I would differ on the advice suggesting that the NST cases be connected to the AC ground when using the paralleled NST's. That's fine from a personal safety perspective, while testing the phasing. But during actual TC operation, no one should be touching the NST case. As the NST case is tied to the secondary midpoint, there is a LOT of high voltage, high frequency transients present on it. If you tie the case to your AC ground line, you will be routing those transients into your AC wiring. I believe that the NST case should only be tied to the RF ground, not the AC ground.
Regards, Gary Lau
MA, USA