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Question about electricity towers and poles

  1. Jan 25, 2015 #1
    I have several questions about poles and towers.

    a. Does all of transmission towers carries three-phase powers? Is there no transmission towers designated for single-phase?

    b. Does all towers are for transmission purposes only? Is there any towers in the world used for distribution instead of poles?

    c. How do I know if a pole carries single-phase or three-phase? Does the look, style, or shape of a certain pole dictates what it carries?

    d. How to tell if the wires I am seeing between poles are primary voltage line or secondary voltage line?

    e. I saw this phrase in one website : "Grounded wires on the utility side of the system do not generally have insulation."
    >> I. what does the utility side means?
    >> II. why does it has no insulation? it would be risky right?

    1. Please answer the question according to number.
    2. Label your answer according to what number you are answering
    3. This question generally ask about some "real" aspects which might be too obvious to some of you. Please don't be rude
  2. jcsd
  3. Jan 26, 2015 #2
    a. Most of them,yes. However, there are towers for HVDC transmission (they carry two wires of opposite DC polarities). And there are "small towers" carrying a 25 kV AC single phase line to power railways. Some other examples of single phase HVAC transmission are in existance too.

    b. No, some of them carry links for communications as well. Don't understand second question... big electrical towers are usually called pylons.

    c. Count the wires or number of insulators per tower, for HV range usually it works. Yes, it does.

    d. You can't tell. Primary and secondary sides are determined by direction of flow of the energy.

    I. In that context, a LV side where energy is consumed.Utility line has a broader meaning though.
    II. Is "grounded" wire in 4-wire cables insulated or not?
    At place of actual grounding it must be uninsulated (otherwise, grounding wouldn't have a purpose)
    Last edited: Jan 26, 2015
  4. Jan 26, 2015 #3


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    I gotta a question. Is this for homework?
  5. Jan 26, 2015 #4
    No it was not a homework. I am currently student of engineering course and all of those questions came from myself and there as part of my curiosity
  6. Jan 26, 2015 #5
    What I am trying to say here is, does majority/all of pylons are dedicated for transmission purposes only? I believed that poles served as way of distributing voltages ( in other words I believed that poles are used in distribution system).

    I am asking about "How do I know if a POLE carries single-phase or three-phase?" not towers/pylons.
    To emphasize this is when I walked outside of our house ( to be specific, on the streets), I saw several electrical poles. Some of them are made of wood, steel, and concrete. Then how would I know if that particular pole carries Three-Phase power or Single-Phase power?

    I don't get this part. Are you asking me Sir? Well, I don't know the answer.

    Then I ask Sir why there is no insulation? It would be risky right?
  7. Jan 26, 2015 #6
    Yes, pylons are dedicated to transmission of electrical power, to carry HV lines. Sometimes they carry fiber optic cables for communications purposes too.Poles and smaller towers serve to carry overhead lines of smaller voltages and for distribution. Sometimes you see 3 bare wires for medium and low voltage hanging to a pole, and sometimes, in the case of a low voltage, poles may carry just one thick overhead power cable. In that cable there are wires of all 3 phases, and they must be insulated.

    See previous answers. Vast majority of electrical poles carry all 3 phases.
    When you see bare wires on pole, insulators on poles and surrounding air is only insulation, and no other insulation is needed in that case. Advantage is that uninsulated overhead wires can carry more current than insulated cable given same cross section of conductors.
  8. Jan 26, 2015 #7


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    am questioning that answer ... if the conductor cross-section is the same, the current carrying capacity will be the same
    regardless of if it is insulated or not

    The real advantage is ... the uninsulated power lines save huge amounts of money in the cost of the plastic insulation

  9. Jan 26, 2015 #8


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    I think he's thinking about the lower heat dissipation of insulated cables leads to a higher temperature rise given all the other factors are the same. I'm not sure how much that would lead to derating the cable, but your point is also valid that it makes the wire cheaper when it has no insulation.
  10. Jan 26, 2015 #9
    It isn't. Aerial bundled cables (and power cables in general) harder dissipate heat than separated naked conductors in air. Besides their upper temperature due to overloading is lower since it can hurt their insulation. Interestingly, seems that fact isn't listed in the linked wiki page

    There are advantages as well as disadvantages.
  11. Jan 26, 2015 #10


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    You have only now brought up bundled cables ( which is irrelevant) ... the general conversation was just about insulated or uninsulated conductors

    Berkeman covered that nicely :)

    cuz for bundled cables you MUST have insulated anyway lots of sparks otherwise ;)

    As I saw in my neighbours dropper cable early in the morning last week
    I got woken up by bangs outside ... thought some one was breaking into my car (which is parked outside the bedroom window)
    nope all OK , went back to bed and the bangs continued, next time I looked out I saw a flash of light hmmmm

    got dressed and investigated ... the 240V dropper cable ( contains a phase and neutral) to my neighbours home was arcing and showing sparks
    with the appropriate bangs. Adding to the mix was rainwater running down the outside of the cable till it got to the damaged section and provided an even better path between the phase and neutral conductors :)

