According to SpamFree :

Further, do you realize that there are hundreds of thousands (perhaps
millions) of feet of old style (non-follower-wire) AC installed in
thousands of buildings much of it dating to the forties and earlier?

Yup. Here too.

And that old-style AC is being used as a ground by its very nature.
The metal box with the switch or the device is connected to the armor
of the AC by a screw (not a clamp), the armor of the AC is connected
to the load center metal (probably your denigrated steel) by another
screw, and the load center is connected to the ground (likely the
water main). Whether it actually forms an E(quipment) ground depends
on the connection between the device and the box but in the case of
direct wired lamps (isn't this Horne's dramatic case) it's almost
certainly electrically bonded.

But of only minimal use as a ground. Those outlets were two prongs.

Just how good do you think 50 year old BX armor is as a ground, given
the oxidation it, the boxes, and clamps have gotten over the years?

And it's proposed to utilize this as a classic three prong outlet
grounding system. I've seen too much of that stuff to trust it.

Like the perfectly well panel-grounded chunk of BX in my friend's basement
that recently raised an enormous bang when a contractor slung a grounded
metal trouble light on it. You see, the end had a hot-armor short,
and the armor didn't have enough conductivity to trip the breaker.

Contractor: "we see this all the time...".

The house is about 100 years old.

So applying your horror scenario, if a fault develops in the lamp or
the box or in part of the cable the weak and ineffective armor will
not be able to carry enough current to blow the circuit breaker and
the home owner (or tenant), presuming a defective bulb, will be
electrocuted when he investigates. Alternatively the AC will heat to
incandescence (your earlier assertion and presumably Horne's dramatic
case) and burn the building down.

It happens. My co-author has seen it. And, IIRC, it was the cause
of the major casino fire in Vegas.

OK, I'm being sarcastic but the point stands. If old-style or
new-style AC represented any significant danger then there'd be moves
afoot to replace it (mandatorily) and there certainly wouldn't be any
more installed. Isn't this what's happening with Knob & Tube?

No. Very few jurisdictions, if _any_, are requiring mandatory
replacement of K&T, except in renovation work to new code. It's
the insurance companies in the private sector making K&T difficult
to insure. Despite the fact that aside from burial in insulation,
existing K&T is usually quite safe. Better than some later wiring
systems.

Similarly aluminum. Proven track record of hazard. Is there a mandatory
recall? No.

Asbestos.

UFFI.

Lead paint.

PT lumber.

Etc. Except for few cases (mostly commercial or local rules), none of these
are required to be removed. Mitigated in many cases, but rarely
mandatory removal.

You can argue that they're not as hazardous as some claim all you want, but,
they have been deemed hazardous, and _none_ of them are subject to mandatory
complete removal. Because it costs too much.

There's a reason that current carrying conductors are copper, not steel.

Aw come on! The current carrying conductors have to be wound around
screws and bent in tight arcs. I'd be the first to agree that steel is
not suitable for this use.

Bus bars and terminal tabs aren't steel either, they don't have to be bent
at all. Steel isn't suitable for that use either. Steel simply isn't
suitable as a conductor (except perhaps conduit, and even that we don't
permit).

[Canadian code, and I suspect US too, even doesn't really permit you to
use a metal box as part of a ground conduction path - you gotta bond
those copper ground wires together in NM, rather than run them
under different box screws. Except with US AC...]
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.