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?