How come the big metal towers that hold them up don’t short them to ground?
The towers don’t touch the wires. The towers touch glass, or glass-like, pieces that hold the wires. The glass peices don’t conduct electricity so there is no connection to ground. Next time you see a high tension line, take a look. You’ll see what looks like a rod with rings on it connecting the wires to the towers. That is what is made of glass.
I’m sure someone will come along with a more comprehensive answer full of interesting facts. I don’t any interesting facts related to this topic, just the basics.
Billy is correct. IIRC, the “glass” (or ceramic) pieces are better known as “insulators”, as a general term. Sometimes, when you look at these towers, you may notice a thick, short wire running between two sets of insulators. IIRC, this is known as a “jumper” to route the electricity past the tower itself. But, I don’t know why it seems that a “jumper” is not always needed.
Also, the height of the towers is often directly related to the voltage they carry. Higher voltage lines are elevated at a greater height above the ground. One exception you may see is when the lines have to cross above a bridge, for example…you’ll see taller towers to aid the wires in safely clearing the bridge, but the other towers in this “string”, away from the bridge, will be more typical.
Hope this helps.
- Jinx
Those “glass like pieces” are called “insulators” for those who like technical terms.
They are made of high strength ceramics. I’m not sure there’s much more interesting aspects of this for most people.
Ah, ceramics, nifty. I figured there had to be some good insulator seperating the wires from the steel towers, but never could see what it was. (Mind you, I have never been very interested in venturing a closer look. 
)
I’m sure Google will help you with the image bit. I have some nice old-fashioned glass insulators from a camping trip where we came across some old power poles where they ran by the railway tracks (they’d um… fallen over decades ago and there were no wires, FYI. Don’t just take any old ones ya find…).
Essentially the same principal used today.
Good gawd! People collect these things. Makes me wish I had an account on e-bay…
To clear this bit up:
There are two types of towers, “suspension” towers and “tension” towers.
Most towers are of the suspension type. In this type, there is one insulator string per conductor, and the insulator strings hang down vertically. The conductors are suspended from the insulator strings.
Every mile or so, or wherever there is a change in the line’s direction, there will be a tension tower. These towers have two insulator strings per conductor. The insulator strings in these towers don’t hang vertically, they’re strung “in line” with the conductors. Each conductor has an insulator string leading to the tower, and one leading away. The “jumper” piece is needed to connect the ends of the two insulators, each of which terminates a length of conductor.
To recap: a suspension tower supports a continuing length of conductor; a tension tower terminates one length of conductor, and starts a new one.
Why do High-Tension lines cause so much noise on AM radio? After all, the are only radiating at 60 Hz-so how do they generate noiseon the AM band?
Yeah but you still get something coming off of them. For fun go out at night with a fluorescent light in your hand (the long light bulbs are best for this) and stand underneath a high voltage power line. The light bulb will glow in your hand. Kinda cool!
For whatever reason, high voltage produces radio waves. I was taught it is these radio waves that cause fluorescent light bulbs to glow - as Whack-a-Mole mentions. IIRC, neon tubes will glow, as well. - Jinx
And they have to be cleaned now and then. We see a helicopter spraying them on a power line north of San Bernardino occasionally. This site is a short [summary of insulator contamination"]](http://psalserver.tamu.edu/~web/students/chris.html)
That is precisely why an insulator is formed by a chain of individual ceramic insulators shaped like an umbrella: to avoid dirt on the surface creating a conductive path.
I’m not sure, but I would guess that the reason that the lines mess with AM radio is the noise from corona discharge, which happens right around the conductors.
By the way, I’m an EE, although not in the power transmission part of the field. I have the impression that the term “high tension” is not used by people in the field, similar to how “tarmac” is not used by airplane/airport people. It’s like a common lay belief that this is the correct term. I’ve seen high-voltage lines labelled “haute tension,” which I assume is French for “high voltage,” but AFAIK the proper term in English is “high voltage transmission line.”
The “tension” in Desmostylus’s explanation is something else.
In case anyone was wondering why insulators have that odd ribbed shape, its to prevent surface arcing across the inductor.
Correct, and well spotted. The tension I was referring to is the force in Newtons in the conductor, caused by the weight of the conductor.
Ceramic insultaors are used in most HV cable applications but there are exceptions where glass is used instead.
The Ceramic insulators are actually porous and so are sealed with a type of varnish.If this layers of sealing is comromised water will get into the insulator and over time it will break down.
Some areas are more vulnerable to damage, Joe Public seems to like taking potshots with air guns and the like.
In such places glass insulators, though not as physically strong, will be used due to their lack of porosity, of course it goes wothout saying that glass insulators are somewhat more expensive.
I wonder if anyopne can recall the last time much of the north-Eastern US and adjacent Cnada had a blackout ? This was caused by one bboy with his air rifle, must have been over 20 years ago I’d say.
Ceramic insulators aren’t varnished, they’re glazed. Glazed porcelain in fact, just like a decent teacup.
Botherness, you’re right of course, I jumped in too quick.
That glaze is sharp too and impossible to see when it gets into your skin.