Re: To Bury or Not to Bury [Telecom]

Vehicles can't crash into power poles that aren't there, winds can't > affect power lines that are underground, and the visual pollution of > underground power distribution is limited to the access ports on the > pavement.

Are "access ports" in Australia the same things that we call "manhole covers" here in the USA -- round steel plates about a meter in diameter?

Where are equipment enclosures (transformers and switchgear) located? Are they also underground?

"T > I've also read reports that indicate trouble shooting and repairing > underground power lines near the end of their life is very expensive.

Troubleshooting and repairing any kind of underground cables -- power, telco, or CATV -- is expensive at any point during the life of the cables. Physical damage due to excavation work can occur at any time.

To the extent that repair work is required near the end of life, it's usually caused by failure of the > As to burying power lines, obviously individual lines to houses and > lines to a block of houses can be buried. But is there a limit to the > amount of voltage on a line that can economically be placed > underground?

Depends on what you mean by "economical." There's no inherent "limit" imposed by voltage, but there's certainly a tradeoff between voltage and cost. Given enough money, you could probably put anything underground. As Will Roberts noted in TD 28:69, a 345-kv line exists in Boston. By my calculation, that line cost about $12 million/mile. But even that cost was (presumably) economical when compared to the cost (both in dollars and PR issues) of building it overhead.

An even bigger problem is conductor heating. For any given power, current is the inverse of voltage, and the resulting I-squared-R loss produces heat. That heat must be dissipated through insulation. If the insulation does not dissipate it safely, the insulation will fail.

The best insulation provides high electrical resistance and low thermal resistance -- in other words, air. Which is why almost all overhead power conductors are uninsulated -- just bare metal swinging in the breeze. Only customer drops (typically between 115 and 460 volts) are insulated.

For obvious reasons, underground cables have to be insulated with something else. There are two ways to control I-squared-R heating: by reducing current (increasing voltage) or by reducing resistance (using larger conductors). Both ways are employed, but larger conductors are more obvious: take a look at a primary riser, and you'll see an abrupt transition between the relatively small uninsulated conductors and the much larger conductors sticking out of the insulation at the top of the riser.

I'll note that while > One newspaper report I just read stated that underground power lines > cost from 4 to 10 times as much as overhead lines.

A lot depends on the costs of right-of-way acquisition and restoration requirements.

Construction in a new residential neighborhood (before lots are developed) is relatively inexpensive: the power company digs a trench, dumps in a "sand bed" (4"-6" of sand), all three companies (power, telco, and CATV), lay their cables, power puts in another layer of sand, then backfills the trench with the spoil and compacts it. Except for backfill, there are no restoration costs. Right-of-way (usually by easement) is dedicated in the original plat.

C > Our electric monopoly, long-ago privatized, said in September and > repeated in January that it would cost ratepayers a million dollars a > mile to bury the lines. Overhead lines were said to be one tenth of > that.

That's a common rule-of-thumb for moving existing overhead to underground. But as Tony Toews noted:

So I'd want to see some detailed cost estimates and real world > experiences before agreeing that underground power lines are a "good > thing".


Randall also wrote:

No mention was made of what it would cost to cut the damned > trees that took out the lines both times.

Or the wrath of local homeowners if it actually tried to do it?

John Levine

That is all industry bovine effluent. First of all automatic > trenching equipment is available these days. Cut and cover and bury > it all. Second of all you [can] run everything through massive > conduits.

Sure, if you're putting the cable in an open field. But try running your automatic trenching equipment down a backlot easement in an established residential neighborhood, working your way around tree roots, swing sets, fences, gardens, swimming pools, lawn-sprinkler pipes, dog houses....

Or try running it down a congested city street, working your way around sewer pipes, catch basins, water mains, gas mains, steam pipes, telco conduits, telco manholes, abandoned coal bins, abandoned streetcar rails, colonial-era cobblestones....

I agree that all conductors should be in conduit (although I must admit that I've been responsible for many miles of direct-bury CATV cable). Power companies routinely install transmission and cables in conduit, typically 4- or 6-inch Schedule 80 PVC, one conduit per conductor (and sometimes they provide extra conduits for telco, CATV, or whatever). Then they encase the entire bundle of conduits in concrete. The end result looks like a buried sidewalk three or four feet deep.

