Hey to N8WRL from WA1YYN !
All this stuff is interesting to me because of amateur radio stuff, being an electrical engineer and having to do too much work with metallic pair (coper wire) protection stuff, and being a volunteer firefighter and thus having seen the high cost of failure. So merging these areas produces psychotic split on bad days, and OK results on good days.
I view lightning events as one of three flavors: Tickle, Slap, and Kick. The Tickle is what you see on your TV and hear on the phone. It can easily be dealt with via suppression devices on your AC and telephone. The Slap is a very near by strike. I had one of these when generator was running and it blew out a protector on the start relay. If the protector hadn't been there, the entire generator control circuit would have been fried. This can also be where a tree close to your house gets hit, and you blow up a few appliances. Some of that is caused by ground getting kicked around (called "ground bounce"), and some by picking up energy. This is the every wire (including fences) is an antenna and there's a lot of energy in the RF (radio frequency) range with lightning. Whole house suppressors and a good ground can help a lot. If you are relying on the telephone company's gas tube protectors for these, you will loose- you need additional solid state protectors of the type found in some of the surge protector strips.
These surge protector strips are only as good as the ground system in your house. If you've got the weenie 4' copper plated ground rod that the telco's put into disturbed soil under your eave (i.e.dry) then you're gonna loose. If you've added 2-3 ground rounds, and maybe even put one where the gutter dumps out so it's wet then you've got a much better ground and might win. The biggest reason I put all 20 amp outlet circuits in the new house was for the better ground. I have about 1200 feet of copper in the ground for my ground system, some have accused me of overkill.
The only thing I've lost to lightning in 20+ years is a telephone PBX. It was in 2002, and we had an incredible drought in NC. The nearby strike that took out the generator start relay fried the PBX. I attribute this to dry ground making my ground system less effective, which is why I have such a large ground at the new house.
That leaves the Kick. This is a direct strike either on your house or onto any utility drops going to your house. The definition of surviving a Kick is that your house doesn't burn down. You might fry a lot of electronics, but that's what insurance is for. So things you do to survive getting Kicked are:
1) Try to get the AC into your house so it's buried. Yes, you can have a strike at the pole but if it's real bad it will blast out of the cable in the ground and you won't get the full brunt of the strike. Your whole house protector will sacrifice itself to save your house.
2) Try to get the phone line into your house buried. Same idea. My phone line is buried 5 feet deep and is in conduit. It's a 800 foot run. It's that deep because the gophers only go about 2-3 feet deep and because I have a backhoe (had to tie this back into tractors somehow). It's also buried at the street. The conduit give me over 10KV of isolation, and even on a typical ground strike you don't typically see that much voltage that deep. Did I mention my love of overkill?
3) Figure out how a strike on the roof will get to ground. A metal roof makes this a bit easier. I've seen 3 houses with asphalt shingles that got struck and the lightning went around and found the vinyl coated aluminum facia. It ran around the house and found the coax for the satellite antenna and followed it to the grounding block. The coax was vaporized. All three houses had lots of internal appliance damage, but none burned. Houses that burn had the lightning find it's way inside and then it was on.
Note that protection is a divide and conquer approach. You put the bulk to ground, some gets in, you may have secondary protection to deal with that, and then the end device might have some protection.
So we've seen tips on grounding a metal roof, the OP's concern. I'd add to those idea the possibility of adding a few ground rods out in undisturbed soil near a down spout if you can. Put your satellite dish on a pole in the ground. Get a whole house surge protector.
I apologize for the long post. This is something I'm passionate about (or crazy about, depending on your perspective). The science behind the nature of lightning is as well understood as economics, batteries, and women. So I've focused on real things that anyone can do. Since a strike can be as small as 2000 amps and as large as 500,000 amps, there are no guarantees. The last piece of any lightning protection plan is insurance.
I end with all the normal disclaimers and statements that your millage may vary, there's no promise any of this will work, consult a professional for your particular situation, and please don't be part of the problem and sue me if something goes wrong and you're unhappy.
Pete
