The Different Types of Transmission Lines and Keeping Them Safe

The electrical grid relies on the bulk movement of electricity from a generating site to a network of substations. If you are a provider of this crucial service, you appreciate the importance of moving that power along transmission lines smoothly and without interruption.


Many power customers are not aware of this transmission network, which moves electricity to substations before it moves on to their homes and businesses through the distribution network. Their higher voltage makes transmission lines particularly dangerous and important. Proper clearance through vegetation management is required to protect transmission lines.


Major Types of Transmission Lines


Transmission lines carry voltages starting at 110kV but they can range to more than 700kV. Transmission lines have greater clearance requirements than distribution lines due to their higher power load. Loads will vary by location — for example, urban areas typically require a heavier load than rural areas. The lines may travel underground or overhead, and each type has its own potential issues. 

Overhead Transmission Lines

Overhead transmission lines, towers, and other above-ground components are where clearance comes into play. Conductors themselves are usually uninsulated, with only the surrounding air and insulators providing protection against electric discharges.

Subtransmission Lines

Subtransmission lines carry lower voltages, travelling to regional distribution substations. Large commercial or industrial clients may tap into these along the way — meaning, those customers would feel the effects of any damage to this type of line.


Risks to Electrical Transmission Lines


The North American Electric Reliability Corporation (NERC) outlines their clearance regulations in the Electric Reliability Standard FAC-003-3. NERC has mandated that utilities operate with a zero outage policy on their bulk transmission systems. Failure to properly maintain vegetation around transmission lines can incur expensive sanctions and fines. 

Threats from Wind

Since overhead transmission wires lack insulation, minimum clearances for trees and other objects are essential. All kinds of weather conditions, including snow, ice, high winds and extreme temperatures, can lead to power outages. Wind speeds as low as 23 knots (43 km/h) can blow tree branches into transmission lines. Falling or broken tree limbs can pull lines down from supporting towers or poles leading to outages, down lines or fires. If downed lines remain energized, they can initiate fires, particularly in dry conditions.

Other Weather Threats

During warm weather and heavy loading, power lines heat up and stretch, sagging closer to any vegetation or objects underneath. At the other end of the thermometer, snow and ice can bend or break branches, bringing them close enough for electricity to arc.

Trees and Other Objects Close to Lines

Any of these weather conditions pose a threat when objects are too close. Swimming pools, buildings, irrigation equipment, fences, and trees can all be dangerous when they’re too close to transmission lines.


Trees don’t have to physically touch an energized power line to be dangerous. Electricity can arc from the power line to nearby trees given the right conditions, such as a voltage surge on the line from a nearby lightning strike. 


Arcing can kill anyone near the tree, cause a fire, or cause a power outage. Tree-related power outages disrupt power to homes as well as critical locations like hospitals and emergency services. 

Tree Maintenance is Critical 

Keeping trees pruned, and removing them when necessary, reduces the risk of a tree contacting a transmission line. FAC-003-3 requires the following power line clearance:


  • Trees and vegetation growing in or adjacent to the transmission power line must be trimmed to avoid contact.
  • Because trees and other vegetation are constantly growing, they should be trimmed beyond the minimum clearance specified.
  • Every utility company is required to implement a vegetation management plan to comply with these regulations as well as state and local laws.


A combination of vegetation management methods is needed to manage fast-growing, tall, woody species. The goal of integrated vegetation management is to prevent outages and 

assure the integrity of the line. Establishing a stable, diverse, low-growing plant community on the right-of-way also reduces the potential for soil erosion and benefits local wildlife.

A reputable utility vegetation management company can also recommend shrubs, grasses and low-growing trees suitable for an easement, including trees that reach a mature height of less than 15 feet.


How NG Gilbert Works Around Transmission Lines


It’s important that clearance around the lines is maintained by professionals who understand how to work around them safely. Working in or around trees near high-voltage, energized power lines is highly dangerous work that requires ongoing training. 


At NG Gilbert, we serve investor-owned electric utilities, municipal systems, rural electric cooperatives and independent transmission companies. Our highly trained industrial vegetation management team trains to work around transmission lines safely and effectively. We’re committed to the safety of our employees and the community.


Contact NG Gilbert if you need a team who can expertly manage trees and other vegetation to maintain safety and reliability of your power grid.