Ruling Span

[rubendar]

What value of ruling span to take to spot tower to a new distribution electric line?. it´s necesary to assume one value?.
To validate ruling span chosen may be calculating error between ruling span assumed and real (once tower are spotted). if error is lower for example 5%, then ruling span initially assumed is acceptable, on the contrary start again taking now ruling span calculated, iterate until to get the error acceptable. it´s this procedure correct?. are there another methodology?.

Best regards,


Ruben

 

[cranky108]

I would be asking our Civil/structural engineer. As so many of us electrical engineers likely have not worked on transmission lines.

 

[jghrist]

Each section between conductor deadends has a single ruling span. The ruling span best represents the average tension of the individual non-uniform spans when calculating tensions at different loading conditions. Acceptable error depends on the clearance and tension deviations.compared to the design. Once towers are spotted, use the actual ruling span to recalculate sags and tensions. If clearances are acceptable, then you won't have to re-spot towers.

 

[bacon4life]

The amount of acceptable error somewhat depends on how closing the installation is designed to any actual limits. In my region, suburban distribution wood pole are often build quite robustly to allow for the easy future addition of transformers, secondary service wires, and communications lines. Often pole placement is determined more by where a pole can be placed without blocking a driveway than by ideal ruling spans.

In some areas utilities choose to install the absolute minimum size pole to minimize upfront costs. With this design philosophy, any changes to pole placement will require redoing the engineering design analysis.

For long distribution spans and for transmission, an iterative approach is common if you are running calculations by hand. Most new transmission lines are designed in PLS-CADD. Using PLS-CADD allows the use of more exact calculations.

There is also a big difference between wood pole designs versus manufactured structures like lattice towers, steel poles and cement pole. Wood has a large amount of variation, so it is typically installed with large safety factors and low engineering effort. Manufactured materials have lower variability, so these materials are often used with lower safety factors and more engineering effort.

In the USA, 'tower' would typically refer to a lattice tower rather than a single pole. Other than river crossings, lattice towers are uncommon at distribution voltage levels in the USA. Other parts of the world do use lattice towers for distribution. Lattice towers are quite rigid, so the towers take significant horizonal loading from non-uniform spans. Wood poles are quite flexible, and having the top of the pole move just a few inches eliminates much of the horizonal loading.

 

[Palletjack]

Ruling span is theoretical. All spans same length, all elevations the same, etc..
It’s not practical.
The math becomes too involved for pole loading analysis when spans and elevations are different than ruling span in the real world.

I assumed everyone uses software now like spidacalc for line design.

 

[dik]

https://electricalengineerresources...tension-computation-using-ruling-span-theory/

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik