Re: Safety Gap Question

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hello Jason,

On 8 Oct 2001, at 11:55, Tesla list wrote:

> Original poster: "Jason by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<jasonp-at-btinternet-dot-com>
> 
> Malcom,
> 
> > If the safety gap is placed directly across the main gap, why is it
> > needed at all (think about it)?
> 
> I have thought about it... I think what you are referring to is a
> non-grounded gap, i.e. just an alternative route for the cap to discharge if
> the RSG stops functioning for any reason. I am talking about a three horned
> gap with the centre point grounded to RF ground so that the current will go
> to ground rather than through the primary.

Not an alternative path for the cap to discharge to. Charged cap 
energies are not involved. See below:
 
> > The real problem occurs when the
> > transformer is positioned some distance away from the main gap. The
> > long leads running between the gap and transformer form a
> > transmission line which is quite capable of generating high voltages
> > with very little energy given the high shunt impedance of the
> > transformer terminating it at the far end. Ergo, the safety gap
> > should be placed directly across the transformer secondary terminals
> > and secondary ground if the secondary is grounded.
> 
> Point taken - I use an RSG with a neon, and i know how important it is to
> have an SSG :) I use a Terry style protection filter that incorporates large
> resistors. If I put the SSG across the transformer then I would run the risk
> of damaging the resistors as well as overcharging the caps. My cap bank is
> also practically resonant (10/140 35nF) so I always put the SG across the
> past of least resistance and inductance. The high impedance of the neon plus
> the impedance (and inductance) of the resistors actually makes the SSG more
> effective when it is across the gap as less gets back to the neon. I am also
> not sure what you mean by the transmission line 'generating high voltages'
> though... as far as I am concerned it's just a conductor.

Your resistors won't be damaged. As far as the transmission line 
goes, consider the TC secondary: it has a high impedance termination 
at one end and a low impedance at the other. It oscillates primarily 
at its 1/4 wave fundamental. But long lines behave in a similar 
fashion as does any circuit having distributed capacitance and 
inductance. At frequencies generated by the gap breakdown (see some 
of Terry's notes if you want to get an idea of how high they can go), 
any distributed circuit so terminated can ring at its characteristic 
frequency and caused voltage magnification by resonant rise. Tuned 
circuits and filters manufactured as printed circuit board striplines 
have the same properties. If you have witnessed a safety gap firing 
at the transformer end of the power feed line to the TC primary 
you'll know what I mean. The safety gap can fire at a higher voltage 
than the primary gap connected directly across one end of the 
"conductor" and with much lower energy. D.C. Cox used to refer to 
this as the "Blumlein Effect". I witnessed a series of such 
discharges prior to the demise of one of my NSTs. I also caused an 
arc to form in the outer portion of a transformer which has given 
nothing but reliable operation when run with leads between it and the 
main gap of just a few inches length in similar circumstances. I 
learnt my lesson on the second occasion.

     You can use the safety gap to forestall widening the main gap 
too far but this other problem is much more insidious. One instance 
in which I would use a safety gap to avoid widening the main gap too 
far is to plant a static gap across a rotary in a NST-driven system. 
Contrary to popular belief in some quarters, rotaries do not politely 
wait for their electrodes to align before firing. They will fire as 
soon as the electrodes are close enough for the voltage presented 
across them. You can easily witness this phenomenon at work as you 
turn the variac up.

Regards,
malcolm