Grounding vs. bonding
- Grounding: connecting the system (typically the neutral) to the earth via a grounding electrode system (NEC 250 Part III).
- Bonding: connecting enclosures, raceways and equipment so they share the same potential and give a low-impedance fault return path.
- Main bonding jumper: the single neutral-to-ground connection at the service. Downstream, neutral and ground stay separate.
System grounding methods
| Method | How | Pros | Cons |
|---|---|---|---|
| Solidly grounded wye | Neutral bonded directly to ground | Fast fault clearing, neutral for 120/277 V loads | First ground fault trips; arc flash on faults |
| High-resistance grounded (HRG) | Resistor limits ground fault to a few amps | Continue operating on first fault; reduces arc damage | No line-to-neutral loads; needs fault-location procedure |
| Ungrounded delta | No intentional ground | Continuity on first fault | Transient overvoltages; legacy; requires ground detection |
| Corner-grounded delta | One phase grounded | Low cost (legacy) | Unusual; special breaker ratings needed |
Separately derived systems
A transformer secondary, such as 480Δ to 208Y/120, creates a new system. It must have its own system bonding jumper and grounding electrode connection (NEC 250.30). Generators may or may not be separately derived, depending on whether the transfer switch switches the neutral.
Ground fault protection
NEC 230.95 requires ground-fault protection of equipment on solidly grounded wye services of more than 150 V to ground and 1,000 A or more, which covers typical 480Y/277 V services. GFPE detects low-level arcing faults that a standard breaker might not see.
Testing and maintenance
- Ground resistance testing of electrodes (fall-of-potential or clamp-on methods)
- Inspect main bonding jumper and grounding electrode conductor connections
- Check for objectionable neutral-to-ground connections downstream of the service
- Test GFPE systems at installation and periodically, per manufacturer instructions
Related reading: Safety Handbook and Standards & Regulations.