Understanding the Differences Between Bonding, Grounding, and Earthing

Jan 1, 2009 12:00 PM, By Larry Ray and S. Frank Waterer, Square D Engineering Services/Schneider Electric

Grounding electrode (earthing)

This term refers to the method by which the facility grounding system is connected and referenced to earth. The most common grounding electrode for small facilities is a metallic ground rod, but earthing systems for larger buildings can — and should — be more elaborate and include the means by which to inspect and test these systems periodically. A grounding electrode system that is buried in earth or encased in concrete and then forgotten is often the source of increasing problems as the building ages and the grounding electrodes deteriorate.
Lightning abatement

Some facilities use air terminals (also known as lightning rods) to direct lightning strikes away from power equipment, but these devices are often connected to the grounding system in such a way that they have the opposite effect — unintentionally bringing lightning energy into facility structural steel, low-voltage transformer windings, and, subsequently, sensitive building loads.
Signal-reference grounding

Sensitive electronic machines rely on the grounding system for reference of low-magnitude signals. Therefore, it's often crucial to provide multiple grounding paths, rather than rely on a single equipment grounding conductor between the power source and the sensitive load. This ensures that spurious voltages on the grounding system are maintained well below the level at which they might be confused with sensitive machine reference signals. The best guide for signal-reference grounding is IEEE Standard 1100-2006, “Recommended Practice for Powering and Grounding Electronic Equipment.”

Note that earthing is not required for sensitive machine operation. Modern aircraft, for example, are packed with sensitive computers and electronic devices, which operate correctly without an attachment to earth. They rely on a bonded metallic system — the airplane framework, skin, structural supports, raceways, and grounding conductors — to serve as the ground reference. If this bonded system rises in voltage with respect to earth, all machines onboard experience the increase together. The net result is that the machines see no voltage potential differences with respect to each other. Once the airplane lands, any voltage potential between the plane and earth must be discharged by an electrode that bypasses the rubber tires.

Resolving the issue

The immediate solution to the example plant's illegal ground rod (click here to see Fig. 2) was to remove the shock hazard. This was done by connecting a grounding conductor (1/0 copper) from the ground rod to the nearest part of the building grounding system — in this case, the structural steel. This connection eliminated the shock potential during storms by reducing the resistance between the ground rod and the building grounding system.

The next step was to eliminate the wiring errors and install a ground wire from the source to the CNC machine (click here to see Fig. 3). The primary reason that the isolated ground rod was effective in decreasing operating problems was the building's bonded system experienced voltage transients, imposed on it due to wiring errors. One common error is the improper connection of neutral wires to ground buses or ground wires to neutral buses. This error allows neutral currents to flow on the bonded system, thereby creating voltage transients. Neutral wires are only allowed to be connected to the bonded system at a service entrance or at a step-down transformer (called a separately derived source by the NEC). Notice in Fig. 2 that the plant had installed both a voltage regulator and a noise suppression device ahead of the CNC machine. These devices are often applied to solve the nuisance operating problems brought on by ground system transients. Suppression devices are not a cure-all, however. In fact, they're sometimes unnecessary when wiring and grounding problems are corrected first.

Once the spurious ground rod had been connected to the rest of the bonded system, operating issues had to be addressed, which involved correcting the wiring errors identified in the site survey. For the example facility, these steps were adequate. For other situations, you should refer to the following checklist:

Connect the ground rod to the bonded system and install a grounding conductor from the power source to the sensitive load to eliminate the safety hazard and allow an effective ground-fault return path.
Correct wiring and grounding errors on the power system serving the sensitive machine.
Install a step-down transformer (i.e., a separately derived source) to serve only the process machine. Derive a new neutral to the ground bonding point at the load side of the transformer.
Any remaining operating problems are probably caused by communications ground loops. Ground loops, which are introduced by communication wiring between sensitive machines fed from different power sources, may require more elaborate correction schemes, such as optical isolation.

Taking the next step

In summary, the plant in the example had installed a CNC process machine in accordance with the manufacturer's recommendations. Unfortunately, those recommendations included the requirement for a separate ground rod to serve as the only means of equipment grounding. While this practice may reduce data errors in sensitive process machines, it violates the NEC, creates a shock hazard for employees, and causes a potential difference that may damage sensitive electronic components.

Electrical engineers and contractors can help customers avoid situations like this by providing proactive counsel in this area. The best place to start is to gather as much information as possible — from the 2008 NEC, seminars/conferences, trusted electrical equipment manufacturers, and online sources. With that knowledge in hand, you have yet another reason to call on a customer and resolve an issue of critical importance.

Ray, P.E., is director of Schneider Electric's Square D Engineering Services, Raleigh, N.C. He can be reached at larry.ray@us.schneider-electric.com. Waterer is an Engineering Fellow for Schneider Electric's Square D Engineering Services, Norcross, Ga. He can be reached at frank.waterer@us.schneider-electric.com.

Sidebar: Knowledge is Power

An electrical engineer or contractor who understands the various elements of proper grounding, bonding, and earthing systems is best positioned to counsel customers on appropriate practices in this area. A keen understanding of NEC requirements could also help you develop a reputation as being the one to contact with any bonding/grounding-related questions. Such expertise could also lead to future business.