Sag Selection From Sag-Tension Table

You use the sag without wind, you would not be able to install the cable in a high wind. The wind sag is horizontal, the dead load sag is vertical,two different things.

I can't understand the reason why wind load is neglected?

suppose today sag is 2.5m witout wind load and after 50m/s wind speed it becomes =2.5+(2.5-2.07)=2.93m which is beyond the limit. Therefore considering the bad condition tension should be such that sag is 2.07m. Am i true?

RE: Am i true?

No. First of all, as passingtongreen has pointed out, the wind load / sag is in a horizontal direction, and the "gravity" sag is vertical. The resultant sag, in a 50 m/s wind, and in worst case design conditions (well-I'm assuming that--is an ice load considered?), would be the vector resultant of the forces in the two directions, not 2.93 meters.

Furthermore, the sag tables are primarily (or at least initially) for installation purposes--you want a sag table with entries that correspond to conditions that are likely to be present when the cables are installed.

The installers of the cable all hope this will be still air with no ice load. The table probably needs a few temperatures from which the sag at the current temperature can be interpolated.

Reiterating, the sag table does not (typically) show the worst case sag conditions. The engineer / designer has determined the worst case loads that need to be designed for, then determined the sag required at normal conditions which will insure that the allowable tension in the cable will not be exceeded in the worst case conditions.

How you consider that wind flowing horizontally because it may flow at any direction with at any angle. Therefore considering the worst case of 90 deg down words. Then sag will be max. So, why we can't consider wind load for sag-tension adjustment.

Plz, told the exact physics behind it.

The force wind exerts on a cable varies with the square of the wind speed. The movement of the cable during light winds is within the accuracy and precision that are normally available for determining the sag. Part of the clearances and separations specified in system design is to allow for the normal and real inability to get to design criteria. In complex terrain system designs, I use to look at multiple wind speeds at various ice loads and the cable's galloping envelopes so that I would be comfortable saying it was a good design. Another factor is that almost all cables stretch over the life of the system. You design for the expected stretch, realizing that it will not be accurate. If I had been asked, I would have been able to give a table of design sags for given wind conditions. I actually discussed this with an installation contractor one time, and his response was that he had too much paper already that he ignored as effectively meaningless, and he did not want any more. ( His language was actually somewhat saltier.) in answer to the original question, use the windless value. If the cable is blowing out far enough that you are uncomfortable using this value, get an experienced person to look at the set up and give you advice for that particular installation.

I get your point. Then what is the requirement of providing the max sag value corresponding to wind load?

The person who does the calculations may or may not know the actual terrain and trees in the system. the Owner of the system and especially the installer will see it in person. one of their jobs is to evaluate if something has changed which would invalidate the plans. I have done plans for a system based on as builts from previous installs and when I got out to check the system (about a 1000 miles from my business) Buildings, roads, new telephone lines, and slag piles had changed the profile of the land under the cables. I had to resag severa sections to meet the new situation. Please note that the first design would have installed fine. It would naot have had problems until the cables was loaded and stretched. An experienced installer will always second guess anything on paper in front of him; and the designer wants him to. If it looks to him like the max sag will have problems, it is the installer's responsibility to question it. By having the max expected sag in a loaded end of system life situation, he can better judge when there might be a problem.

Additionally the blow out distance is also calculated and should be provided. This gives an envelope where the cable might be under different conditions. when the cable is being blown to the side, it will not have as much vertical sag as when all of the load is straight down. When you have different cables with different weights and diameters in the same system, such as when fiber optic cables are attached to high tension power lines, they blow out at different angles You need to know both vertical and horizontal positioning to ensure they do not make contact during wind conditions. Again, if the installer sees something flakey that he can check the numbers on the installation direction and see if this was espected.

Still, I never found any blow out distance calculation. So, if you have such data plz, provide me and the final conclution over the discussion will be that wind load sag is to catter right-of-way i.e. horizontal/ vertical clearence from sourrounding trees, buldings etc. Am I true?

Is ther any good books or technical papers regarding stringing of transmission line(long span)/ substation over head line(short span)

What you say is correct. in that right of way issues are part of the blowout calculations.

One place to find uses and descriptions is at www.powline.com. this company wrote PLS-CADD one of the premier power line design programs. You can also Google "SAG10", a program used by many people in sag and tension calculations. Neither program is really intuitively obvious as to how to use and what data to input to do the calculations. I have never found a simple program, that I believed the results. Parabolic equations especially with the creep of strength element added in, and different strength elements with differring COTEs and creep factors provide much area for confusion. Many of the formulas are not numerically solvable so iterative procedures are needed to get to the correct answer, Many of the approximations, such as parabolic approximations are OK for short simple systems, but fail miserably for more complex systems. Going back to the original question, the person should check with the person who gave him the initial information as to why both sets of information were given.

May u provide me SAG10

I do not have a copy of sag10. If you google sag10, you will find many sites which say you can download sag10 from them. It looks like there might be many versions calling themselves sag10. I have never downloaded a version, and would not know what version to recommend. Sag and tension calculations are based on catenary equations. Over the years, I have written sag and tension programs in basic, fortran, excel and even lotus notes. I do not have a copy of any of them. I have run sag and tension programs written in C+, Math Cad, and Mathmatica by other individuals I have worked with. I have run purchased versions of sag and tension programs from several different companies. Most worked well in restricted situations, but each had its limitations as to when it was really appropriate. Amung the frustrations, were the facts that most suppliers of materials used in cables did not know the values of the properties needed for the the sag and tension programs. Each of the programs I used needed a different set of parameters, some of which I was never able to identify. For some of them, as a fiber optic cabler, i was asked to provide values and was never able to find a definition that described the parameter.