Re-Commissioning After Hail Damage and Inverter Replacement
Following a hailstorm, a 740 kWDC commercial rooftop array lost 38 modules on two strings, and one 60 kW inverter was replaced under warranty. The EPC's crew installed same-model replacement modules and re-terminated 14 MC4 pairs, then re-landed the DC feeders at the new inverter and updated the string plan. The system is now mechanically complete, and the owner wants it re-energized before the weekend. Your firm is asked to perform a 'quick commissioning check' consisting of a visual inspection and an insulation resistance test on only the two repaired strings. As-builts show 11 strings per inverter, 16 modules per string, 550 W modules, and a 1500 V DC system with module-level rapid shutdown. Overnight rain left about 20 cm of standing water near two roof drains, and the forecast is dry for the next three days.
1. What is wrong with the proposed scope, and what should a proper re-commissioning include?
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The scope is too narrow. Visual inspection plus insulation resistance on two strings does not cover the new inverter, the 14 re-terminated connector pairs, the 38 replacement modules, or hail damage elsewhere on the array.
A proper re-commissioning should follow IEC 62446-1: verify protective earthing and bonding continuity, string polarity, open-circuit voltage, and short-circuit or operating current for every string; insulation resistance for every string, or at minimum all repaired strings plus a documented representative sample; inverter functional and protection settings; monitoring communications; and rapid shutdown. Confirm the replacement modules match the original model's electrical class for series-string current, check string fuse ratings, and verify that the new inverter's DC/AC ratings and firmware are correct.
Hail can also cause microcracks that are invisible from the roof edge, so include a thermal scan of the affected areas.
2. How should the standing water affect the work plan?
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Standing water near drains is both an electrical and a slip hazard. De-energize and remove or isolate the water before working, inspect conduit and junction boxes for water ingress, and consider roof loading. Do not energize wet equipment to 'test' it.
Use the dry three-day forecast to sequence the work: water removal and moisture inspection first, then DC tests, then inverter start-up and rapid shutdown verification. Document the conditions and any water found in enclosures.
3. What test voltages and acceptance criteria apply, and how should rapid shutdown be verified?
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For a 1500 V DC system, insulation resistance test voltage is commonly 1000 V DC per IEC 62446-1, but never exceed module or equipment ratings, and follow manufacturer instructions. Minimum acceptance is typically 1 MΩ, although the contract may specify a higher value; record actual readings and compare with sibling strings.
For rapid shutdown, test at the array boundary using the initiator: measure voltage and time to confirm compliance with the NEC edition adopted by the AHJ and the manufacturer's listing - commonly 30 V or less within 30 s under NEC 690.12, or the applicable UL 3741 hazard-control threshold if that listing is used. Document results, issue a punch list, and schedule the AHJ and utility witness tests only after repairs and retests pass.