A clear understanding of grounding and bonding is essential to the safety of your co-workers as well as the reliability of your equipment.
During its first three months of operation, a new high- tech manufacturing wing incurred 191 hours of downtime. Throughout this episode, the maintenance department racked up an additional 60 hours of overtime, $38,000 in repair parts, and $7000 in outside consultant fees. Yet, problems persisted at the same furious rate - with no end in sight!
An electrician not assigned to that wing of the building heard about the problems over lunch. The discussion piqued his interest. He'd recently attended a National Electrical Code (NEC) seminar, so when the phrase "isolated ground" came up, he felt confident he could fix the problem. Why did he have such insight? He simply had a basic understanding of bonding and grounding principles and saw through common misconceptions.
In the electrical industry, one common misconception is the idea that earth and ground are the same. They are not. Aircraft have power systems and sensitive electronics that require grounding. Yet, you will not find an electrode made to earth on any aircraft - they don't touch the earth while in flight. Spacecraft also have grounding systems. However, at 240,000 miles from the earth, there is obviously no earth connection. A ground is simply a common reference plane. To make it work, you need bonding.
What does bonding do? As stated in NEC Art. 100, bonding establishes "an electrically conductive path that will ensure electrical conductivity and the capacity to conduct safely any current likely to be imposed." How does bonding do this? By permanently joining all metallic parts of a power-distribution system with a bonding jumper. You use the equipment-grounding conductor to complete the chain of bonding conductors.
As you can see in Fig. 1 (right), bonding jumpers tie the grounding system together through a reliable low-impedance path. If you remove an equipment-bonding jumper, that equipment no longer has a low-impedance path to ground. If you remove the equipment grounding conductor at the service entrance, no equipment has a low-impedance path to ground.
What happens if your path is unreliable, or the impedance is high? We know electricity prefers the path of least resistance. So, if your path to ground is a high-resistance path, where do you think undesirable current will primarily flow? Not into ground, because equipment and people are paths of lesser resistance.
Do you need uniform resistances? We know the relationship of resistance to voltage from Ohm's Law. Thus, we know if you have different resistances to ground from one piece of equipment or structure to another, you have a difference in voltage, too. That difference in potential could easily render the equipment unsafe because of the potential for electrical shock. If a person makes contact between two objects having different voltage potentials, current will flow through the person.
Does earth measure up? Does earth provide a low-impedance path that reduces this potential? If you measured the resistance, end-to-end, of 100 ft of No. 4 copper wire, you'd read about 0.03 ohms. Based on a table in the IAEI Soars Book on Grounding, that same 100 ft (if consisting of gravel) would measure more than 100 megohms! A more typical soil type would still read about 1 megohm. Even ideal soil won't read much below 10 kilohms. Obviously, earth is no substitute for copper wire when it comes to providing a low-resistance path to ground.
What about a reliable connection? After all, you can remove a copper wire but you can't remove the earth. True, but the point where an electrode contacts the earth is not a true connection. You can't weld, crimp, and bolt a conductor to the soil. All you really have is the surface-to-surface contact with the edges (asperity points) of soil particles.
Is the resistance reliably consistent? Electro-Test, Inc., a NETA member firm, makes soil resistivity tests all over the world. The company's experience is that the resistance of soil varies - even in the same spot on the same day. In fact, temperature and humidity can cause wide variations in the resistance of the same plot of soil.
What if you configure your grounding system as shown in Fig. 2 (right)? This common configuration has problems. First, you already know the resistance between the two electrodes (connected only by the earth) is neither low nor reliably consistent. This means you have a high potential between the two ground rods. That freestanding electrode, though buried in earth, is not grounded. As electricity seeks the path of least resistance, it will seek a path other than the earth bonding jumper between that freestanding rod and the service-entrance ground rod. Because of the high resistance in your earth bonding jumper, that electricity will build up potential before it can start to flow. Instead of the volt or two you'd have on your copper wire, you may have several hundred volts of difference building up and suddenly discharging through equipment or people. So, you can see the grounding arrangement in Fig. 2 is unsafe. However, it would be safe if you bonded the two electrodes with a copper wire. Does that bond cause other problems that justify the safety risk of not having it?
Sin of omission. According to one myth, bonding those two electrodes allows undesirable current to flow from the main grounding system into equipment we must protect from undesirable current. This misconception describes deliberate omission of a bonding jumper as an isolated ground, which is incorrect. Fig. 3 (right) shows a properly configured "isolated ground." Notice how similar it is to Fig. 2. All we did to fix Fig. 2 was bond that isolated rod to the grounding system.
Think of an isolated grounding conductor as a directly wired bonding jumper, not a separate grounding system. Rather than bond this conductor to the equipment-grounding conductor or the building frame at the point of use, you run it to a point much farther upstream - usually to the service entrance. And you bond to ground there. Not bonding this isolated grounding conductor to the service grounding electrode results in a voltage potential between it and your grounding system. That potential could be lethal.
We know omitting the bonding jumper inhibits the protection of equipment and people. What do we lose by including it? In other words, what are the advantages of using earth as a bonding jumper? The short answer is: There are none. Are installation manuals' claims that manufacturers can void warranties if you don't omit the bonding jumper true? No.
Let's look at Fig. 3, again. This configuration provides some isolation from the rest of the load side, by connecting to ground so close to the main electrode. This means that noise generated by a particular device will propagate into the mass of the grounding system and the various low paths of resistance before it even gets to your connection. The impedance of that length of isolated grounding conductor also helps. For nearly all installations, this arrangement has proven to be a more than adequate defense against noise coming from the grounding system.
Because omitting that bonding jumper is an open invitation to electrocution and equipment damage, omitting it is ethically unacceptable. It's also a violation of Art. 250 - which makes it unlawful in most jurisdictions.
If it is unlawful in your jurisdiction, a warranty clause requiring you to use earth as a bonding jumper is automatically void under the Uniform Commercial Code because of the underlying illegality. If you have such a clause in your warranty, consult your company's legal department. But don't just cave in and use earth as a bonding jumper. You could be personally liable for the consequences, and you'd have no defense in court.
Sidebar: A Grounding Primer
By the intentional connection to one system conductor at the power supply, grounding limits the voltages caused by lightning, line surges, or unintentional contact with higher voltages. It also stabilizes the voltage to ground during normal system operation.
One important component of grounding is the equipment-grounding conductor. In a typical grounding system, this conductor connects the noncurrent-carrying metal parts of equipment, raceways, and other enclosures to the system grounded conductor (neutral), the grounding electrode conductor, or both, at the service equipment or at the source of a separately derived system. The equipment-grounding conductor removes dangerous voltages that could exist during a ground-fault condition by opening overcurrent protection devices. The greater the current, the quicker the overcurrent device clears the fault.
To do this, the equipment-grounding conductor must have sufficiently low impedance. An effective grounding path must be mechanically and electrically continuous, and capable of safely conducting fault-currents without damaging itself. For the equipment-grounding conductor to do its job, you must bond all metal parts within an electrical distribution system to each other.