In answering a question on alt.home.repair about why BX cable with no bonding strip is unsuitable as a grounding electrode conductor I gave the answer that I was given in apprentice school low these many years ago. A follow up posting say's that my answer is nonsense. Does anyone here have the straight info on this question so I can provide the original poster with good information.
-- TIA Tom Horne
Tom Horne wrote:
> snipped-for-privacy@noISPwhasoever.com wrote:
> It is either true or can become so without warning. The problem >> is that without a bonding strip the individual turns of the >> interlocking metal tape armor become insulated from each other >> by a thin layer of corrosion.
That's a pile of hogwash. You can't assume the presence of corrosion. And even if it is present...
> This causes the armor to behave >> as a coil rather than as a straight conductor. The increased >> reactance of the coil effect in the Equipment Grounding >> Conductor retards the current flow and delays the operation of >> the Over Current Protective Device long enough for heating and >> arcing to occur.
Total nonsense. At 60 cycles, the impedance (or more correctly the Inductive Reactance) is meaningless. Start to transmit RF over that wire and it's a different story. Sheeeesh.
>>> BTW this is a very poorly understood area.
I'll say. Just look at the wizard "educating" you.
>>> In subsequent research I found at least six websites that >>>> ignore the problem of impedance altogether.
Uhhh, at 60 Hz, so they should. It's not an issue.
Your totally correct. Flexiable conduits USED to be accepted as a ground. First local authorities out lawed them and then finally the NEC. Flex or the sheith was considered a ground path 30 odd years ago. Has not been accepted her in the SW for more than 20-25 years. My journey man always said. "If you run a ground wire and connected it first and correctly you do not have to worry about being killed or killing someone else."
I run ground wires in metal conduit. Just my way of knowing it will always be safe.
I have seen research data on this, but I can't put my hands on it at the moment. It was a grounding study done by Georgia Institute of Technology. They tested a number of raceway/conductor combinations. There were inductive effects for some combinations. I know the impedance of BX was higher, but I don't recall how much was resistance or inductive reactance. I will try to find it.
Consider this, however. If you assume that in the worst case the coils are effectively insulated from one another, then the grounding path is along a helix. I just measured one at 4 turns/ inch. If you assume .375 mean dia, then you get approximately 4.7" effective path in a 1" run. The cross-sectional area may also be smaller than other raceways , so you would have a grounding path resistance possibly over five times that of a rigid metal raceway for the same length run. Any reactance would makes things even worse.
Ben I agree with the first one. It is the impedance of a "thousand turn choke" that the MC/FMC helix becomes. The bonding strip in type AC bleeds off the leading edge of the current fault wave.
Are you talking about BX or are you talking about flex armor used for motor leads, etc.? BX here is a helix of one continuous strand of metal suitable for grounding that does not rely on touching the adjacent strand because it has just one strand, and the other is interlocking rings that are only mechanical protection, limited to 6 feet, and requires a ground wire.
maybe, theoretically there is something if corroded so badly the edges can't touch, , but in reality how much of a coil is a one strand coil at 60 hz ? Nil
- Use the coil equations and figure the inductance. zip.
if corroded so the metal can no longer touch and is effectively one long helix, theoretically there is something - but even if corroded, how much reactance in reality is there in a one wrap 1/2 inch coil at 60 hz? Nil.
and delays the operation of
What, 50 microseconds in a several hundred millisecond response time residential breaker? Adding a ground conductor in a conduit has ten thousand times the slowing of that of the BX "coil" in a 50,000 amp short.
long enough for heating and
heating of a half a degree on a 50,000 amp short, maybe. The sun spots affecting the bathroom GFI is more probable.
by someone who has never designed a coil, maybe. The coil and impedance equations are over a hundred years old. And well understood by engineers and technicians who work in frequencies above a few thousand hertz.
They also ignore the effect of dinosaur snorts - that doesn't mean they forgot them - it means their effect is non-existant.
BX has its weaknesses just like other types - but the coil effect is not one of them - (esp that the box termination for BX needs a protective sleeve that often is left off - and yet so do romex connections.)
And there are local regulators that create limits because of potential theory rather than from any reality.
Doesn't mean their theories hold water when you put numbers to them.
1) as to the short circuit and drain capacity of BX, specifically the resistivity of BX ground - the numbers -
Hold up a #14 ground wire and look at its end - a #14 wire is 0.003 inches^2 -
Hold up the helix strip carrying #14 and look at its end. Is the helix on the BX rated for #14 at least ten times as big -- .030 inches^2? (Soft steel being 10 times the resitivity of copper.) You could theoretically use it as a conductor, if you insulate it.
As to a ground - indicated by the conductor comment above, a one on one comparison is not really technically fair . That #14 wire is sized with its continuous current rating, to not overheat the particular insulation in its anticipated environment after hours of heavy use. - as to what it really can carry in a dead short - a hell of a lot more, since even the best protection will let thru 4000 amps, and the usual is closer to 50,000 to 150,000 amps -for parts of a second only, however, and that is important when comparing grounds. So a ground, i.e., a non-current carrying wire that is used to drain charge and to carry appreciable amounts of current only a few times in its life, can normally be smaller than the conductor it serves since it can get hot to near the point of combustion of adjacent materials as it works while a conductor may not - as long as the ground does not start a fire itself while the fuse is opening the circuit. So you need to compare the thermal capacity of steel and copper, the melting point of each, rise-time resistance and arc limiting, etc. to fairly compare. But IMHE, it would be more fair to compare the BX helix strip area to a #16 or 18 AWG copper. And that is about the size of that extra thin wire that used to be in the BX, the paranoid's strap. .
Second - alert! Rant coming -- beware of rant! beware of rave! ---
- the obvious but often overlooked - a pet peeve of mine about some regulatory/codes committees - those on the committee who checked and approved BX for the NEC inmtitally and at each revision were not stupid, in spite of what some present regulators seem to think of their predecessors. It is likely that the approvers looked at BX with a wary eye and tested it and found it as reliable as most. Not perfect, since none are. Grounds, vermin, heat, mechanical, etc. If some today looked at emt and conduit and romex like they look at BX, they would ban all conductors - Fortunately, they don't get into some areas like cranes, or flexing machinery where the weaknesses of conduit as grounds show quickly, and where the lengths and levels cause grounds inside conduit to show why we don't use them. And on the flip side - what works in one area may not work in another - what works in Biloxi's humid salt air and 50 degree swing ( 50 F to 100F protected) will fall flat on its ass and pull off the south wall of a commercial building in Frostbite Falls Minnesota with fuel gas contamination, pine-forest ozone, driving snow, freeze-thaw and building shifts, and a 200 degree F swing ( outdoor -40 ambient to 160 F sun-heated surface). (FWIW, as to the yes and no of regulators - it is amazing the number of people I see on the committees who are as dumb as a fence five minutes before they get on the committee, and suddenly are geniuses five mintues after they have been voted on. Osmosis, I guess. And they suddenly know that those who went before them were really dumber than a rock. We're lucky to have them. :>
Some valid points. You're right of course about the thermal issues. But I hope most short circuits in residential wiring don't reach the 50,000 to
150,000 amps you're thinking. A lot of service panel breakers are only rated for 10K interrupting rating. A 100,000 amp fault would not be cleared at all. ISTR that the available short circuit current is supposed to be taken into account when selecting breakers.
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