Watch out: do not ignore the NEC requirement to label the GFCI receptacle on a two-wire un-grounded circuit as "GFCI Protected" and "No Equipment Ground.
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Honestly though I would never trust one 6 screw and a thin ground strap on our plastic boxes to handle fault current.
to an existing 2-wire circuit? A non-grounding type receptacle can be replaced with a grounding type receptacle if an equipment grounding conductor is installed and connected to any accessible point on the grounding electrode system.