosting birds, raptors, and woodpeckers.

Nesting birds commonly build their homes on lattice towers, poles, and in substations. Nesting materials can cause faults, and bird excrement can contaminate insulators.

Removing nests sites can be difficult since the instinctual nature of certain birds will cause them to rebuild nests in the same location. To further complicate matters, nesting birds can attract predators (e.g., raccoons, snakes, cats) that can be a worse reliability problem than the birds themselves.

Roosting birds use electrical equipment to rest on or to search for prey. They cause faults by bridging conductors with their wings and contaminating insulators with their excrement.

A small number of roosting birds is typically not a reliability concern, but the instinctual nature of certain birds can result in many thousands of birds roosting in a single location.

3.1 What is bridging?

Bridging is where a bird or animal makes contact between phases or between phase and ground creating a short circuit. The illustration below makes it clear how easily large birds can cause problems across all distribution voltages in substations and on overhead lines.

Phase bridging usually results in the bird or animal being electrocuted and depending on how the dead bird or animal falls will determine whether the auto-reclosing system operates successfully. Often the bird or animal will fall away allowing the auto-recloser to re-energize the circuit as intended.

Although this may not normally require investigation by emergency crews it usually leaves un-noticed damage. Figure 3 left shows typical “telltale” sign of wildlife induced damage.

Left: Damage to conductor known as “stranding”; Right: Dead bird fallen away from contact area.
Figure 3 – Left: Damage to conductor known as “stranding”; Right: Dead bird fallen away from contact area.

Although a circuit may be operating normally a series of these events can lead to a build-up where the damage gets worse with
each trip and finally causes a hardware failure, for example where each trip burns out a single strand of a conductor eventually dropping the line.

If the dead bird or animal falls between its points of contact and remains there it will result in the circuit not being able to be re-energized and will stay like that until an emergency crew can remove the fault (bird or animal) and re-energies that circuit.

At least this type of event can be logged with some certainty of what happened whereas the bird or animal that falls to the ground or on top of a transformer is quite likely to be removed by some predator. (Figure 3 right shows a typical occurrence – before being removed).

Most bridging problems occur on Medium Voltage systems (typically <36 kV) designed where bare conductors, bushings, busbar systems and associated equipment have “air-spaced” clearances that under normal conditions operate without issue.

3.2 Prevention

To prevent birds from roosting, anti-roosting devices can be placed on attractive perches (see Figure 4). For locations that cater to thousands of roosting birds, more extreme deterrent methods such as pyrotechnics and falconry may be required. Some of the larger roosting birds have a tendency to empty their bowels when they take off (referred to as streaming).

In addition to contamination, streaming can result in faults if it bridges phase-to-phase or phase-to-ground potentials.

Table 2 – Typical wingspans for large birds.

BirdTypical Wingspan
Golden Eagle8 ft
Bald Eagle6 ft
Vulture6 ft
Gray Owl5 ft
Osprey5 ft
Blue Heron6 ft
Hawk4 ft

Long wingspans can bridge conductors and result in a fault.

Many large birds have threatened or endangered status, requiring utilities to take measures to reduce the number of electrocutions regardless of reliability concerns.

The left figure shows a bird perched on a distribution pole
Figure 4 – The left figure shows a bird perched on a distribution pole

This behavior can be deterred through the use of anti-roosting devices like the cone shown in the center figure. Phase-to-phase wing contact can be avoided by protecting the center phase with an insulated covering like the one shown on the right.

Raptors are birds of prey such as eagles, hawks, ospreys, owls, and vultures.

In addition to the types of reliability problem associated with roosting and nesting birds, raptors have large wingspans that can bridge phase conductors during takeoff and landing. In addition to causing a fault, this can kill the raptor – a legal problem since most raptors are protected by federal statute in almost all countries.

Typical raptor wingspans are shown in Table 2 above.

