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Re: Strike Rail

Tesla List wrote:
> 
> Original Poster: "Malcolm Watts" <malcolm.watts-at-wnp.ac.nz>
> 
> Hi Bart,
> 
> > Original Poster: "Barton B. Anderson" <tesla123-at-pacbell-dot-net>
> >
> >
> > >
> > > Original Poster: "Malcolm Watts" <malcolm.watts-at-wnp.ac.nz>
> >
> >
> > Ok John and Malcolm,
> > This is interesting  - comment below:
> >
> > >
> > > > There is some indication that in many coils, a strike to the primary
> > > > is unable to damage the cap because the low impedance of the
> > > > primary "kills" the voltage.  Also overvoltages will be shunted across
> > > > the safety gap.  I do not use a strike rail on my coils....I just let
> the
> > > > sparks hit the primary.
> > > >
> > > > cheers,
> > > > John Freau
> > >
> > > My reaoning exactly. I don't use strike rails either and the primary
> > > gets hit plenty.
> > >
> > > Regards,
> > > Malcolm
> >
> >
> >
> >
> > I was thinking that a high powered system may be better off with a strike
> rail.
> > Would either of you say a high powered (14.4kv/694mA pole pig driver)
> strike to
> > the primary have no ill effects on the cap? It just seems a 1.6MVp-p
> strike may
> > find it's way across the gap to the cap and possibly back to the
transformer.
> > Does the low impedance of the primary actually stop this from occurring
> > regardless of power levels?
> 
> The cap - unlikely as it can only climb back up to its original
> charged voltage if all energy from the secondary somehow  got
> transferred back into it - unlikely in view of transfer losses. This
> would happen with no breakout anyway so I can't see a problem
> there. The gap is essentially still in a conductive state when
> secondary sparks burst forth so the transformer terminals are still
> shorted by the gap so differential voltages don't seem likely if the
> transformer is electrically close to the gap (which mine always are
> these days).
>        The only thing I could see happening is a high common mode
> voltage racing back towards the transformer terminals but I haven't
> seen any evidence of it in the systems I fired recently. If anything,
> pigs should be more immune to damage from that kind of event
> because of their BIL ratings.  I could always be proved wrong
> however so if in doubt, don't try it.
> 
> Regards,
> Malcolm

Malcolm, Bart, Gary and all,

This is one of the more interesting threads, since it goes against some
"accepted wisdom". I have little doubt that you are basically correct,
but I still think a strikerail makes sense. Safety gaps must provide the
"weakest" path to RF ground, and they should be on each incoming HV legs
to RF ground. But I still see some problems...

The instant a streamer connects to the primary, the _entire_ primary
tank circuit will be suddenly elevated to a comparativley high common
mode potential. If there were no safety gaps, this voltage would be
limited by the ratio of topload C versus the overall primary and tank
circuit parasitic capacitance to ground. Prior to the safety gap firing,
the common mode voltage can be MUCH higher than the limit imposed by the
ratios of Ctopload versus Ctank, since the parasitic primary circuit
capacitance to ground will usually be much lower than Ctank. And, the
fact that the main spark gap may still be conducting is irrelevant to
this common mode transient, and provides virtually NO protection. The
actual common-mode peak will be (eventually) clamped by the breakdown
voltage of the safety gap... as long as the safety gap is truly the
"weakest link". This is similar to the insulation coordination problem
that power companies face when sizing/placing arrestors so that they are
always the weakest link.

However, prior to safety-gap breakdown, the outermost turns of the NST
or pig winding will bear the brunt of this voltage transient. Since
breakdown of the safety-gaps may not occur immediately, the entire
primary circuit will likely be stressed by high common-mode voltage
transients which are of significant amplitude, containing VHF and UHF
components. In this light, it seems to me that a strike ring is cheap
insurance, since it stops this particular transient at the source. Of
course, your mileage may vary...

-- Bert --
 
Bert Hickman
Stoneridge Engineering
http://www.teslamania-dot-com