They float, which is to say they have no earth ground reference. That's why they're battery powered and if they do have communications it's usually done optically.
If you need isolation from this `150-250V, it's going to boil down to a "DC current clamp in a chip" like solution (eg LEM current sensors) or "Floating multimeter with an isolation barrier" like solutions (jereon79, frozenfrogz, etc).