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Anonymous Poster #1

### Transmission Line Sag Template

01/02/2012 5:05 AM

I've prepared a parabolic sag template for a 132kV line using a basic span of 335m. However, the alignment of the route goes through a very steep terrain which is unavoidable. At some places the towers are located too close with too much level difference. Is the sag template is applicable here or is there some other form of calculating sag here?

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#1

### Re: Transmission Line Sag Template

01/02/2012 5:42 AM

First off, get the shape right. It's a catenary, not a parabola.

The wiki should tell you all you need to know.

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#2

### Re: Transmission Line Sag Template

01/02/2012 8:16 AM

Frustrating, isn't it?

It is a catenary as JohnDG says. The trick with sagging is that the ends of the curve must be level, and you must find the portion of the curve that fits. You may have to place some dead end structures at each end of the hilly section to change the tension.

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#3

### Re: Transmission Line Sag Template

01/05/2012 1:24 AM

The factors that affect sagging of transmission lines are conductor load per unit length, span, temperature, conductor tension and level of supporting towers. The sag is calculated at a temperature of 120F (50C) with no wind, 60F (15C) and a wind force of 6lb/ft (290Pa), and at 32F (0C) with no wind and ice loading, which is appropriate to the location of the transmission line. Refer to a Loading Data, Sag & Tension Data Table, NESC, IEEE 1990.

Another important factor is the required safety clearance of conductors. Safety clearances are of the following;

1. Vertical Clearance: Distance between conductors and ground.

2. Horizontal Clearances: Distance between conductors and distance between conductor and structure.

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#5

### Re: Transmission Line Sag Template

01/05/2012 1:29 AM

Passingtongreen,

My apologise, I mistakenly replied to your post.

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#4

### Re: Transmission Line Sag Template

01/05/2012 1:25 AM

Data from the above factors are collected and analyzed upon the designing of transmission lines to determine the required sag and tension properties to be employed.

The template is cut as a parabola on the maximum sag (usually at 49C) of the ruling span and should be extended by computing the sag as proportional to the square of the span for spans both shorter and longer than the ruling span. By extending the template to a span of several thousand feet, clearances may be scaled on steep hillsides. The form of the template is based on the fact that, at the time when the conductor is erected, the horizontal tensions must be equal in all spans of every length, both level and inclined, if the insulators hang plumb. This is still very nearly true at the maximum temperature. The template, therefore, must be cut to a catenary or, approximately, a parabola. The parabola is accurate to within about one-half of 1% for sags up to 5% of the span, which is well within the necessary refinement.

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#6

### Re: Transmission Line Sag Template

01/05/2012 1:42 AM

There are two subdivisions of a transmission line, with which each end of these subdivisions are terminated by a tension tower. Between these tension towers 5 to 10 supporting towers are used to carry the phase conductors. At the supporting tower insulators are positioned vertically and are aliened with the conductor angle of the tension tower. With respect to load conditions, the tension tower carries both vertical and horizontal loads while the supporting tower carries only the vertical loads. This construction has the advantage that damages that can be impacted by storm is limited to one section.

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#7

### Re: Transmission Line Sag Template

09/03/2012 7:26 AM

Hi RON,

Can you help me by explaining the procedure in calculating the Sag-Tension chart / Sag-Tension table.

Regards,

Devine Davidson.