Service grounding vs. Lightning grounding

[Noooorm]

I have a ground loop around a small rectangular building, ground rod at one corner of the loop. Lightning protection provided on roof. Presently, the down conductors from the air terminals connect to the ground loop, and the ground bus in the panel connects to this same ground loop. Inspector insists that the lightning ground system and the power ground system need to be seperate systems, but connected together. I am looking at NFPA 780, sections 4.13.1.1, section 4.13.1.3, and NEC 250.106, which all seem to confirm what the inspector is stating. But, if these two systems need to be bonded together anyway, why can they not be the same system (why can't I have one grounding system that both the power grounding and the lightning protection tie in to)?

 

[charlie b]

I have often wondered the same thing myself. I am looking forward to seeing if anyone has an answer, for I do not. To me, the tricky part is that for the power system grounding, we have to connect to any and all electrodes that are available. So if there is a grounding electrode system that serves the lightning protection system (LPS), then that grounding electrode system is "available," and we have to connect to it. So I don't see the difference between, (1) Connecting the ground bar in the main electrical room to a single ground rod, and then running a bond wire to the LPS's ground ring, and (2) Connecting the ground bar directly to the LPS's ground ring.

Welcome to the forum.

 

[jghrist]

The reason is to avoid having a lightning surge enter the building grounding system at one point and leave another point, creating high voltage differences within the system. A lightning surge is a high frequency current and causes high voltage drops across the inductance of ground wires.

 

[bphgravity]

I believe you intended to reference section 250.60 of the NEC in addition to 250.106.

I do not believe there is an "electrical" reason for this requirement.

I believe the requirement is to ensure that NEC required grounding systems are installed per the NEC and that NFPA 780 required grounding systems are installed per the NFPA 780 & that neither is relying on the other.

Also, it allows for either system to be altered or removed from service without affecting the other.

There really isn't any more to it than that...