Grounding and Bonding

Grounding and bonding are two words that cause a lot of electricians to flinch or roll their eyes. One can hardly blame that kind of reaction due to the complex nature of the extensive rules for "earthing," "grounding," and "bonding." In fact, Article 250, "Grounding and Bonding" in the 2008 NEC is the single most largest section in the entire code book! There were 40 significant code changes from the 2005 NEC edition, making it also one of the most changed articles as well. With 81 separate sub-sections, there is a tremendous amount of material to absorb and understand. It is a foregone conclusion that anyone preparing for ANY type of electrician's exam must study and know Article 250 fluently. There have been many books, articles, and studies published on this subject. There is an entire industry in and of itself surrounding this subject alone. So don't feel bad if you are a little shaky on this subject. Over the next few weeks you will find several blogs on our site dealing with various parts of Article 250.



Last week I visited a job site where an older residential home was being converted into a commercial type workout gym. One of the electricians on the site had made himself a pretty ingenious device to trip circuit breakers. It was a single pole toggle switch mounted in a single gang bell box with a cord cap attached to about 36" of SO cord. He'd simply plug it in and flip the toggle switch, thus causing a purposeful direct short. (he must have had the equipment grounding conductor on one side of the toggle and the ungrounded conductor on the other side) He had also left one end of the bell box unplugged to allow the arc flash or heat to have a convenient path for discharge.



In this building, he mentioned to me that every time he employed his shorting device, he'd noticed a much louder and larger arcing inside his device. I immediately had a suspicion that the system may have a poor grounding system, thereby creating a much larger potential resistance to ground to overcome, "forcing" the circuit breaker to function. There were two separate 100 amp panels in the basement. Each panel had its own service conductors feeding it from an outside utility transformer. (a very rare and odd set up) After removing the dead fronts, I discovered that there was NO grounding electrode conductor what-so-ever! I was extremely surprised and I would have chalked it up to simply being a case of an old service that had not ever been brought up to code. BUT, these were obviously newer panels and it was very apparent that at least one panel was less than two or three years old. I have to assume that it must have just escaped notice, however, this was a serious safety over-site by those technicians in the past!


One panel had a neutral/ground bond, the other did not. The two grounding bars between the panels had been bonded together with a #8 AWG copper conductor. We must treat these two panels as two separate services. Our controlling Articles would be: Art. 250.52, 250.53, 250.56, 250.64, and finally Table 250.66. Next week we'll go into detail on how these specific articles apply to this installation.