Another response from a colleague.
Another response from a colleague.
In terms of grounding new and existing structures, I call your attention to NEC-Section 250.58. This section actually addresses establishing the grounding-electrode system(s) for initial service-entry and separately-derived electrical-power distribution systems in the same building or other structure, but it also applies to separate buildings (other structures) that both have grounding electrode systems as addressed by NEC-Section 250.32 ? each building must have its own grounding electrode system.
Without any electrical-power distribution system within the new structure, there are no concerns for ground-fault return of any electrical circuit back to the power source: The grounding-electrode system is being established for static discharge as you have indicated. The main concern of static discharge would be for lightning dissipation ? I?m assuming this is the purpose of the 20-foot ground rods being driven at each steel beam (or column?) in the structure.
I am also assuming we are relying on the interconnection of the overhead steel super-structure as our bonding means for all the steel columns in terms of our lightning arrest system or must the individual steel columns be interconnected with bonding jumpers as well? This same interconnection is technically the main discharge path for any static buildup along different parts of the new structure. Remember, any friction between dissimilar materials, whether by wind, air-handling, pressurized piping, moving fork lifts or other hoisting conveying systems, constitutes many static sources of electricity.
At this point in the discussion, there seems to be no need to interconnect the two grounding-electrode systems of the two building/structures since only one contains an electrical power distribution system. But can any of the workers touch any grounded surface of the existing building/structure and any separately-grounded surface of the new building/structure at the same time? If the answer is yes, then the two grounding-electrode systems have to be interconnected.
In terms of a ground fault to the existing equipment-grounding system, there is going to be a voltage potential on the effected parts of the equipment-grounding system as well as the existing-building super structure in reference to earth ground at the ground-reference point of the supply transformer until the ground-fault is relieved by the opening of the affected OCPD. This voltage potential is due to the voltage drop along the length of the grounded service-neutral conductor which serves the dual role of both the grounded imbalanced load-current return conductor and the ground-fault overcurrent return conductor when the building bonding grid or electrical-equipment grounding system becomes victim to a ground-fault.
This neutral-conductor voltage drop is not a safety hazard in the existing building/structure because the service neutral conductor is NE Code-required to be re-grounded to the building/structure grounding-electrode system at the point of entry of the electrical-power distribution system into the building [NEC-Section 250.24(A)(1) & (2)]. Re-grounding of the service neutral conductor in this manner brings the entire equipment-grounding system, the distribution/building (structure) ground-electrode system, and the surface of the earth adjacent to the building outside, up to the same potential as the neutral conductor ? in reference to earth ground at the transformer supply point ? until the fault is relieved.
Now the possible shock hazard if the separate building/structure electrode systems are not bonded or interconnected: What if the new-structure grounding-electrode system offers a much lower ground resistance to the fault return path through the earth back to the supply transformer in the existing building as compared to the existing-structure grounding-electrode system / grounded-neutral conductor return? Since current must take every path in a parallel circuit even when the majority of the load or fault current will take to the path of least resistance, a worker touching any part of the grounding-grid of each building at the same time at the instant of the fault will serve as a bonding jumper between the two separately-grounded systems. The degree of electrical shock, which could lead to electrocution, depends on the individual?s resistance to the crossing current flowing through his or her body.
Assuming the new-building/structure grounding electrode system still has a lower-value of ground impedance in comparison to the existing building/structure grounding-electrode system and the worker is in a place where he or she is toughing both grounding systems that are not interconnected, and instead of a ground fault on a feeder or branch circuit in the existing building/structure, the existing super-structure takes a direct hit from a bolt of lightning. The worker(s) again is (are) subject to an electrical shock or possible death from electrocution because he or she is the only bonding jumper (in this scenario) between the two grounding-electrode systems.
In terms of not interconnecting the two building/structure grounding-electrode systems with one being a lightning-arrest system, I call your attention to NEC-Sections 250.53(B), 250.60 with both INs (Informational Notes), and NEC-Section 250.106.
NEC-Section 250.53(B) requires a minimum of 6-foot or 1.83-meter spacing between the grounding electrodes of the two separate systems. With the amount of concrete you described about the slabs in the area, it might be hard to determine where the end of the existing building/structure grounding-electrode system is. Notice the last sentence to this section: ?Two or more grounding electrodes that are bonded together shall be considered (as) a single grounding-electrode system.?
NEC-Section 250.60, which will not allow a grounding-electrode system installed/designed for/as a lightning-arrest system to serve as the required service or separately-derived electrical-power distribution system grounding electrode system for a building or other structure, technically does not apply to your described installation, but the INs are helpful in understanding why your two separate grounding systems should be bonded together. IN No. 1 is a steering reference to NEC-Section 250.106 (discussed in the next paragraphs). IN No. 2 indicates reduction in the shock hazard we are discussing is accomplished by bonding the two grounding-electrode systems together which ?will limit the potential differences between their associated wiring systems?.
Most folks involved with the NE Code interpret NEC-Section 250.106 as applying to a single building or other structure where it stipulates that the ?lightning protection system ground terminals shall be bonded to the building or structure grounding electrode system)?, because ?building or structure? is singular ? not plural. But the concern here is the danger to human life (or any life for that matter ? electricity is not a choosey phenomenon), therefore this ruling applies to adjacent buildings or structures as well.
NFPA 780-2011, Standard for the Installation of Lightning Protection Systems, referenced by NEC-Section 250.106, IN No. 1, which contains detailed information on grounding, bonding, and sideflash distance from lightning protection systems, is a must for reading if you are still not convinced the existing building/structure and the new building/structure grounding-electrode systems should be bonded together.
NEC-Section 250.106, IN No. 2, makes reference to the same standard and points out that the noncurrent-carrying metal parts of the existing and the new building/structure equipment-grounding / grounding-electrode systems must either be bonded together to create a common grounding-electrode system, or else when installed in close proximity of each other; a minimum spacing of 6 feet or 1.83 meters must be maintained between any noncurrent-carrying metal parts of the two separate grounding systems ? same spacing as required for separate-systems grounding electrodes in NEC-Section 250.53(B).
Closing:
Well Jason, I hope I answered you questions in a way that will be helpful. By the way, the ?25 ohms or less? reading you quoted from NEC-Section 250.53(A)(2), Exception only applies when the service grounding-electrode system is limited to a single rod, pipe, or plate grounding electrode. The 25-ohm restriction is lifted by NEC-Section 250.53(A)(2) when two or more grounding electrodes ? interconnected ? are installed.
