Odd primary caps??

From:  Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent:  Friday, June 12, 1998 11:56 PM
To:  Tesla List
Subject:  Re: Odd primary caps??

Tesla List wrote:
> ----------
> From:  Stefan Bishay [SMTP:stefan00-at-gte-dot-net]
> Sent:  Friday, June 12, 1998 1:29 AM
> To:  tesla-at-pupman-dot-com
> Subject:  Odd primary caps??
> In my 4" coil (now being rebuilt) I used four 7500vac .0066uf caps in
> series-parallel for a 15kv rating, and used a 12kv 30ma nst. I got about 8"
> arcs out of it, but I am wondering why the caps did not have problems. They
> are metal-can, case ground, GE "Dielectrol" units, picked up for five bucks
> at the power company surplus yard. The protection circuit is a pair of
> chokes using ~75 turns of #22 wire on  3/4 x 4" PVC pipe forms, and a
> safety gap consisting of two nails about 1/2 inch apart. The safety gap
> rarely fires, the coil works fine, I am just wondering the caps have not
> exploded yet, or why the nst has not fried.
> Regards,
> Stefan Bishay
> Seattle, WA
> ICQ 2141646
> "Hell, we all die in the end"
>         -Jello Biafra


Nice catch! Power Factor Correction (PFC) caps that power companies use
are considerably more robust than most other caps you'll encounter,
since they have to withstand high amplitude switching spikes, lightning
transients, and tough environmental conditions. The units you obtained
are rated at 95 kV(!) BIL (Basic Insulation Level). Because of their
high surge capability, they also tend to be considerably "bulkier" when
compared to other capacitors you'll typically encounter. 

While many older PFC's used paper-oil dielectric, plastic film
dielectrics are also beginning to be used in some of the newer units.
The GOOD news is that GE's Dielektrol caps are of this genre, and use a
polypropylene dielectric system. That's why you're not seeing any
problems in using these capacitors! Congratulations - it sounds like
you've got a pretty bullet-proof system!

-- Bert --