UL Approval Requirements for Non-Isolated Electronic Transformers With 24-120VAC Output

I'm looking at a project to make a electronic tranformer for use within a product, and driving its teathered external devices for a control system. I want to make the output AC voltage configurable between 24VAC and 120VAC. This can easily be done with PWM of the AC signal and a feedback path. In its simplest form it is just back-to-back MOSFETs in a AC switch with a little filtering, and the PWM controls.

I'd like to get UL approval on the device. Electronic transformers typically empoly a small transformer for isolation (smaller than typical transformer due to high freq operation). However in this application where 120VAC is an output option, the transformer would be 1:1… and thus be seemingly of dubious value for isolation.

It is not clear when UL requires isolation. There are lots of non-isolated AC-DC converters that are UL approved. I also noticed that there are new rules that allow non-isolated PV inverters that are high-voltage DC-AC connected the mains (high stakes consqeunces).

So the basic questions are:
1. Under what circumstances is non-isolation acceptable to UL?
2. What safty measure would be required for this class of control device?

Troy, are the external devices able to be powered directly from non-isolated 120 VAC and meet UL?

Thanks for the good reponse. There are two parts to this question:

In 120V mode, yes, anything that would be plugged into it are things that could be pluged into an outlet and are UL listed. I'm guessing this is less of an issue, as they could be powered by an relay without question and without isolation as long as basic saftey is observed (over current, etc)… true?

In 24VAC mode, though things may be different(?) While the external devices are still UL listed, the 24VAC I'd like to supply to the external components (e.g HVAC equip) from our control box would be from an internal 24 VAC non-isolated electronic transformer. I note that many non-isolated AC-DC converter circuits are common amongst IC mfgs. What I don't know is when non-isolated transformers are acceptable (AC or DC) and if they are acceptable when power and signaling connected to devices outside the envelope of the widget?