I have recently encountered in a new building installation, lightning protection down leads coming from the roof air terminals, and running through the ceiling sleeved in PVC until it is cadwelded on to the building steel structure, I was called in to answer the question is this to code. My first answer was no due to the plastic of the PVC being in a plenum ceiling. but the question is if the PVC is taken out, what is the rules for re-supporting the wire that is left hanging? I was under the impression that the wire is not allowed to hang on or touch anything else in the ceiling space, because the lightning could spread, but I cannot find a code that states this. Please advise. thank you.
When routing lightning down conductors through conduit, the preferred material is PVC. The magnetic properties of metallic conduit can actually impede the flow of current, if the conduit is not bonded to the same potential as the conductor. This is why grounding bushings are required on both ends of metallic conduit runs, and bonding jumpers are required around conduit junctions. It is also the same reason why 8 to 12 inch radius bends are required for lighting down conductors. When lightning propagates through a conductor it forms massive magnetic fields. Metal conduit and close radius bends can cause the formations of what is called a self-induced coupling, resulting in an impedance choke, and ultimately the conductor can burn open like a fuse.
The problem with PVC, is that fire departments tend to disapprove its use inside of buildings. When PVC burns it produces toxic smoke, which escaping victims really don’t need to worry about breathing as they generally have plenty other things to focus their attention on. But, if you can get it approved (which is generally the case for small uses under 1-foot in length), then PVC is the correct choice.
Our concern is more in the use of the building steel structure as the ground terminal device. Generally speaking, lighting protection systems are designed to protect buildings, and tying the lighting system into your concrete foundation is a really great way to destroy it. When you bond your grounding systems, weather that be a lightning ground or your NEC ground, to building steel, you are technically using the concrete foundation as your grounding electrode. This is called a “concrete encased electrode”.
Why is this not ideal? The water that is inherent to concrete will rapidly expand when heated. If you have ever seen someone take a blow torch to concrete, you will know that the concrete will literally explode from the inside out as the water turns into steam. Lightning is more than capable of doing this same thing as it will heat the water in the concrete as it passes through it on its way to earth. While the lightning down conductors certainly need to be bonded to building steel, we believe that they should have dedicated grounding rods (NFPA 780 18.104.22.168).
The NFPA 780 does allow concrete encased electrodes for use as a ground terminal. However, they have some very specific rules for its use.
- They may only be used in new construction. You cannot use building steel as your ground terminal (electrode) when installing a lighting protection system on an existing building. NFPA 780 4.13.3
- The concrete encased electrode must be installed near the bottom concrete foundation or footing of the building. This means that it must be a spate electrode and cannot be the foundation itself. NFPA 780 22.214.171.124
- The concrete encased electrode must be indirect contact with the earth and be at least 20-ft (linear) in length. There are additional rules as to the copper vs. steel content of the electrode. NFPA 780 126.96.36.199 & 188.8.131.52
For the record, E&S Grounding Solutions does not ever recommend the use of concrete encased electrodes under any circumstance, as they are generally more expensive to install, perform less effectively, and are good for only a single electrical fault before the concrete cracks open and makes the electrode ineffective.
In conclusion, the real issue here is in getting some grounding rods installed in compliance with NFPA 780.
If you should have any further questions or concerns, please do not hesitate to call us at 310-318-7151 and someone will be glad to speak with you about your project, free of charge.
The Engineering Team at E&S Grounding Solutions
Photo credit: http://www.flickr.com/photos/idovermani/3832520688/sizes/l/in/photostream/