SpamFree wrote:
John Hines wrote:


wrote:


Personally I like the dedicated equipment grounding conductor, but
then I have to remind myself, BX(AC) has a dedicated equipment
grounding conductor, and unlike romex(NM) it also shields the
conductors from accidental abuse.


BX isn't AC.


The modern AC is repeatedly referred to as BX by electricians and
electrical supply stores. I was in HD a couple of days ago and they
had a special on 250ft 12/3 ***BX***. DIY books seem to use the BX
designation more frequently than AC. Like Romex and Kleenex and Xerox,
BX was a trade name used by the inventors, GE, and referred to the
location of the plant where it was invented: the Bronx, i.e. BX.
Similarly Romex was invented (!) in Rome NY, hence the name.


BX doesn't have the grounding conductor, which means it
acts like an inductor when the cladding is used as a conductor. This is
why the bare grounding conductor was added, and the name changed to AC.


This has been discussed here many times.


To which discussions you obviously didn't pay attention. The
discussion of induced current in the spiral sheathing resulted in the
conclusion that at 60hz any induced current is infinitesimal. In
present day AC the small gauge follower wire is not the ECG and is
electrically not connected to anything except the outside sheathing.
The outside sheathing IS the ECG.

Although it's difficult to find out the exact history the consensus
seems to be that BX was introduced not primarily to prevent damage to
the conductors from nails and the like but to stop rodents gnawing on
them. The ability to use the outside sheathing as a ground was simply
serendipitous.

As to where the "induction" idea came from I heard an interesting
comment from an old guy who was certainly around at the time of the
introduction of the follower wire (early sixties). He said that the
problem with the old no-follower-wire BX was that it caused
interference with the TV sets which were becoming popular at the time.
The solution was the follower wire which stopped the spiral from
becoming an antenna. This may of course be a crock-of-****. If anyone
has any firm documented information from the era, I'd be interested in
hearing about it.

Underwriters Laboratories conducted test back in the sixties on the use
of the armor of metal jacketed cable as an Equipment Grounding Conductor
(EGC). The results of that testing revealed that the spiral wound
metallic tape armor was unsuitable for use as an EGC unless the spirals
were shorted to each other by an internal bonding strip. Even on newly
manufactured cable the absence of a bonding strip more than tripled the
time to open of some Over Current Protective Devices (OCPD). The effect
this had was to allow an arcing fault to continue long enough to result
in the ignition of combustible structural elements. The UL report led
to the requirement for a bonding strip inside the armor of cable
assemblies were the armor would serve as the EGC for the circuit. The
accelerated aging tests showed that the problem was much worse in cables
that had time to develop a layer of corrosion between the spirals and
that it happened well before the corrosion was visible to the naked eye.
I have attended fires that were caused by the failure of older "BX"
cable to conduct enough fault current to trip the OCPD prior to ignition.

Several posters have said here that impedance or rather the reactive
component of the total impedance could not possibly make a difference at
60 hertz. I don't know how to explain away the testing that was done by
UL that showed that a wireman's failure to cut a slot between two
knockouts of a metal box that each pass a conductor of an AC circuit
into that box will result in inductive heating and that this inductive
heating can, over time, cause the pyrolysis of the supporting structural
element to which such a box is attached resulting in the eventual open
flaming ignition of the structure. That process takes months rather
than hours so it is not well understood in the industry because the
effect is not readily discernible over a short time.

I do not lay any claim to fully understanding the physics involved and
according to some posters that makes me a "codebot." If my
unwillingness to agree with the newsgroup's self appointed experts over
the work of UL and the National Fire Protection Association makes me a
"codebot" then I will except that more readily then the task of carrying
out the dead from fires of electrical origin. I guess that means that
"codebots" don't think they know all there is to know about electrical
safety and are willing to depend on the professional judgment of the
engineers at UL and NFPA more readily than the "experts" here.
--
Tom H