I have a situation where a customer wants to replace some of the original two-prong receptacles in a house from the 20's. I understand the options available in Article 406.4 (D) and although AC does satisfy the requirements for an equipment grounding conductor in and of itself, I am concerned that it would be better to simply replace with two-prong devices rather than to install grounded devices and rely on the integrity (continuity, effective impedance, etc.) of the original AC wiring method. The cables are connected to the boxes with a snap-in connector (no lock-nut) and I just don't think I want to count on these to provide a very good bond (I don't even know if they were intended to provide an effective grounding means when they were designed or installed). If i were to install grounded devices, I couldn't even use a grounding pigtail because there are no tapped holes in the boxes to attach it. I would have to use a clip or self-grounding devices. I just don't like it and don't feel good about it. I will ask the Dearborn, MI electrical department, but I wanted your opinions first.