Correct way to define a cable sag 1

[Costin Ruja]

Good evening,

I have to work myself through some electrical line sag calculations and the norm states that it shouldn't have more than 6% sag.

In this particular case where the supports at different levels, how is the sag defined? S1 and S2 are pretty self explanatory and S4 comes into mind from simply supported beams.



Regards,
Costin

 

[JAE]

Not sure if this helps: https://resources.wolframcloud.com/FormulaRepository/resources/Inclined-Span-Catenary-Sag

 

[JAE]

Or this: https://www.fhwa.dot.gov/publications/research/infrastructure/bridge/05083/appendg.cfm

 

[JAE]

Sorry - one more: https://www.southwire.com/medias/sys_master/related-pdfs/related-pdfs/hf2/h66/8874318168094/AppJ.pdf

 

[BAretired]

6% of what? horizontal span? I would think it should be (S1+S2)/2, but the norm is not clear.

 

[Costin Ruja]

Tons of material for calculations, thank for the links @JAE.

Also one from my side:

https://www.researchgate.net/public...rhead_Line_Based_on_a_Given_Catenary_Constant

The issue, as @BAretired pointed out is that the norm does not state exactly how to measure the sag. Ah, yes, and the 6% are related to the total length of the wire. Guess I'll have to ping pong some emails to the code body that oversees these calculations.

 

[BAretired]

If 6% relates to the length of wire, then I would say the sag to be used is S4.

 

[Denial]

I have no idea how various Codes of Practice express themselves. However I have read a lot of academic and semi-academic papers on the subject of cables, and they pretty much universally define sag as the VERTICAL distance from the MIDPOINT of the straight line between the cable's end points to the appropriate point on the cable. So unless the cable's endpoints are at the same height the cable's actual lowest point is irrelevant in this context.

This is an irresistible opportunity to refer you to a spreadsheet I have developed over quite a long period, which calculates the forces and displacements for a single cable with (or without) a single point load applied anywhere along it. The spreadsheet can be downloaded from my website, https://rmniall.com.