At Ningbo Changshi, we understand that the sag of overhead transmission lines is not a sign of poor installation, but a critical, engineered design feature. The downward curve, or sag, in a power line is a natural and necessary result of two primary forces: the conductor's own weight and the effects of temperature and weather.

Key Factors Causing Sag

1. Conductor Weight and Gravity: The most fundamental reason for sag is gravity. The conductor, even when under significant tension, is a flexible material. Its own weight pulls it downward, creating a parabolic curve between support towers. This is an unavoidable physical property that must be accounted for during the design phase.

2. Thermal Expansion and Contraction: This is the most significant factor affecting sag. Like all metals, the aluminum or copper in a conductor expands when heated and contracts when cooled.

   • In Hot Weather: As the conductor heats up from direct sunlight, ambient temperature, or the electrical current flowing through it, it expands and lengthens. This increased length causes the line to hang lower, increasing the sag.

   • In Cold Weather: Conversely, a drop in temperature causes the conductor to contract and shorten, pulling the line tighter and reducing the sag.

3. External Loads: Weather conditions like high winds and the accumulation of ice or snow can add substantial weight and lateral force to the lines. These loads increase the effective weight on the conductor, which in turn increases the sag and the tension on the support structures.

The Importance of Proper Sag Management

Proper sag is essential for the safety and longevity of the power line.

• Safety Clearance: Sag ensures that the conductors maintain a safe vertical clearance from the ground, buildings, trees, and other structures. Excessive sag can lead to dangerous contact, causing short circuits, fires, and electrocution hazards.

• Preventing Mechanical Stress: By allowing for sag, the line is not pulled taut, which would put it under extreme tension. Without sag, the contraction in cold weather would create immense stress on the conductor and support towers, potentially causing the line to snap or the towers to fail.

Our specialized conductor tension stringing equipment is designed to precisely manage sag and tension during installation, ensuring that the lines are erected with the correct amount of slack for safe and reliable long-term operation under all environmental conditions.