Robert said:
All bets are off in a direct strike. Common grounding only
reduces the damage from a nearby strike.
....
I still doubt this was induced _unless_ 1) Phone was
run parallel to the floodlight circuit for some distance
and 2) Phone and power were on the same groundstake.
When I built my house, I wanted some wiring to be available
in all rooms, so I ran loops of 25 pair CAT3 telephone cable
around the outside perimeter of each floor in the house. The
loops end at punch down (66) blocks in the basement. There
are no sections of the loops that run parallel to the power wiring
separated closer than 1 foot. There is only one area where the
loops run perpendicular to the power wiring, and that is where the
power cables drop to the service panel. At these points, the loops
are a couple of inches away from the power cables.
One of the damaged modems was connected to the loop that ran
nearest to the lightning strike. It was a zoom modem ISA bus
modem, and its protection circuitry shorted out. No damage
was done to the computer.
The other damaged modem was a $15 Centdyne. It had a chip inductor
in series with one side of the line that was completely blown away.
The MOV that was across the line was still OK. The current that entered
this modem would have had to pass through about 100 feet of
#24 gauge wire, yet it still had enough zap to burn that inductor
off of the board (and destroy the modem's chip set, and the computer's
RS232 drivers and uart)
Most likely, phone was on it's own groundstake, so anything
bonded to power ground would get fried by the ground
differential between power and ground.
Since I did the power and telephone work, they are both grounded
to the UFER ground at the service panel, which is where they enter
the house. The nearest power pole is 600 feet away. So there is
no point in the power grid within 600 feet where lightning could
enter the system... other than my house.
Hard to say. A lightening rod system could be dangerous
if the insulation was insufficiently heat-resistant.
What seems to happen, anecdotally, is the rods conduct the
bolt, and burn the roof where the ground wires run along
the ridge... even though the ground wires for the rods typically
are held 1 foot above the roof.
What I am thinking of doing is adding a lightning rod to the
masonry chimney structure. It is the highest point on the house,
and in the vicinity. I will give it a decent UFER ground
of its own, a few dozen feet from the house. My hope is this
will protect the chimney, but will not endanger the rest of
the house.
-Chuck Harris