HV puncture of schedule 40 PVC

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Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>

Hi all,

I ran into this problem today and thought that I would
run it by the resident geniuses of this forum ;^) I de-
cided to placed a piece of PVC pipe over the naked
end leads of my two x-ray transformers for safety. One of
them has been modified for AC output but the other one
is still putting out HVDC. They are both approximate-
ly 150 kV output and the HV secondary outputs are
mid-point grounded for -75 kV -- 0 -- +75 kV. With the
DC output one I put a piece of 2.5" dia. schedule 80
gray electrical PVC conduit over them and this really
reduced the corona hiss to almost zero. When a
grounded wand was brought up against the exterior
of the PVC, there was hardly even any increase in
the audible hiss, even with the 0 - 280 volt variac turned
to about 75 on a 0-100 scale. The wall thickness is
a little over  1/4" thick.

However, I took a 4" dia. piece of white schedule 40
white PVC drain pipe and placed it over the HV elec-
trodes of the AC output transformer. 4" was the smal-
lest ID that would fit over them. When I brought a
grounded wand near the exterior of the PVC pipe, the
HVAC would immediately puncture through the 1/4"
thick wall of the white schedule 40 drainage pipe. The
variac input was turned up to about the same 75 on the
scale as with the DC output tranny. The AC mod tranny
is about twice as heavy and draws nearly 5.5 amps of
magnetizing current before the 13" seperation of the elec-
trodes fianlly breaks down. The DC output transformer
only draws about 2 amps of magnetizing current at the
same input voltage level.

My questions are does AC have a higher puncture
coefficient than an equal DC voltage? Or is the white
schedule 40 PVC pipe not near as puncture resistant
as the grey schedule 80 electrical grade PVC, even
though the wall thickness is about the same? Remem-
ber, only half of the rated voltage was being felt at the
PVC since the  copper tube wand was grounded (mid-
point grounded secondaries on the x -ray trannies -
just like a huge NST). Any thoughts or comments
are welcome.

David Rieben