Assessment: DC Arc Fault in MC4 Connector Causes Cable Fire — EUR 127,000 Damage to 180 kWp Rooftop System
Assessment type: Fire cause investigation / ForensicsSystem size: 180 kWp polycrystallineRegion: Rhine-Main areaPeriod: March – May 2025
Assessment summary:
Cable fire on 180 kWp PV rooftop system (commercial building, built 2019) caused by DC arc fault
Fire cause: Improperly crimped MC4 connectors at the string transition
Contact resistance at the damage point: 2.8 ohm (limit: <0.5 milliohm for new installations)
Temperature development up to an estimated 850 degrees C — cable fire with bead formation
Total damage: EUR 127,000 (system + roof membrane + business interruption)
What was the starting point of the investigation?
On March 12, 2025, the Frankfurt-East fire department was called to a fire on the flat roof of a commercial building (logistics company). The fire could be traced to a section of the PV system. The building insurer immediately commissioned a forensic expert assessment for fire cause investigation.
Situation found during initial inspection (March 14, 2025)
During the initial inspection three days after the fire event, the following was observed:
The copper melt beads found at the damage point (diameter 1.2–2.8 mm) showed the microstructure typical of serial DC arc faults under scanning electron microscopy (SEM): uniform solidification front, fine dendritic structure, and absence of oxide layers in the interior. The melting temperature of copper (1,085 degrees C) was locally exceeded — consistent with arc temperatures of 3,000–6,000 degrees C at the core.
Resistance measurement on the damaged connector
The remaining MC4 connector on the opposing lead (negative pole, not destroyed by fire) showed a contact resistance of 2.8 ohm upon measurement. The normal value for a correctly crimped MC4 connector is <0.5 milliohm — the deviation is thus a factor of 5,600.
Forensic finding: The crimp sleeve had not been fully pressed onto the stripped conductor. An air gap of approximately 2 mm remained, in which over 5 years of operation through micro-vibrations (wind) and thermal cycling, increasing contact resistance developed. At full load (Isc approx. 9.8 A, Voc approx. 720 V DC), a serial arc fault formed that ignited the cable insulation.
What timeline led to the fire?
June 2019
Installation of the 180 kWp PV system by a subcontractor of the general contractor
2019–2023
Normal operation, no regular maintenance or thermography inspections performed
Summer 2024
Monitoring registers slight yield decline in String 7 (-3.2%) — not investigated
March 12, 2025, 1:42 PM
Fire alarm on the upper floor triggers, fire department arrives at 1:51 PM, extinguishes smoldering fire on roof surface
March 14, 2025
Initial inspection by expert assessor, securing of evidence
April 2025
Laboratory analysis of melt beads, SEM examination, preparation of assessment
May 2025
Assessment submitted to insurer, settlement initiated
What was the total economic damage?
Damage calculation (expert determination):
Item
Amount
Replacement of 23 modules + DC cabling String 7+8
EUR 34,500
Roof waterproofing membrane repair (3.4 m² + connection work)
EUR 12,800
Scaffolding + crane for roof access
EUR 8,200
Lost yield (6 weeks total system shutdown)
EUR 14,300
Inspection of all 168 connectors (thermography + resistance)
EUR 6,400
Electrical specialist + recommissioning
EUR 4,800
Assessment costs (forensics + laboratory)
EUR 22,000
Business interruption logistics operation (partial area)
EUR 24,000
Total damage
EUR 127,000
What recommendations were issued?
The assessment recommended:
Complete inspection of all remaining MC4 connectors using thermography under load and resistance measurement
Installation of an AFDD (Arc Fault Detection Device) per DIN EN 63027 in the generator junction box
Introduction of an annual maintenance routine with thermography inspection per DIN EN 62446-2
Liability claim against the installing subcontractor (installation error — faulty crimp)
Expert assessment: The fire was in all probability preventable. A single thermography inspection (cost: approx. EUR 800–1,200) would have detected the elevated contact resistance as early as 2023. The lack of maintenance of DC connector joints is a widespread but underestimated risk in the industry. Statistically, faulty MC4 connections account for approximately 35% of all PV fires in Germany.
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