But doing all that requires a lot of hand work. Even if automatic trenching equipment is used to dig the trench, workers still have to place the conduits, build forms for the concrete, and guide the wet concrete into position. When the work is done, the trench must be backfilled, compacted, and the surface restored to as-was condition. Workers are also needed to pull the conductors through the conduits and terminate them.

Then there's the problem of finding space for transformers, splices, and switchgear. In suburban areas, you can put this stuff in big steel boxes above ground and hope that nobody complains. But in congested urban centers, there's no place for it above ground. So you either have to construct manholes or put it in the basement of a nearby building. You can imagine the difficulties these options would encounter.

You can put a lot of transformers and switchgear on poles, and most people don't even notice.

What's been your experience with NYSEG there in Trumansburg? Are they converting overhead to underground? If so, how are they placing the cables -- in conduits or direct burial? Where are they putting the transformers and switchgear? Who's paying for it? Are they burying telephone and CATV at the same?

NYC is probably the worst case example, where the city is built on a > slab of rock, and digging the tunnels involves jackhammers and > blasting. They flood, too. But given the density of wires there, > overhead simply stopped being practical in the heart of the city, > although there's still plenty of overhead in the outer boroughs.

For only ten times the cost of putting the stuff on poles....

Neal McLain

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Neal McLain
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In the street near me which was converted to underground a few years ago, the circular power covers (concrete) are only about 0.5 metre dia.

Actually that street is "almost" converted as there is one property who must have refused to pay their share of the costs of conversion and they are served by an overhead feed from the single remaining pole in the whole street (apart from the street lights). The one remaining pole highlights how ugly it is in comparison to the cleaner and far more open street-scape where they have been removed.

I know a lot of power infrastructure is underground in the city here, out in the 'burbs a lot is still up on poles.

The city ones are usually covered by large rectangular covers that pop up to create barriers when work is done in them (as do a lot of the telco pits). ........

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David Clayton

We were told our 40 year old power distribution lines, which were not buried in conduit, would "wear" from the power line heating and cooling and expanding and contracting against the earth. Apparently conduit lines are better protected from that. We had to replace the lines. $$$

I saw in a new community how they built underground lines, as you describe. So much better than what we originally had. (Thank goodness telephone and cable aren't our problem.)

The local water company had an easement through our community. They needed to build a high volume new water line. Technically the easement ran under our driveway, it was to mutual advantage for them to run it under a wild growth area (so our driveway wouldn't be disrupted and they could dig in soil instead of concrete). We had a problem afterwards in that they wouldn't restore as many trees as existed before, as they promised. Of course we screwed up by not getting the agreement in writing in advance.

Constructing a few miles of street running track for a new light rail line cost more than building 25 miles in open area, due to all the stuff you describe in an old industrial city. Don't forget telegraph lines, too.

When the cable company upgraded to fibre from coax, they needed to install "steamer trunk" boxes at frequent intervals. In our community we arranged to locate such boxes behind shrubs. But in other communities, they just put them along sidewalks. Suburban homeowners are VERY fussy about their lawns and screamed like crazy. (A big cable rate increase at the time didn't help.)

In our community, the building transformers were originally underground. As they expired (sometimes violently) we replaced them with aboveground boxes. The community objected to them but we were told below ground transformers were no longer available and had shorter lifespans. Due to PCB fears, we had to replace all of them. $ $$

In Chicago, they built a tunnel network to handle phone lines. But then they used the tunnels to move freight around. Eventually the freight system was retired, but then the tunnels reverted to their original goal--utility lines were installed in them.

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Access ports can be manholes but they can also be metal doors about 6x8 feet. There are several of those in downtown Providence.

Not to mention the vent stacks that stick up occasionally. That's what happens when you build a city on a site that was once called the Great Swamp.

Which is why you run inexpensive nylon lines through the ducting in order to pull through new cable.

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Even laying underground telephone cable / fiber or even CATV coax in an existing neighborhood has it's issues. You can use plows to put the cable underground without too much work, but you have to directional bore at each paved driveway, sidewalk, or road crossing. That's time consuming and expensive. Imagine a line of 20 houses, each with a driveway and sidewalk going from the house to the road.

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