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Re: Capacitor Help

Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Malcolm,


Original poster: "MalcolmTesla" <tesla@xxxxxxxxxxxxx>



> The safety gap is usually a three terminal gap.  Center terminal is
> connected directly to NST chassis. The two outer terminals connect to
> the NST bushings (if no Terry filter) or to the main gap side of the
> Terry filter if you are using one.  The purpose of the safety gap is
> to protect the NST and is adjusted with the TC primary disconnected
> from the circuit so the NST is unloaded.  One sets each side of the
> safety gap to "just not fire" with the NST at maximum voltage.

My original understanding was that the NST connected to mains ground and the
safety gap center terminal connected to RF ground with only the strike ring
and secondary coil.  That way if a streamer struck the primary coil the high
voltage would jump across the safety gap to the RF ground and leave the NST
alone.  Or does that now work unless the NST's chassis is also connected to
RF ground?
OR - is this one of those things that can be done multiple ways and I just
need to pick one?
You ask great questions. Im thinking Im modifying my opinion on this. If the NST chassis is grounded to green wire and the TC is grounded to a separate RF ground, what is the most effective use of the safety gap? If you use only one safety gap, you need to weigh the tradeoffs between protecting the NST and keeping any arc to the TC primary off of the mains distribution. Given the choice, I would give higher priority to the mains protection and ground the safety gap center terminal to RF ground. The consequence of this as far as protecting the NST is that any noise difference between RF and MAINS ground will show up as an extra voltage on the NST bushings - above what the safety gap protects at. This disadvantage (if considered significant) can be circumvented by adding two safety gaps -- one at the TC where the center terminal is RF grounded and one at the NST where the center terminal is tied to NST chassis. However, if the NST chassis is grounded to RF ground (for the case where the NST is physically located with the TC), you get the best of both worlds as far as protection. 



> The safety gap ground terminal should always be tied to the
> NST chassis directly (as short as possible since the NST is what it
> is protecting).  Many tie their NST chassis to RF ground to help keep
> noise off of the mains when the NST is located with the TC and is not
> a safety consideration.  Since you will be operating inside and dont
> have a RF ground available and the TC and NST will be inside this
> chicken wire tree cage (if I interprete what you say correctly),  you
> may want to ground the tree cage so it acts like a faraday cage.  It
> would be grounded to your "RF ground" that in turn would be grounded
> to the mains green wire.  The whole TC and NST would then be
> surrounded by a grounded cage and sparks eminating from the TC will
> probably hit the tree cage.

Humm OK.  So I should move the NST off of mains ground and tie that to the
RF ground along with the strike ring, and secondary's ground.  Then when I
fire the unit up at home I can hook up my RF ground to a copper pipe in the
ground.
Yes, this is what I would do for outside operation if the NST is located with the TC. Keep mains ground away from the TC. 


However at work I'll hook up my RF ground to the large chicken wire cage
(and I'm also going to run a wire up to the over head steel beams above the
suspended ceiling).  Yet you also say for the indoor usage at work I should
join the mains ground to the RF ground.  Would this not defeat the purpose
of having the seperate RF ground and cause lots of noise to go back to the
110 mains ground?
For inside operation at work (given that your Christmas tree will act like a faraday shield). I would tie the safety ground to NST chassis and make the NST chassis, strike rail, and secondary base a common ground that is grounded to the mains green wire ground. Since everything is enclosed in a faraday cage, there should not be any RF currents that will affect mains ground (the cage will keep everything in except the very highest frequencies that are above the cutoff frequency of the cage mesh). A one inch mesh, for example, will shield things below 1GHz quite well. Connecting to the steel beams should not be necessary. You may need to retune with the NST inside the cage. 


> In general, indoor operation can be done with low power TC's using a
> one ground system (green wire) and a counterpoise (a ground plane
> everything sits on with a radius at least equal to the height above
> the ground plane that the coil and topload are).  The counterpoise
> can be chicken wire, AL foil, etc.  It will be very important for
> indoor operation to NOT let the sparks hit anything that is not
> grounded directly to the base of the coil. Especially avoid hits to
> the walls and ceiling.   It will also be important to not have any
> secondary to primary hits (your strike rail should prevent
> this).  For this configuration, the NST chassis, secondary base,
> strike rail, and counterpoise are grounded to green wire.

Again, won't the RF ground interfer with the mains ground if I join them?


Not if the TC is enclosed in a faraday shield.


Humm a whole office full of computers.  I won't be able to make everyone
unplug their PCs.
I think you will be OK given the shield. If you have equipment on a floor beneath the TC, you may want to extend the chicken wire to include the floor so the TC is fully enclosed. Just dont put the TC primary too close to the ground mesh underneath it. From you pictures, I think you will be OK. 

Gerry R.