Kort svar: Nej, du behöver ingen jordledning. Koden låter dig ersätta två-trådsbehållaren med ett GFCI.
Från 2011 års nationella elektriska kod
406.4(D)(2)(b) A non–grounding-type receptacle(s) shall be permitted to be replaced with a ground-fault circuit interrupter-type of receptacle(s). These receptacles shall be marked “No Equipment Ground.” An equipment grounding conductor shall not be connected from the ground-fault circuit-interrupter-type receptacle to any outlet supplied from the ground-fault circuit-interrupter receptacle.