Grounding versus Bonding – Understanding the Difference in Building Electrical Systems



Expertise Includes:

    • Electrical Equipment Failures
    • Electrocution and Electric Shock Accidents
    • Fires and Explosions
    • Industrial Electrical Accidents
    • Lightning Strike Analysis
    • Scope of Damage - Estimate of Repair

While a teenager might be very familiar with being “grounded”, there is confusion over the meaning of the word in the electrical sense. In building electrical systems, “grounding” and “bonding” are two terms that are often misunderstood. Improper application of the concepts of grounding and bonding may create lethal shock and fire hazards. “Earthing” is a term which comes from the European International Electrotechnical Committee (IEC). Earthing is synonymous with grounding but often thought to have a different meaning.

The National Electric Code (NEC) is the standard which guides electrical installations in the United States. This standard is revised every three years to incorporate new technologies and best practices to improve the safety of electrical systems and installations. Article 250 of the NEC, currently titled “Grounding and Bonding”, contains requirements for providing a permanent, low-impedance “effective ground-fault current path”. Installed correctly, this ground path is one method to facilitate the operation of a circuit overcurrent protection device (OCPD) such as a fuse, circuit breaker, or ground fault circuit interrupter (GFCI) when a fault to ground occurs. In 2005, this article underwent a major update. In prior editions, the article was simply titled “Grounding”.

Ground Symbol

Ground Symbol

Grounding in simple terms is the connection of an electrical system to the earth to provide a path to dissipate surges from lightning and similar events. The attachment to the earth, made by an electrode driven into the ground, is connected to a specific point in the electrical system depending on the source of the electrical power. This single point becomes the ground reference to the rest of the branch circuits in the installation.


Ground rod installed in the earth with clamp.

Ground rod installed in the earth with clamp.

Bonding is the attachment of electrically conductive materials and other equipment throughout the electrical system to provide that same reference point to the system ground. Bonding must be done in such a manner as to provide a path for fault current if an energized conductor touches a surface in an unintended manner. Such a fault will be cleared by the OCPD because of the path that the bond to ground creates. The appropriate sizing of bonding conductors is defined in tables within the NEC.

The addition of other grounding electrodes to an electrical installation does not necessarily make a safer system. While these electrodes provide additional paths to dissipate high energy, high voltage surges like lightning, they create artificial reference points. Without proper bonding of the additional electrodes to the system ground reference point, potentially dangerous differences of potential between the different electrodes in the system can develop. At low voltages, below 600 volts, the resistance of the earth between these electrodes can vary with changes in soil conditions from several ohms to several hundreds of ohms.

Electrical Panel showing ground and neutral bond.

Electrical panel showing ground and neutral bond.

The NEC covers a number of special electrical installations as listed under additional articles. For instance, areas with livestock that have sensitivity to stray electric fields are further defined in Article 547 of the NEC. Marinas and boatyards have requirements beyond Article 250 to ensure safety given the proximity of electrical services to water and watercraft tethered to a dock structure. Table 250.3 lists additional grounding requirements for different applications.

Each electrical system installation is unique and requires careful evaluation of the relevant NEC articles to ensure compliance to the current code standards. Changes and additions to an electrical system need to be evaluated for compliance. Shortcuts made during modifications can compromise the performance and safety of the original system as it was designed.

Tom Kelly has a Bachelor of Science in Electrical Engineering and a Master of Science in Electrical Engineering from Florida Atlantic University, Boca Raton, Florida, along with a Master of Business Administration with emphasis in strategic leadership from Winthrop University, Rock Hill, South Carolina. Tom’s 25-year career in electrical engineering includes forensic engineering investigations involving industrial electrical accidents, electrical equipment failure analysis, control system failures, robotics and automation components, and scope of damage assessments. He has conducted investigations for fires, arc flash incidents, electrocution and electric shock accidents and lightning strike evaluations.

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