Why are Overhead Power Lines Loose on Electric Poles & Towers?

Why are Overhead Power Lines Loose on Utility Poles and Transmission Towers?

Transportation of people, goods, and services today is no longer a problem, as technology is rapidly advancing. Distance is no longer a barrier or an issue. Just as people, goods, and services are easily transported across long distances, electricity is no different. However, unlike goods, electricity requires a medium through which it is transported. Often, you may notice that the power transmission lines hanging on electric poles are loose. You might have wondered, Why aren’t the lines fastened tightly to the poles?

Let’s find out why power lines are loose on electric poles and why sag is needed in distribution and transmission lines. But before we delve into that, let’s take a look at some important tips that will help us better understand.

Power Transmission

Power is the rate at which work is done. Work is done when electrical energy covers a distance. From this, we can deduce that power is the amount of energy delivered per unit of time. Electricity can be transported over long distances via power lines, which serve as the medium for transporting it.

Active power is typically measured in watts. When transmitting power, high-voltage transmission is preferred to save energy. Electric current generates heat, which can be detrimental as it causes the electric lines to wear out and fail. To conserve the transmitted power, electric current, which produces heat and contributes to line wear, should be transported in smaller quantities, while voltage is transported in larger quantities. This approach is known as high-voltage power transmission.

Conduction and Linkage 

During power transmission, some electrical power is lost to our surroundings as the transmission lines are not insulated. According to Ohm’s law, resistance (R) varies directly with the length of the conductor (L), meaning that as the length of the conductor increases, its resistance also increases. Air is not a good conductor, so it cannot efficiently dissipate the heat generated by the electric lines.

This is why electric lines  are engineered to have larger diameters, which in turn reduces their resistance to the flow of electric current. Resistance (R) varies inversely with the area of the conductor, so the larger the diameter of the conductor, the lower the resistance, and vice versa.

Electric Wires and Cables

Electric cables and wires are conductors, mostly made of copper wires, through which electricity is transmitted. However, these wires are not composed solely of copper. To impart mechanical properties, the conductors are alloyed with another element. The conductivity of the conductor is not affected by the addition of this other element. Instead, the other element enhances the copper’s mechanical properties without impacting its conductivity.

Joule’s Law of Electric Heating

There is no such thing as a pure metal. The degree of purity of any metal is never 100%, and as a result, they possess internal resistance. The energy expended or heat generated when current flows through the conductor is calculated using the Joule’s law of electric heating as follows:

• P = VI.t
• P = I²Rt.

Other forms of Joule’s Law

• P = I2Rt
• P = VI.t … (R = V/I)
• P = W.t … (P = W = VI)
• P = V2t/R …. (I = V/R )   using Ohm’s Law

As can be seen from the equation above, the heat (P) generated by the moving electrons is proportional to R, t, and I². When an electric current flows through a conductor, it dissipates electric current into the surrounding environment in the form of heat when overcoming resistance, which acts as a barrier to the drifting electrons.

Effect of Weather & Temperature on Electric Lines

The resistance of a conductor increases with an increase in temperature. This occurs because, as the temperature of the conductor rises, the electrons within the conductor gain more energy and move randomly, leading to collisions with other atoms, which ultimately results in the generation of heat.

Excessive heat produced by the conductor can potentially cause it to melt. In hot weather, the wires tend to become looser as the conductor expands, whereas in cold weather, the wire contract.

Tension in Lines

Tension is a force that exists in a string when it is under the action of two forces in opposite directions. Therefore, a wire hanging on a pole is under tension and would experience even more tension if the wires were to be made tight, which could lead to the wires cutting easily when they undergo slight contraction or expansion.

Why is Sag Required in Distribution and Transmission Lines?

Sag in transmission lines refers to the downward dip or curvature of the cables between support structures (poles or towers) due to the influence of gravity. It occurs as a natural consequence of the wire’s weight and tension.

During the transmission and distribution of electricity over long wires, heat is dissipated. The heat generated by conductor is minimized through high-voltage transmission. Weather conditions and the internal temperature of the wire make it necessary to leave the wire lines somewhat loose.

If the power lines were tightened, and the weather became cold, it could cause the transmitting lines to contract, thereby creating more tension in the lines, which might result in damage. Therefore, the wires are intentionally left loose so that even if contraction occurs, there won’t be excessive tension that could lead to wire and cable damage.

Sag is mandatory in transmission line conductors to prevent overheating and reduce tension. It ensures the safety, reliability, and longevity of the electrical transmission system. It plays a crucial role in maintaining the proper functioning of the system and preventing accidents and damage.