Re: HV puncture of schedule 40 PVC

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

Hi David,

On 15 Dec 2004, at 18:10, Tesla list wrote:

> 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

I can't speak for the relative puncture voltages of the two colours
of pipe but to my way of thinking, applied AC is a far more brutal
test since you are subjecting the material to severe voltage
reversals and charge/discharge currents which are probably causing
some heating, if not of the material then of the ionized air in the
vicinity.

Malcolm