Hi, when you are powering the Arduino with the capacitive dropper, is the Arduino circuit safe to touch? The Arduino ground is no longer at the same potential as the earth ground right? Isn’t that hazardous?
No. As stated in the cautionary note at the top of the article, the circuit is not touch-safe.
Any circuit powered using such a method should be properly enclosed in a manner suitable for AC-mains powered equipment. Additionally, the capacitor used should be a X-rated safety type, and a suitable over-current protection mechanism such as a fuse should be incorporated in keeping with typical safety standards.