[TCML] Are safety gaps necessary?

[bartb]

Hi Aaron,

Yes, Gary does bring up a point I had not considered (and I'm still 
rummaging through my head on it). On my pig, I run similar to David 
Reiben with one of my ears at RF ground. However, with PIG coils were 
usually talking rotary gaps and **ALL** rotary gaps should use a safety 
gap! This is very different than a static gap that has a natural 
clamping voltage action. Rotary gaps can and will miss a firing here and 
there and since timing is in control, there is no clamping action 
occurring. Thus, a safety gap serves this specific purpose.

I agree with the best practices comment. But some do have problems with 
safety gaps and I suspect many remove the problem by making the safety 
gap useless (too wide to do any good). I think if we are to keep with 
best practices, we need to work on this issue. I'm sure there's a simple 
solution. Set the safety gap to just not fire and the main gap to just 
fire with the NST and gaps alone. Then while running, if the safety gap 
is firing too much, rather than  increase it, reduce the main gap until 
it's acceptable. This would actually be a "best practice" solution. 
Spark lengths will suffer a little, but maybe that is acceptable for a 
long life NST.

Take care,
Bart

jaholmes at silicon-arcana.com wrote:
> 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.
>
> Cheers,
> Aaron, N7OE
>
> --- On Wed, 6/18/08, bartb <bartb at classictesla.com> wrote:
>
>   
>> From: bartb <bartb at classictesla.com>
>> Subject: Re: [TCML] Are safety gaps necessary?
>> To: "Tesla Coil Mailing List" <tesla at pupman.com>
>> 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
>> being.
>>
>> 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,
>> Bart
>>
>> 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
>>>       
>> gap.
>>     
>
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