[TowerTalk] SPG (Lightning) problem (long post)
w2lk at bk-lk.com
Fri Jul 31 14:47:27 PDT 2009
I have a SPG question which I would like advice on.
There are four structures on my property; the newly built main house,
about 1 1/2 years old; a separate garage with living quarters above,
about 6 years old; a workshop whose electricity comes from the panel in
the garage and has its own sub panel, and a radio tower. The main house
and garage have separate electrical service feeds and meters which
ultimately connect to the same utility company transformer which has no
other connections. Connections to all structures are via underground
conduit including utilities and CATV. The main house and garage are on a
security system which connects between the two buildings via conduit.
CATV also runs from the main house to the garage through conduit.
The main house has a single point ground consisting of a U shaped copper
wire Cadwelded to 4 or 5 ground rods, all of which is underground. One
end of the U is the safety ground for the electrical service and the
other enters the building foundation about 25 feet away and is connected
to an aluminum strip which is the station ground. All radio equipment
and computers are connected to this ground. At the electrical service
entrance there is a large, heavy aluminum plate connected to the service
ground. Coax, control lines and AC to the tower each run in their own
conduit, all of which terminate near the ground plate. The AC lines are
completely enclosed and connect to the supply. The other conduits end at
the foundation wall and all cables run through lightning arrestors
bolted to the aluminum plate. The CATV hardline also enters at this
point in its own conduit but it is not connected to the ground inside
The garage does not have its own safety ground. It has a copper water
supply and the service neutral is connected to the water lines. The
water lines are fed by a plastic pipe from the well, i.e. no connection
to the earth. The ground here seems to depend on the electrical system
ground at the pole transformer and meters which are about 300 feet from
The workshop does not have its own safety ground.
The tower is grounded at each leg by a heavy copper wire which runs
horizontally about a foot below ground for about twenty feet. I think
one of the legs has two wires so there may actually be four ground
wires. There is also a ground rod next to the concrete base connected to
heavy copper wire which runs up to a PVC box and connects to a metal
plate. All cables from the antennas run through Alpha Delta lightning
protectors which are connected to the metal plate. All control wires run
to Polyphaser surge protectors which are grounded to the metal plate at
its connection to the ground wire. All cables run underground about 300
feet to the house.
Lightning recently hit a tree about 400 feet from the house. Electrical,
telephone and CATV lines run on poles about thirty feet away from this
tree. The resulting surge did the following: damaged the CATV lines
along the road (blew lots of fuses and melted several connectors);
destroyed the cable modem and router in the main house; destroyed the
master board of the security system in the main house; damaged the
auxiliary security board in the garage (still waiting for an assessment
of this damage but it's probably fried); popped most of the GFI outlets
in the garage and destroyed the master CCTV controller in the main house
and one of the CCTV cameras attached to the garage; destroyed two of
three air conditioning thermostats in the main house which are connected
to the security system for remote control purposes; destroyed a CATV
amplifier in the garage. No radio or PC equipment was damaged except for
an Elecraft band decoder and a brand new Green Heron rotator controller,
both of which were connected to lines that were supposed to be
protected. The Green Heron damage was minimal and has been repaired and
I have yet to figure out how to fix the band decoder. There was no
damage to any other electronics in either building.
Based on the damage, my assessment is that an electrical surge probably
hit the the main house and garage at the same time. The SPG in the main
house worked like it should have but the lack of SPG in the garage
caused the surge to find the path of least resistance through the
security system and CCTV links, destroying them. The cable modem and
router were probably destroyed directly by the surge on the CATV system.
To prevent these problems in the future, I think the CATV hardline
should be grounded where it enters. This is easy but probably won't stop
any surge on the inner conductor. What can be done for that?
Adding a SPG will help dissipate surges in the garage, but I am
concerned that there may still be a voltage differential between the
structures in case of another strike, which could cause similar damage.
My vote would be to connect the SPG of each building together with heavy
copper wire and ground rods externally underground but this would entail
a run of some 125-150 feet of heavy copper and lots of digging. It is
not possible to get to the SPG of the main house without a large amount
of damage to landscaping. The other alternative might be to run a heavy
cable through a spare conduit between the two buildings to connect their
grounds together but I am uneasy with this approach.
Do I need to connect the two buildings? If so, would a cable through the
conduit be safe? Any other suggestions?
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