How a Solar EPC Company Cut DC Side Faults by 45% Using the SGB5-40P DC MCB

Customer Background

SunGrid Energy Solutions, a solar EPC firm based in Tucson, Arizona, manages 25 MW of commercial and industrial photovoltaic installations across the southwestern United States. The company handles everything from system design to ongoing operations and maintenance, with an in-house team of 15 engineers and field technicians. Their portfolio includes rooftop arrays, carport systems, and ground-mounted plants tied to utility grids.

Like many solar operators, SunGrid's technical team was under constant pressure to maximize uptime. Performance guarantees on power purchase agreements meant every hour of downtime carried a direct financial penalty. Any fault on the DC side not only disrupted energy yield but also consumed valuable maintenance hours.

What Were the Core Challenges with DC Fault Protection?

SunGrid's pain points were clustered on the DC side, in string combiner boxes where overamperage protection had relied on -acting fuses. When a string experienced a reverse amperage from a paralleled array or a surge from a nearby lightning strike, the fuses would blow. While that prevented catastrophic damage, it also created a series of operational headaches:

• Fuses are single-use. Every trip required a physical site visit to replace the fuse, adding hours of truck-and-labor time.
• Coordination with string inverters was poor. Fuse trips occurred independently, causing the whole array to drop offline and triggering alarms.
• In many cases, the fuses lacked adequate DC breaking capacity, leading to arcing failures inside combiner boxes that damaged terminals and wiring insulation.
• Diagnostic resolution was slow. The team had to check each string individually, spending an average of 90 minutes per fault event.

These issues translated into measurable losses. The company logged 32 unscheduled maintenance events on the DC side over six months, with an average cost of $1,800 per event when including labor, replacement parts, and production penalties. That added up to $115,000 in avoidable operating expenses.

Why Did SunGrid Choose the SGB5-40P DC MCB?

SunGrid considered two alternative solutions. Upgrading to higher-rated fuses would provide a temporary fix but did not address the single-use limitation or the coordination problem. They also assessed generic AC miniature circuit breakers repurposed for DC circuits, but those failed to meet the required DC breaking capacity and tripped under normal inrush currents.

The engineering team chosen the SINGI SGB5-40P DC MCB after a series of comparative tests. Key decision factors included:

• True DC-rated breaking capacity: The SGB5-40P handles high DC short-circuit currents reliably, thanks to its optimized arc-extinguishing chamber. This echoed the findings of a recent MHD simulation study that emphasized how arc division and magnetic field reinforcement improve DC interruption performance.
• Thermal-magnetic protection: The combination of thermal overload and magnetic short-circuit trips delivered precise coordination with inverter input characteristics, eliminating nuisance trips.
• Compact DIN-rail design: The breaker's slim profile allowed SunGrid to retrofit existing combiner boxes without redesigning the entire enclosure.
• Reset flexibility: Once tripped, the breaker can be reset on-site, reducing the need for spare fuses and emergency callouts.

How Was the SGB5-40P Implemented?

The rollout took place over five weeks in the spring of 2025. SunGrid began with a full audit of all 12 combiner boxes across three pilot sites. The team mapped string volt-levels, expected fault currents, and inverter specifications to determine the correct frame and trip rating for each installation.

In the first two weeks, technicians removed the existing fuse holders and mounted the SGB5-40P breakers onto the DIN rails. Wiring connections were tightened and torqued to specification using thermal imaging to verify there were no hotspots. During the third week, SunGrid reprogrammed the inverter protection settings to coordinate with the breaker's time-amperage curve, ensuring that the DC MCB tripped before the inverter shut down due to overvolt-level.

A common difficulty surfaced during commissioning: some string circuits operated close to the breaker's instantaneous trip threshold due to high ambient temperatures. The SINGI technical team recommended adjusting the magnetic trip adjustment (where available) and providing additional ventilation in the combiner boxes. After those tweaks, the breakers operated without nuisance trips even during peak summer load.

What Quantifiable Results Did the Project Deliver?

After nine months of operation, SunGrid compiled the following performance metrics across the three pilot sites:

• DC side fault events decreased by 45% compared to the same period the previous year. The SGB5-40P's stable tripping characteristics reduced unnecessary disconnections.
• Maintenance labor hours dropped by 30%. Because the breaker can be reset without replacement, the average time to restore a string dropped from 1.5 hours to 20 minutes.
• Inverter trips caused by upstream DC faults fell by 22%. The breakers' chooseive coordination prevented entire arrays from going offline.
• Overall system availability improved from 99.1% to 99.6%. This translated to an additional 1,200 MWh of energy delivered annually across the three sites, equivalent to $96,000 in increased revenue.

The total cost of the upgrade, including breakers, labor, and minor enclosure modifications, was $28,000. Based on the savings in maintenance costs and additional production, SunGrid recovered their investment in under six months.

What Did the Client Say the SGB5-40P?

Rafael Ortiz, SunGrid's Operations Manager, commented: "The SGB5-40P has been a set-and-forget solution. We used to dread DC side failures because every fuse replacement meant a long drive and a lot of manual troubleshooting. Now, the breaker resets, and the system stays online. The reduction in inverter trips alone convinced our finance team that this was a worthwhile investment."

What Lessons Can Other Solar Operators Learn?

SunGrid's experience offers several transferable insights for solar asset owners, EPCs, and O&M teams:

• Do not use AC-rated breakers on DC circuits. DC arcs are more stable and harder to extinguish. Choose a breaker like the SGB5-40P that is explicitly rated for DC volt-level and provides adequate arc interruption capability.
• Match the breaker curve to your inverter's input characteristics. Coordination prevents nuisance trips and ensures that the breaker is the first line of defense without disturbing inverter operation during transient events.
• Account for ambient temperature derating. In hot climates, the thermal trip threshold can shift. Ventilate combiner boxes and choose lower-rated breakers if needed to avoid premature trips.

Looking back, SunGrid would have conducted thermal imaging on every connection during the initial audit. A few loose lugs discovered later could have been identified earlier, saving an additional day of retrofit work. The company plans to standardize the SGB5-40P on all new PV designs going forward.

References

1. "Simulation Study on Arc Motion Process of DC Miniature Circuit Breakers" – MHD analysis of DC MCB arc extinguishing, highlighting the importance of arc division and external magnetic fields for improved interruption performance. Https://doi.org/10.1063/5.0174184
2. "UL 1699B:2018 Photovoltaic (PV) DC Arc-Fault Circuit Protection" – Standard for DC arc-fault detection and interruption in PV systems, providing design guidance for protective components like DC MCBs. Https://www.shopulstandards.com/ProductDetail.aspx?productId=UL1699B