What caused capacitor failure?

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

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



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