Original poster: "Gerry Reynolds" <gerryreynolds@xxxxxxxxxxxxx> Hi Malcolm,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.
If you want to also set you main gap this way (really not necessary if you use a safety gap), then you can set it to the maximum setting possible (also with the TC primary disconnected) so the main gap fires reliably. Dont trust physical separation measurements as the geometry of the gap helps determine the breakdown voltage.
The main gap is a two terminal gap where as the safety gap is a three terminal gap. You can think of the safety gap as really being two gaps one for each bushing of the NST. If you do use a safety gap that is properly set, then you can experiment with the main gap setting and dont need to worry about setting it too wide.
Regarding grounds, most operate their TC's outside and tie the secondary base and strike rail to RF ground (copper rod staked in the ground). 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. I hope this wont be a fire hazard and I'm not sure how people would see the sparks if the cage was covered by tree stuff. I may not be visualizing this correctly.
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. You may also want to make sure that any electronics plugged into the mains are turned off or better yet unplugged.
Gerry R.
Original poster: "MalcolmTesla" <tesla@xxxxxxxxxxxxx> I still don't understand. How does the safety gap in parallel with the regular spark gap help any unless it's connected to something else as well?