Lightning Protection Down Conductor

[muhandas]

A seven story apartment building has a complete Lightning Protection System installed. The bare copper down conductor, however, is run on the inside of the concrete building wall (rather than along the outside wall.) I can't find anything in the NEC that would forbid that and I'm not familiar enough with NFPA 780 to make a determination. Also, the installation bears a UL Master Label. Nonetheless, having been in the electrical field for a long while my gut feeling leaves me uncomfortable with that construction. What do you folks think about it? Does it violate any code?
I'd appreciate your opinions.
Heinz R.

 

[bphgravity]

This is purely a design consideration and a violation of neither the NEC nor the NFPA 780.

There are some considerations that need to be made however. See 4.9.9 of the NFPA 780. For example:

Down conductors coursed on or in reinforced concrete columns or on structural steel columns shall be connected to the reinforcing steel or the structural steel member at their upper and lower extremities.

 

[sc57ford]

We usually run the downcomers inside for Architectural reasons. The Owners and the architects don't like them on the outside. I have done several design projects using this approach and as pointed out already, they were attached at the top and bottom of the steel columns. Also, the rods on top of the building should be addressed for personnel safety (If maintenance personnel are to be on the roof for any reason), as in the height and spring loaded so if someone falls they won't be stabbed. Exiting the building through pvc conduit to the external ground loop (or rods) should be properly detailed at each point.

 

[don_resqcapt19]

Aren't there some bonding requirements that say you have to bond all metal ojects that are within 3' or 6' of the down conductor? If so how is this accomplished?
Don

 

[muhandas]

Tx. for responses. Good to know that running the down conductors on the inside of the building is not that unusual.
250.106 talks about about bonding the Lightning Protection system ground terminals to the building grounding electrode system and FPN 2 talks about "typical" spacing of 6' or 3' (through dense materials) but inasmuch as it is an FPN it is only informational and not an enforceable requirement. The Florida building code, however, does invoke NFPA 780 (as do both FPNs to 250.102.)
I don't know NFPA 780 thoroughly enough to allow me to reference a particular article.
Being aware of the tremendous energies (both voltages and currents) that that down conductor will be exposed to in the event of even a partial strike I just got uncomfortable. Maybe Architects have a greater sensitivity to the esthetics than I, an Electrical Engineer, have but I got sort of gun shy at this construction asking myself whether I would be comfortable laying on a bed during a lightning storm with my head probably less than 2 ft. from that conductor.
I Appreciate all your responses.
Heinz R.

 

[bphgravity]

...The Florida building code, however, does invoke NFPA 780...

This is not true. There are several occupancy types that require a LPS installed per the requirements of the NFPA 780. See Chapter 35 - References and look under NFPA. You will see several references to Chapter 4.

In the event the particular occupancy does not require a LPS, once installed, the system is one that is REGULATED by the FBC therefore it must be installed per the Chapter 35 reference to the NFPA 780 and be engineered per the FAC.

 

[Jpreverso]

Aren't there some bonding requirements that say you have to bond all metal ojects that are within 3' or 6' of the down conductor? If so how is this accomplished?
Don

Things may have changed but 6 years ago when was an installer I dont remember a 3' or 6' bonding rule for the down conductor. Aa the cabel entered the building through the roof we would bond the nearest colum or beam (usually within 3' so that extra cable supports would not have to be used) then ran the cable through pvc down to the botom floor where we bond steel once again before the cable exited the building to tie into the electrode. We made our bonds using either a bronze beam clamp or bronze bonding plate. Both required that you sand 8 square inches of paint off the beam or colum so that entire surface area of the fitting touch exposed steel. The beam clamp mecanically clamped to beam( see attachment for old style of clamp) where as the plate required drilling and taping the colum twice per plate or welding or brazing it in place.

I do remember a 6' bonding rule for the cable on the roof. Any where the cabel passed within 6' of a metalic object that object was bonded to the system create an even potential between the two.