Why are power lines strung far above ground?

Factual Questions
1

This thread reminded me.
http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=152138
It’s not relevant to that conversation, so here we are.

Power lines (usually accompanied by telephone wires) are usually found aboveground, and are sometimes buried when the neighborhood is rich enough to not want to see the ugliness.
Why are they suspended on poles?
I’ve thought that it’s a waste of wood, steel, and work to put up those things and have to keep putting them up whenever they fall. I mean, why not keep them along the ground, perhaps inside a tube of something insulative?

I’ve been told this - 1) electricity is lost easily when wires have constant contact alogn miles of roads. Rather, an imperfection in the wire will not be noticed when it’s laid flat along the ground, and will put its energy straight into the earth, but if it’s mostly just touching air, and you can carefully check each ceramic insulator on the poles to see if electricity is being lost to the ground, then it’s more effective.
2) Petty couriosity is deterred when power lines are beyond human reach. Riiight. Gimme a stick and I can knock one of those things down. Of course, then I’ll die, but that’s another matter…
3) Animal curiousity is deterred. Only squirrels and birds can be killed by high power lines, but one on the ground will kill anything ANYTHINg that gnaws it.
4) It’s just been done this way. Yah, you have plastic NOW, but wood was all they had when power distribution got popular, and that’s the wa it will stay.

Does anyone know? Gven my druthers, I’d see power lines be strung along inside strong plastic tubes perhaps six inches in diameter with two-inch walls, pulled tight and rarely touching the walls of the tubes or anything, just held up a tregular intervals.

2

I think pulling it that tight would make it more fragil – ever notice that power lines are kind of loose?

And I definitely think that ground level lines would have people – kids, especially – messing with them constantly. Plus, they would be in our way much more than they are now, which would encourage people to mess with them even more than curiosity would.

3

The power lines strung up on poles are relatively taut. It’s just the weight of the wire which makes them sag. If you pulled them any tighter, they’d snap.

I’ve heard that a lot of power lines are buried these days, which cuts down on the work involved in maintaining poles, and eliminates human/animal curiosity and meddling.

4

High Voltage power lines can breakdown the air gap between you and the line, if the gap is not sufficiently high. Besides, putting them in the ground has the following disadvantages :

1> If there is any fault it becomes harder to detect.
2> People digging up power lines accidently and ,…
3> High Voltage lines will need more spacing even if you put them underground.
4> Insulation rots inside ground easier. No Insulation is needed on above ground high poles
5> For low voltage lines - you can have street lights and power on the same pole.

I’m sure teeming millions will put forth other reasons too.

5

Once you hear how 20,000 homes lost power because some jerk ignored the signs and used his backhoe to dig up a power line, you’ll understand why they are placed out of reach of idiots - either on poles (wood is cheap, btw) or under pavement. Lots of pavement.

and still, they manage to destroy them - on the plus side, doing so is sometimes fatal, so there is a self-limiting feature to this behaviour.

6

Driveways.

7

I think SmackFu is right. You can’t have wires interfering with traffic, so it must be either above or below the traffic.

For long-disance transmission, traffic isn’t a big concern but those use higher voltage so you want to insulate the wires from the ground. Air is a cheap and reliable insulator.

8

Personally, I find the towers appealing.

9

  1. Power lines operate at lethal voltages, so you don’t want people coming in contact with them. The only safe places are high overhead, or underground.

  2. When most of the first power distribution lines were installed in residential/commercial areas, there wasn’t a viable way of putting the cables underground. The polyethylene/polyvinyl chloride/poly tetrafluorethylene/etc insulation materials we have now simply didn’t exist. Cotton and oil-soaked paper were available, but unreliable/expensive/required high maintenance.

  3. Underground cabling was and still is more expensive than overhead wiring. Lots (probably most) new housing estates use underground cabling despite the extra cost, for esthetic reasons. But it’s difficult to justify the cost of ripping out existing overhead wiring and replacing it with underground cabling.

10

The higher the poles are, the better, but there are practical and financial limitations.

One significant factor is the cable to ground capacitance.

If the whole network were simply run over ground in an infinately strong insulator this would become a very significant problem.

Capacitance is inverseley proportional to the distance between the electrodes, the closer they are the more it is, and running overground in a tube means that the tube itself would have to be a very good insulator indeed, which also makes it an ideal dielectric, the better the dielectric the greater the capacitance.

Underground cables are run as individuals in high voltage applications in order to reduce the effect of intercable capacitance.

