I'll chime in a bit here:

No surge-protection device can guarantee protection of electrical and electronic equipment against a direct strike. The voltages and current are just too great, even with the typical short duration.

Lightning rods, now called "air terminals", are mainly meant to reduce lightning strikes, because they help "drain off" excess electrons from the ground up. The sharper the point on the rod, the more effective they are.

The secondary benefits of rods may help divert a portion of the current around the outside of a building, and may even reduce the liklihood of fire, but there will still be plenty of induced currents in any metal around.

Often, the nails in a house produce charring in the wood surrounding them, and they're not even part of a circuit. Imagine the induced currents in your house wiring, where one end of every circuit is grounded (the neutral).

The best TVSS setups use main-service voltage clamping cascaded with point-of-use secondary clamping combined with current-limiting for sensitive electronics. A certain portion of surges are generated within the building by other equipment.

In addition, every wire entering the premises should be grounded via a device with a voltage gap for each conductor, as well as directly grounding any shielding and metal (masts, dishes, etc.) Not easy to do, but important.

Finally, all grounding points should be interconnected (bonded). Remember, current requires a difference of potential (a.k.a., voltage), and bonding reduces potential differences. Unfortunately, that can lead to ground loops, but them's the breaks.

When lightning strikes (or high-current short circuits, a.k.a. ground faults or "bolted faults") occur, the extreme currents cause voltage drops along conductors, metal, and even the earth itself (called "gradients"). Bonding minimized these gradients.

Finally, while it's said that "electricity follows the path of least resistance", the truth is that electricity can have many parallel paths. Do your lights go out when you turn on the stove or A/C? Of course not! Same for lightning.

I could go on spouting for a while, but it's time for dinner. I hope some of this makes sense.

Larry
www.fineelectricco.com