How To Make SAG Template for Tower Spotting – Part 1

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Route Plan and Profile

It is assumed by now that you already have a “route profile” or simply “profile” that depicts the elevation of the ground level from a reference datum. Details of all crossing such roads, rivers, forest area, ponds, railway lines and power lines along the route should be included. Objects within the width of Right of Way (ROW) shall be noted.

Incase of overhead crossing, the height above ground of the earthwire or OPGW should be measured. Likewise for crossing under existing power lines.

Typical scale in plotting of the route plan and longitudinal profile is 1cm =20m horizontal and 1cm= 2m vertical. The sag template should have the same scale with the route plan.

What is a Sag Template?

A sag template is a tool in spotting manually the position of towers or poles on the route profile. Today, transmission design engineers are using sophisticated design softwares.

A sag template is specific for the particular line voltage, the conductor used and local weather conditions. It consist of a set of curves drawn on a transparent celluloid or acrylic clear sheet. The set of curves in the sag template consist of:

  • “Cold or Uplift Curve”:
    • showing sag of conductor at minimum temperature and still wind.
  • “Hot or Maximum Sag Curve”:
    • showing maximum sag of conductor may experience. It may be due to creep or heavy loading due to ice and wind loads or due to maximum temperature.
  • “Ground Clearance Curve”:
    • which is drawn parallel to the “Hot or Maximum Sag Curve” and at a distance equal to the specified minimum ground clearance for the relevant voltage.
  • “Tower Footing Curve”:
    • which is drawn parallel to the “Ground Clearance Curve” and separated by a minimum distance equal to the maximum sag at the ruling span.
Illustration of Sag Template.

This method can provide the following:

  • Maintenance of proper clearance from conductor to ground and to crossing conductors
  • Economic Layout
  • Minimum possibility of errors in design and layout
  • Proper grading of structures
  • Prevention of excessive insulator swing or uplift at structures
  • Exact and correct quantity of materials purchased and delivered to the proper site

The primary goal of this post is how to make a sag template. Then the succeeding articles on how to use this template in locating transmission towers or commonly called “tower spotting”.

How to make a Sag Template?

Step 1: Gather the design data.

The design data will include the following:

  • The scale of the route plan or profile used.
  • Appropriate ruling span.
  • Conductor data for sag and tension calculations.
  • Stringing tension limits of the conductor.
  • Ground clearance standards for different voltage levels.

Step 2: Determine the weather conditions that may affect your conductor.

These data are necessary for calculating the characteristics of conductor curves.

  • Cold Curve : minimum temperature at no ice and still wind.
  • Maximum Sag Curve:
    • maximum temperature of the conductor due to electrical loading
    • maximum ice loading with no wind
    • maximum ice loading with concurrent wind

For areas in the US, refer to the NESC loading zone. Otherwise, refer to the local standard in your area. For example, here in the Philippines we don’t experience ice loading hence maximum sag will be determined by the maximum temperature due to electrical loading.

Step 3: Calculate the sag and tension of the conductor.

It is recommended to include the plastic elongation factors in the calculation. However, for simplicity in our examples, we will use the linear elongation model which do not include this factors. We will use the excel spreadsheet file that I developed for sag and tension calculations.

Calculate the sag and tension at different weather conditions and tabulate them.

Step 4: Plotting of the parabolic/catenary curve.

Plot the four curves into a graphing paper with the same scale with the route profile. Take note of the offset of the “ground clearance curve” and “tower footing curve” as described above. The usual practice is using a parabolic curve.

Step 5: Transfer the plotted curve to a transparent paper or celluloid.

Photocopy the plotted curved to transparent paper or celluloid.

On the next post…

On the next post, I will present an example calculations in making a sag template.


References:

  • NS220 Overhead Design Manual
  • Principles of Mechanical Design in Overhead Transmission Lines
  • Standard Handbook For Electrical Engineers
  • UEP Bulletin 1724E 200. Design Manual for High Voltage Transmission Lines
  • Construction Manual for Transmission Lines