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Black HDPE testing



I used a 8kV 50mA NST to test my 0.8 mm (31 mils) thick black HDPE sheet.

I put a 3 x 3 cm slice of HDPE between two bolts connected to the NST
without a variac. THERE WAS NO ARCING. The HDPE I have got is NOT
conductive!

In complete darkness I could see a slight violet corona around the bold
contacting the HDPE. The nice thing is that as I connected/disconnected
rapidly the NST from the mains, it generated extravoltage that was capable
to jump OVER the HDPE and reach the other bold! This accounted for a
distance of about 3 cm.

I could also jump 3 cm of air between bolds (with no HDPE) by doing the
same connect/disconnect trick.

This tells me that the 0.8 mm HDPE exceeds 8kV AC of isolation (forgetting
the corona effect, of course) but tells me also that it is safe to have a
safety gap on each cap.

I have also decided to have only a layer of HDPE, not two. My cap design
is now as follows:

- paper    INNER LAYER
- Cu
- paper
- PE
- paper
- Cu
- paper
- PE      OUTER LAYER

Paper is 0.05 mm (40g/m2) kraft paper not waxed.
PE is 0.8 mm (31 mil) HDPE 
Cu is 0.1 mm copper sheet
capacitor is filled with oil

I am confident this cap will stand 10 kV DC and connecting two of them in
series I should be able to use them in a typical TC. I will also mount on 
each cap a safety gap set as, say, 1 cm. Leaving 2 cm of PE border without
copper inside the cap should ensure that the safety gap fires BEFORE I get
arcing between the copper electrodes edges.

Comments?





________________________________________________________________________

 Marco Denicolai                   Vista Communication Instruments, Inc.
 Hardware Development Manager      www.vistacom.fi   

 marco-at-vistacom.fi                 Kaisaniemenkatu 13 A
 fax:    +358-9-622-5610           SF-00100 HELSINKI
 phone:  +358-9-622-623-15         Finland

   Remember, Murphy was an optimist! I am not...
________________________________________________________________________