Jim Brown wrote:
> On Wed, 24 Dec 2008 21:04:26 -0600, firstname.lastname@example.org wrote:
>> b) History, practice and more importantly, mistakes in
>> practice (accidents) have made important contributions
>> to the NEC than simple theory would or could predict;
>> take for iinstance grounding practices. What _is_ the
>> rationale for grounding the secondary circuit (the
>> service to the home) of a power transformer for instance?
>> This isn't strictly neccessary for that service to
>> function. (I am asking this for the purposes of bringing
>> forth an example; I can cite cases where, in practice, this
>> has saved property and/or life, but that is different than
>> a statement or the treatment a textbook might give.)
>> Could you cite a textbook wherein that rationale is brought
>> to light?
> One reference is the tutorial cited below. While it's written
> for sound and video contractors, the fundamentals apply to
> It all comes down to safety. The secondary is bonded to provide
> both lightning safety and, with the equipment (safety) ground
> conductor (green wire) provide a solid path to blow a fuse or
> breaker if a fault develops that could shock someone or start a
> fire. The lightning protection comes from because the bond to
> earth limits the potential that can exist in the event of a
> strike. The concept behind lightning protection is to have the
> whole building rise in potential by the same amount.
Although it's usually phrased this way, it's pretty much impossible to
have an entire building rise to the same potential at the same times due
to the dV/dT (rise and fall) for lightning strikes, BUT is it possible
with proper, single point grounding AND the proper routing of all wiring
going into any particular area in a building to have voltage rises that
are "close enough" within all of the rooms.
However the voltages between one end of a building and the other can
differ dramatically particularly when the lightning strike is very
close, or say when it hits a tower adjacent to the building.With a 60 to
80' long building the instantaneous voltages between one end and the
other can be well over a 1000 volts, BUT if the the system is properly
grounded, the services feeding the building are properly protected, and
properly routed, the voltages on different wires in any particular area
should differ little. But, for instance, IF you have a ham station that
is properly grounded, the separate AC feeds follow the same route, the
same distance you are in about as good a shape as you can expect.
However say you use a computer or computers with the ham station and
they are on a different circuit that may arrive from an entirely
different direction there can be substantial differences in potential
from the computers to the ham station. This is not uncommon,
particularly with older homes where circuits feed multiple rooms such as
a room on the center of the West side has the circuits on the West and
North walls coming from the North going clockwise to the breaker panel
while the outlets on the South side of the room may go to the South
through several rooms counter clockwise to get to the breaker panel
through a path that may be twice the length of those on the North and
An inspection of the wiring in older homes can be surprising and
sometimes a bit scary.
> NEC can be purchased in annotated form -- that is, with
> explanations and commentaries interspersed with each paragraph.
> Jim Brown K9YC
> RFI mailing list
RFI mailing list