Open-circuit voltage (Voc) measurement in a PV combiner box involves safely disconnecting the PV strings from the inverter and measuring the voltage across the string terminals to verify expected output.

Purpose of Voc Measurement

Measuring the open-circuit voltage of PV strings ensures that all modules in a string are connected correctly and functioning properly. It helps identify issues such as bypassed panels, faulty modules, or poor connections. Voc measurement is also required for compliance with standards and for documenting system performance during commissioning or maintenance .

Procedure

1. Disconnect the PV string from the inverter: Open the DC isolator or remove the shunt to isolate the string from the system.
2. Ensure safety: Use insulated leads and appropriate personal protective equipment (PPE) to prevent electric shock, as PV strings can generate high DC voltages even under low light conditions .
3. Measure voltage across terminals: Connect a suitable DC multimeter or specialized PV tester across the positive (+) and negative (−) terminals of the string.
4. Compare with expected Voc: Calculate the expected voltage by summing the rated Voc of each module in the string, considering environmental factors such as temperature and irradiance. For example, a string of 10 panels rated at 38 V each should produce approximately 380 V under standard test conditions .
5. Record results: Document the measured voltage for compliance and trend analysis. Any significant deviation from the expected Voc may indicate a missing or faulty module .

Considerations

• Voltage fluctuations: PV module voltage can vary with temperature and irradiance, so measurements should account for environmental conditions .
• Combiner box design: Ensure the combiner box is rated for the maximum Voc of the system and that components such as DC breakers, diodes, and surge protection devices can handle voltage fluctuations .
• String monitoring: Advanced combiner boxes may include integrated voltage and current monitoring, which can simplify Voc verification and provide ongoing performance data .

Safety Notes

• Always follow local electrical codes and standards.
• Only qualified personnel should perform Voc measurements.
• Use insulated tools and verify that the string is properly isolated before connecting measurement equipment . By following these steps, Voc measurement in a PV combiner box can be performed safely and accurately, ensuring system reliability and compliance with operational standards.