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22. Grounding, Bonding & Rapid Shutdown

Learning objectives

  • Distinguish system grounding, equipment grounding, and bonding.
  • Size and route the EGC and GEC correctly and use listed bonding hardware.
  • Execute the 690.12 rapid-shutdown requirement by either compliance path.

22.1 Grounding vs bonding (the distinction that confuses everyone)

  • System grounding: establishing a reference to earth for the electrical system (stabilizes voltage, helps clear faults).
  • Equipment grounding / bonding: connecting all normally non-current-carrying metal (module frames, racking, enclosures) together and to a low-impedance fault-return path. The point is that a fault then trips protection instead of energizing metal someone can touch. Two conductors do this work:
  • EGC (Equipment Grounding Conductor): the fault-current return path bonding the metal parts; sized by NEC Table 250.122 based on the circuit’s OCPD rating.
  • GEC (Grounding Electrode Conductor): connects the grounded system to the grounding electrode system (ground rods/building electrode); sized by NEC Table 250.66.

22.2 The Article 690 Part V sections

PV grounding/bonding lives in 690.41–690.47, read alongside Article 250:

  • 690.41: ground-fault protection; PV circuits at/above 30 V or 8 A require a ground-fault protection device (GFPD), now typically built into the inverter.
  • 690.43: equipment bonding: exposed metal bonded to an EGC; module-securing devices must be listed and identified for bonding.
  • 690.45: EGC sizing.
  • 690.47: the supporting building/structure must have a grounding electrode system per Article 250.

22.3 Functionally grounded systems and listed bonding hardware

Functionally grounded (non-isolated): an array that is not solidly bonded to earth. Instead the inverter supplies the system's voltage reference and continuously watches for ground faults, so no separate DC grounding system is needed. Almost all modern grid-tied arrays work this way.

Most modern grid-tied arrays are functionally grounded. For equipment bonding, UL 2703-listed racking provides the bonding path through devices like WEEBs (washer-type bonding jumpers), grounding clips, and lugs. A fully bonded array has hundreds of redundant low-resistance paths. That gives it lower ground-path resistance and better fault detection than a single bare-copper EGC. ⚠️ Never lay bare copper directly against aluminum (galvanic corrosion); use listed, compatible hardware.

22.4 Rapid shutdown (690.12): protecting firefighters

Rooftop DC conductors stay energized whenever the sun shines, a hazard to first responders. 690.12 requires a rapid-shutdown function that, on initiation, controls conductor voltage:

  • Array boundary = 1 foot from the array in all directions (since the 2017 NEC).
  • Outside the boundary: controlled to ≤30 V within 30 seconds.
  • Inside the boundary: controlled to ≤80 V within 30 seconds (effective January 2019), which forces shutdown at the module level.
MLPE (Module-Level Power Electronics): per-module devices, namely microinverters or DC power optimizers, that can shut down or limit each module's output individually (Ch 6.2).

Two compliant paths satisfy the inside-boundary limit:

  1. MLPE: microinverters or DC optimizers that drop each module’s output on initiation.
  2. PV Hazard Control System (PVHCS) listed to UL 3741. This evaluates the whole array (modules + racking + wiring) as one firefighter-safe system. It enables string-only designs (no per-module electronics) on larger commercial jobs, provided every component matches the listing.

Initiation and labeling: the system needs a single, readily accessible initiation device. For one- and two-family dwellings it sits outside, reachable with no locks, ladders, or tools. It carries the placard “PHOTOVOLTAIC SYSTEM EQUIPPED WITH RAPID SHUTDOWN.” The 2023 NEC added exceptions for PV on non-enclosed/detached structures (e.g., ground mounts, carports), where firefighter rooftop operations don’t apply.

22.5 Rapid shutdown: the array boundary, drawn

                  ┌─────────── ARRAY ───────────┐
   ≤80 V in 30 s  │ [mod][mod][mod][mod][mod]    │  ← INSIDE the boundary:
   (module-level) │ [mod][mod][mod][mod][mod]    │     module-level shutdown (MLPE)
                  └──────────────┬───────────────┘     OR a UL 3741 PVHCS
        ┌─── 1 ft boundary ──────┘
        ▼
   ≤30 V in 30 s  ──── conductors leaving the array ───►  [initiation device]
   (OUTSIDE)                                               readily accessible,
                                                           no locks/ladders/tools
                              placard: "PHOTOVOLTAIC SYSTEM EQUIPPED WITH RAPID SHUTDOWN"

Two zones, two limits: ≤80 V inside the 1-ft boundary (forces module-level control) and ≤30 V outside, both within 30 seconds of initiation, all to make a roof safe for firefighters.

22.6 The grounding picture

 module frames ─┬─ racking (UL 2703 bonded, WEEBs/clips) ─┐
                │                                          ├─ EGC ─► fault-current
 inverter/encl ─┘                                          │        return path
                                                           │        (Table 250.122)
                       building grounding electrode ◄── GEC (Table 250.66)

EGC bonds all the metal together for fault return; GEC ties the system to earth. Functionally grounded inverters monitor for ground faults and eliminate a separate DC grounding system.

Chapter 22 summary

Bond all metal to an EGC (sized by Table 250.122) and tie the system to earth via a GEC (Table 250.66), per 690.41–690.47 and Article 250. Modern arrays are functionally grounded with UL 2703 bonding hardware (WEEBs/clips). Rapid shutdown (690.12) drops conductors to ≤30 V outside / ≤80 V inside the 1-ft boundary within 30 s, achieved with MLPE or a UL 3741 PVHCS, with a readily accessible initiator and the required placard.

  • EGC (Equipment Grounding Conductor): bonds metal parts together for fault-current return; sized by Table 250.122.
  • GEC (Grounding Electrode Conductor): ties the grounded system to earth; sized by Table 250.66.
  • Functionally grounded: an array referenced by the inverter rather than solidly earthed.
  • MLPE: module-level power electronics (microinverters or DC optimizers) that enable per-module shutdown.
  • Rapid shutdown (690.12): drops conductor voltage on initiation to protect firefighters.
  • PVHCS (UL 3741): a listed hazard-control system that makes string designs firefighter-safe.

Full definitions: Appendix A (glossary).

Practice Problems: Chapter 22

  1. What voltage limit applies inside the 1-ft array boundary on rapid-shutdown initiation, and within what time?
  2. What limit applies outside the boundary?
  3. Name the two compliant ways to meet the inside-boundary requirement.
  4. What is the purpose of rapid shutdown, and who is it protecting?
  5. Which conductor provides the fault-current return path, and which table sizes it?
  6. Why is bonding through listed UL 2703 racking often electrically better than a single bare-copper EGC?
  7. For a one/two-family dwelling, where must the rapid-shutdown initiator be, and what does “readily accessible” forbid?

Solutions: Chapter 22

  1. ≤80 V within 30 seconds.
  2. ≤30 V within 30 seconds.
  3. MLPE (microinverters/optimizers) or a UL 3741 PV Hazard Control System (PVHCS).
  4. Firefighters / first responders: so they can work on/around the roof without high-voltage DC shock risk.
  5. The EGC (Equipment Grounding Conductor), sized by NEC Table 250.122.
  6. A fully bonded array has hundreds of redundant low-resistance paths, giving lower ground-path resistance and better ground-fault detection than one wire, and it survives single-point failures.
  7. Outside the building, readily accessible: no locks, ladders, or tools required to reach it.