How We Fixed a 0V DC String Fault & Arcing Event: A 120 kW Solar Plant Case Study

If you have ever checked your system and wondered, "Why is my solar string showing 0V DC at the inverter terminals?" you are dealing with a critical system fault. In grid-connected solar PV plants, failures within DC protection systems—particularly surge protection devices (SPDs)—can completely collapse string voltage, lead to abnormal operating conditions, create severe fire hazards, and cause extended downtime if not identified early.

This technical blog presents the exact troubleshooting process and root cause analysis of a DC-side fault encountered at a 120 kW grid-connected solar PV plant in Nerul.

Why does a solar string drop to 0 V DC while others operate normally?

During a scheduled preventive maintenance check at the Nerul plant, abnormal voltage behavior was observed on MPPT-1 of Inverter-1. One string was operating normally at approximately 720–760 V DC, while the second string indicated 0 V DC at the inverter input terminals when measured using a digital multimeter.

While accessing the fuse of the affected string using insulated tools, a high-energy DC arcing event occurred, resulting in localized fire damage inside the DCDB. This proved that a serious internal fault was suppressing the voltage and waiting to discharge.

Key Technical Learning: DC arc energy is significantly higher than AC, which dramatically increases fire risks during manual handling if a component has degraded.  

How do you troubleshoot a DC-side fault in a solar PV plant?

To safely locate the root cause without causing further system damage, the Avishakti engineering team followed systematic diagnostic procedures in accordance with international standards like IEC 62446-1 and IEC 62548:

• Step 1: Safe Isolation: Inverter DC isolators were switched OFF, and strings were isolated at the DCDB to verify a zero-voltage condition before manual intervention.

• Step 2: Visual Inspection: Inspection of the DCDB revealed localized thermal discoloration near one SPD terminal and carbonization marks around the associated fuse holder.

• Step 3: String Health Verification: Disconnecting both PV strings from the protection circuitry and measuring open-circuit voltage showed 880–900 V DC per string. This confirmed the PV modules and cabling were completely healthy, ruling out the solar array as the fault source.

• Step 4: Protection Path Testing: We systematically checked the fuse and MCB continuity (both were healthy).

• Step 5: The SPD Isolation Test: The moment the SPD connected to the affected string was disconnected from the circuit, normal string voltage (~720–760 V DC) was restored immediately toward the inverter terminals.

Can a single failed DC SPD cause a solar string failure?

Yes, a single failed or degraded DC Surge Protection Device (SPD) can completely disable an entire PV string. In this case study, the root cause was conclusively traced to an internal component failure of just one DC SPD. The degradation caused an internal short that collapsed the voltage on that specific protection path. Furthermore, because DC arc energy is significantly higher than AC, handling the fuse under these degraded conditions aggravated the system, resulting in the localized fire damage within the DCDB.

What corrective and preventive actions stop DC arcing in solar plants?

To restore safe, stable operation, the Avishakti team replaced the faulty component with new, IEC-certified DC SPDs compliant with IEC 61643-31 and renewed all adjacent damaged wiring.

To reduce the risk of recurrence for long-term asset management, we implemented these preventive protocols:

1. Periodic SPD inspection and active condition monitoring.

2. Scheduled torque verification of all DC terminations to prevent loose connections.

3. Real-time string-level monitoring for early anomaly and voltage drop detection.

   
   

Is Your Solar Asset Truly Protected?

Clean solar panels are only 10% of the maintenance puzzle. As this case study shows, a single failing protection component inside a DCDB can bring down an entire string and create severe fire hazards.  

If your current AMC provider is only washing panels and skipping technical, standard-compliant electrical audits, your investment is at risk.

Secure your plant’s generation and safety. Contact Avishakti Rooftop Solar today to book a comprehensive Solar Plant Health Audit and discuss our expert MAC (Maintenance and Asset Management) contracts.