electrical calculation question

[gymboy34]

hi all,
dont know if in correct section ofn forum
but here we goes
calculate the diameter of an aluminium busbar which is 24m long and whose resistance is 0.00139
formula a= pl/r and then convert the csa area into a diameter which i dont know how to do.

a = 2.84*10-8*24/0.00139 =4.9

I f im going down the correct pah i need to know my csa formulas.
any help greatly appreciated.

cheers
gymboy

 

[telectrix]

use A = ​ΠΓ²

 

[Richard Burns]

or d = √(A/(π/4))=√(A/0.78539816)

 

[ipf]

A round busbar, eh?

 

[davesparks]

Yeah, next time you drive past a bigger substation you'll probably be able to see some linking things together overhead.

 

[Besoeker]

Yeah, next time you drive past a bigger substation you'll probably be able to see some linking things together overhead.

But are they Al?

 

[davesparks]

But are they Al?

I doubt it, the ones I've noticed are often a shade of green which suggest copper, or the pine trees are dropping their pollen again.

Whether round Al busbars exist or not the mathematics are going to be the same.

After all this is surely a hypothetical question designed to test someone's maths rather than actual electrical work. I'd have thought a real round bussbar would be hollow, otherwise why not just make it square/rectangular?

 

[Besoeker]

I doubt it, the ones I've noticed are often a shade of green which suggest copper, or the pine trees are dropping their pollen again.

Whether round Al busbars exist or not the mathematics are going to be the same.

After all this is surely a hypothetical question designed to test someone's maths rather than actual electrical work. I'd have thought a real round bussbar would be hollow, otherwise why not just make it square/rectangular?

I agree. Round is the worst geometry for getting best rating for the given cross sectional area.
That's why, in general, busbars are rectangular. Surface area makes the difference.

 

[Silly Sausage]

I agree. Round is the worst geometry for getting best rating for the given cross sectional area.
That's why, in general, busbars are rectangular. Surface area makes the difference.

That'll be the old 'Skin Effect'.

 

[Besoeker]

That'll be the old 'Skin Effect'.

Not really at 50Hz.
It's surface area for cooling that makes the difference.

 

[Silly Sausage]

Not really at 50Hz.
It's surface area for cooling that makes the difference.

Interesting!
A quick google...at high currents, as you point out, the surface is cooler than the core, ie lower R, so larger current flow. So a skin effect to some extent!

 

[davesparks]

Not really at 50Hz.
It's surface area for cooling that makes the difference.

At 50Hz skin depth is approx 8mm, so for big busbars and the national grid hollow conductors make sense.

Also a hollow bussbar can have coolant pumped through it.

 

[Knobhead]

At 50Hz skin depth is approx 8mm, so for big busbars and the national grid hollow conductors make sense.

Also a hollow bussbar can have coolant pumped through it.

Ever worked on a water cooled system? It’s fun and games to say the least.

 

[Besoeker]

Ever worked on a water cooled system? It’s fun and games to say the least.

I've done quite a few. Mostly for high current rectifiers.
Semiconductor heatsinks and transformers were water cooled as well.

One limb of a 7kA unit at the build stage:

 

[davesparks]

Ever worked on a water cooled system? It’s fun and games to say the least.

Nope, I doubt I'll ever get to play with anything as interesting as that.

Unless the indoor rain that happens in houses just after the plumbers have arrived counts?