On 10/19/17 1:46 PM, Jim Brown wrote:
On 10/19/2017 12:26 PM, jimlux wrote:
Interesting.. I'll have to go look up the history of that change.
There must have been some problem.
NEC has ALWAYS prohibited more than one neutral to ground bond in a
system, and has always required earth electrodes for every building. As
you noted earlier, the addition of a transformer establishes a new
system, where there must be a neutral to ground bond. I have great
respect for those who produced and update NEC (although the section on
Antennas is pretty dated, and seems focused solely on the possibility of
wires falling on power lines).
Dated is the word - I think it was last revised when people were using
toploaded "aerials" with copper clad steel.
But yes, if there was a change to require ground bonding between
buildings, it must have been for some good reason. Perhaps the
increasing use of coax, which then became the "de facto" bonding
conductor, which is generically a *bad thing*.
At some point, though, the ground bonding requirement goes away -
there's no bonding conductor between successive transformers on
residential service along a street. The individual houses are "sort of"
bonded because the neutral conductor is connected to earth ground at
each house, and all connected together at the distribution transformer
on the pole. (and, of course, "broken neutral" causes all kinds of
interesting phenomena - when you turn your lights on and off, the
voltages on your neighbor's two phases change)
As a member of the Standards Committee of the Audio Engineering Society,
I've been part of many hundreds of discussions on the fine details and
possible repercussions of almost every word in a Standard. Like the
engineers who produce NEC, the AESSC includes membership from a very
broad range of disciplines, and the resulting perspectives yields many
"what if" scenarios. Likewise, as the sound system consultant to
architects designing buildings, more "what if" scenarios.
That's why I'm interested in why it changed - I'm sure it's an
interesting scenario.
Things might
have been very different if only such collaborative design and the right
"what if" questions had been raised during the design meetings that put
the generators at the Fukushima nuclear power plants in the basement.
Chicago learned this lesson 20 years ago when basement generators
flooded after an accident on the Chicago River caused it to back up into
downtown buildings. Cities in New York, New Jersey, Florida, and Texas
have all learned it after major hurricanes.
sort of - they moved the generators up, but not the fuel tanks, or vice
versa.. The code evolves, though.
Bottom line is that good engineers (as well as business and political
leaders) keep open minds and learn from history, and from each other.
Indeed they do.
73, Jim K9YC
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