Violations of the NEC, Article 250, Part V are nearly always present in a facility experiencing power quality problems. What kinds of violations are we talking about?
First, it helps to know what Part V is about. Article 250 provides a nice pictorial guide to what’s in it [Figure 250.1], and from that we see it’s about bonding. We also see the bonding requirements are tied to Parts II, III, IV, VI, VIII, and X. That is, all but three of the 10 Parts of Article 250.
What is bonding? We find its definition on Article 100: “Connected to establish electrical continuity and conductivity.” In other words, connected with metallic jumpers. Contrast this to the definition of grounding, also found in Article 100: connected to the earth.
Suppose you had two electrical enclosures, and you wanted them at the same electrical potential. How would you connect them? Let’s say you use a #4 copper conductor; it has negligible resistance. If you draw out the circuit and apply Ohm’s Law, you’ll see these enclosures are at the same voltage.
Now let’s say you connect them with earth instead of copper wire. You drive a ground rod next to each enclosure. Look up the resistance of your soil type, and you see that, no matter what it is, the resistance is much higher than for copper wire. Applying Ohm’s Law, you now find these enclosures are different voltages. You can measure a difference of potential between them.