One of the most difficult parts of NFPA 70, National Electrical Code, to teach and for students to understand is Article 250: Grounding and Bonding. This is also a great area of the Code to focus on as a follow-up to the last few articles of this column because the conversation on grounding and bonding was left out of the navigation example we had worked through before. Now seems like the perfect time to dive deep into Article 250 and learn a bit more on how the NEC handles this important aspect.

Starting at Article 100

Before we get into the specific requirements found in Article 250, understanding grounding and bonding starts in a different article. That’s right, we are headed to the definitions in Article 100 to take a look at specific terminology critical to applying Article 250.

The article’s title is simply “Grounding and Bonding,” and in all my years of teaching the NEC, just the meaning of these two words can cause a tremendous amount of confusion. Let’s look at bonding first, or as the NEC puts it, “bonded (bonding).” This is one of the simplest definitions in the Code, and it is defined as, “Connected to establish electrical continuity and conductivity.” In other words, when something is bonded, it simply means that it is connecting two conductive objects. Notice there is nothing yet about being grounded.

When we dig a little further into the definitions and get to “grounded (grounding),” this term is defined as, “Connected (connecting) to ground or to a conductive body that extends the ground connection.” 

However, we still need to know what “ground” is, and this is defined simply as “the earth.” For a conductor to be grounded or to be a grounding conductor, it means that it is either connected itself or connecting something else to the earth. This is very different from the term “bonded (bonding),” yet these terms are often misunderstood and incorrectly applied.

Back to Article 250

Heading back into Article 250, we can sort out the requirements so they make a little more sense. A great place to pick up our journey into better understanding this critical concept is to dissect the general requirements of grounding and bonding found in Section 250.4. This section is divided into two subsections that either cover the grounding and bonding requirements for grounded or ungrounded systems. Yes, even ungrounded systems still have requirements for grounding and bonding, and we will get to that a little later.

Let’s examine Section 250.4(A), which deals with the general grounding and bonding requirements for grounded systems. The very name of this type of system indicates that one of the conductors is going to be connected to the earth, or to a conductive object that extends the earth connection. 

However, Section 250.4 goes one step further to explain why that system conductor needs to be grounded. This section is often referred to as the “performance requirement” for grounding and bonding. 

Section 250.4(A)(1) states that grounded systems must be connected to the earth in a manner that will limit the voltage imposed through events such as lightning strikes, high-­voltage crossovers and other types of surges and will stabilize the voltage to ground throughout the system.

In addition to grounding of the system, we can see that Section 250.4(A)(2) also requires us to connect all electrical equipment to the earth so that all normally non-current-carrying metal parts of electrical equipment have a limited voltage­ to ground. In other words, we are going to connect the frames and enclosures of electrical equipment to the earth so that when a voltage to ground is referenced in the system, such as 120V to ground or 277V to ground, that it is the same reference and potential difference on normally non-current-carrying surfaces.

What are we missing?

However, there is still something missing. At this point, we have connected our system conductor to the earth and connected our equipment to the earth, but if we are not careful, these two connection points could be far enough away from each other that the ground reference is different and could cause a difference in voltage potential between varying surfaces. 

To remedy this, sections 250.4(A)(3) and (4) will spell out that we must also connect the whole system of earth connections and normally non-current-carrying metal parts together so it is all interconnected, or more aptly named, bonded.

To summarize, electrically connect all normally non-current-­carrying metal parts together, connect this interconnected system to the earth, and make sure that one system conductor is also connected to this grounding system to stabilize the voltage in the system. But what happens if there is a ground-fault in this system? Does this just energize everything to the system voltage? Does this system of connections bleed off the power into the earth? 

Well, Section 250.4(A) isn’t quite done with us yet. Maybe the most important safety aspect of Article 250 can be found in Section 250.4(A)(5), and that is the requirement that this grounding and bonding system do so in a manner that creates something called an effective ground-fault current path. The last sentence is very important here: “The earth shall not be considered as an effective ground-fault current path.”

Back in Article 100, we can find the definition of an effective ground-fault current path: “An intentionally constructed, low-­impedance electrically conductive path designed and intended to carry current during ground-fault events from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault detectors.” 

Now we have that missing item. The system is grounded to stabilize the voltage and bleed the overvoltage events, and the system is all interconnected to ensure there is not a difference in potential between any two normally non-current-carrying metal surfaces. This connection must be done in a manner that can safely handle the amount of current that can flow during ground-fault conditions until the overcurrent protective device can open the circuit and shut it down. 

For ungrounded systems, it is essentially the same concept except we will not be connecting a system conductor to the earth.

Section 250.4 outlines the purpose of grounding and bonding of electrical systems. The rest of the requirements are all aimed at meeting this very important section. For example, when we get into sizing the equipment grounding conductor, you can see how the purpose is to create the effective ground-fault current path and connect the equipment to the earth. Once we understand this, the rest of Article 250 begins to come into focus. 

However, now an even bigger question remains: when do we need to ground our systems? Stay tuned for next month’s article, when we start to unpack the requirements around what systems must be grounded and why. 

Until next time, stay safe and always remember to test before you touch.

Read Connecting Up to Code, part 2 >>

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