Les Kalmus wrote:
> I have a SPG question which I would like advice on.
>
> Background:
<snip>
To quickly summarize: 4 independent structures, all connections between
bldgs are in conduit underground. Independent electrical services to 2
of the bldgs, third bldg fed from one of the others with a service.
>
> 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 building.
>
> 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
> either building.
That's probably not code compliant for a residential structure, but
since it's a garage (and perhaps in a rural area, so considered an ag
outbuilding or something) maybe there's an exception (I don't think so,
though.. If it has a separate service drop, it needs its own grounding
electrode)
>
> The workshop does not have its own safety ground.
It needs one.
>
> The tower is grounded at each leg by a heavy copper wire which runs
> horizontally about a foot below<snip>
I assume the tower is also bolted to the base, so it's probably grounded
that way, too.
>
> Problem:
> Lightning recently hit a tree about 400 feet from the house. Electrical,
> telephone and CATV lines r
<snip>
> Questions:
> Based on the damage, my assessment is that an electrical surge probably
> hit the the main house and garage at the same time.
And the tower and the workshop too.
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.
The transient can also be coupled electromagnetically. It's not
necessarily a matter of what's the lowest impedance path.
>
> 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?
You've got two issues.. one is the difference between shield and chassis
potential (i.e. the voltage of the shield vs the voltage on the
powerline grounding) and the other is any voltage between center
conductor and shield. The latter is likely to be quite small (it is a
shielded cable, after all). The former is what bonding the shield to
the power supply ground (at the same point.. not 20 feet away) helps.
Then, if there IS a transient, the equipment goes up and down with all
the connected voltages together.
Say you have a cable TV box and coax and TV connected it. If the cable
box chassis and the shield coax coming in are connected to one reference
point and the TV is referenced to another, then when the transient comes
in, you'll still have problems.
>
> Adding a SPG will help dissipate surges in the garage,
Single reference points don't dissipate surges. What they do is provide
a commmon point to which a bunch of things are connected, so that there
is no surge, within the bunch. The whole bunch might have a lot of
voltage relative to some other distant point, though.
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.
Yes. Exactly.
> 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.
You don't need to go to that extreme. The bonding conductor needs to be
appropriately sized (I don't have my code in front of me, but it's like
AWG 8 or something). The key is that the bonding conductor AND all the
wires that it parallels, all move together. The bonding conductor has
inductance and resistance, just like the interconnecting wires. A
transient will propagate down it just like the on the victim wiring. So
everything rises and falls together (at a given point).
--- bonding the two systems together still might not help. Here's why:
Consider the power feeders and your bonding conductor as a big
triangular loop. Get a strike, and the magnetic field from the stroke
will couple into that loop, inducing a potential around the loop.
All you can do is arrange your gear so that any such "loop" currents
don't flow through the delicate insides of the boxes.
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