Hi, I had a rotary spark gap coil probably similar to your setup. The spark gap and capacitor bank was under the primary, and I had wire mesh around this lower deck. I used a commersial variable frequency motor speed control unit and an asynchronous three phase motor. The motor was grounded to mains ground, as well as the screen of a screened power cable for the motor. Around this cable I had another screen connected to Tesla ground. The HV transformer and the motor control was at perhaps ten m distance from the coil. I used heavy coaxial cable for HV, screen connected to Tesla ground.
For Tesla ground I used the rebar mesh of the concrete floor, as well as a few sheet metal pieces on the floor. I had no connection from Tesla ground to mains ground. The transformer core was connected to mains ground. I had a mains filter before the transformer, but I doubt that it made any difference, the core of the inductors in that were probably overwhelmed by HF.
This was a big coil with 20 kVA input, and the rotor motor, HV transformer and motor control unit held up. If I connected one end of a fluorescent tube to Tesla ground, and the other end to mains ground, it flickered, so there was probably a few hundred volts difference between grounds.
If I understand you right, my grounding setup was similar to yours.
Regards,
Jan
Stockholm, Sweden
Skickat från min iPad
> 17 okt. 2026 kl. 03:28 skrev George Schramm <georgeschramm5@xxxxxxxxx>:
>
> Ok that is one scenario. Another i've theorized goes like this and I would
> value your input. Say i wrap the bottom deck (containing mots, doubler, rc
> low pass filter, safety spark gap, and asrsg) with wire mesh and connect
> that to rf ground along with secondary base, safety spark gap, strike rail,
> mot cores (in this case using copper ground rod as rf ground) near coil.
> Power input wires for mots and rotary spark gap are wrapped in copper emi
> tape that connects to rf ground, (only at that end!) And goes until it
> meets control box (to protect against top load strikes)
>
> Does this sound
>
> As an aside, I, rather optimistically, build a dual scr phase control
> circuit for speed control to the asrsg that sits inside the control box.
> Could I build a faraday cage around this(to protect scrs) that is
> completely encased and not grounded? This would be completely separate from
> rf ground scenario theorized above. The input (from wall) and output (to
> rsg) both have microwave oven emi filters in line.
>
> I have some pictures I could provide if that helps!
>
> Thanks again!
>
>> On Fri, Oct 16, 2026, 12:39 PM Jim Lux <jimlux@xxxxxxxxxxxxx> wrote:
>>
>> Connect counterpoise to cage. There’s really no inside or outside at these
>> frequencies.
>>
>> Sent from my iPad
>>
>>> On Oct 15, 2026, at 18:51, George Schramm <georgeschramm5@xxxxxxxxx>
>> wrote:
>>>
>>> Ok, good to know on the emi filters. I'll look them up to be sure. The
>> coil
>>> is designed to run at 131khz, although hopefully they'd have some use
>>> blocking hf (mhz) transients generated by the spark gap firing.
>>>
>>> As far as the faraday cage, should the inside be grounded to the
>>> counterpoise, and the outside to safety ground? Or leave the cage inside
>>> disconnected from the rf ground and just surrounding the coil?
>>>
>>> Thanks for your help!
>>>
>>>> On Thu, Oct 15, 2026, 3:54 PM Jim Lux <jim@xxxxxxxxxxxxx> wrote:
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> Faraday cage or simulacrum is he way to go. You could use something
>> like
>>>> 4" reinforcing mesh for both the counterpoise and the walls and roof of
>> the
>>>> cage. Or whatever is structurally, aesthetically, etc suitable.
>>>> You'd want to have that cage connected to "green wire" earth ground for
>>>> safety - if someone touches it, with bare feet on floor, you don't want
>> a
>>>> shock.
>>>> Isolated things near a tesla coil running can pick up a surprising
>> charge.
>>>>
>>>> I'd not worry about a ground rod.
>>>> RF choke on power wires is a good thing. The Corcom filters *might* be
>>>> ok. You need to look up the data sheet and see what frequencies they
>> are
>>>> good for. If your coil is running at 100kHz and the filter is a low pass
>>>> with a cutoff of 1 MHz, that doesn't do much good. Those filters are
>>>> designed for particular applications - some are good for low
>> frequencies,
>>>> others are looking for 10s of MHz (e.g. to keep computer noise out of a
>>>> power line or box)
>>>>
>>>>
>>>>
>>>> On Thu, 15 Oct 2026 12:33:00 -0700, George Schramm <
>>>> georgeschramm5@xxxxxxxxx> wrote:
>>>>
>>>> Hi all,
>>>>
>>>> I'm currently building a dual mot asrsg 6" tesla coil, and i'm debating
>>>> different grounding scenarios and am looking for advice.
>>>>
>>>> Ideally i'd like to run the coil in my garage, with a, say, 4'x4'
>>>> galvanized metal roofing counterpoise, surrounded by a Faraday cage that
>>>> also connects to the counterpoise, as well as the strike rail and mot
>> cores
>>>> and safety spark gap. The coil would be completely encased.
>>>>
>>>> Another option I was thinking is running a standard copper ground rod
>>>> outside, no faraday cage, in this instance I was wondering if I can
>> connect
>>>> my mot cores to mains ground, maybe with a rf choke on the mains ground
>>>> line to discourage rf from the power supply flowing back in through the
>>>> mains ground wiring. Is this acceptable?
>>>>
>>>> The coil will run dual mot with a voltage doubler, 500pf 40kv bypass
>> caps
>>>> and 2x 1kohm 225watt resistors forming a low pass rc filter. Along with
>> a
>>>> safety spark gap connected to rf ground.
>>>>
>>>> I also have two 30a Corcom emi filters, one at the input of the control
>>>> box, and one on the output of the 20a variac.
>>>>
>>>> Power input is limited with a 120v 20a circuit, (us)
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>>>>
>>>>
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