How a Solar Installer Cut DC Circuit Faults by 62% with DC MCBs

A solar EPC and O&M contractor running 60 MW of commercial rooftop and small ground-mount arrays across Arizona and Nevada replaced fuse-based DC string protection with Soutya DC miniature circuit breakers in 210 combiner enclosures. Fault-related downtime fell 62%, and the retrofit paid for itself in under seven months.

Customer Background: Who Took On the DC Protection Upgrade?

The client is a solar engineering, procurement and construction firm with an in-house operations and maintenance division. Headcount sits at 120. The fleet covers 60 MW across 340 commercial rooftop systems and six ground-mount arrays in Arizona, Nevada and southern Utah. Asset owners include retail chains, cold storage operators and two municipal utilities.

Fleet size grew 40% in 18 months while O&M staffing stayed flat. Service agreements carry a 48-hour response window for production-affecting faults, with penalties attached to missed windows.

What Problems Did the Existing DC Protection Create?

Each combiner box protected 12 to 16 strings with 15 A and 20 A DC fuses. That arrangement produced three recurring problems.

• Diagnosis took longer than repair. A blown fuse says nothing which string caused it. Crews isolated the whole combiner, then tested strings one at a time with a clamp meter. Mean time to isolate a faulted string was 74 minutes, and two technicians were needed on every call.
• Spares multiplied. Twelve fuse SKUs rode on the trucks to cover voltage ratings, ampere ratings and holder types, at $26,400 in annual spend.
• Downtime stacked up. String outages removed 1,480 MWh of generation across the fleet in a year, worth roughly $77,000 at contracted rates, and drove 620 unscheduled truck rolls at $310 each.

Hail in the Phoenix metro area during one spring produced 90 fuse failures in a week. Crews worked through the weekend, and two asset owners filed service credits against the response window.

Why Choose Soutya DC Miniature Circuit Breakers?

The engineering team compared four paths. String-level monitoring with the existing fuses carried a capital cost above $900 per combiner. Indicating fuse holders solved diagnosis but kept the fuse inventory and the downtime. Molded case DC breakers exceeded the depth of the existing enclosures, which meant new boxes and new concrete pads at the ground-mount sites.

The Soutya DC miniature circuit breaker matched the enclosure and the workflow on four points:

• Resettable operation with a visible trip indicator, so a technician identifies the faulted string on sight.
• A two-pole DIN rail footprint that drops into the existing 35 mm rail without enclosure changes.
• A DC-specific arc chamber with splitter plates. Direct current has no natural current zero, so arc extinction depends on chamber geometry and trip speed than on a zero crossing.
• A 6 A to 63 A ampere range that covered the entire fleet with three SKUs instead of twelve.

Ratings were checked against IEC 60947-2 and the combiner listing, including DC voltage rating, breaking capacity and polarity marking.

How Was the DC MCB Rollout Implemented?

The project ran 14 weeks across two states.

1. Fleet audit (weeks 1–3). Crews recorded string counts, open-circuit voltage, operating current, enclosure dimensions and internal ambient temperature at each combiner.
2. Specification and bench validation (weeks 4–6). Twenty units were tested on a 1.2 MW array with thermal imaging during peak irradiance, plus trip verification at the inverter's DC limits.
3. Pilot on three sites (weeks 7–9). Field crews timed the reset procedure and logged every trip event.
4. Two-wave fleet rollout (weeks 10–13). Work ran inside scheduled maintenance windows, so no generation was lost to the retrofit.
5. Documentation and spares (week 14). Single-line diagrams, torque specifications and labeling were updated, and each truck received a standardized spares kit.

The difficult moment. Two combiners in the pilot tripped between 13:00 and 15:00 on clear days. String current measured 8.6 A against a 10 A breaker inside an enclosure sitting at 68°C internal ambient. Thermal derating inside the box pushed the effective capacity below the string output. The fix was upsizing those positions to 16 A breakers and adding a 20 mm ventilation gap inside the enclosure door, after which the sites ran eight weeks without a nuisance trip.

What Did the Switch to DC MCBs Deliver in Numbers?

• Unscheduled truck rolls: 620 to 260 per year, a 58% reduction, saving $111,600 in labor and vehicle cost.
• Lost generation from string outages: 1,480 MWh to 562 MWh per year, down 62%, recovering roughly $47,800 in contracted revenue.
• Mean time to isolate a faulted string: 74 minutes to 22 minutes, down 70%.
• Annual protection component spend: $26,400 to $17,400, down 34%.
• Total identified savings: $187,000 per year against a retrofit cost near $96,000, a payback under seven months.

The crew safety profile changed as well. Fuse replacement under load disappeared from the standard procedure, and the DC-rated work instruction for a string fault is now a lockout, a visual check and a reset.

What Does the Client Say the Project?

“A string fault used to mean two technicians, a fuse kit and most of an afternoon,” said the director of O&M. “Now one technician opens the enclosure, reads the tripped breaker, repairs the string and resets it. Afternoon callouts dropped by half within the first month.”

The head of engineering added: “The audit was the part that mattered. Sizing breakers to enclosure ambient instead of nameplate string current is what kept the nuisance trips away.”

What Can Other EPCs and Asset Owners Learn From This Project?

• Size for the box, not the datasheet. Internal ambient temperature on a rooftop enclosure can sit 25°C above outside air and derate a breaker well below its nameplate rating.
• Pilot on the hottest, highest-irradiance site. A single cool-weather trial hides the thermal behavior that creates nuisance trips in July.
• Treat diagnosis time as the main saving. Replacement parts delivered a third of the savings; faster fault location delivered the rest.
• Reflection. The team would start the audit before hail season and negotiate a pre-assembled retrofit kit with the enclosure supplier, a change worth $12 per enclosure in installation labor.

References

• IEC 60947-2:2019, Low-voltage switchgear and controlgear — Part 2: Circuit-breakers. webstore.iec.ch/publication/62482
• UL 1699B:2018, Photovoltaic (PV) DC Arc-Fault Circuit Protection. shopulstandards.com/ProductDetail.aspx?productId=UL1699B
• Simulation Study on Arc Motion Process of DC Miniature Circuit Breakers. doi.org/10.1063/5.0174184

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Specifications and ampere ratings for the Soutya DC miniature circuit breaker are available on the DC miniature circuit breaker product page.