To me, the "concept" behind an SPG is simply that all of the equipment
in a room, or on a system, in a building, or how ever you wish to group
or organize it will tend to rise to the same potential from lightning
strikes, be they direct hits or nearby strikes. There may be thousands
of volts difference between the equipment in different rooms, but the
equipment in each room will have roughly the same voltages. The true SPG
is the easiest way to at least to successfully approach this goal. I say
approach, and I think Vic said it well as you will at some point reach
the point of diminishing returns or ROI is no longer viable.
I also think more is involved in lightning protection than the SPG as
this assumes the pulse or pulses will come in on some line or lines when
a nearby strike can induce as much as 1000 volts per meter from a mile
More is involved in lightning protection than the approaches many of us
take with grounding, lightning arresters, and SPGs, but everything is
based around the concept of the SPG. Unfortunately we can easily undo
much of the protection we have gained from the SPG without ever knowing
how or why.
Different systems be they cable, telephone, satellite, AC power need to
follow similar paths yet keep the necessary isolation from each other.
An example is say most of the lines closely follow the same paths except
the phone line (it could be a hard wired network as well). Rather than a
direct hit, there is a nearby strong strike that induces 1000s of volts
in all lines. Those following the same paths, or close to the same
paths will have similar voltages, but the phone line which may have
entered through an SPG follows a path that differs by many feet. This
longer or shorter path, maybe through the walls on the opposite side of
the room (homes are wired that way) will end up with that phone line
having a far different voltage on it than the other lines even though
they ALL came through the same, grounded entry point. In this case the
damage will still be done. The phone line wipes out the computers which
are connected to the ham rig(s) and they now have substantial voltage
differences. The SPG will work for most items in the home except the
phone line in this case unless the strike comes in on the phone line.
The above case is also true with what I call "installation creep".
Initially the wiring in a house will follow some sort of pattern.
Typically the circuits leave the breaker box and head *up* through the
walls, across the ceilings where the junction boxes are located, and
from there the circuits are daisy chained up and down through the walls.
Often the outlets in one room are on the same circuit as the lights in
another. The thought being that if a breaker blows due to a short in one
of the outlets that room will not go dark. BUT, say we need another
circuit in the ham shack. Will it be run up through the now closed walls
which are probably full of insulation (likely foam), or will it now run
across the basement ceiling (through the floor joists) and up to the new
outlets for say a new amp that needs 220, or maybe it's 110 because more
outlets were needed for additional equipment such as computers, their
accessories and accessories for the ham station? Now we have AC powering
interconnected equipment in the same room, but whose AC follows a far
different path length. Likely much shorter in this case. It's not just
the grounds rising and falling at the same voltages, it is all of the
lines rising and falling together.
I'm a firm believer in using conduit and that is metal conduit inside
buildings, not PVC. That allows the wiring ...ALL wiring TO a particular
room to closely follow the same path. It also shields said wiring "to
some extent", but the conduit itself will have induced voltages.
So the point of diminishing returns not only includes cost, but complexity.
Add to that the idea that we can assure a SPG for a given area, and we
can carefully route all the wiring to minimize differences in length and
location, BUT we can not prevent all damage. The better thought out and
carefully implemented ground systems and wiring the more we minimize
the potential for significant damage, but that potential (no pun
intended) never reaches zero and particularly so where the super strike
or positive lightning is concerned.
Often it is impossible to maintain a true SPG. My ham station is such an
example. I have a station in the house and one in the shop. The system
is set up for SO2R with these stations BUT the stations are in different
buildings with DIFFERENT AC FEEDS. The control leads for the 6-pack are
common to both stations, the coax feeds are common to both stations, AND
the computer network is not only common to both stations, it ties into
the phone line, the cable feed, and it runs within 10-12 feet of the
tower base. That is a total of about 350 feet of CAT5e and CAT6 cable
into which voltage can be induced. There are two 130 foot runs from the
8 port switch in here to the computers in the shop and nearly a 100 feet
total to the three other computers. However the system grounds for all
towers, stations, and AC feeds are all tied together with over 600 feet
of bare #2 cad welded to 32 or 33 8' ground rods and the common lines
between systems closely follow the ground system.
So far with all the strikes the system has taken I've had no damage.
However I was surprised to see the plastic caps on all of the AV640
stubs were burned.when we took it down today and they were all burned
more on the side away from the mast.
TowerTalk mailing list