    Had to phone the fire service who came out and they contacted the power authorities. and here comes the worst part of it .... the fire brigade left BEFORE the power guy arrived ... I happened to glance out my window and saw neither there and went to investigate. The cable had now broken right through and the live end was now sitting /dangling down in the middle of the footpath and still arcing away happily, ready to zap some poor unsuspecting early morning jogger ( in the dark)
    Another call to fire service saw their return and securing the area till the power guy arrived :eek::rolleyes:

  12. Jan 26, 2015 #11
    Well, as concerns current carrying capability it is relevant not just if they are insulated or not, but also if they are bundled or not (active surface to dissipate heat comes into play), materials, cable designs, enviroment etc. That's one separate big topic. Anyway, OP was merely interested what electrical poles, consoles, pylons/towers carry, and I hope my answers were of some help. And there is much more to tell and if he/she wants to know more, googling key words like "transmission of electrical energy" will be helpful undoubtly.
  13. Jan 27, 2015 #12


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    Utility side: The wires between the consumer and the nearest transformer.
    Grounded wires are at ground potential. Therefore no risk.
  14. Jan 27, 2015 #13


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    A good reason not to insulate (from http://www.oppd.com/residential/trees-power-lines/transmission-lines-trees/)

  15. Jan 27, 2015 #14


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    It sounds like you have some strange ideas about the difference between transmission and distribution. I don't know what you mean by "distributing voltages." I suggest you read both of these articles on Wikipedia. Power Distribution, and Electric Power Transmission. You will l not find universal agreement on the definition of where transmission ends and distribution begins.

    Your questions are also confusing regarding what is seen sometimes, versus seen primarily. Those terms are hard to define.

    I am in The Florida Keys right now. Here, there is only one highway, and one set of poles/towers. They are forced to carry transmission and distribution and communications on the same poles/towers. On a global scale that is unusual, but in some places it is normal.

    zoki85 gave you the best practical advice: look overhead at the number of wires, and the number of insulators, and try to figure out their function and the voltage level.
  16. Jan 28, 2015 #15


    Staff: Mentor

    Here is a picture of a pole/tower in my vicinity. I show it not to overwhelm you, but to illustrate just how much you can teach yourself by looking carefully and trying to reason out the functions one step at a time based on what you actually see.


    From top to bottom:
    1. A single wire with no insulators. This is the shield wire that protects the pole from lightning. This wire carries no power.

    2. Three wires with many insulators each. This is a three-phase power transmission circuit.

    3. Two crossbars with three wires each, and fewer insulators. (fewer insulators means lower voltage) These are two three-phase distribution circuits.

    4. A single mystery wire. You can not be certain by looking that its function whether power rather than communication. I believe it is a single-phase low voltage distribution cable with two insulated conductors inside a plastic sheath and spiral wrapped on the outside with a bare wire. Zoom in and look carefully and you can see that the spiral wire is grounded to the tower.

    5. Two wires going off to the left. Look carefully and you can see that they have no insulators at all. They are attached directly to the metal tower. These are not electrical wires, they are guy wires helping to support another pole on the other side of the highway.

    6. Two black sheathed sets of cables for communications. These carry telephone, and cable TV, and Internet. Possibly, there are both wires and fiber optic cables inside. In all cases (I believe) you will find communications cables lower on the pole/tower than electric power cables. That is required for safety of linemen, and it greatly reduces the chances of those wires getting struck by lightning and thus endangering people talking on the phone.

    7. Not shown in the picture. If you look around the base of the metal tower, you will see that it is attached to an earth ground.
    In the second picture below, you see another tower. Three wires from one of the distribution circuits go to nearby wooden poles that supports three distribution transformers (barely visible at the bottom of the picture). Cables from those transformers go to nearby buildings. A black communication cable also branches off to the buildings on the right.


    Just stop, look, think, and be patient. The purpose of the things you see may become clear.
  17. Jan 28, 2015 #16

    jim hardy

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    That's the Fla Keys - I remember when those metal towers went in, just after Hurricane Andrew blew down the wooden ones bringing in power from Miami.
    We were a couple weeks with very limited power , from diesel generators in Marathon.
  18. Jan 28, 2015 #17


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    Do I see a Florida Power Engineers Association in the making? Or maybe a party in Fl? Can I come? :olduhh:
  19. Feb 1, 2015 #18
    You really get what I am asking and you answered it amazingly! Thank You for elaborating it to me! As well as other kind users that help and try to explain it to me. Thank you very much all of you Sir/Maam

    As far as I know. the poles are used in distribution and towers.pylons are used for transmission. I am very amazed in that pole that carries both transmission and distribution line :)
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