It is good to hear from you again.
The IEC Lightning Protection Codes (IEC 62305-3) is by far more stringent than the NFPA 780. You should have no concerns meeting US Requirements for Lightning Protection Systems (LPS) by using the IEC 62305-3 Standard.
One bit of advice when using the IEC 62305-3 is that the standard gives you three (3) choices when it comes to selecting an Interception Model, we recommend you use the Rolling Sphere Model (RSM) or sometimes known as the Electro-Geometric Model (EGM). The Protection Angle Method (PAM) and the Mesh Method (MM) should not be used. The PAM and MM are legacy methods that have been left in the standard due to historical reasons, you will be far better served by using the RSM.
One standard that you may not be aware of and is in common use throughout the US Military in regards to their data centers, is the ANSI-TIA-EIA-J-STD-607-A “Commercial Building Grounding (Earthing) and Bonding Requirements For Telecommunications”. This standard has some very specific requirements in regards to grounding / bonding of data centers and has been adopted by the US Government. Included in this standard are provisions to avoid proximity to lighting down conductors. While the standard does not specifically forbid the use of building steel as a down conductor, it does have requirements to avoid ferrous materials for grounding and has the proximity issue already mentioned. This could quite easily be interpreted as negating the use of building steel in the facilities Lighting Protection System (LPS).
That said, while many standards do in fact allow the use of building steel in its Lightning Protection Systems (LPS), E&S Grounding Solutions does not recommend it. Ever. Especially in the case of a data center like yours. Copper is at least 12 times more conductive than steel, and 250 times less magnetic. Not only does directing the lightning energy into the building generate significant electromagnetic, harmonic, and transient noise that is harmful to electronics, but the massive current can also damage the concrete foundations due to the heat (and steam) that is generated. It is also far more dangerous for Human Safety. It just makes no sense when you are trying to protect a building and its occupants, to intentionally force lighting energy into the structure. We recommend copper down conductors routed to a buried ground ring that circumnavigates the entire perimeter of the structure with grounding electrodes capable of handling the lighting current.
A grounding study can determine the current levels on any one conductor segment, Resistance & Impedance-to-ground, Ground Potential Difference (equipotential), Human Safety (Step & Touch Voltages), Electric & Magnetic Fields, and the Time Domain & Frequency Spectrum of a Lighting Strike on your structure. These studies can prove out and validate your design and/or make recommendations for improvements. Often, additional grounding is required that exceeds the listed requirements in the standard.
If you do choose to use your building structural supports as lightning down conductors, this standard from the BS EN 62305:2006 may provide a good guideline for you: Concrete columns that are used for down conductors must be tested at 0.2 ohms or less continuity, and rebar must be welded with 20x diameter overlaps. These must be bonded to the floor slab.
In conclusion, E&S Grounding Solutions recommends using copper down conductors as this will provide the best protection for your critical electronic systems, protect the building itself from damage, and will provide better Human Safety protection for the occupants working in the structure. We further recommend that you review the ANSI-TIA-EIA-J-STD-607-A standard in regards to protecting your electronics. Additionally, the proposed grounding system should be studied and analyzed using computer modeling systems. E&S Grounding Solutions is of course more than happy to help you with this critical function. But if not us, please get someone with the proper engineering software and expertise to analyze these critical parameters.
If you should have any questions or concerns, please do not hesitate to contact one of our engineers during regular business hours California time, we will be glad to speak with you about your project, free of charge.
The Engineering Team at E&S Grounding Solutions
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