On Sat, 23 Dec 2017 23:14:03 GMT, Iggy
m wrote:

replying to trader_4, Iggy wrote:
You're absolutely right IF it's actually flexible conduit for just in the
crawlspace. However, If the entire place is actually BX, then the jacket can
be very successfully used as a ground conductor between a grounding outlet and
a grounding rod. I've done it and have had it done, the ground will be fully
confirmed by any and all Wiring Testers, GFCI Affirmers and other testing
methods.

Sure, a separate ground wire MAY be better, but only due to matched or
slightly better impedance though at much less capacity. 1st the system's
ability needs to be confirmed, then each outlet needs confirmation and
necessary correction. But, he needs to understand that new wire isn't better
nor a guarantee, replacing sound 14-gauge with new 14-gauge is no upgrade and
completely wasted effort for no benefit.
There are several versions of "bx" cable. The original was just the
spiral jacket - and is no longer considered a ground. The next version
had a "ribbon" or "tape" conductor running the length inside.. The
third version got a bare copper wire ground.

I believe the current versions are called MC and AC. AC is "armored
cable" - steel? armor and aluminum ribbon bonding strip that allows
the armor to be used as a ground (250.118(9)).

MC is "Metal Clad" and has no bonding ribbon, and a full circuit
ground conductor. (article 330) It is an aluminum sheath IIRC.