Woodpeckers peck holes in wood with their beaks as they search for insects or build nests. This does not harm trees (the bark regenerates), but can cause devastating damage to utility poles. Woodpecker reliability problems can be mitigated by using steel or concrete poles, by placing new poles next to damaged poles, and by using woodpecker repellent.

Utilities addressing woodpecker problems should be aware that certain types are considered threatened or endangered and are protected by federal statute.

Birds make a power transformer explode

If a huge flock of birds sitting on a hydro line leave all at once they make the line swing (known as a swing short) which causes a cut out fuse to blow.

Surprise babies in the 115kV substation!

Doing some maintenance in a former factory electrical substation, I ran across an unusual sight. Surrounded by 115,000 volts, mom picked a strange place for this. Wait for it.

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4. Snakes

If you ask me, I admire them very much, but don’t like them. Snakes are major reliability concerns in both substations and underground systems. They can squeeze through very small openings, can climb almost anything, and have the length to easily span phase conductors (see Table 3 for the lengths of different types of snakes).

Snakes cause more substation outages than any other animal except birds and more underground outages than any other animal except mice.

Table 3 – Common snakes and their typical maximum length

Snake NameLocationVenomousLength (ft)
Royal PythonSoutheast AsiaNo30
AnacondaSouth AmericaNo25
King CobraSouth AsiaYes18
Black MambaSub-Saharan AfricaYes14
BushmasterCentral AmericaYes12
Boa ConstrictorLatin AmericaNo9
RattlesnakeAcross the USYes8
BlackNortheastern USNo8
Water MoccasinSouthern USYes6
RacerEastern USNo6
Indian CobraSouth AsiaYes5
KingEastern USNo5
CopperheadEastern USYes4
Rosy BoaSouthwestern USNo3
Eastern CoralSoutheastern USYes 2.5
GarterNorth AmericaNo2.0
Rubber BoaWestern USNo1.5

The length of a snake is an important reliability consideration since many faults occur when snakes bridge phase conductors.

Snakes are cold-blooded animals focused on body temperature regulation and obtaining food. During cold weather, snakes may seek out warm electrical cabinets. While hunting, snakes may seek out rats and mice in underground distribution systems and birds and nests in substations.

Snakes problems can usually be mitigated if electrical cabinets are well sealed and if food supplies are removed.

Another common solution is to install specialized snake fences around substations and critical pieces of equipment. Some snake fences use a low-voltage electric shock to increase their effectiveness.

Dead anaconda found electric panel

You definitely don’t expect to find fried giant anaconda in a switchgear. That must’ve been a huge shock for this guy!

Large rat snake entered 13kV substation

This a video of the explosion that resulted when a large rat snake entered the 13,000 volt North Athens substation. Video provided by Athens Utilities Board, Athens, Tennessee.

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5. Fire Ants

Maybe they do not deserve adjective ”Beautiful” as it states in the title above, but what and how they live and work is beautiful for sure!

Fire ants, originally from South America, are small but have a severe sting. They have proliferated across much of the Southern US and it is becoming common in these regions for fire ants to build nests in pad-mounted equipment.

Nesting materials can cause short circuits, the ants can eat away at conductor insulation, and their presence can make equipment maintenance a challenge.

Fire ants nesting inside transformer
Figure 5 – Fire ants nesting inside transformer (photo credit:

There are actually two types of fire ant: black and red. The black fire ant, accidentally imported from South America into Mobile, Alabama, was first reported in 1918. Its distribution is still restricted to parts of Mississippi and Alabama. The red fire ant was imported around the 1930s and has spread across most states in the Southeast.

This species has become very abundant, displacing many native ant species. Fire ants have the potential of spreading widely across the South, the Southeast, and the Pacific Coast.

Fire ants were accidentally introduced into the US from South America in the early 1900s. Since then, they have spread widely across the southern US, displacing native ant species and causing reliability problems by building nests in electrical equipment.

Movement of entire colonies into electrical boxes

Fire ants can be found in electrical housings during all times of the year. Air conditioners, traffic signal boxes, and other devices can be damaged. Ants nesting in these units are highly defensive of their colony and can be a medical threat to maintenance personnel.

