Power Line Sag

The lines are often heavily loaded because of increased power consumption, and the conductors, which are generally made of copper or aluminum, expand when heated. That expansion increases the slack between transmission line structures, causing them to sag.

How do you calculate sag on a power line?

y ( x ) = H w cosh ( ( w H x ) − 1 ) = w ( x 2 ) 2 H . For a flat span, the low point is at the center and the wire sag D is found by substituting x = S/2. Exact and approximate formulas for the sag calculations is shown in Equation (10): D = H w ( cosh ( w S 2 H ) − 1 ) = w S 2 8 H .

Why do power lines need to sag slightly?

The lines are often heavily loaded because of increased power consumption, and the conductors – which are generally made of copper or aluminum – expand when heated. That expansion increases the slack between transmission line structures, causing the lines to sag.

Why are overhead power cables hung with a sag?

Tension provides the other two forces, which should be tangent on the cable and the same magnitude at the bottom. Seeing this, you understand why the cable must droop. Because the cable has mass (and therefore a gravitational force pulling down), there must be an upward component of the tension to keep it at rest.

What is SAG limit?

The maximum sag (dip) is represented by the value of y at either of the support points. At support point A, x = l/2 and y = S (sag) therefore, sag S = w(l/2)2 / 2T. therefore, sag S = w.l2/8T.

What is sag and tension?

Tension. The sag and tension are inversely proportional. As the tension on the conductor increases, the sag decreases. The tension on the conductor increases by wind pressure, ice loading and natural disasters.

What is SAG formula?

Sag = [WLA2 / 8TA] where LA = Actual span.

What is SAG and its formula?

In a transmission line, sag is defined as the vertical difference in level between points of support (most commonly transmission towers) and the lowest point of the conductor. The calculation of sag and tension in a transmission line depends on the span of the overhead conductor.

How do you know your sag?

To calculate your sag percentage, divide the static travel by the total length of the shock and multiply by 100. Most manufacturers recommend that you set your sag somewhere between 25 to 35 percent.

Do power lines sag more in summer than winter?

A line sags more in hot weather and less in cold weather. The reason for this is because conductors expand in hot weather; in other words the length of the conductor increases as the temperature increases.

Why do electric wires sag in summer?

Metals expand in heat and shrink in cold temperatures. Electrical wires being made of metals become warmer due to rising temperature and expand in both length and diameter. That is why they sag in summer. In winters, they are cooler and hence they shrink.

What is sag point?

The sag point is defined as the temperature at which a "lass fiber 0.5 to 0.8 millimeter in diameter, horizontally supported at . H-inchintervals, will sag under its own weight in 25 ± 5 minutes.

How do you know if a cable is sag?

The sag relative to a support equals cable weight per unit length times the distance to the low point squared divided by twice the horizontal tension.

What is safety factor in Sag?

Normally the tension in conductor is kept one half of the ultimate tensile strength of the conductor and therefore safety factor for the conductor is 2.

Why are there 3 power lines?

Because they carry 3-phase alternating current. When there are just 3 cables, the circuit is closed by ground.

What are the factor affecting sag?

The SAG is directly proportional to the weight of the conductor. The SAG is directly proportional to the span length. The SAG is inversely proportional to the working tensile strength of the conductor at constant temperature.

How do you prevent a sag in a transmission line?

Present techniques to compensate for such sag caused by undesired increase in length of a cable include:

1. (i) Shortening the distance between adjacent towers to reduce span length and thus reduce line sag; this is shown in FIG.
2. (ii) Erecting taller transmission towers to accommodate line sag; this is shown in FIG.

What happen to tension if the sag in an overhead line increases?

Tension is inversely proportional to sag. Sag S ∝ 1/T As sag increases, tension will decrease.

What are the factors affecting sag in an overhead line?

The sag of the line increases when the line elongates due to increasing temperature, mechanic load and time. The temperature of the conductor is dependent on conductor properties, line current and ambient conditions.

How do you reduce sag?

Tighten the bottom lock nut to decrease sag distance. Use your hammer and punch to knock the bottom lock nut clockwise at least 1 full turn. This will increase the shock spring's preload, which shortens the spring and decreases the sag.