Grounding and Bonding
Grounding and bonding questions are usually about paths and points. The system grounded conductor (neutral) is grounded at the service; the equipment grounding conductor provides a low-impedance fault path back to the source so the overcurrent device opens. These are separate functions and must remain separate downstream of the service.
Concepts to understand
- The grounding electrode system ties the service to earth through electrodes and a grounding electrode conductor sized from the service conductor size.
- Main bonding jumper connects the neutral bus to the enclosure at the service only.
- At sub-panels, the neutral and ground buses are isolated and the enclosure is bonded by the equipment grounding conductor.
- Bonding jumpers are required around water meters, at metallic raceway terminations, and wherever the fault path could otherwise be interrupted.
- Equipment must be bonded to provide a fault path; a grounding electrode alone will not open a breaker.
- Special rules apply to separately derived systems, isolated receptacles, and patient-care areas.
Traps
- Re-bonding neutral and ground at a sub-panel (objectionable current on the ground path).
- Sizing the grounding electrode conductor from the load rather than the service conductors.
- Assuming a ground rod clears faults; the earth path is too high-impedance.
- Lifting grounds to reduce noise.
Rule of thumb
Trace the fault path from the faulted enclosure back to the source: it must be metallic, continuous, and bonded at every joint. If any joint could open, a bonding jumper is required there.