Sag templates & Curves

Posted on by Engr. Hamid Ali

Sag templates are a critical tool in the design and construction of transmission lines. We usually use these to determine the vertical position of conductors (and ground wires) under specified design conditions, ensuring that the transmission line provides adequate clearance to the ground and other facilities. Below is a detailed explanation of sag templates based on the information available in the TES-P-122.07, Rev.02 document:

Sag Template Curves:

The sag template includes several curves, each representing the conductor’s position under different design conditions. These curves are based on the ruling span (a representative span length used for design purposes).

Key Curves in a Sag Template:

Following are the Key Curves in a Sag Template

  1. Cold Curve:
    • Represents the initial sag of the conductor at the minimum temperature (typically -1°C) with no wind or ice.
    • Used to check for uplift (when the conductor exerts an upward pull on the structure) and insulator swing.
  2. Normal Curve:
    • Represents the final sag of the conductor at everyday temperature with no wind.
    • Used to check normal clearances and insulator swing.
    • Everyday temperatures vary by operating area (e.g., 25°C for Central and Southern areas, 30°C for Western areas).
  3. Hot (Maximum Sag) Curve:
    • Represents the final sag of the conductor at the maximum design temperature with no wind.
    • Used to check for minimum vertical ground clearances.
    • Maximum design temperatures vary by conductor type and operating area (e.g., 80°C for ACSR conductors in Central and Western areas, 93°C for ACSR/AW Condor in Eastern and Southern areas).
  4. Ground Wire Curve:
    • Represents the initial sag of the overhead ground wire at the minimum temperature (-1°C) with no wind.
  5. Ground Clearance Curve:
    • Represents the specified ground clearance over open terrain from the maximum temperature (Hot) curve.
    • This curve is used to ensure that the conductor maintains the required clearance above the ground.
Key Curves in a Sag Template:

 Use of Sag Templates:

Sag Templates are used for the following reasons

  • Structure Spotting:
    • The sag template is used to determine the height and location of structures by ensuring that the conductor maintains the required ground clearance.
    • The template is placed vertically on the plan and profile drawings, and the ground clearance curve is aligned with the ground profile.
  • Checking for Uplift:
    • The cold curve is used to check for uplift conditions, where the conductor exerts an upward pull on the structure.
    • If the conductor support is below the cold curve, uplift occurs, and corrective measures (e.g., adjusting structure locations or using higher structures) must be taken.
  • Special Cases:
    • For steep slopes or uneven terrain, the sag template may need to be adjusted, and sag values recalculated to ensure proper clearances.

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