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Capacitor Explosion




6/23/96

To All,

I just finished a new spark gap made with two 2" diameter by 1/2" thick 
flat polished stainless steel disks.  The faces are parallel and the 
gap is variable about 3/8".  I have a shop vac blowing between the gap. 
I upgraded my 15/30 neon to a 12/60 neon.  I also have a new 0.025 mF 
20 kVAC Commercial Capacitor from Scott's first order.  
Coincidentially, it measured 0.033 mF today.  Externally the cap 
appeared new and in excellent shape.  I had never fired the cap before 
today.

I fired my coil with this new set up today.  At about 45 seconds and    
the Variac at 120 VAC there was a tremendous explosion.  One end of the 
capacitor, the rolled first section and capacitor oil blew out striking 
me on the legs.  I was standing about 10' - 12' from the cap.  Oil hit 
the wall about 17' from the cap.  My 15 Amp circuit breaker did not 
trip and a arc struck from the from the blown end of the cap to high 
voltage lead that had parted from the end terminal.  I killed all power 
as soon as I was able.  There is a tremendous mess.  I immediately 
examined the capacitor.  It was room temperature.  Each rolled section 
is about 3" high and 4" diameter.  The one section blown out the end 
was also room temperature.  Each section and the end terminals are 
connected by two soldered flat metal straps.  I'm by no means an 
expert, but it appears that the strap weld between the first and second 
section may have been faulty.  Probably an internal arc developed in 
the oil in a closed space.  The rest is predictable.

My system is still a small Tesla Coil, 720 Watts.  If memory serves me 
correcly, Commercial Capacitors rated these caps very concervatively 
and built them specifically for Tesla Coil use at very high power 
levels.  Now there have been two catastrophic failures that are 
probably the same, ie. the ends blew out of the capacitors.  In 
addition, I believe there was a whole thread on one posters 
intermittent failure and another with decreasing capacitance in these 
caps.  There are probably major design/engineering or manufacturing 
defects in these caps.  They all have serial numbers and should be 
recalled by Commercial Capacitors.

If anyone chooses to continue using them, be advised, they are very 
dangerous even at relatively low power levels and may cause great harm 
if not protected.  Build a containment enclosure around them and have 
all shut down measures and proceedures operational.  Don't stand in 
front of an end terminal as the metal bolt and terminal lug are blown 
free and propelled with considerable force.  

My advise is don't take any chance using them.


Nursing my wounds,

RWW