RE: What caused capacitor failure?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-compaq-dot-com>

Did you really mean that your gap has ten 0.28" gaps?  In series, that's
a total of 2.8" !  Might as well not even have a gap - the mains
resonant rise will ring up and kill almost any cap you have. 

But assuming that you really meant that each gap was 0.028" and the
total gap was 0.28", there still may be a problem.  Assuming that your
gap is properly set and that you're not getting mains resonant rise,
a12kV transformer will give you a peak voltage of 1.4X or 16.8kV.  An
8-long string of 1.6kV caps is good for 12.8kV.

Regards,
Gary Lau
MA, USA

===========================================================
Original poster: "Brett Error by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <bretterror-at-hotmail-dot-com>

Thanks to all on this list.  You've been tremendously helpful, and this 
evening I finally got some sparkage as the fruits of my labor.  Worked 
great, and even my wife was awed.  She finally understood why all the 
effort.

Unfortunately, I've already had a capacitor failure.  Seemingly relevant

specs of the coil:

12kV 60mA NST
MMC Built almost identically to the one Terry describes:
http://users.better-dot-org/tfritz/site/MMCinfo/MMC.html

I used the identical capacitors described ordered from DigiKey.
However, I 
only used eight in series as I only needed to support 12kV.  I built
three 
strings, but only used two of them in parallel for a total capacitance
of 
.014 microFarads (the transformer matches to .013 microFarads).

I have a RQ-style cylinder spark gap with 10 .28 inch gaps in series.

She ran beautifully for probably 10 minutes of total run time during the

course of several hours (15-second or so at a time).  However, shortly
after 
lighting her up the last time one of the capacitors split and caught
fire.  
I immediately shut down the coil.  When I examined the capacitor bank
more 
closely I found that ALL the capacitors have suffered stress.  They have
all 
at least cracked their outer coverings, many of them have completely
broken 
away chunks exposing the foil inside.  Unfortunately, I didn't check the

caps as I went to see if they were gradually failing, or if this was an
all 
at once, catastrophic thing.  But based on how they look I would guess
they 
were gradually failing.

Right after failure I checked the other caps and they were not hot... I 
don't think the failure was from heat break down.  Instead, from the
look of 
the other capacitors, it looks like they actually sort of blew
themselves 
up.

Any ideas what would cause this?  My first guess would be too much
voltage 
and they really did blow themselves up.  However, the caps are rated at
1600 
VDC each, so with 8 in series I should be able to handle 12.8 kV.

How can I prevent this problem in the future?  Needless to say, that was
an 
awfully expensive 10 minutes (although immensely satisfying).  I'd like
to 
figure out what I can do to prevent this failure before trying it again.

Thanks in advance for your help.

Brett