HorneTD wrote:

SpamFree wrote:

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.

"Always question." "Always be wary of asserted authority."

In none of the above do you make reference to anything we can check or
to anything which explains the apparent anomaly of a perfectly good
steel strip connecting the source of the fault with the building
ground not working properly if wound in the form of a spiral but being
OK if a ridiculously small gauge wire follows along the spiral. The
proffered explanation of inductive resistance seems to be definitively
crushed by Chris Lewis who in the earlier discussions was supported by
other posters who had presumably done the same calculations. In any
event no one pointed to errors in his reasoning.

You (or UL) seem to think that the EGC depends on the contact between
each of the spirals. Why? AC is a continuous length of steel that just
happens to be in a spiral form. If one were to pull apart the spirals
thereby extending the steel to its full length and properly connect
both ends wouldn't that form an adequate EGC? Corrosion between the
spirals is really immaterial. The only questions would seem to be: Is
the steel armor of sufficient gauge to carry the current and are the
ends properly connected? In the latter case you can badly connect the
NM ground too and connections can work loose over time so how does
that differ?

And please spare us the drama. "Carrying out the dead" indeed!