Part V of NEC Art. 250 provides the requirements for bonding. So what is bonding, and why do we do it?
The definition in Art. 100 is short: “Connected to establish electrical continuity and conductivity.” This seems vague, unless you think about it or read related definitions such as “bonding jumper.” It means using a metallic conductor to make a conductive path between metallic objects. Basically, you are shorting them together. You can’t do this with a ground rod driven into the dirt (the conductance is not nearly enough).
Say you have shorted together six enclosures, using metallic raceway. You’ve now eliminated dangerous differences in potential. That is one reason we bond.
What else have you done? If these enclosures are connected to a bonding system that is connected to the source (as required by Art. 250, Part V), you’ve created an effective ground-fault current path back to the source. Why would this matter? That low impedance path back to the source allows fault current to quickly rise high enough to open the overcurrent device for the circuit that faulted.
