[TowerTalk] [LP/PQ] Lightning Damage Case

[George Kauffman]

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