Safety gap in pole pig coiling

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Original poster: "Robert Hanford" <pomnept@xxxxxxxxxxx>

I have a basic question concerning my pole pig coil. I have found that many 
schematics, mostly of NST type coils, have a three ball safety gap, the 
middle ball being connected to ground and the NST case. This could be RF 
ground or its own dedicated ground. But my pig is different because its 
case is not a center tap for the secondary winding. I have connected the 
mains ground from my 240v plug to the case of the pole pig, also one of the 
HV bushings, as per recommendations from Dr. Resonance in a recent post. Is 
a three ball safety gap necessary, and if so, what to connect the middle 
ball to? By the way I am using a sucker gap for now as my main gap, set 
rather close. It does seem to be its own safety gap, but I want to 
experiment with rotary spark gaps as well. Any reply would be very welcome.
Robert Hanford