[{"data":1,"prerenderedAt":207},["ShallowReactive",2],{"guide-inverter-rapid-shutdown-arc-fault-isolation-detection-en-US":3},{"guide":4,"related":189},{"title":5,"seoTitle":6,"description":7,"summary":8,"sections":9,"faq":100,"cta":116,"date":118,"updated":118,"service":119,"order":120,"sources":121,"slug":187,"locale":188},"How Solar Inverters Protect People and Property","Rapid Shutdown, Arc-Fault and Riso Explained","How rapid shutdown, DC arc-fault detection, insulation checks, residual current monitoring and anti-islanding work in a PV system, per NEC and IEC.","Solar panels produce voltage whenever light reaches them, so a PV system carries its own protective functions. Rapid shutdown (NEC 690.12) lowers roof conductor voltage for firefighters, arc-fault detection (NEC 690.11, UL 1699B, IEC 63027) stops series arcs that fuses cannot see, insulation resistance and residual current monitoring (IEC 62109-2) catch leakage to ground before and during operation, and anti-islanding (IEEE 1547-2018, UL 1741 SB) stops the inverter from energizing a dead utility line. Each addresses a different hazard, and none should be disabled or bypassed. When a fault keeps returning, the system needs inspection by a licensed electrician.",[10,45,58,64,70,76,83,88,94],{"heading":11,"paragraphs":12,"table":14},"Four protective functions and the hazards they address",[13],"Opening the main breaker de-energizes a house wired to the grid, but it does not stop a solar array. Every module in sunlight produces DC voltage, and a residential string of modules in series commonly reaches several hundred volts. The protections below exist because of that fact. They are built into inverters, module-level electronics and switches, and they are required by codes that have tightened over the past fifteen years.",{"caption":15,"head":16,"rows":21},"PV protective functions in US and international rules, as of October 2026",[17,18,19,20],"Function","Hazard addressed","US requirement","International counterpart",[22,27,32,37,40],[23,24,25,26],"Rapid shutdown","Shock to firefighters from energized roof conductors","NEC 690.12; UL 3741 for PV hazard control systems","No direct equivalent in IEC rules covered here; German VDE-AR-E 2100-712 guidance for firefighting",[28,29,30,31],"DC arc-fault detection","Fire from arcing at a broken conductor or connection","NEC 690.11; UL 1699B","IEC 63027",[33,34,35,36],"Insulation resistance check","Leakage from array conductors to ground before connection","NEC 690.41(B) ground-fault protection","IEC 62109-2",[38,39,35,36],"Residual current monitoring","Shock and fire from leakage during operation",[41,42,43,44],"Anti-islanding","Energizing a utility line that workers believe is dead","IEEE 1547-2018, tested under UL 1741 SB","National grid connection rules",{"heading":46,"paragraphs":47,"list":49,"after":55},"Rapid shutdown under NEC 690.12",[48],"Rapid shutdown reduces the voltage on roof conductors after a single action at ground level, so that firefighters working near the array are not exposed to dangerous DC. [The requirement has changed with each code cycle](https:\u002F\u002Fiaeimagazine.org\u002Felectrical-fundamentals\u002Fnec-rapid-shutdown-requirements-and-ul-3741\u002F).",[50,51,52,53,54],"2014 NEC: conductors more than 5 feet inside a building or more than 10 feet from the array had to drop to 30 volts and 240 volt-amperes within 10 seconds. There was no array boundary yet.","2017 NEC: the array boundary was defined as 1 foot from the array in all directions. Outside the boundary, or more than 3 feet inside a building, conductors must drop to 30 volts within 30 seconds; inside it, to 80 volts within 30 seconds from January 1, 2019, or the array must be a listed rapid shutdown PV array.","2020 NEC: the listed-array option became the PV hazard control system (PVHCS), evaluated under UL 3741, which was published in December 2020.","2023 NEC: inside the boundary, the options are a PVHCS listed to UL 3741 or the limit of 80 volts within 30 seconds; the third option of the 2020 edition, for arrays without exposed wiring or conductive parts, was removed, its approach largely absorbed into UL 3741. IAEI notes that UL 3741 may allow firefighter contact with as much as 160 volts or more, a level UL considered in developing the standard.","2026 NEC: the voltage and time limits are unchanged, and the initiation-device text in 690.12(C) was reorganized.",[56,57],"The initiation device is the switch that starts the shutdown. Under the [2020 NEC text](https:\u002F\u002Fwww.mikeholt.com\u002Ffiles\u002FPDF\u002F20SOLAR_690.12.pdf), placing it in the off position initiates rapid shutdown. In the 2026 edition, as revised by [Tentative Interim Amendment 26-2](https:\u002F\u002Fdocinfofiles.nfpa.org\u002Ffiles\u002FAboutTheCodes\u002F70\u002FTIA_70_26_2.pdf) (issued August 20, 2025, effective September 9, 2025), initiation devices must be readily accessible and consist of the service disconnecting means, the PV system disconnecting means or listed switches, and for one- and two-family dwellings they must be at an outdoor location. The TIA restored that outdoor-location sentence, which