Grounding treatment of photovoltaic combiner box branch circuits

Proper branch grounding in a PV combiner box ensures safety, fault protection, and compliance by connecting each string and the enclosure to a low-resistance grounding system.

Branch Conductor Grounding

Each PV string in a combiner box typically has a positive and negative conductor. While the positive conductors pass through overcurrent protection devices (fuses or DC breakers) to the positive busbar, the negative conductors connect directly to the negative busbar without fusing, as per NEC 690.9(B) requirements . For grounding purposes:

• Equipment Grounding Conductors (EGCs) from each string should be routed to the combiner box grounding busbar.
• Bonding jumpers may be required to connect metallic parts of the combiner box and busbars, establishing an equipotential plane .
• Proper conductor sizing is critical: grounding conductors must be sized to safely carry fault currents without excessive voltage drop, following NEC and IEC standards .

Combiner Box Housing Grounding

The metal enclosure of the combiner box must be grounded to the system ground busbar to prevent shock hazards and protect against lightning or transient overvoltages . Key practices include:

• Remove paint or coatings at grounding points to ensure metal-to-metal contact.
• Connect all internal components that require grounding, such as SPD modules, monitoring modules, and DC busbars, to the main grounding bus .
• Use corrosion-resistant hardware and compatible metals to avoid galvanic corrosion .

Inspection and Maintenance

• Ground continuity testing should be performed with a true-RMS multimeter to detect intermittent faults .
• Inspect grounding connections at least annually or after extreme weather events to maintain system reliability .
• Ensure all screws and terminals are properly torqued to prevent overheating or contact resistance .

Best Practices

• Maintain separate conductors for grounding and bonding; grounding provides a fault current path, while bonding establishes an equipotential plane .
• Follow standardized wiring topologies to simplify troubleshooting and ensure consistent protection across all PV strings .
• Integrate dual-layer protection where possible, combining traditional grounding with ground fault detection for enhanced safety . By following these guidelines, PV combiner boxes can safely aggregate multiple strings, protect equipment from electrical faults, and ensure long-term operational reliability.