High capacitance limts the ability of a network to carry current, you might construct the cable to carry 10MVA but circuit condions such as capacitance may well limit this to much less than 9MVA, or worse in extreme cases.

Think about this, the network needing to carry 10MVA would have to be overdesigned to carry perhaps 12MVA and this would add lots to the cost, all the switchgear would have to be designed to this level, and there is a strong possibility that everything would have to be pyshically larger, HV switchgear can take up a large amount of land as it is.

11

I don’t know about you, but I sure wouldn’t want to be anywhere near a backhoe that digs up a high voltage underground line. It’s bad enough digging up water mains, sewer lines, telephone lines, fiber optic cables, and the other sundry underground utilities (all of which happens more often that you may realize) without having to worry about high voltage electricity. Digging into one of those would almost certainly be fatal.

12

Look, casdave, I can see that you are knowledgable on the topic, but the way you’ve expressed it makes it pretty much incomprehensible to the average reader.

13

It is way faster and easier to run the HV cables above ground. Also the trouble-shooting and or re-routing of the lines is much easier.

We wouldn`t have all these videos of bears, parachuters, small aircraft, etc. getting caught in the lines if they were buried under ground.

14

No. Fatalities may happen ocassionally. I can’t remember any recently in this country, but I don’t deny that they may happen. But in all the cases that I can remember the backhoe operator not only survived but had to pay the cost of repair.

15

CASDAVE - why/how is the capacitance a bad thing?

16

Hmmm…

Capacitance is the ability of a circuit to store a charge, which property can be crucial, or disastrous depending upon the circumstances.

In the case of a supply network (by which I mean the system that puts the power right from the generator right the way down to your house) we have both capacitance and inductance which are both fairly undesirable outside of certain design parameters.

Taken together, inductance and capacitance form a property called reactance.
You could look at inductance and capacitance as the two opposites of a similar thing.

In the case of capacitance, what happens is that as you turn on a circuit, there is a flow of current that is just charging up the capacitor, it is not doing any useful work. This current flow starts off high but reduces over time as the capacitor gets closer and closer to being fully charged.

In virtually all high power distribution sytems the supply is being continuously reversed so current travels in one direction, and then it goes the other way, - alternating current.

When you reverse the supply voltage then the capacitance in the network has to be first discharged, and then charged up again the other way around, but, as I said, this type current is not doing any useful work.This is called reactive power, and is dependant upon the amount of reactance in a circuit.

EE’s have decided to seperate out the current that is doing useful work, from that which is not (even though both are transmitted down the line at the same time), this makes design much, much easier.

What we say the is that there are two currents travelling in a network at any given time, the real power, and the reactive power.

The supply cables have to carry both the real power, which is what is doing useful stuff like running tv’s and dishwashers etc, and reactive power which does nothing useful at all for you.

If you could completely get rid of reactive power, then you could use smaller cables for the same power at the consumer, ie you.

Designers try to minimise undesirable elements such as reactive power, and this is why capacitance could be a bad thing.

In real systems you do need some sort of reactive power and circuit elements are built in to provide for a specific amount of it, but you can have too much of it.

17

If HV lines were to have insulation around them for the pupose of stringing along the ground then the insulation would be many times the width of the cable. Im guessing that currently there is not a cable making machine with the specifications to make this gigantic wire. If every overhead line was scrapped and replaced (in the USA)with this impractible wire (and pylons removed and underground tracks dug) then I would be interested to see how long Bill Gates fortune would last if he was the guy paying for it. Im guessing he would be bankrupt in under 2000 miles.

18

Telco checking in.

About once evey two weeks I have to send out a repair crew because a contractor cut through one of my customer’s T1 or T3 lines, or god forbid one of our IOF trunks. If 1 cable cut per month per tech is typical, that works out to 120 per month. That might sound excessive to laymen, but it sounds about right to me.

19

Another factor - Trees. Trees swaying in the wind can break lines.

Its true that most poles are not above the tree line. And many utilities spend a lot of time trimming those back. But being up higher gets you more out of the range of a lot of the low growth.

I think they’d go higher if they could get longer poles at a reasonable price, and service equipment (bucket trucks) to operate safely at higher heights.

The present height of poles seems to be an optimum tradeoff of the many factors described in earlier responses, for distibution from the substation outwards (4KV to 20KV) and end-user distribution (typically 220V). And BTW the 220V is always lower on the pole. (I’m using typical US Voltages for all this.)

However, for distribution between substations and ‘overland’ at 30KV to MV’s they use those steel contraptions that go much higher up.

Bob K