3 Topic: Conductors (1.5h)
Instructions:
- Cover the following content as a group (either reading out loud or independently) then give an opportunity to answer any questions.
- Have students do the review questions independently, then take up answers.
Types of Conductors
The Construction Standards Manuals separate conductors into two categories: overhead and underground.
Overhead
Utilities use several types of overhead conductors. Each has advantages and disadvantages which dictate where they are best used.
Copper
Advantages and disadvantages of using copper as a conductor include:
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Some advantages of copper are:
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Some disadvantages of copper are:
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Copper conductors come in three different tensile strengths:
- Hard drawn
- Strongest tensile strength of the coppers
- Used for overhead circuits
- Medium Drawn
- Soft Drawn (annealed)
- Made when hard drawn copper is heated and cooled to soften it (more flexible)
The use of copper in overhead lines is old construction and is now replaced with other conductors. Copper is mainly used on ground wires, risers, and jumpers, as it can handle high current flow and repeated flexing (cut-out risers).
Aluminum
Following are the advantages and disadvantages of using aluminum as a conductor.
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Some advantages of aluminum are:
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Some disadvantages of aluminum are:
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Straight aluminum conductor is used in short spans and is designated with a “flower” name in the Construction Standards Manual. For example, “266 Daisy”
ACSR (Aluminum Conductor – Steel Reinforced)
ACSR is used almost exclusively on transmission, distribution, and secondary lines. It is an aluminum conductor that has one or more strands of steel in the middle. The steel gives the lightweight aluminum a much greater tensile strength, making longer spans possible. It is designated with a bird name. For example, “3/0 Pigeon”
Some advantages of ACSR are:
- Lightweight
- High tensile strength
- Low cost
Steel (Hi-con)
Following are the advantages and disadvantages of using steel as a conductor.
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Some advantages of steel are:
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Some disadvantages of steel are:
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Hi-con was used almost exclusively on rural lines, but the advent of ACSR, with its excellent ampacity and long spans, brought about its demise. It is mainly used on shield wires (transmission lines) to protect conductors from lightning.
Copperweld
This conductor consists of either two copper strands with one steel strand for strength or copper coated steel strands. While having a higher capacity than Hi-con, it also could not compete with ACSR.
Coated Conductors
Overhead coated conductors make up the secondary conductors (Duplex, Triplex and Quadruplex).
These conductors distribute secondary voltages and are insulated to provide protection. They consist of various sized aluminum conductors wrapped around a bare ACSR conductor. This bare conductor is the neutral and is usually one size smaller than the conductors.
- Duplex – 1 insulated conductor and 1 neutral = 2 wires
- Triplex – 2 insulated conductors and 1 neutral = 3 wires
- Quadruplex – 3 insulated conductors and 1 neutral = 4 wires.
The neutral is smaller on some overhead secondaries because it can be cooled by the surrounding air. The coated conductors will hold more heat, thus increasing resistance.
Line Conductors
When working with conductors, you will learn that sag is one of the most important factors besides resistance. Many factors will affect the sag of a conductor, which in turn, can increase or decrease ground clearance.
There are five factors which affect sag:
- Elasticity
- Span Length
- Weight of the conductor
- Temperature of the conductor
- Tension
1. Elasticity
If you have sagged wire before, you will know there are two sag charts for each condition: initial and final. Anytime a new conductor is sagged, it is pulled a little tighter to allow for the “stretching” a conductor is subject to (wind and ice loading.) This is the initial sag.
After the wire has been subject to the forces that will stretch it, sag will increase a given amount and then stretch very little after. This is called the final sag. Anytime a used conductor is utilized, it must be sagged to this chart.
2. Span Length
Span length is the distance between two supports. A line with long spans has more conductor so the weight is increased. This weight will cause the sag to increase.
3. Weight of Conductor (or Type)
A heavy conductor will sag more than a light conductor. Therefore 3/0 ACSR conductor will have much less sag than 3/0 copper.
4. Temperature of conductor
Metals expand when hot and contract when cooled. This is also true with conductors.
5. Tension
The higher the tension a wire is subject to, the less sag a line will have. One may consider pulling the conductors extremely tight to increase clearance, but there are several reasons why this is not done. First, tension must be well below a conductor’s breaking strength. If a conductor is too tight and severe loading occurs, it will break.
Review Questions: Conductors (30m)
Match the statements on the right with the insulators on the left.
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____ It has a very high resistance |
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____ This is a coated steel conductor. |
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____ It is an excellent conductor |
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____ Long spans and good conductor |
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____ A low tensile strength limits it to short spans. |
- Copper risers are stranded and annealed to make them:
- Resistant to corrosion
- Stiff and firm
- Flexible
- All of the above
- ( Steel / Copperweld / ACSR ) conductor is most often used to protect lines from lightning.
- ( Hi-con / Copperweld / ACSR ) is the preferred conductor for rural distribution.
- ( T / F ) A steel core is used on secondary neutrals to support the coated conductors.
- ( T / F ) Conductor tension must not be exceeded, or conductor damage can occur.
- Answer Key
- 1.c, d, a, b, e . 2. C, 3. Steel, 4. ACSR, 5. T, 6. T