Why do power lines sometimes have different amount of cables on them?

I thought there should be 5: three hot wires, a neutral wire, and a ground wire. But some of them have 4, and some have 3. I've never seen 2 or 1.

Usually 3 of them are very high, and the 4th or 5th one are lower. I always assumed the 3 high ones are the hot wires, and the lower 2 are neutral and ground wires. It is possible that one of the lower wires could be a telephone wire because they seem darker and hence insulated, or it could be a guy wire between two poles, but I don't know.

I sometimes see poles with just 3 wires, all at about the same height. If they are all hot wires, where did the neutral wire go?

 

Maybe a good starting point:
http://en.wikipedia.org/wiki/Overhead_power_line

under "circuits" and "ground wires"

 

if we are talking distibution and higher there will always be at least 3. 3 wires for 3 phases. usually there will be a neutral/ground. so typically 4. when you get into the big pylons you will commonly see 6 wires. it is two separate 3 phase circuits. usually you know its the 3 phases because they will have the largest insulators compared to smaller insulators of a ground/neutral. commonly the neutral/ground are on the same line. hope that helps!!

 

Why are there two separate circuits on the big pylons? If you have transformer substation, do they only use one circuit from the big pylon, or both circuits? They only need one, but then why are there 2?

Doesn't the neutral wire come out of the power-plant? So how can you not have a neutral wire, and only have 3 wires for 3 phases?

Also, is the neutral wire grounded near the power-plant? I know the neutral wire is grounded at the service panel in homes, and I know that on low voltage lines (I guess I should call low voltage lines distribution lines, and high voltage ones power lines) sometimes the ground wire is combined with the neutral wire, sometimes they are separate wires (what's the point of having a separate ground and neutral wire between poles?), and sometimes there doesn't seem to be a neutral/ground at all, and instead you just have 3 wires which I assume are the 3 phases. But how can you not have a neutral? For example this is a picture of what I sometimes see in the neighborhood:

http://dizorb.com/wp-content/uploads/2010/02/Power-line_Dizorb_dot_com.jpg

and there doesn't seem to be a 4th wire.

 

Had an uncle who retired from his very first job as a young graduate engineer. He worked for the local offices of Kansas Gas and Electric Co for his entire career. I used to browse through his books and I can tell you that there are no "standards" when it comes to configuring the cross-country pipes for electrons at the local levels. When major companies have to tie into each other, there's a great deal of cooperation but at the 'neighborhood' level you can see a variety of philosophies as you travel across the country.

It's likely that the photo you posted is 3-phase power that probably has one leg earth grounded at both ends of the run (for lightning protection) but it could well be totally floating on all three legs too. You'd have to probe each line and measure voltage with respect to earth to figure this out.

In any case, a "neutral" or grounded line is not necessary until AFTER voltages between phases is applied to a transformer and a center-tapped secondary provides 240 single phase power to a customer's facility. At that time, the center tap of the secondary is tied to earth ground and to the 'neutral' bus bar in your breaker box.

You're provided with single phase 120/240 by using power between either outside leg and neutral (120) or between both outside legs (240). The white wires (neutral) and bare wires (safety bond wire) all go to the same ground bus in the breaker box. The white wires are power returns and carry the same current as the protected feeder (black). The safety bond wire is never expected to see current flow . . . current flow in that wire is a 'ground fault' failure in wiring or a powered appliance which is expected to trip any GFI protection systems that may be installed.

The stuff on the poles outside can be any of several combinations but if you see a 4th wire, it's probably the top wire on the array and is earth-grounded at every pole for lightning protection. This wire is not a part of the energy distribution system. Next time you see some new power poles laying on the ground awaiting erection. If they have a copper wire stapled down the side of the pole ending in a little spiral array on the bottom, that's the 4th-wire ground. I've seen 4-wire systems on metal poles where the top wire has no insulator. It simply travels from pole-to-pole with the hope of tagging a lightning strike and conducting it directly to the adjacent poles.

 

In essence, if the AC line currents are balanced (equal in magnitude) then you don't need a neutral conductor. Supply authorities would try to ensure the line currents are balanced in a 3-wire transmission system.

Also if you have a delta connected three phase alternator there's no place to make a neutral connection.

The neutral becomes important at the consumers' end of the distribution network where single phase supply is used. Most modern systems employ multigrounded common-neutral distribution methods at the consumer end of the network.

 

Why are there two separate circuits on the big pylons?

My guess would be weight of the wire. To transmit the amperage in one set of wires the wire would need to be heavier. Then the pylons would need to be closer together to support it (to prevent sag of wire) Two separate smaller wires would carry the amperage and be lighter.

 

Why are there two separate circuits on the big pylons? If you have transformer substation, do they only use one circuit from the big pylon, or both circuits? They only need one, but then why are there 2?

just for simplicity and to put things into perspective, think of a power plant having two generators. one generator feeds one side of the pylon and 2nd generator feeding the other side. then at the opposite end, one side could feed half a big city and other side feed the other half of the city. both sides are independant of each other. its easier than constructing two seperate pylons for each circuit.