What blowout temperature to use

[designdraft]

When calculating blowout for a span what is the most appropriate temperature to use in the calculation? Some references use a high temperature eg 50°C intending that to give an upper limit for the blowout, others use an everyday temperature on the basis that under blowout wind pressure any heat will be blown out of the line. Thoughts?

 

[jghrist]

NESC Rule 234, Clearance of wires from buildings and other structures, calls for using a 6 psf wind at 60°F for wind displacement. If you want to use high wind, I would still use 60°F, based on the loading requirements of Table 251-1.

 

[bacon4life]

A 48 mph wind will keep the conductors very close to ambient. Since I am in the USA, I use the NESC criteria jghrist mentioned. Are you designing to British standards? What standard references using 50 degrees C? Be very cautious of mixing and matching criteria between different codes.

 

[designdraft]

I am in Australia where we use AS/NZS7000:2016. The current release of that standard does not give a specific temperature for blowout calculations, however there are comments in the Standard and associated handbook recommending using the everyday temperature. Different utilities use varying temperatures in their design manuals without consistency, one of them being 50°C. Also, different approaches are used for calculating the midspan sag for blowout - some start with the no wind sag which is then rotated sideways, some use the wind loaded sag.

 

[cuky2000]

See the info below regarding the wind blowout on T. Line an animation of towers having fun.

https://www.youtube.com/watch?v=NDW8Scl37Yw

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[designdraft]

I made this graph using the Poles 'n' Wires software for a typical span which shows that most of the heat in a conductor is blown away with very little wind.