# Aerial and subterranean electric lines

1. Sep 18, 2004

### Clausius2

I've read in a newspaper aerial electrical lines (conductors) have less diameter than subterranean lines. Also, the typical weight of subterranean lines is 30Kg/m, while aerial ones is 2Kg/m. Why do they have such differences of diameter and weight? Must the subterranean lines be more isolated than aerial ones?

Another question: do you think subterranean lines causes less electromagnetic intensity field in the surface road than aerial ones?. Why?

2. Nov 19, 2004

### wwtog

The ampacity of cable is mainly due to two things, how much heat it can take and still remain strong enough not to stretch (areial). Or how hot it will get before the thermoplastic insulation will flow, or otherwise stop becoming an insulator (buried). Which case do you think will require more heat?, neglecting thermal heat transfer into air or soil.

3. Nov 19, 2004

### brewnog

Even thinking of it as a purely mechanical problem, aerial cables have to be self-supporting to some extent, and the transmission towers (pylons) have to be able to bear this weight too. If the cables were 15 times heavier, the towers would have to be so much stronger (and more expensive).

The other thing I thought of, - perhaps subterranean lines are more frequently used over longer distances and carrying more current, but with a bit more thought I guessed that this (at least the distance bit) is probably not the case, as it is much more expensive to tunnel a cable than to string it up... Just a thought!

4. Nov 19, 2004

### Clausius2

Thanks to brewnog and wwtog.

I see what you mean, brewnog. But nowadays the local governments make the effort of burying electric lines to avoid electromagnetic radiations, as they say. Subterranean lines are more expensive, due to the proper process of burying, but I got surprised when I realized that external diameter of those lines are larger too. Why is it needed more material?.

Talking about electromagnetic radiation, the relative permitivity of usual dry terrains is $$\epsilon_r=4$$, the electric conductivity is about $$10^{-5} S/m$$ and I haven't found any figure for the permeability.

Is it true that burying the electric lines enhances less electromagnetic radiation in the surrounding?