I think one of those responses was referring to ISOLATION TRANSFORMER:
Those are special, some look roughly like a Variac, in that the medium power devices have a regular mains AC outlet, for plugging your device to under test. It is, for example in the US, 120 VAC both for input and for output; that is, that isolation transformer is simply wound as a one to one turns ratio. You get NO voltage change, with only modest power capability (small ones maybe 200 watts).
But advantage is that the secondary (transformer) windings are voltage-isolated. That gives some increased measure of 'safe' isolation. Of course, that's a great, fairly affordable way to go, when repairing stuff, generally. Bench TECHs use an isolation transformer, just for one-time or temporary use.
Reminded me of some older friends, (now gone), engaging in self-employed 'COLOUR TELEVISION' repair / service folks.
Try search on 'Isolation Transformer'