had been dropped in error.","Rapid shutdown lowers voltage; it does not create working space on the roof. Fire code setbacks and pathways are a separate requirement, and our guide to [rooftop solar setbacks and rapid shutdown in New York](\u002Fguides\u002Fsolar-fire-setbacks-rapid-shutdown-nyc) covers the FDNY rules, the 2025 NYC Electrical Code and how the city coordinates the initiation device location with the fire department.",{"heading":59,"paragraphs":60},"How module-level shutdown works",[61,62,63],"Meeting the 80-volt limit inside the array boundary means acting at or near each module, because a string of modules is far above 80 volts. Three approaches are common: microinverters, DC optimizers that reduce each module to a low voltage, and shutdown devices that disconnect modules. A UL 3741 listed PV hazard control system can reduce the number of module-level devices required, according to its listing.","Many shutdown devices use the [SunSpec Communication Signal for Rapid Shutdown](https:\u002F\u002Fsunspec.org\u002Fwp-content\u002Fuploads\u002F2025\u002F01\u002FSunSpec-Specification-Communication-Signal-For-Rapid-Shutdown-APPROVED-20210420.pdf), an open specification first published in 2017 and revised in 2021. A transmitter, usually in the inverter, sends a permission-to-operate signal over the DC conductors themselves, a form of [power-line communication](https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FPower-line_communication). A receiver at each module allows the module to produce power only while that signal is present and initiates shutdown when the signal is absent. The design is fail-safe: operating the initiation device or a loss of utility power ends the signal, and the modules drop out. Proprietary signaling schemes from individual manufacturers work on the same principle.","[Microinverters](https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FSolar_micro-inverter) meet rapid shutdown by design. Each converts its module's output to AC at the module, so DC exists only on the short lead between module and microinverter, well under 80 volts for typical modules. When grid voltage disappears, a UL 1741 certified microinverter ceases to energize its output under its anti-islanding function, and the roof conductors are left without power. That is why microinverter arrays are common on New York roofs, and why owners who later want a battery often look at [AC coupling the array to a hybrid inverter](\u002Fguides\u002Fac-coupling-microinverters-hybrid-inverter), where the hybrid inverter forms a grid for the microinverters during an outage.",{"heading":28,"paragraphs":65},[66,67,68,69],"An arc fault is an electrical discharge across a gap, hot enough to ignite nearby material. In PV systems the concern is the series arc: a break in a conductor or connection, such as a loose or poorly crimped connector or a cracked solder joint in a module, across which current keeps flowing as a plasma arc. As [SMA's white paper on PV arc-fault protection](https:\u002F\u002Fwww.sma-america.com\u002Ffileadmin\u002Fcontent\u002Fwww.sma-america.com\u002Fdocs\u002FAFCI_White_Paper_130811_2_.pdf) explains, a series arc sits in the normal current path and can carry no more current than normal operation, so fuses and breakers do not see it. A parallel arc, between conductors or from a conductor to ground through failed insulation, is different: opening the circuit does not extinguish it and can make it worse.","The 2011 NEC introduced section 690.11, requiring PV systems on or penetrating a building with a maximum system voltage of 80 volts or more to detect and interrupt series arc faults, disable the system, show a visual indication and require a manual restart. The certification standard, UL 1699B, at the time an outline of investigation, required detection of an arc of 300 watts or more and interruption within two seconds. Current editions apply the requirement to PV DC circuits operating at 80 volts or more between any two conductors, with exceptions for some circuits not on buildings; confirm the wording of the edition your jurisdiction has adopted.","Detection works by listening to the current. An arc injects broadband electrical noise into the DC circuit, and a PV [arc-fault circuit interrupter](https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FArc-fault_circuit_interrupter) filters the current signal and analyzes its spectrum for the pattern an arc produces, increasingly with trained classification models. The difficulty is that a PV system is electrically noisy: the inverter's own switching, other power electronics and power-line signals such as the rapid shutdown keep-alive all occupy parts of the same spectrum. Long cable runs, many strings and added devices raise the noise floor. Detectors are tuned to reject this noise, and when the tuning meets an unusual installation, the result can be a nuisance trip.","Internationally, [IEC 63027:2023](https:\u002F\u002Fwebstore.iec.ch\u002Fpublication\u002F27362) defines requirements and tests for DC arc-fault protection devices in PV systems up to 1,500 V DC. It addresses series arcs and does not cover