lightning protection question

[mshields]

I have been told that separately derived systems should not use building steel as ground electrode if the building steel serves as the down conductor of a lightning protection system. And that this was particularly true for a hospital. Does this have merit?

 

[bphgravity]

I am not aware of any section of the NEC, NFPA 99 or NFPA 780 that indicates what you are stating to be true. In fact, it would violate Section 250.30(A)(7)(2) of the NEC if (1) were not present. Even if (1) was utilized, (2) would have to be bonded to the system anyway.

There would be NO compliant method to ground a SDS without that system being bonded to the LPS grounding system at some point. A SDS isolated from the LPS system would likely be a greater potential hazard than any reason one might have come up with this idea...

Keep in mind the purpose of the NFPA 780 is much the same reason an SDS must be grounded as indicated in 250.4(A)(1).

 

[G._S._Ohm]

The resistance of steel I-beams can be estimated, and using 30,000 amps as a standard lightning strike current the voltage on the building steel can be calculated to be at some elevated level with respect to ground.

I'd hope the rules allow for this and don't contradict each other.
In principle you could work backwards from these rules to the simplified equivalent circuit and current magnitudes that they must be assuming.

It should all fit together, logically and mathematically.