safety gaps on capacitors

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

Original poster: "Area31 Research Facility by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rwstephens-at-hurontario-dot-net>

All,
 
Simple safety gaps (not impedance current limited) placed across your tank
capacitor can cause the capacitor to fail due to a mechanism known as Blumlein
Inversion Generator.  The capacitor is usually a long pair of foil strips with
a dielectric between.  The fact that it is rolled up is not important.  This
structure is a form of parallel wire transmission line. If you fully charge the
line to some voltage X, then if you place a low impedance short circuit across
one end of this line (the safety gap firing) you will create a pulse of reverse
polarity which will travel down the transmission line, adding to the original
charge and appearing as 2X voltage at the other end.  You have just subjected
your capacitors innards to the highest possible discharge current it can
possibly produce with all the associated hydrodynamic stresses, and you have
doubled whatever overvoltage condition you normally subject the capacitor to in
Tesla use.  This is a proven formula for capacitor failure!
 
Safe use of a capacitor safety gap requires carefully matched series impedance
to neutralize the Blumlein effect.
 
Rob
>
> ----- Original Message ----- 
> From: <mailto:tesla-at-pupman-dot-com>Tesla list 
> To: <mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com 
> Sent: Saturday, August 04, 2001 17:52
> Subject: Re: Awsome first light !! but...
>
> Original poster: "Edward Wingate by way of Terry Fritz
> <<mailto:twftesla-at-qwest-dot-net>twftesla-at-qwest-dot-net>"
> <<mailto:ewing7-at-rochester.rr-dot-com>ewing7-at-rochester.rr-dot-com>
>
> Tesla list wrote:
> > 
> > Original poster: "Lau, Gary by way of Terry Fritz
> <<mailto:twftesla-at-qwest-dot-net>twftesla-at-qwest-dot-net>"
> <<mailto:Gary.Lau-at-compaq-dot-com>Gary.Lau-at-compaq-dot-com>
> > 
> > While your safety gap may have been set for 12kV, what was your main gap
> set
> > to?  It sounds like your main gap was set to something higher and allowed
> > the safety to fire.
> > 
> > As a side issue, it's generally not recommended to use a safety gap alone
> > across your cap.  With no safety gap current-limiting, the discharge
> current
> > could potentially be bad for your cap.  Also, it's impossible for the cap
> > voltage to ever be any higher than the voltage across your main gap.
> > Whatever your main gap is set to will also limit the cap voltage, and the
> > cap-gap is redundant.
> > 
> > Gary Lau
> > MA, USA
>
> Gary,
>
> Do you current limit your primary to keep the discharge current from
> damaging your cap?
>
> I have NEVER seen a cap destroyed by a safety gap yet. I HAVE however
> seen many caps go to the great beyond for the lack of a safety gap!
>
> The first question I ask someone who has just blown a capacitor is "were
> you running a safety gap on the cap?", and almost invariably the answer
> is no. I personally will use a safety gap every time!
>
>  
>
> Ed Wingate RATCB