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Or it might be better stated: digging holes or planting poles. What I'm talking about is the differences between overhead and underground electrical cabling. I've been doing some research on this of late and it's interesting. So, which is better – overhead or underground power transmission? The answer is a resounding … Yes.

Digging Holes and Planting Poles – Part Two

Steve Maurer, IME
Or it might be better stated: digging holes or planting poles. What I'm talking about is the differences between overhead and underground electrical cabling. I've been doing some research on this of late and it's interesting.

So, which is better – overhead or underground power transmission? The answer is a resounding …

Yes.

Or perhaps a better answer is … it depends. It does depend on many factors, including but not limited to cost. In this article (which is far from exhaustive), we'll look at some of the aspects of this longstanding debate.
The electric grid is comprised of two main sections: transmission and distribution. Transmission starts at the power plant and ends at a substation. From there, the power is distributed to the end user, whether industrial, commercial, or residential consumer. There may be a section between the substation and end user designated as sub-transmission, depending on the voltage output at the substation.

According to many experts, high-voltage transmission lines should be carried overhead. For long distance runs, this is almost a necessity. It's more cost effective to place poles and run the conductors overhead. They can be smaller, and the process is cheaper and more efficient in most cases.

Trenching for long distances, particularly over 20 miles, almost demands overhead transmission instead.

We see an increasing number of underground installations in urban and city scenarios. It is more aesthetically pleasing, cutting down on clutter. It's also safer for the public. In rural settings, you'll find overhead transmission, which morphs into substations for underground distribution. The substation supplies housing additions with underground distribution to residential dwellings and commercial facilities.

Because overhead conductors are bare (excluding the actual connections at the pole), it's cheaper to install  than insulated conductors required in underground applications. There is also less prep work (dig a hole and fill it) than trenching for underground distribution. Ideally, underground cabling should be encased in conduit to make future repairs easier. Often an additional empty conduit is placed in the trench. This also adds to the cost.
According to some estimators, underground installations can be 2 to 10 times more expensive than equivalent overhead installs. By the way, the additional cost is normally passed on to the consumer in their utility bill.

So, what is the appeal for underground distribution? Surely it can't be for its prettier look, can it?

Let's look at some comparisons made in a report by NEI Electric Power Engineering.

In the section titled "National Trends," NEI mentioned that municipalities have passed law requiring new distribution facilities to be placed underground. Keyword is new. The reasons were for aesthetics and to increase property values.

Another reason was to provide better protection from storm damage and improve reliability of the power supply.

Note that while underground distribution can provide protection from high winds, ice and snow storms, and falling trees, it's not impervious to all situations. Earthquakes, flooding, hurricanes, lightning, rodents, and other disasters can still damage underground systems.

Transmission systems carry 69,000 volts and above. Even so, the report states that less than 2% of all outages occur there. Most power failures come from the distribution system (25,000 volts and below). So anything you can do to shore up that system helps.

Overhead systems are easier to troubleshoot and repair. However, the underground system isn't affected as easily by weather conditions in most cases.

Future changes to the landscape or service area should also be considered. Overhead lines are easily rerouted. Underground installation may have to be abandoned in the ground is some cases.

The report also mentioned that the laws of physics limit how physically long a power line can be. This has more impact on high voltage underground systems than overhead lines. A lot of this is due to the effect of capacitance, creating line charging current. Underground lines have 20-75 time more line charging current than overhead lines.

Furthermore, to counteract some of the other issues that underground distribution creates, it may be necessary to move from a radial configuration to a looped system.

While underground distribution systems may be 2 to 10 times costlier than overhead systems, they can be cheaper under some situations, such as in new construction where dirt work is being done, allowing the underground system to be put in place for less cost.

The upshot is this. Underground distribution systems do have some significant advantages. If those advantages outweigh the potential higher cost, it might be worth investigating.
Photo courtesy of BURNDY
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