You can understand grounding by overcoming common reasons for confusion.
When people are tripped up by the requirements for grounding, it's often because they don't realize Art. 250 covers both grounding and bonding. An unfamiliarity with the fundamental differences between the terms bonded, grounded, and effectively grounded makes the situation even worse. Not knowing these definitions makes proper application impossible.
How do grounding and bonding differ? When you bond something, you're permanently joining metallic parts to form an electrically conductive path that can safely conduct any fault current likely to be imposed. Proper bonding creates an effective, low-impedance, ground-fault current path for the purpose of removing dangerous voltage from a ground fault by quickly opening the related overcurrent protection device (OCPD).
When you ground metal parts of electrical equipment you're intentionally connecting the equipment to earth. Failure to properly do so could result in high voltage on metal parts if lightning enters the structure. Lightning doesn't necessarily strike only grounded items when seeking a path to the earth. If the metal parts aren't effectively grounded, much of the high energy from the lightning strike will dissipate into the structure, which can result in electric shock or fires inside the premises.
Sometimes metal parts should be grounded to earth to help prevent the build-up of high-voltage static charges where the discharge (arcing) could cause failure of electronic equipment or a fire and explosion in a hazardous classified area.
System grounding is the intentional connection of one terminal of the power supply to earth for the purpose of stabilizing the system line-to-ground voltage during normal operations. According to IEEE-142 (Green Book), “arcing, restriking, or vibrating ground faults on ungrounded systems can (under certain conditions) produce surge voltages as high as six times normal.”
To effectively ground something, you must intentionally connect it to earth through a ground connection or connections of sufficiently low impedance and of sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazards to connected equipment or to persons.
Art. 100 is clear about what grounding and bonding mean, but in some cases these definitions get lost along the way. For example 404.9(B) requires snap switches to be effectively grounded. This doesn't mean you must intentionally connect the switches' metal yoke to the earth via a ground rod. Instead, it requires you to bond the metal switch yoke to a low-impedance effective ground-fault current path so dangerous voltage from a ground fault can be removed by opening the circuit protection device.
Perhaps the NEC says it best. Bonding is the “permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed.” On the other hand, a ground is a “conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth or to some conducting body that serves in place of the earth.”
Any time you encounter confusion about grounding and bonding, just refer to Art. 100 and carefully read the definitions. Then any action you take will be solidly grounded in the proper understanding of these important concepts.