parallel arcs. Germany adopted it as DIN EN IEC 63027 (VDE 0126-27) in 2024. Whether arc-fault detection is mandatory varies by country, and we do not state a European requirement here.",{"heading":71,"paragraphs":72},"What happens after an arc-fault trip",[73,74,75],"The manual restart in the NEC is deliberate. An arc points to a damaged conductor or connection, and an automatic restart would re-energize the same defect. SMA's US documentation shows the typical sequence: the inverter detects an arc, interrupts feed-in and shows an event code, and the [reset procedure](https:\u002F\u002Fmanuals.sma.de\u002FSBx1SPUS40\u002Fen-US\u002F832627595.html) begins with a qualified person disconnecting the inverter from all voltage sources and checking the modules, the DC cables and the DC terminal block for defects, repairing or replacing what is damaged, and only then recommissioning the inverter.","Outside the US, some inverters offer a mode without operation inhibition. SMA documents that its [ArcFix function](https:\u002F\u002Fmanuals.sma.de\u002FSTPxx-80\u002Fen-US\u002F11618874123.html) complies with IEC 63027 and that, depending on the country data set, arc-fault detection is activated or deactivated by default. Such settings belong to the installer under the rules of the country and the product's listing; in the US, the listing and NEC 690.11 call for a manual restart.","A repeated trip is information, not an inconvenience. It can come from a real intermittent connection that only arcs in heat or wind, or from noise that the detector misreads. Telling the two apart means testing the array, which is work for a licensed electrician.",{"heading":77,"paragraphs":78},"Insulation resistance and residual current monitoring",[79,80,81,82],"Many residential string and hybrid inverters are transformerless. Without a transformer there is no galvanic isolation between the array and the grid, so the inverter itself must watch for leakage from the array conductors to ground. It does this in two stages, similar in purpose to an [insulation monitoring device](https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FInsulation_monitoring_device) in other electrical systems.","Before connecting to the grid, typically each morning, the inverter measures the insulation resistance (Riso) between the array conductors and ground. IEC 62109-2, the inverter safety standard, sets the minimum threshold at the maximum array voltage divided by 30 mA; for a 1,000-volt array that is about 33 kΩ. Below the threshold, the inverter raises an alarm and does not connect. Manufacturers usually set their thresholds well above that minimum and retest several times before reporting a fault.","During operation, a residual current monitoring unit (RCMU) compares the currents flowing out and back, in the way a [residual-current device](https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FResidual-current_device) does. Under IEC 62109-2, for transformerless inverters up to 30 kVA, the inverter must disconnect within 0.3 seconds if continuous residual current exceeds 300 mA, and faster for sudden changes: 30 mA within 300 ms, 60 mA within 150 ms and 150 mA within 40 ms. [SMA describes its RCMU](https:\u002F\u002Fmanuals.sma.de\u002FSBSExx-50\u002Fen-US\u002F13316496907.html) as all-pole sensitive, monitoring AC and DC residual currents and disconnecting redundantly on jumps above 30 mA. The continuous limit is higher than the step limit because a large array has normal capacitive leakage to ground through the module surfaces; a sudden change is what signals a person or a fault.","In the NEC, section 690.41(B) requires ground-fault protection for PV system DC circuits above 30 volts or 8 amperes, which detects the fault, interrupts or isolates it and indicates it; verify the wording against your adopted edition. The same inverter insulation check also protects battery circuits on some systems, a point that matters for the [second-life EV battery projects](\u002Fguides\u002Fsecond-life-ev-battery-home-storage) that rely on it.",{"heading":84,"paragraphs":85},"Why wet mornings cause isolation faults",[86,87],"An array is many modules, connectors, junction boxes and conduit runs in parallel to ground, and every one of them leaks a little. Dew, rain and humidity lower the resistance of each leakage path, and because the paths are in parallel, the total insulation resistance of the array falls further than any single module would suggest. The morning Riso test runs at exactly the hour when the array is wettest.","Manufacturers describe the same pattern. Tigo advises that [if the error always happens in the morning](https:\u002F\u002Fsupport.tigoenergy.com\u002Fhc\u002Fen-us\u002Farticles\u002F4410273802387) and clears by the afternoon, moisture is most likely the cause, and suggests looking for discoloration or corrosion. Solis attributes most [PV isolation faults](https:\u002F\u002Fsolisinverters.com\u002Fglobal\u002Fdocumentation\u002F38816.html) to environmental and system factors, especially in rainy weather. A fault that clears on its own as the array dries usually points to moisture somewhere in the system; a