?D? is for danger? The requirements found in Section 690.47(D) have been controversial since they were first added to the Code in 2008. (See ?Additional Electrodes for Array Grounding,? SolarPro magazine, October/November 2008.) Some industry stakeholders pointed out that the requirements did little to improve system safety and they significantly increased costs, especially on smaller PV systems. Others complained that the section was poorly written, included language ripe for misinterpretation and contained requirements that were difficult to enforce or comply with. As a result, many industry veterans were pleased when this language was removed from NEC 2011. Unfortunately for them, Section 690.47(D) reappears in NEC 2014, only with a twist this time?a one-word change that has inspired a new set of detractors.
Whereas the 2008 version of Section 690.47(D) required ?additional electrodes for array grounding,? the 2014 version requires ?additional auxiliary electrodes for array grounding.? According to a substantiation in the ROC, the intent of adding the term auxiliary was to signal that ?this grounding electrode is not required to be tied into the premises? grounding-electrode system; and if multiple grounding electrodes are installed, they do not need to be bonded together by a dedicated bonding conductor.? The substantiation concludes by noting that the dc EGC ?will serve to bond the electrodes in the PV array together. A separate bonding conductor would be duplicative.?
Not everyone agrees with this assessment. Mike Holt, the founder of Mike Holt Enterprises, is a Code expert and electrical training specialist. According to a 26-minute YouTube video presentation (see Resources), Holt?s opinion is that ?690.47(D) needs to be immediately removed from the Code. It is the only Code rule I am aware of that was added to the Code that makes an installation unsafe.?
The safety problem, as Holt sees it, is that the 2014 version of Section 690.47(D) references Section 250.54, ?Auxiliary Grounding Electrodes,? meaning that system integrators do not need to incorporate the required auxiliary electrode into the premises? grounding-electrode system. As a result, in the event of a lightning strike there could be a difference in potential, or voltage, between an auxiliary electrode and the grounding-electrode system. If that occurred, the difference in potential would induce current in the EGC path. Not only is equipment in this path at risk of damage from lightning-induced surges, but also people could be exposed to a shock hazard, either due to direct contact with an inadvertently energized circuit component or due to a side flash, which occurs when lightning jumps from one object to another.
Brooks concurs with Holt?s assessment: ?If the AHJ requires an additional electrode on a building with an existing electrode, then you should bond the new electrode to the existing electrode following the requirements of Section 250.53. You should make this bond at ground level, not over the top of the building. Making the bond any place other than ground level is asking for lightning damage.?
Brooks continues: ?My basic interpretation is that all buildings with an existing electrode do not require this additional auxiliary electrode because of Exception 2. The building electrode applies to the whole building. So regardless of the physical placement of a ground rod, there is no place on a structure with an electrode that can be farther than 6 feet from the premises? wiring electrode. An uncontroversial example of this is an Ufer ground that is attached to reinforcing steel in the concrete. A reasonable person would agree that the whole slab is part of the grounding system.?
