Grounding and bonding are two different jobs

The trade uses these words interchangeably. The code does not. One sentence in Article 250 settles most of the questions people get wrong.

The sentence

Article 250 contains one line that answers more exam questions than anything else in it. In the performance requirements, under Use of the Earth, the code states flatly that the earth shall not be considered an effective ground-fault current path (NEC 250.4(A)(5)(c)). Open your code book and read that sentence in place.

Sit with it. A ground rod does not clear a fault. Dirt is not a conductor in any useful sense at the currents a breaker needs to see. If an answer choice implies fault current travels through the earth back to the source, it is wrong, and it will be offered on more than one question.

So what is the ground rod for

Grounding, the earth connection, does a real job. It is just not the job most people assume.

• It holds the system at a stable reference relative to earth, so the voltage on a conductor is a known quantity rather than a floating one.
• It gives lightning and utility-side surges somewhere to go that is not through the building.
• It limits the voltage the system can be pushed to if a higher-voltage line contacts it.

Notice that none of those is clearing a fault. Clearing a fault is a different job, done by different hardware.

Bonding is what clears the fault

Bonding is connecting the metal together. Conduit to enclosure, enclosure to equipment grounding conductor, all of it back to the source through metal.

That metal path is what carries fault current. It is low impedance, it goes back to the supply transformer, and it lets enough current flow to open the overcurrent device fast. The code describes the effective ground-fault current path as low impedance and as capable of safely carrying the maximum ground-fault current likely to be imposed on it (NEC 250.4(A)(5)).

When a hot conductor touches a metal box, the fault current runs through bonded metal back to the source and trips the breaker. The ground rod is not involved in that sequence at all.

Grounding connects to earth. Bonding connects metal to metal. Bonding is what saves someone.

Where this shows up on the exam

Once you hold the distinction, a whole category of questions gets easier.

• Anything about a rod, pipe, plate, ground ring or concrete-encased electrode is a grounding question.
• Anything about jumpers, bushings, locknuts, continuity across a fitting, or a raceway crossing an expansion joint is a bonding question.
• Anything about clearing a fault is bonding, no matter how the question is worded.

A standard locknut is a good example. At a service, a standard locknut is a mechanical connection and is not accepted on its own as the bonding means (NEC 250.92(B)). That is why bonding bushings exist. Nothing about that question involves the earth.

Two traps built into the wording

The first is objectionable current. If multiple grounding connections cause current to flow where it should not, the code permits four specific remedies, and lifting the equipment grounding connection is not among them. It says so directly, and it adds that electrical noise or data errors in electronic equipment are not the objectionable current this section is about (NEC 250.6(D)).

So the field habit of lifting a ground to cure a hum has a section written specifically to forbid it.

The second is the isolated ground. An isolated equipment grounding conductor is permitted, and an informational note under the section says plainly that using one does not relieve the requirement for an effective ground-fault current path (NEC 250.96(B)). The insulated conductor is in addition to the fault path, never instead of it.

How to study Article 250

It is long and it is dense, and reading it front to back does not work well.

Read the performance requirements at the front first. They explain what the rest of the article is trying to accomplish, and most of the detailed sections make sense once you know which of the two jobs they serve.

Then, for any section you are working through, ask one question: is this about the earth, or about metal-to-metal continuity? Almost every rule in the article sorts cleanly into one of those, and the ones that sort into bonding are the ones that matter when something goes wrong.

What this page cites

• NEC 250.4 Performance requirements for grounded and ungrounded systems. Read this section first.
• NEC 250.6 Objectionable current, and the four permitted remedies.
• NEC 250.92 Bonding at services, including the methods and the locknut question.
• NEC 250.96 Bonding other enclosures, including isolated grounding circuits.