Good Day,
I have no intentions to start any long discussions here, so I decided to stay away from any.
However, I would want to give a recommendation to David: Start your own internet research on safety isolation transformers and then talk to an experienced electrician to understand the benefits of these units.
On that subject: Your VARIAC very likely provides no isolation. Here is a simple set-up how to test this: Unplug the unit completely and use cables with alligator clips to shorten the input's LIVE/NEUTRAL and another pair to shorten the output's LIVE/NEUTRAL conductor.
Now measure the resistance between both points - the shortended LIVE/NEUTRAL on the secondary side and the shortened LIVE/NEUTRAL on the primary.
Unless you measure a high resistance path, your VARIAC provides no isolation.
Cheers,
Magnus
