[TCML] Ground Voltage Gradient

[David Dean <deano@xxxxxxxxxxxxxxxx>]

Hi

Often Tesla coilers connect everything on the power source side (control cabinet etc.) to the power 
source ground, and everything on the "business" end connected to a dedicated RF Ground.

In the case of 240V 60Hz supply (assuming a dedicated service with no other loads attached) both L1 
and L2 would be referenced to ground only through the secondary windings of the distribution 
transformer supplying the service. 

Additionally, let us assume that:

The distance between the ground at the source/control point and the load/TC is 1000 feet (makes 
impedance calcs easier for the wire used for L1, L2)

The Tesla coil uses a charging choke on the secondary of the high voltage transformer located at the 
base of the Tesla coil (eliminates ballast)

The Tesla coil uses a SISG (eliminates additional wiring for RSG, control wiring, etc.)

The Tesla coil is powered by a "pole pig" and the center LV bushing has been left floating because 
after all this is a Tesla coil, and we don't want that connected to RF ground. The case is bonded to 
the RF ground.

The "pole pig" is housed inside some sort of metal cabinet, also bonded to RF ground (personnel 
safety)

Now we know that an operating Tesla coil can produce a fairly high differential between RF ground 
and Mains ground. No question there. That is the point of using a separate ground, to keep high 
voltage spikes etc. off of the mains ground. So lets say that the demonstration or display went 
well, everything is hunky dory and the coil has been disassembled and put into storage. For some 
reason, which does not matter because this is a hypothetical question, the power is restored to the 
now unloaded "pole pig" all happy in its protective cabinet. Now the case of the pig, and its 
cabinet as well, are bonded to the RF ground out in the field, and the center low voltage bushing is 
still floating, and should have a potential of 0V with respect to the ground at the source, I.E. 
mains ground.

So if someone were to put a voltmeter between the pig case and the center LV bushing, what would it 
read?

Are there natural ground currents that might cause a potential difference between two ground points 
separated by 1000 feet?

If such ground currents exist, are they constant or do they vary from time to time, and if so, what 
affects them?

What happens if a thunderstorm were to pass nearby?

TIA for any opinions, insight, etc..

later
deano

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