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Power Quality Basics, Part 3

Power Quality Basics, Part 3In Part 2, we said that Figure 250.1 shows that Article 250, Part V is tied to all but three of the 10 Parts of Article 250. There is more to this story.

One of the excluded Parts is Part VII, “Equipment Grounding.” Recall what we said earlier about the difference between grounding (connecting to high-resistance earth) and bonding (connecting via low-resistance metal conductor).

Nowhere in Part VII do you find any requirement to drive a ground rod. This is really about bonding. The very first section [250.31] has three subsections. Subsections A and B explicitly say to bond. Subsection C lists 5 methods, and each of them is a means of bonding.

So did the NEC err in not connecting Part V to Part VII in Figure 250.1? No. In the other instances, it’s about eliminating differences of potential between metallic objects by bonding them together. In Part VII, it’s about connecting to that grounded circuit conductor and/or the “equipment grounding conductor.” This latter is not the grounded conductor, but is another conductor run for the purpose of extending the ground (earth) connection. It’s typically run with the power supply conductors [Article 100].

What you don’t want to do is create another ground connection on the load side; it will be at a different potential from the one on the supply side. You want to extend, via bonding, the ground connection.

Getting this wrong is a common cause of power quality related equipment failure.

« Part 2 | Source: Mark Lamendola | Mindconnection