At 07:32 AM 7/27/2007, jeremy-ca wrote:
> >
> > 73,
> >
> > Jim Brown K9YC
>
>
>What I find strange is that the prior NEC grounding codes were sufficient
>for decades upon decades. So why the sudden change?
hardly sudden.. the deprecation of "water pipe" grounds goes back to
the 70s, when plastic pipe started to be used extensively (especially
for repairs). The increased emphasis on Ufer types is more recent
(90s), and every revision of the code clarifies things or adds
explanatory notes.
>Do they qualify their
>reasoning?
yes... there's extensive discussion of changes in the codes with lots
of people weighing in with written opinions, backup documents,
etc. A lot of it is becoming available online. Any change in the
code has pages of rationale to back it up.
> Is it via a "concensus" which is like trying to make sense of
>climate change? Or could it be that the NEC must make regular changes to
>justify their existence?
Neither. The code changes to reflect building practices that become
more common, changes in usage patterns and the kinds of equipment
being hooked up. A big change about 15 years ago was in the rules in
sizing the neutral for 3 Phase 208/120Y Wye connected loads because
of the increasing use of switching power supplies. Another example
was a lot of changes to accomodate the increased use of grid-tie
inverters or the new rules for solar panel installations.
There's also changes in what the expectations are of the utilization
equipment. The IEEE Emerald book used to be called "grounding for
sensitive equipment", but the word sensitive was removed because mfrs
who wanted their stuff installed in accordance with those specs
didn't want people to think their gear was "sensitive" (even though
it is... computers compared with, say, 1000 HP electric motors in a
rolling mill).
But, taking the Emerald book as an example, there have been numerous
changes over the years because of two big things:
a) better understanding of the interference and fault issues, based
on analysis and testing and user experience.
b) changes in the susceptibilities of the equipment being used. 50
years ago, there weren't computers using CMOS circuitry, and people
didn't string data comm cables hither and yon.
There are also changes when some egregious event points up a problem.
The grounding problem in the MGM Grand hotel fire is a good example.
It prompted a lot of changes, particularly for wiring practices in
plenum spaces, since the original codes were written in an era before
the widespread use of dropped ceilings and use of that space for the
HVAC return air.
>I for one dont like one size fits all mandates. What may be correct for
>Florida may be severe overkill for Minnesota or Arizona.
WHich is why the AHJ has the final say. The NEC is a minimum
standard (and interestingly, does not really address lightning protection)
>Copper eventually disolves in active soils, it will last for millenium in
>the desert.
But extensive testing in all sorts of soils indicates that copper
embedded in concrete has very good longevity, which, by the way, is
one reason why the traditional driven rod isn't as favored.
>The difference between a 8' and 10' rod may be important in some locations
>but in others 2-4' would be adequate. My 8 footers are driven horizontal in
>12-24" of poor soil; typical for this granite ridge. The power company has
>constant problems in the area with grounding; neighborhood lights dim with
>even moderate household inductive or high current resistive loads.
But is that a grounding issue or a distribution problem? In my area,
there's noticeable light blink when neighbors' air conditioners kick
on, but that's more likely from excessive resistance on the primary
side of the distribution transformers, according to what the Edison
engineer said when I asked her. There shouldn't be any current
flowing in the ground circuit.
>Why should a 40' Rohn 25G for an example be treated the same way as a 2000'
>TV tower?
It's not. It's your insurance company that probably sets the
rules. The NEC required grounding (safety oriented) is pretty
trivial to meet. It's the NFPA lightning grounding rules are
probably the sticky point.
>Unless a person is being forced into compliance then IMO common sense should
>prevail. There is no mandate to upgrade existing residential grounding; the
>term is grandfathering.
That's strictly a pragmatic economic decision. It doesn't mean that
it was safe before or even safe now, it might reflect an acceptance
of some level of risk in exchange for the financial impact. Consider
things like grounded receptacles (3 prong outlets) or GFCIs in wet
locations. Nobody would argue that non-GFCI circuits are as safe as
GFCI circuits, in general, but regulators are happy to let some
number of people kill themselves rather than force millions of people
to upgrade all at once. Instead, they mandate that wet location
appliances like hairdryers and curling irons include the GFCI in the
appliance, essentially spreading the upgrade cost out over time, and
more quickly (since people buy new appliances more often than they
replace their house wiring).
And, in many jurisdictions, if you make any changes, that triggers
some level of changes to bring other parts into newer compliance.
>The successful ones know when to quit computing and hand the product to the
>manufacturing engineers.
Well.. isn't that the very definition of success?
Jim, W6RMK
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