Hello Les;
I will try to explain some of what might be going on at your location. Please
do not take my observations or possible conclusions as a suitable solution or
design. There are probably several other factors, which you might not have
mentioned, that could have a substantial effect upon the opinions expressed
here.
Your situation is much more complicated than a normal house by two factors:
1. There are several structures, with interconnecting cables. These can act
as independent grounded entities.
2. There is a tower, that can increase the likelihood of direct strikes.
Unless exceptional bonding and lightning grounding establishes an
equi-potential plane, it is possible that you will have to suppress all
external input and even inter building wiring. Just running grounds in
parallel with power or signal conductors may not be able to
reduce damaging current flow. I have seen numerous cases of wiring in buried
metal conduit be unequal to the task of reducing damaging energy in the wires.
>From your description, the most likely problem that you encountered was a
>ground potential rise involving the pole servicing your location.
The voltage of the location where lightning enters into the soil is elevated
substantially, together with all nearby equipment. Despite our efforts, with
the best ground electrodes, the resistance is almost always too high to avoid
creating this high driving voltage. Most electrical codes require either an
earth electrode resistance of less than 25 Ohms, or use of two ground rods, at
the service entrance. The value of 5 Ohms might be achievable in a Florida
swamp, while over 50 Ohms might be typical in a dry Colorado location. A small
lightning stroke current of just 10kA will tend to produce 50kV or 500kV at
that poor electrode. I say tend, because there are usually parallel paths to
share current, and arcing (side-flashing) and other breakdown can also occur
to limit the voltage. Keep in mind that double insulated power equipment is
designed and hypoted at only 2kV to 4kV, so this level of driving voltage is
easily enough to
cause failure to most equipment. Communication wiring is usually susceptible
at much lower voltages.
Architectural lightning protection, which includes rods and wires on the roof
ridges, needs to include adequate ground bonding to make voltage gradients
small across the span of equipment and interconnecting wiring. This is why
lightning grounding and bonding usually includes a peripheral ground wire
system. This seems to be installed at the house, but not the other
structures. The next technique is to suppress all wire entries to
the ground system. Finally, use adequate, however imperfect, earth electrodes
to drain the current into the soil. Using the tower grounding as well as
the power service entry electrode and grounded wire also can augment the
earthing connections.
Les, you ask "Do I need to connect the two buildings? If so, would a cable
through the
conduit be safe? Any other suggestions?" You are already connecting the
building, with cable. More connections might increase protection, however the
following parameters are involved in whether two buildings are separate or
grounded as one: Distance of separation of the buildings, the resistivity of
the soil, the location of earthing electrodes and peripheral grounds, the
susceptibility of interconnection wiring, a tower, the height of the
building or other factors increasing the likelihood of direct strikes. Off
hand, with the experience of damage, it is likely that these factors are not in
your favor, and just installing cables in conduit will very most likely not
protect the equipment.
Not only the lack of a SPG, but the possible lack of earth connection at the
garage might be a problem for power faults as well as lightning currents.
Also, with a tower, I would would hesitate to assume that there was only one
strike, or that it occurred only at the tree location. Or will not strike in
the near future.
In response to your comments
"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?"
NexTek makes an excellent protector with F connectors for 75 Ohm video and CATV
lines. Please refer to:
http://www.nexteklightning.com/products/Arrestors/Standard/Cable.html
This product does contain voltage limiting on the center conductor!
The minimum code requirement in most locations is to ground a coaxial cable at
the entry to a building. While this does an adequate job of reducing fire and
electrical voltage risks, it does not prevent damage in many cases. You might
actually need protectors when cables transit one building to another, if the
voltages cannot otherwise be controlled.
I hope this helps explain some of the possible factors in your situation.
George
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