If you are new to grounding and bonding, or just interested in learning all that is available, I encourage you to come with me now, and we will look a little deeper into the science behind grounding and bonding. We will also complete the electrical installation that we have already begun, and in the process, reveal another real-world grounding and bonding scenario; I feel certain it will add to your growing arsenal of knowledge.
In Part 1 of this series you will recall that we made a clear distinction between the act of grounding, which is the process of providing a conductive path in the form of an equipment grounding conductor for current to flow (to trip a circuit-breaker or fuse) in the event of a ground fault condition, and the act of bonding, which is the process of creating continuity between metal electrical components, but not providing a conductive path in the form of an equipment grounding conductor for current to flow (to trip a circuit-breaker or fuse) in the event of a ground fault condition.
You can see that the obvious and most critical difference between these two procedures is the installation of the “equipment grounding conductor,” which is only installed during one of the two wiring methods. The equipment grounding conductor (EGC) provides the essential return path necessary for fault-current to flow, so that the overcurrent protection device will trip during the event of a ground fault.
Now, let’s take a moment to look at what is happening on the other end of that equipment grounding conductor, and what it is that enables this conductor to effectively carry current at such a rate that it will instantaneously trip a circuit-breaker or fuse in the event of a ground-fault. It is normal to assume that a conductor will carry current, but in the matter of fault-current it is important to know why an equipment grounding conductor is able to do its job.
We know that grounding a metal electrical component, such as a piece of EMT conduit or a metal 4-square box, physically connects that component to the Earth. But “earth” itself, being composed mostly of soil, rock and some moisture, does not serve as a good enough conductor of electrons that it will allow current to flow fast enough to cause an overcurrent protection device, such as a circuit-breaker or fuse, to trip in the event of a ground fault. That is why the NEC requires that all equipment grounding conductors throughout the electrical system be connected (bonded) to the neutral conductor of that electrical system. It is that connection, that “bonding” of those equipment grounds to the service neutral conductor, (which is also connected to the Earth through the grounding electrode) that provides the low-impedance path required for current to flow at a rate fast enough that it will trip an overcurrent protection device during a ground fault. That service neutral, available at the first means of disconnect of an electrical service, runs uninterrupted from the service drop or service lateral feeding the building’s electrical service all the way back to the utility provider’s substation, somewhere in or near your City. We will be covering this subject in depth in a later article when we examine the difference between a “Grounding” and “Grounded” conductor, as well as the difference between a “Neutral” and “Grounded” conductor.
In the meantime, to keep this lesson easily understood with clear dividing lines, we shall focus on the fact that “grounding” a component provides a path back to ground for current to flow- for the purpose of tripping a circuit breaker during a fault, where bonding a component, only connects that component to other electrical components.
Now, let’s pick up where we left off: We had three metal 4-squares boxes, mounted 10’ apart on a non-conductive, wooden wall. These three boxes have been connected, one to the other, using 10’ sticks of PVC (plastic) conduit. We then pulled a green insulated conductor between the boxes, and properly fastened the conductor to the interior side of each metal box. We had effectively “bonded” our boxes together.
Now, let’s illustrate how dangerous this configuration can be, if we fail to GROUND it.
We shall now introduce a 120 volt circuit to our empty metal boxes. First, to get the 120 volt circuit to our boxes, we shall install 30 feet of non-metallic flexible conduit from our first metal box, all the way back to our electrical panel. Remember, this is 30’ of NON-METALLIC flexible conduit. Now, with that new flexible conduit installed, we shall take advantage of the path that it has provided from our metal boxes to our power source. Let us now push one colored #12 conductor and one white #12 conductor from the DE-ENERGIZED electrical panel, through our non-metallic flexible conduit, and all the way into our first metal box. We shall now leave the panel area and proceed to our first metal box, where our conductors are now visible. From here, we can then feed the colored and white conductors on through the PVC conduit, and into the second and third metal box. Now, that that is complete, let’s connect our colored and white conductors to a receptacle, one receptacle for each box, along with an appropriate cover that fits our 4-square box. Now that that is done, let’s go back to our electrical panel and terminate our colored conductor onto a new circuit-breaker, and our white conductor onto the neutral bar that is located within that same panel.
