Use of EMT or PVC in lightning systems

[marcosgue]

Hi all, we've new construction school projects with lightning protection system, the conductor down to ground ring system was installed in 1" EMT from the roof to ground change to PVC at 3 ft high before landing in the ground ring, now we've to change the EMT conduit to pvc.
I was looking but the nec is lacking about this type of system and I don't have access to nfpa 780. Can someone expert in the forum explain why the EMT or GRC or any metallic conduit can't use and other basic elements to consider in the installation of lightning protection system. Blessings

 

[winnie]

I don't know lightning protection systems, so this is a guess:

This probably has to do with the 'choke effect' where the inductance caused by the magnetic material surrounding the conductor acts to impede the flow of current.

For example, the NEC permits grounding electrode conductors to be installed in metallic conduit, but requires that the conductor be bonded to the conduit at both ends of the conduit, to prevent the choke effect from being a problem. The choke effect still occurs, but the bonds permit the choked current to flow in the conduit itself.

I'd guess that if this happened when the down conductor was carrying strike current that the choke effect would either block the current flow or shunt it into the conduit.

Jon

 

[tom baker]

Lightning protection is a specialized field. In your area there should be contractors who have some type of LPS certification.
For protection of the GEC, a metallic raceway must be bonded at each end, otherwise it impedes 97% of the lightning current. All the LPS systems I have seen, use a braided down conductor, never in a raceway.
All the NFPA standards are available onlne after you register

 

[LarryFine]

Nost likely for the same reason we must bond both ends of any metallic raceway containing electrode conductors.

Otherwise, lightning current can cause a high-impedance (choke) effect and reduce the current and thus protection.

Added: What they just said, but in fewer words.

 

[brantmacga]

All the LPS systems I have seen, use a braided down conductor, never in a raceway.

same.... we have always sub'd this work out, and I've never seen them put anything in a raceway. I've always seen them strap that cable directly to the building coming off the roof.

 

[synchro]

... All the LPS systems I have seen, use a braided down conductor, never in a raceway.

When I visited the Chicago Air Route Traffic Control Center operated by the FAA, I saw quite a few large braided copper cables coming down the building for the LPS. It was evident that they were prioritizing functionality over any aesthetics.

 

[marcosgue]

Thanks you for the responses that should be the same argument, but still they consider PVC better option. Thanks Tom for give me the way to access nfpa standards.