Re: [TCML] Are safety gaps necessary?

[drieben@xxxxxxxxxxx (David Rieben)]

Aaron, all,

Actually, I now use a single eared pig to drive my
Green Monster Tesla coil. For the "safety gap", I
use a 36 kV rated distribution arrester across the
"ear" and the case of the 14.4 kV pig. The pig is
sitting on a plywood deck inside the control panel
and is mounted via its two pole mount brackets
to wooden 2x4's that are set inside the control
panel as well. This allows the case of the pig to
"float" at RF ground inside the control panel w/
out allowing the mains grounded outer surface
of the control panel to ride on the RF. I simply
use high voltage x-ray cable for the transmission
line for the high voltage from the control panel to
the base of the Tesla tank circuit. Of course I run
the "hot" lead from the pig's single bushing through
the the inner conductors of the x-ray cable and run
the RF ground return via the outer braided sheathing
of the cable. There is also a thin plastic sleave over
the outer RF grounded mesh braiding of the cable
and this allows for  reasonable isolation of the RF
ground return from physical electrical contact with 
the ground surface, although there are a few places 
where it has "burned through", probably from direct
streamer/power arc hits from the coil :^0 I'm sure 
there are some who would condemn this setup be-
cause of possible Blumlein effect voltage surges but 
I have about a 50 ft. run of x-ray cable transmisison
line and I have never had an issue (that I can tell) with
this. 

One time a few years ago, I ran it on a windy day 
and the wind "blew" the streamers back into the
bottom of the secondary coil and into the primary tank 
circuit. This caused the 18 kV rated distribution arrest-
er that I was using at that time to fail as a dead short
and that's the reason I now have a 36 kV rated arrester.
I think the pig is still safe since it's rated at 110 kV biL.
That tought me that even a pig driven system needs some
sort of primary circuit overvoltage protection as I dread
to think what may have happened to my pig or primary
cap had the distribution arrester not been in place on
that fateful windy day. Also, I do NOT run it anymore
outdoors (which is currently the only way that I can run 
this monster) during excessively windy conditions!
Here is a short, poor quality video clip of it running
before I got all of the "bugs" out.
http://youtube.com/watch?v=eWZD3M-nye8

--
David Rieben

-------------- Original message -------------- 
From: jaholmes@xxxxxxxxxxxxxxxxxx 

> 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 
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