A.George Flach, former chief electrical inspector for New Orleans, responds: The answer depends on what kind of BX cable you actually have. General Electric’s original BX cable was manufactured without a bonding strip — a thin metal strip within the outer metallic jacket that runs the length of the cable — or bonding wire. Even if the cable is properly fastened to the metal box, the spiral metal tape jacket alone can’t be considered a reliable equipment-grounding conductor. The only way to upgrade a two-prong outlet to a three-prong outlet with this type of wiring is by providing GFCI protection, either with a GFCI breaker to protect the entire circuit, or with a GFCI receptacle at the junction box. Outlets on the same branch circuit and those connected downstream of the GFCI receptacle would also be protected. (However, be sure to test the GFCI after installation: If the test button doesn’t trip, there’s no ground-fault protection.) These receptacles must be clearly marked “GFCI protected. No Equipment Ground.”

Later versions of BX cable are equipped with a small-gauge, uninsulated conductor; you may also have AC (armored cable) with a bonding wire and interlocked metal jacket. In either case, if the cable is in good shape — and there is a good connection between the outlet, the metallic junction box, and the bonding wire — a standard, properly installed three-prong outlet would be grounded. An alternative would be to install a GFCI receptacle or breaker, as above.

Before doing anything, though, I’d remove the existing receptacle’s faceplate and examine the wiring without disturbing it or removing any receptacles. If the rubber insulation on the branch circuit conductors is brittle and falling off, don’t remove the existing receptacle unless you intend to replace all of the wiring, too.