[TowerTalk] SPG

[Roger (K8RI)]

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 
away.

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.

73

Roger (K8RI)