If you were to look at the UL Standards, 'BX' is standard # 4. The fourth standard developed, that was a long long time ago. 'BX' is the common name based on a company that manufactured the cable from the Bronx - using the B and X from the name.
If you were to try and find 'BX' in any documents such as the NEC, or NEMA or UL you would find no references, All you will find is AC cable.

'BX' did not have a grounding wire manufactured with it and has not been produced in many many years. AC - Armoured Cable has a grounding conductor in the form of a bare (usually aluminum) 16 AWG conductor manufactured with it. The wire that holds the redhead in place [Linked Image]

'Bx' will most likely show continuity when testing for an equipment ground conductor, but the armour cannot and should not be relied upon for equipment grounding purposes.

When working with existing 'BX', one should not install a grounding type receptacle to that circuit as the equipment ground is unreliable and has in many situations started fires or created shock hazards.

For a long time, AC cable and conduit were the most common wiring methods. I would say that NM cable has become the most widely used method, but AC cable is still installed in large quantities.

Pierre

Pierre Belarge