Why Does Clearance Between Phases Increase with Altitude?

The flash-over distance (i.e. the distance a spark can jump for a given voltage difference between two conductors) increases as air pressure decreases.

Without looking it up, I suspect this is due to the increase in mean free path of the ions as the pressure decreases. (Mean free path is the average distance a particle can travel before colliding with another particle).

The first sentence by JohnDG answers the question completely.

There are more variables in "Relative Air Density".

Dan

After 60 years in the business, I did not know it did.

Voltage, capacitance and corona considerations and are the main criteria in the US.

Where does your information come from ?

Of course the higher voltages are on higher poles.

Snakers

Physics degree from many years ago.

Sorry if I was a bit short there - I was doing a quick check on CR4 before dashing out to scrape the frost off the car & make an early meeting for a service call-out.

If my memory still works, this is the sort of reasoning behind having low pressure gasses in discharge tubes. Too many gas molecules in there, and the ions can't get a decent run at it.

I still haven't looked anything up, and it's a long time time since I needed to know about such things. No doubt someone will put me right if I'm out of it.

John.

Check out www.corona-technology-course.com for more info and resources.