Re:[TCML] Black polyethylene pipe ok for secondary?

[Jim Lux <jimlux@xxxxxxxxxxxxx>]

Greg Peters wrote:
Agreed. That was one thing I was afraid of. I have seen a black poly 
pipe secondary fail in Tesla Coil duty but that was a coil producing 
over 3x the winding length so I figured it might have happened with any 
form. I'm cautious enough to avoid it altogether though I suspect it 
can't be any worse than cardboard!

On another note, I was thinking that filling the inside of the secondary 
with expanding polyurethane foam (cheap, comes in a spray can) might be 
a good way to prevent internal arcing and may allow us to make ground 
terminals with a bolt through the form. I find making the ground and 
toroid terminal the hardest part of secondary construction. It would be 
nice if we could just use a nice solid bolt through the side! Surely 
filling the form with polyurethane foam would make the inside of the 
form less conductive than the outside, in which case the coil could 
always be expected to fail externally first right?! Any thoughts?

---

Foam isn't a particularly good HV insulator, it's sometimes worse than 
open air.

What happens is that the field doesn't distribute evenly.  THink of a 
thin section through the foam, and what happens to the electric field. 
It's like a series combination of big and small capacitors (big where 
the dielectric is the plastic, small where the dielectric is air).  The 
voltage distributes according to the inverse of the capacitance; that 
is, if you have 100V across a 1 uF and a 9uF capacitor in series, you'll 
have 90V on the 1uF and 10V on the 9uF.

So what happens is that you have a bunch of high and low fields in 
series, and the field in the air bubbles is *higher* that it would have 
been if it was just air with no plastic, so it breaks down sooner.

Where foam *might* help is once the spark starts propagating, because 
there's "stuff" in the way of the propagating spark in air.

The other thing where putting something in air helps is when it 
increases the surface length between the two electrodes (e.g. grooves or 
fins on an insulator), but there, the idea is to get the along the 
surface length to be 3-4 times the "through the air in a straight line 
length", so that the insulator doesn't breakdown along the surface.

If you've ever taken your NST and put a couple wires on a sheet of 
insulator (glass/plastic) you'll note that the distance over which it 
breaks down is much greater than when the two wires are in free air.  A 
15kV NST will typically jump a 1/4" gap in air without much trouble, and 
seeing it breakdown over a 1" gap across plastic isn't unusual.
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