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Posted by Tom The Great on May 17, 2006, 5:49 pm
On Wed, 17 May 2006 03:52:12 -0000, clewis@nortelnetworks.com (Chris
Lewis) wrote:
>> On Tue, 16 May 2006 02:30:15 -0400, gfretwell@aol.com wrote:
>
>> >On Tue, 16 May 2006 05:01:00 -0000, clewis@nortelnetworks.com (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.
You have very very good points, but I'm thinking they reflect on the
older BX cable that didn't have the shorting wire. Infact AC is so
recognized as a good ground, if they toss in an insulated ground wire
you get HFAC. Sure there more qa done, but the jacket becomes the
normal ground fault, and the isolated ground is used for the third
prong for sensitive medical equipment.
Now I doubt this 1953 house has AC with the quality control of HFAC,
but just making a point that AC is good, damn good when properly used,
and installed.
;)
later,
tom @ www.FreelancingProjects.com
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