Protecting One's Assets: Lightning Rod Technologies

The only thing you can do is try to provide and alternate path to/from ground for the lightning all around the structure that is low resistance AND low impedance.

there are many many sources of information. just go down these lists page by page until you find references that suit you

http://www.google.com/search?hl=en&q=lightning+%2Bprecautions+%2Bconstruction&btnG=Search

http://www.google.com/search?hl=en&q=lightning+%2Bprevention

The 11 kA stroke current you mentioned is a little low. The mean stroke current is actually more like 35 kA and is of negative polarity. Statistically, about 90% of all strokes are negative (negative charge on the cloud base with respect to earth). Positive polarity strokes tend to be of higher magnitude with a mean stroke current of around 50 kA. There are also high current short duration and low current long duration strikes with the latter causing most of the damage including fires.

The real problem for insulation and other structures is the associated voltage which typically has a rate of rise of around 1,000 kV per microsecond. When lightning strikes an overhead transmission line or shield wire, for example, traveling voltage waves are initiated in all directions away from the stroke termination with a traveling wave velocity of approximately 1,000 ft per microsecond. The voltage continues to rise at the termination point until it is cancelled by a return reflected traveling wave that results from the incident wave being conducted to ground through the grounding system and footing resistance (which are all hopefully low impedance).

Surge arrester are very effective in discharging surge current to ground and resealing. A typical lightning stroke duration is around 100 microseconds and a modern surge arrester can discharge the surge current and reseal without disturbing the system relays or reclosers (which operate at a given number of power frequency cycles; typically 3 - 5). To put the lightning stroke duration of 100 microseconds in perspective, the time to the first half-cycle crest of a 60 Hz voltage wave is 4,167 microseconds.

The most effective lightning protection scheme for a given area is to enclose it inside the equivalent of a Farraday Cage. Arrange lightning rods, solidly grounded through low impedance grounds (less than 10 ohms) above and around the structure to shield the structure from direct strokes. All portions of the structure should fall within about a 30 degree shielding angle below the top of the rods. The rods don't actually have to be discrete metal rods. They can be trees, provided the trees are also effectively grounded through low impedance wrapped or driven grounds. It's more difficult than with metal rods or poles because trees grow and the ground wires must be attached near the top to be effective and to protect the tree. The key is to provide a lower external impedance path to ground than the tree. Trees are usually damaged by lightning when the lightning stroke temperatures rapidly heat the internal moisture to steam and the associated pressure causes the tree to explode.

Same rationale that metal bodied autos (except convertibles) are safe places to be in lightning storms, provided the occupants are isolated from the metal body.

Firstly do not build tall, or on a hill, then use a non metallic roofing material. Use plastic guttering and down pipes. Protect individual electronics with both VDR's Voltage dependent devices rated by their stand off and trigger voltages. Have spark gaps to earth again correctly rated. The building can be protected by a distributed lightning conductor network. See your local buildings inspector and get his advice. The local soil conditions and the building design all come into play so no one here can advise you in any meaningful way. Several tests have shown that the building has as much to do with how you add protection as does the theoretical calculation made by a simulator, or lucky guess.

Two additional clarifications.

The needed low impedance to ground means a low inductance, given the rate of rise of the surge current. Low inductance means large diameter conductors and large radii turns up to 90 degrees. Do not use 270 degree turns over an architectural facade, for example.

A lightning flash typically has several sequential current impulses with the first on the order of 30 to 50 kA and later flashes often smaller in sequence. The lightning flash originates at the end of a stepped leader and arcs over a gap of 10 to 50 meters, or so. It does not have a driving voltage measured in GV, it is much smaller, on the order of hundreds of kV to tens of MV.

Bottom line. Lower inductance means lower voltages during the current impulse and lower chance of flash over from the down conductor to protected items inside the structure. Consider the NFPA guidelines minimum practice. For more protection, use larger diameter conductors, avoid even 90 degree junctions, etc etc etc.

Good advice often left to be found out when the system fails just when you need it most.