Electrical housings (e.g., outside electrical disconnects, junction boxes, pad mounted transformers, etc.) provide warmth during winter months, a dry nesting site during heavy rains, and an undisturbed nesting site throughout the year.

When a colony moves into an electrical box, worker ants import soil for nesting. This material can cause corrosion and interfere with maintenance

In electrical transformer boxes, oil regulates the temperature in half of the box. When moisture from the mound causes corrosion, the oil leaks, inhibiting temperature regulation and resulting in failure of the transformer. In addition to corrosion, once inside, the ants chew on insulation and can cause short circuits or interfere with switching mechanisms.

Affinity of foraging worker ants to electrical fields and switching mechanisms. Imported fire ant colonies (mounds) are often found at the base (slab) or near housings of electrical units. Worker ants leave these nests in search of food to bring back to the rest of the colony members (larvae, other workers, and indirectly to the queen and reproductive ants).

However, when worker ants enter switching mechanisms of electrical equipment, the ants can become a problem.

Fire ants cause electrical problems for Rowlett neighborhood

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6. Large Animals

Large animals typically cause reliability problems through contact with guy wires and poles. Since they are large and strong, they can do physical damage and cause an immediate outage or make the system more prone to outages in the future.

Most large animal problems are due to cattle, horses, bison, and bears.

Cattle cause reliability problems by rubbing on guy wires. Since they are habit-forming animals, they will tend to keep rubbing on the same guy wires and will eventually cause poles to lean. It is difficult to deter this behavior, and mitigation strategies should focus on barricading or removing guy wires.

Horses can also cause reliability problems by rubbing on guy wires, but more commonly collide with those that they cannot see. Even if these collisions do not cause reliability problems, they can kill or injure the horse and create liability concerns. Collisions can be avoided through barricades or by making the guy wires more visible.

Bison are large and powerful and like to rub their heads on objects. If this object is a utility pole, it may be pushed completely over and cause a major outage. There is usually no practicable way to deter this behavior and mitigation efforts must resort to barricades or pole removal.

Bears are becoming more of a distribution reliability concern as the US population expands into previously unpopulated areas. Most reliability problems are associated with wooden utility poles, since both brown and black bears can destroy wooden poles by using them as scratching posts.

In addition, brown bears can climb wooden utility poles and cause faults by contacting live conductors. Bear related problems can be mitigated by placing fences around wooden poles or by replacing wooden poles with steel or concrete.

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Few more amazing videos of animals

Snake Chases Birds on Power Line

I was at my home backyard and heard the birds chirping and looking up to see this snake balancing along the electrical cable trying to catch the birds. The birds seem to play catch me if you can and the snake did a good job balancing moving along the cable, trying for the birds but fail and return to the palm tree, hungry.

Primate swings from live power lines, falls from electricity pole

The chimpanzee escaped from the Yagiyama Zoological Park in Sendai city, Miyagi Prefecture, in the early afternoon, according to city officials. An escaped male chimpanzee was captured by city officials after falling off an electricity pole in northern Japan.

The chimp was on the loose for over an hour before climbing an electricity pole, where officials shot the simian with a sedative.

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  1. Electric Power Distribution Reliability by Richard E. Brown
  2. Managing Red Imported Fire Ants in Electrical Equipment and Utility Housings Molly Keck, Extension Program Specialist, Bexar County and Bastiaan M. Drees, Professor and Extension Entomologist, Texas Cooperative Extension, College Station, Texas
  3. Protect your connection against rodents by Coats

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Eenrique Rojas
    Apr 05, 2021

    Bravo colega!!
    Very good article covering much of real life cases all over the world with interesting videos. A few years ago I went to a site survey and there was a big hole in the substation fence, a cow entered there and produced a fault moving between the HV breakers.

  2. Okufuwa Luke Temitope
    Aug 28, 2019

    I want to receive daily newsletters on electrical power/ Control systems

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