Undergrounding facts for kids
Undergrounding is when we replace power lines or phone cables that are usually seen above ground with new cables buried beneath the ground. This helps prevent wildfires and makes sure our power stays on during strong winds, thunderstorms, or heavy snow and ice. An extra benefit is that the landscape looks much nicer without all those wires overhead. While burying cables can cost more to start with, it might save money on repairs and upkeep over time.
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A Brief History of Underground Cables
The idea of putting cables underground started a long time ago. In 1812, electric cables were used in Russia to set off mining explosives. Later, in 1850, cables were laid under the English Channel to send telegraph signals.
As electricity became more common, so did underground cables. Thomas Edison, a famous inventor, used underground "street pipes" for his early power networks. These pipes first used jute (a natural fiber) for insulation in 1880, and then rubber in 1882.
Over the years, new ways to insulate and make cables were developed:
- 1925: Cables started using pressurized paper for insulation.
- 1930: PVC (a type of plastic) became an insulation material.
- 1942: Polyethylene, another plastic, was first used for cable insulation.
- 1962: Cables with ethylene propylene rubber insulation became available.
- 1963: Special parts for cables that were already shaped and ready to use became available.
- 1970s: Cable parts that could shrink to fit were introduced.
By the 20th century, burying cables directly in the ground became a common practice.
Why Bury Power Lines?
Power lines that carry high-voltage electricity, often held up by huge pylons, can look a bit messy in the countryside. Underground cables are a great solution for sending power through busy cities or places where land is expensive or needs to look nice. They are also useful for crossing rivers, either underground or underwater.
Benefits of Undergrounding
- Safer from Bad Weather: Underground cables are much less likely to be damaged by severe weather like lightning, hurricanes, tornadoes, strong winds, or freezing rain.
- Less Risk of Fires: Overhead power lines can cause fires if they touch trees or other objects, creating sparks. Burying them greatly reduces this risk.
- Reduced Electromagnetic Fields: While electric currents create magnetic fields, underground cables are grouped closer together, which helps reduce the magnetic field outside the cable.
- Needs Less Space: Underground cables need a narrow strip of land (about 1 to 10 meters wide) for installation. Overhead lines, however, need a much wider area (20 to 200 meters) to be kept clear for safety and maintenance.
- Safer for Wildlife and Aircraft: Buried cables don't pose a danger to low-flying planes or animals like birds.
- Harder to Damage: Underground cables are less likely to be damaged by people, whether by accident, theft, or other activities.
- More Room for Trees: Burying power lines on sidewalks leaves more space for planting large trees, which is good for the environment and can make properties more valuable.
Challenges of Undergrounding
- Higher Cost: Burying cables, especially for high-voltage power, is often much more expensive than putting them overhead. For example, overhead lines might cost around $10 per foot, while underground lines can cost $20 to $40 per foot. In very busy city areas, undergrounding can be 10 to 14 times more expensive.
- Harder to Repair: Finding and fixing a broken overhead wire can take hours, but repairing an underground cable can take days or even weeks. Because of this, companies often lay extra cables underground as backups.
- Hidden Dangers: The exact location of underground cables isn't always clear, which means people digging might accidentally hit them, leading to damage or electric shock.
- More Complex Operations: Underground cables can make it harder to control the voltage in the power system because of something called "reactive power." This can limit how long an AC (alternating current) line can be. For very long distances, HVDC (high-voltage direct current) lines are sometimes used underground to avoid this issue.
- Difficult to Upgrade: It's easier to upgrade overhead lines to carry more power by changing poles or clearances. Underground cables can't be upgraded in the same way; they often need to be replaced or new ones added to increase capacity.
- Vulnerable to Ground Movement: Underground cables can be damaged by earthquakes or other ground movements. For example, in the 2011 Christchurch earthquake in New Zealand, many kilometers of underground cables were damaged, cutting power to large parts of the city.
- Street Damage: Repairs to underground cables often require digging up streets, which can lead to bumpy roads and traffic jams. This also increases costs for local governments to fix the roads.
Even with these challenges, the benefits of undergrounding can sometimes be more important than the higher cost and more complex maintenance.
How Are Cables Buried?
There are a few main ways to put cables underground:
- Horizontal Boring: This method uses a special drill that bores horizontally under the ground, starting at one point and curving back up to the surface at another. It's used when you want to cause very little damage to the surface, like under roads or buildings.
- Trench Undergrounding: This is a simpler method where workers dig long trenches, lay the power lines into them, and then cover them back up. This is done along the entire path of the power line.
- Duct Bank: This method uses parallel pipes (conduits) held together by spacers. Sand or concrete is then filled in around the conduits. These can be placed directly into a trench, or pre-made sections of concrete and conduit can be used.
Rules and Regulations
Europe
In the UK, a regulator called the Office of Gas and Electricity Markets (OFGEM) allows power companies to charge customers for some undergrounding projects. These projects must be in special areas like National Parks or "Areas of Outstanding Natural Beauty" (AONB). For example, in 2021, work began to bury 9 kilometers of 400kV overhead power lines in the Dorset AONB. Similar projects are planned for other beautiful areas like Snowdonia.
In the Netherlands, all low and medium voltage electrical power (under 50 kV) is now supplied underground.
Germany also has a very high percentage of underground cables: 73% of medium voltage cables and 87% of low voltage cables are buried. This helps make their power grid very reliable, meaning people experience very few power outages each year.
California, USA
In the United States, the California Public Utilities Commission (CPUC) has rules that allow for undergrounding electrical cables in certain situations. Some projects are paid for by all customers, while others are partly funded this way, or entirely by property owners.
Japan
Most electrical power in Japan is still distributed by overhead cables. In Tokyo's 23 wards, only about 7.3% of cables were laid underground as of March 2008.
Other Options: Aerial Cables
A middle ground between fully undergrounding and using standard overhead lines is installing aerial cables. These are insulated cables that are strung between poles.
- Advantages: Because they are insulated, there's less danger of electric shock if someone touches them (unless the insulation is damaged). They also avoid the high cost of digging, especially in rocky areas.
- Disadvantages: Aerial cables still have the same visual impact as regular overhead lines and can still be affected by storms. However, if the insulation stays intact during a pole failure or if a tree hits them, the power might not go out. This also makes them safer and sometimes allows for re-hanging the cables without turning off the power.
See also
- Overhead power line
- Direct-buried cable
- List of high voltage underground and submarine cables