Let’s close the panel back up; it’s now time to turn the main breaker back on, so that this panel is energized. We shall now flip that new circuit-breaker on, and we should have a working circuit, that includes 120-Volt working receptacles at each 4-square box. Excellent!
(At this point, I shall mention that our scenario is for informational purposes only, and in no way represents an instructional guide to the proper way to build a code-compliant electrical circuit, or receptacle configuration. A matter of fact, our example is intentionally in violation of the National Electrical Code, for the purpose of demonstrating the necessity of grounding.)
Now we will introduce an electrical fault into the system that we have just built, that fault will quickly reveal the violation of the National Electrical Code that I just spoke of. A “fault” is, just as it sounds, an electrical “problem” within an electrical component or electrical system. But before we turn that problem loose into our new installation, I want you to take a moment and consider what we have built here. Think about the different parts involved, how they are all connected, and whether or not you feel like we have- at any time during our manufacturing of this system, GROUNDED our metal boxes and receptacles?
Now, as I promised a minute ago- the fault
During the installation here, you accidentally nicked the insulation on the colored conductor that is feeding the first receptacle. Nicking that insulation has revealed the bare copper beneath, and that copper is now resting firmly against the bare metal interior side of the first 4-square box. This is the same colored conductor that is currently terminated onto the 120 volt energized (Hot) breaker, back in the electrical panel. So, that energized copper is now resting against the conductive metal 4-square box.
Take a moment and visualize what is occurring here. Once you feel confident that you can see the big picture, continue on, and we will see if you have considered all that is happening, as well as all of the things that could happen!
Let’s now see if your mental picture matches up to the events that are occurring at our new electrical installation.
First: The three metal boxes are connected, or “bonded” together by the green insulated conductor that was pulled between the boxes in the beginning. So, whatever “fault” is occurring at the first metal box, by default, must be occurring at the second and third metal box.
Second: The 120-volt energized, or “Hot” conductor is laying up against the inside of the metal box right now, yet both receptacles are still working, and with no apparent interruption!
Third: The only conductive material that is touching our metal 4-square boxes (other than our new electrical fault caused by the nicked wire), is the stripped ends of the green insulated conductor that intentionally bonds these three metal boxes together. Therefore, even though the 120-volt energized portion of the nicked conductor is laying directly against the metal box, there is no path from the box itself, back to anything that is grounded. That is why the receptacles are still working and that is why the breaker has not tripped!
Fourth: Since our three metal boxes are in contact with, (and are therefore now conductively a part of) the energized 120-volt wiring, if a person touches one of these metal boxes, it is the equivalent of touching the stripped end of an energized 120-volt conductor. OUCH!
Finally: Since we recognize that these metal boxes are now the equivalent of a bare 120 volt energized wire, let us also realize that if a barefoot child (or adult) happens to touch these metal boxes, with their bare and unprotected feet planted on the conductive ground (earth), then the voltage potential sitting on these energized metal boxes will now have a grounded path in which to flow, going through that person and back to the earth.
That is a scenario that we never want to experience in the real world.
What do you think we could do to prevent this accident? I will give you a hint, GROUNDING this configuration will keep everyone safe, even during the kind of “fault” that I have described.
To ground this configuration, we would take a green or bare equipment grounding conductor (or a metal conduit) and then fasten it to one of our three metal boxes, while fastening the other end to our electrical panel that is adequately grounded. This will cause the circuit-breaker to immediately trip, when that exposed energized copper of that nicked wire, touches the bare metal of our 4-square box.
When an electrical system is grounded, fault-current has a path to flow. That unobstructed flow of current along the equipment grounding conductor during a fault condition, will quickly surpass the threshold of any circuit-breaker, causing it to trip long before someone touches that energized metal and gets hurt.
So let’s learn these lessons here, so that we can prevent them out there in the real world. Learn more with a JADE Learning electrical continuing education course.