Re: HV puncture of schedule 40 PVC

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Original poster: "Dr. Resonance" <resonance@xxxxxxxxxx>


The rapidly alternating fields (electrostatic & electromagnetic) places
increased strain on a dielectric than a DC current.  That's one reason why
HV caps that have to withstand AC currents requires additional design work.
At RF frequencies  commonly used in Tesla coils the stress is even greater.

Creepage factors also increase substantially with AC and RF currents over DC
currents.

Dr. Resonance

>
> 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
>
>
>