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(Chris Lewis) wrote:

According to SpamFree :
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.

Let's not start all this again, and look at something else - Canadian CEC
hasn't permitted cable sheath to be a ground for at least 30 years,
has _never_ approved armored cable with that "ridiculously small gauge
[bonding] wire", and has insisted for at least 30 years on a full size
copper ground conductor.

I realize that you're Canadian and have a patriotic reason for
asserting the superiority of the Canadian Electrical Code however I
have an equally patriotic reason for asserting the opposite. In fact,
what Canada does wiring-wise has IMO as much relevance as the
electrical code of ... oh, say Bulgaria g. Canada doesn't allow AC?
Well that's nice to know. They probably do have special problems
wiring igloos g.

Cable armor is physical protection and little more.

The facts are simple: the cross-sectional area of cable armor is small,
perhaps not even as much as a copper wire. It's steel, not copper. Steel
rusts. Even when galvanized, the edges rust. Even if aluminum, the
manufacturing process will not produce long-term high conductance joints.

It's small cross-sectional area to begin with. Rust makes it smaller. It's
really long. It's brittle. An impact can destroy the electrical conductivity
if it cracks the strip. One must NOT rely on the conductivity of cable
sheath or box clamps for ground continuity.

So you say. However NYC, hardly a slouch in the imposition of onerous
regulations, has always insisted on the use of AC (nowadays with the
follower wire) and in using the armor as the EGC. Only in the last few
years have there been the minutest changes to allow NM in some very
suburban style non-rental houses. Commercial and rental buildings are
still conduit or EMT or AC. I believe many of the large older US
cities are in the same situation.

If the problem is as severe as you and Horne assert it would seem
logical that, needing the rodent-protecting capabilities of armored
cable, NYC would have changed to MC long ago. But they haven't.

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?
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.

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.

Wow! I guess the NYC electricians guild hasn't thought of this. Just
think of the work replacing all that old-style AC with
more-expensive-than-new-style-AC MC. Just think of the mega bucks
rolling in.

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?

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.

Tom is right. Cable armor is a lousy ground. It's just that reactive impedance
is _not_ the reason why it's lousy.

[If I recall my calculations right, several hundred feed of cable armor will
have an equivalent impedance of a few microhenries. At 60hz, the reactive
impedance is insignificant - nowhere near enough to affect breaker trip.
It's like saying "skin effect" matters at 60hz. Skin effect exists. But
at 60hz it can be totally ignored.]