Grounding vs Bonding

Currently the NEC in Article 100 defines the terms "ground" or "grounded" as "connected to the earth or to some conducting body that serves in place of the earth." Yet, the NEC often uses the term "ground" when it really means "bond" (connected to an effective ground-fault path to clear a fault) [250.2 and 250.4(A)(5)].

Grounding - Grounding metal parts to the earth in premises wiring is only useful to provide a path for lightning, shunting high-frequency noise, or reducing static discharge.

Bonding - Bonding all metal parts together and then to the system winding (typically to the X0 terminal of a transformer) is done to provide a low-impedance path to the source (system) to facilitate the opening of the circuit-protection device to remove dangerous voltage on metal parts. In addition, bonding the system to metal parts (typically to the X0 terminal of a transformer) stabilizes the system voltage to the metal parts and it provides a zero system reference (to the metal parts).

It took me over 20 years to figure out that when the NEC, as well as books and magazine articles, state "ground or grounded" they rarely intend that the metal parts be connected to the earth or to some conducting body that serves in place of the earth. Typically, the intent is to have the metal parts "bonded" to a low-impedance path so that fault current can return to the system winding. See the definition of "effective ground-fault path" in 250.2 and review 250.4(A)(3), (4), and (5).

Grounding vs Bonding