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Re: [TCML] Are safety gaps necessary?
One other observation on safety gaps. On a 6" X 30", NST-powered static gap coil of mine, I have a Terry filter with 3-ball safety gaps at BOTH the input and output of the filter. The 1K ohm resistors are 200W wire-wound units. The center electrode of both safety gaps is connected to the system's RF ground, which is a 3' length of 3/4" diameter copper pipe pushed into moderately damp soil within 5' of the coil. The NSTs are connected to the tank circuit with leads approximately 24" long. I measured the inductance of the wire-wound resistors at one time, but I don't recall the value right now.
I believe Terry Fritz' first experiments with his R-C filter network used 3-ball safety gaps at both ends, and I just copied his initial design.
Both sets of safety gaps are adjusted so they are just beyond the point of flashover at 140 VAC input to the paralleled 15/60 NSTs. During coil operation, depending on power level, primary tap tuning, number of static gap segments in use (total main gap width), and whether the coil is making air streamers or the streamers are attached to a grounded object, sometimes the safety gaps at the output side of the Terry filter will flashover, and sometimes the gaps at the input side will flashover. I don't recall ever seeing them flashing over simultaneously.
I have no explanation of why the gaps at the input side of the filter should ever be firing while the gaps at the output side aren't. However, it would appear that the gaps at the input side of the filter definitely saw (and shunted to ground) transients that otherwise would have gone directly into the NST secondaries.
Any comments are welcomed.
Gary's point about three-terminal safety gaps makes perfect sense to me when I think about midpoint-grounded NSTs. I'm willing to believe that a three-terminal gap is important in that case. In endpoint-grounded scenarios, such as with lower-voltage NSTs or one-eared pole pigs, it seems like the necessity of a safety gap would depend somewhat on the likelihood of a strike on the *transformer* side of the main gap. In a table-top coil with endpoint-grounded NST integrated into the base, the chances of a strike to the transformer-side primary wiring seems unlikely. And if such a strike *did* happen, the main gap is very close to the transformer. On a larger PT- or pig-powered coil with ten feet of wiring between transformer and coil base (where the main gap might be), strikes to that wiring seem much more likely and potentially damaging. What effect will all that wire and its inductance have on the main gap's ability to act as a safety gap and
prevent the strike from damaging the transformer? That seems like something worth worrying about.
In the end, of course, this is all of little practical importance if even a single scenario can be found where a safety gap is useful. After all, safety gaps are cheap and easy to build, and "best practices" are easier to write down and remember.
--- On Wed, 6/18/08, bartb wrote:
> From: bartb
> Subject: Re: [TCML] Are safety gaps necessary?
> To: "Tesla Coil Mailing List"
> Date: Wednesday, June 18, 2008, 7:26 PM
> Thanks Gary for sleeping on it. In the case of a strike I
> had not
> considered. In the event of a primary strike, there is the
> primary to RF
> ground and there is the MOV's across the Terry Filter
> (and there is the
> NST itself). I think the MOV's would save the NST. But,
> how short would
> the life of the MOV's be? I think maybe it's not
> that big a deal if a
> Terry Filter is involved. I think I should test it out.
> This is one of
> those things that requires good old "strike the
> primary 1000 times and
> see what occurs". Maybe a small on the primary
> pointing up at the toroid
> would suffice for a lot of primary strikes. My little
> 4.5" coil is
> perfect for this test, but I stuffed it away for the time
> That will have to wait until my SISG coil is tested out.
> But, I still
> think a safety gap may not be necessary regardless of
> primary strikes.
> Can't prove it and until then I continue to use a
> 3-terminal safety gap.
> Take care,
> Lau, Gary wrote:
> > I slept on it and it dawned on me this morning. What
> happens when a streamer strikes the primary? If one
> exists, the 3-terminal safety gap fires, channeling the
> streamer's energy safely to RF ground. If one
> didn't have a 3-terminal safety gap, the only path for
> that streamer energy is through the NST. I think the NST
> would be happy that the 3-terminal gap exists!
> > So I stand by the advice to use a 3-terminal safety
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