According to Tom The Great :
On Tue, 16 May 2006 02:30:15 -0400, wrote:

On Tue, 16 May 2006 05:01:00 -0000, (Chris
Lewis) wrote:

While the NEC does permit cable sheath as a ground, the CEC
hasn't for a long time, and I wouldn't recommend relying on
it unless there was no other alternative. Old armor can
get remarkably high resistances...
It would still be plenty to trip the GFCI if you had a ground fault to
the case of attached equipment and eliminate one of the problems
mentioned here. Actually AC cable does pretty well if it was properly
installed. I did a survey of some old WWII buildings that were being
converted and all of the AC runs were 1 ohm under a test load (Ecos
tester)


Ohms is a measure of resistances, but typically measure using DC. The
old AC has inductive resistance, which is the problem. So I've been
told.

About a year back when this issue was raised before, I ran the
numbers can came up with a value of a hundred microhenries of
inductance in 100' worth of AC. That can be ignored at 60hz.
Further, as much of the turns will have shorts to adjacent
banding, that will kill most of the inductance (even a single
winding-to-winding short in a coil makes a huge difference).

The sheath on AC has been made in a variety of ways over the
years. Aluminum, cut ribbons of galvanized steel, and other things.

Cut galv. ribbon has gaps in the coating - moisture = rustout.
Aluminum surface oxidation. Corrosion on connectors/boxes. Etc.

I can imagine that WWII military buildings were made with the good stuff,
and installed rather better than average.

I personally would hesitate to use AC armor as ground in
old systems where it was more of an incidental box-to-box ground
rather than something more actively involved in direct grounding
of devices via third wire grounding systems which didn't exist
at the time these circuits were laid.

My co-author of the electrical wiring faq has seen AC armor
participating in a dead short where it was a poor enough
connection to _not_ blow the breaker, but a good enough conductor
to glow red hot.

There was a major fire in a Los Vegas casino several years back
which turned out to be just this sort of thing.

Heck, another close friend found an AC armor segment to be fully
live, yet, as far as he could tell from the wire segments he
could see, was fully bonded back to the panel. Not.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.