Riser Conductors of Lightning Protection System in a High Rise Building

My experience was in power station stack lightning protection only [even of 300 m height] but we always follow only NFPA 780. So only copper conductor mounted on outside surface of the stack is permissible:

6.4 Conductors.

6.4.1 General.

6.4.1.1 Conductors shall be copper, weighing not less than 558 g per m (375 lb per 1000 ft) without the lead covering.

6.4.1.2 The size of any wire in the conductor shall be not less than 15 AWG.

However BS 6651/1999 ch.16 Down conductors 16.1 General:

"This code of practice covers the use of down conductors of various types including the use of strip, rod, reinforcing bars and structural steel stanchions, etc."

.... "Likewise, large conductors reduce the risk of side-flashing, especially if insulated.

Both codes, I think, recommend a separate grounding electrode for lightning system but connected with the main grounding grid.

BS :

ch.17.1 Resistance to earth

"An earth electrode should be connected to each down conductor."

NFPA:

4.13.1.1 Each down conductor shall terminate at a ground terminal dedicated to the lightning protection system.

4.13.1.3 Electrical system and telecommunication grounding electrodes shall not be used in lieu of lightning ground electrodes; this provision shall not prohibit the required bonding together of grounding electrodes of different systems.

BS:

17.3 Common network for all services

A common earth termination network is recommended for the lightning protection system and all other services. It should be in accordance with the recommendations of this code of practice and should also comply with any regulations applicable to the services concerned.

I just need to correct a possible assumption. Lightning rods are normaly steel with a copper plating surface. Even earth spikes driven in to the ground are steel rods with a copper plating. Solid copper rods would be far to expensive, to soft and they would instantly melt with a strike and core quickly in the ground due to acidic soils. The important thing to check on any copper rod is the plating thickness and the quality of the steel before plating.

The connections between the rods shall be copper, i.e the conductor joining all rods. And, this can be insulated or bare copper. Aluminium conductor can be used to connect all the rods. However, the next hassle is bi-metalic connection to all the rod connections. Also, the fancy multi spiked lightning rod afixed on top of a building does not out perform a straight spike, and a recent discovery is that the spike does not out perform the rod which was used on French sailing ships at Waterloo June 18 , 1815. There lightning spikes were in fact loops fixed to the tops of the sailing ship masts and recent testing showed this loop to be far more efficient than a straight spike.

Remember a small fact, it is better to NOT attract lightning to any building, power line grounded source, but if you are the source of streamers for the area, make sure your system is of quality materials and the grounding is good.

Lightning protection is a very specialized engineering practice. So much depends on the location and application that it is dangerous to make a generic blanket statement.

Having said that, adding a flammable insulating material to these grounded conductors will change how easily these conductors dissipate a static charge without causing an arc over lightning strike. So putting an insulator like a PVC pipe around these conductors could easily make this more dangerous instead of less.

Second, you imply that the plans call for these rods to be directly tied to the building network ground only at the ground floor. Do it! This sounds correct to me and I've seen it done this way the few times I could see all of a structures lightning protection system.