Introduction
In high-voltage transmission line engineering, sag templates are critical tools used to ensure safe, efficient, and cost-effective line design. According to Saudi Electricity Company’s Transmission Engineering Standard , sag templates play a central role in determining the vertical position of conductors and ground wires during the design and structure spotting stages.
Whether you’re a transmission engineer, designer, or student of power systems, understanding the principles of sag templates is key to mastering modern overhead line design and ensuring compliance with international and regional standards.
🔹 What Is a Sag Template?
A sag template is a scaled, graphical device used in transmission line structure spotting. It shows the vertical position of conductors and ground wires under different loading and temperature conditions. Essentially, it helps designers determine the optimum structure height and location to maintain safe clearances and minimize material costs.
Using the sag template on plan and profile drawings, engineers can visually evaluate:
- Vertical ground clearances
- Insulator swing limits
- Span lengths and weight spans
- Conductor uplift or excessive sag
This tool allows multiple layout alternatives to be tested for optimum economy and performance.
🔹 Types of Sag Template Curves
The TES-P-122.07 standard defines five major curves that must be represented on a sag template:
1. Cold Curve
- Represents initial sag at the minimum temperature (−1°C), with no wind or ice.
- Used to check uplift and insulator swing conditions.
2. Normal Curve
- Indicates final sag under everyday temperature (no wind).
- Used to verify normal clearance and insulator swing.
- Typical “everyday” temperatures in Saudi Arabia range from 25°C to 30°C, depending on the region.
3. Hot (Maximum Sag) Curve
- Represents final sag at the maximum design temperature with no wind.
- Used to ensure minimum vertical ground clearance.
- Maximum temperatures range from 80°C to 93°C depending on conductor type and area.
4. Ground Wire Curve
- Shows initial sag for the overhead ground wire at −1°C.
- Ensures the ground wire position aligns properly with phase conductors.
5. Ground Clearance Curve
- Represents required clearance above open terrain, drawn as a vertical offset below the Hot Curve.
- Used to determine low points and maintain safe operating distances.
🔹 Sag Template Design Method
Designing a sag template involves precise engineering calculations. For a given conductor, ruling span, and loading condition, sag values are determined using software or analytical methods.
The standard provides the following formula for sag extrapolation:
Equation :
S = SRS × (L / LRS)2
- S = Sag of other span (m)
- SRS = Sag of ruling span (m)
- L = Length of other span (m)
- LRS = Length of ruling span (m)
Where:
- S = Sag for any other span
- SRS = Sag of the ruling span
- L = Span length under consideration
- LRS = Ruling span length
Sag templates must include allowances (typically +0.6 meters) for plotting errors and field adjustments. Each template is designed at the same scale as plan and profile drawings for seamless overlay.
⚙️ Note: A new template must be prepared whenever voltage, conductor size, ruling span, or design tension changes, as these factors significantly affect sag behavior.
🔹 Materials and Preparation
According to SEC standards, a sag template should be made from dimensionally stable transparent plastic or celluloid (≈1 mm thick).
Key preparation guidelines include:
- Use contrasting ink colors (red/blue) for clear visibility on drawings.
- Curves are first plotted on paper, then etched or printed onto the plastic template.
- Include etched scales for structure height and conductor clearances.
- Each template must be clearly labeled with conductor type, tension, loading condition, and ruling span.
🔹 Applications in Structure Spotting
Sag templates are indispensable during structure spotting—the process of locating towers or poles along a transmission route.
By aligning the sag template with ground profiles, engineers can:
- Determine the height and position of each supporting structure
- Ensure clearances to ground, roads, and crossings
- Avoid uplift and excessive insulator swing
- Optimize line layout for cost and safety
This process ensures that the final transmission line alignment complies with the design criteria for mechanical loading, safety clearance, and reliability.
🔹 Why Sag Templates Matter in Modern Line Design
While advanced software like PLS-CADD automates sag and tension calculations, understanding manual sag templates remains vital for:
- Quality control and verification during design review
- Field modification and inspection
- Training and competency development for transmission engineers
- Compliance with national standards such as SEC’s TES-P-122.07
In essence, sag templates bridge the gap between analytical design and practical implementation, ensuring transmission lines are both efficient and compliant.
🔹 Key Takeaways
| Aspect | Description |
|---|---|
| Purpose | Determine conductor and ground wire position under various temperatures |
| Curves Included | Cold, Normal, Hot, Ground Wire, Ground Clearance |
| Main Use | Structure spotting and clearance verification |
| Design Formula | Equation : S = SRS × (L / LRS)2 S = Sag of other span (m) SRS = Sag of ruling span (m) L = Length of other span (m) LRS = Length of ruling span (m) |
| Material | Transparent plastic/celluloid (1mm) |
| Standard Reference | TES-P-122.07 Rev.02 (Saudi Electricity Company) |
Conclusion
The sag template remains one of the most essential tools in transmission line engineering. Whether used manually or digitally, it provides a precise graphical method for balancing safety, efficiency, and cost in overhead line design.
By following the SEC’s TES-P-122.07 standard, engineers can ensure their line designs maintain proper ground clearances, meet structural constraints, and achieve long-term reliability — especially in demanding environments like the Kingdom of Saudi Arabia.
ALSO READ: Plan & Profile Drawings in Transmission Line Engineering – Preparation, Purpose & Key Features
