The probe will detect that an extension cord is live if it is plugged into a AC outlet but has nothing plugged into it. The probe will detect flowing AC as far away as 8". Yes, it will detect a live condition in an empty light socket. Loose building wire is what I am concerned about. A house may be destroyed in a tornado, but the electric may still be connected. Before I start chopping debris to free a victim, I want to know that the power is off. You might not get a shock, but crossing the conductors with an axe head could start a fire. AFAIK, DC cannot be detected this way.