fault that persists in dry weather, or keeps getting worse, points to damaged insulation that needs to be found. Either way, the remedy is to find the leakage path, not to lower the threshold.",{"heading":89,"paragraphs":90},"Anti-islanding and grid support",[91,92,93],"[Islanding](https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FIslanding) is the condition in which a generator keeps energizing part of the grid after the utility supply has failed. A utility crew could be working on a line it believes is dead. IEEE 1547-2018 requires a distributed energy resource to detect an unintentional island and cease to energize within 2 seconds, and UL 1741 Supplement SB tests inverters against it under IEEE 1547.1-2020. The same standard requires grid-support functions such as frequency-watt response, which reduces output as frequency rises and may not be disabled.","In New York, Con Edison has required UL 1741 SB equipment with utility default settings for applications since April 1, 2023 for systems above 50 kW and June 1, 2023 for systems of 50 kW or less; National Grid since January 1, 2023.","A hybrid inverter that provides backup power meets the same rule differently: when the grid fails, it opens its transfer relay to separate the house from the utility, and then forms its own small grid for the home. Our [series article on hybrid inverters and EV batteries](\u002Fguides\u002Fev-battery-home-solar-storage-hybrid-inverter) explains that arrangement, and the inverter's event log, which some equipment exposes through [local monitoring interfaces](\u002Fguides\u002Fopen-solar-monitoring-rs485-modbus), records every grid disconnection and protective trip.",{"heading":95,"paragraphs":96},"European firefighter practice, and when to call an electrician",[97,98,99],"European practice developed differently from NEC 690.12. In Germany, the application rule [VDE-AR-E 2100-712](https:\u002F\u002Fvde-verlag.de\u002Fstandards\u002F0100500\u002Fvde-ar-e-2100-712-anwendungsregel-2018-12.html) (December 2018 edition) sets out measures for the DC side of a PV installation to maintain safety during firefighting and technical assistance. One manufacturer summarizes its aim as keeping first responders from exposure to DC cables above 120 volts once AC is switched off, and equipment makers sell DC disconnect combiners, often called fireman switches, for that purpose. It is an application rule that many installers and fire services follow; we do not describe it as a legal mandate.","For owners, the practical point is simpler. An arc-fault, isolation or residual current alarm means the system has found a condition it is designed to stop for. Do not reset it repeatedly, and do not open combiner boxes, disconnect connectors or change protection settings. Diagnosing and repairing these faults on a live PV system involves high-voltage DC that cannot be switched off while the sun shines, and it is work for a licensed electrician; in New York City, PV work is performed by licensed master or special electricians. We arrange that inspection with our licensed partner master electricians, review whether an older system predates rapid shutdown or arc-fault requirements, and quote any upgrades.","This guide explains how these protections work. It does not describe how to disable or bypass any of them, and none of them should be disabled or bypassed.",[101,104,107,110,113],{"q":102,"a":103},"Why does my inverter report an isolation fault on damp mornings and then work normally?","The morning insulation test runs when the array is wettest. Moisture lowers the leakage resistance of many connectors and modules at once, and the reading recovers as they dry. Manufacturers such as Tigo and Solis describe this pattern. If the fault persists in dry weather or becomes more frequent, have a licensed electrician find the leakage path.",{"q":105,"a":106},"Why must an arc-fault trip be reset manually in the US?","NEC 690.11 requires it because an arc usually means a damaged conductor or connection. An automatic restart would re-energize the defect. Manufacturers' procedures require a qualified person to de-energize the system and inspect modules, cables and terminals before the reset.",{"q":108,"a":109},"Do microinverters meet rapid shutdown without extra equipment?","In typical installations, yes. DC exists only between each module and its microinverter, and the microinverters stop energizing their AC output when grid voltage is lost. A compliant initiation device and correct labeling are still required, and the installer should confirm the listing for the specific equipment.",{"q":111,"a":112},"What is the difference between the insulation test and residual current monitoring?","The insulation test measures resistance from the array conductors to ground before the inverter connects. Residual current monitoring runs during operation and disconnects the inverter if leakage current rises above set limits or changes suddenly. IEC 62109-2 sets minimum requirements for both.",{"q":114,"a":115},"Is rapid shutdown required in Europe?","Europe has no requirement we can confirm that matches NEC 690.12. Germany has the VDE-AR-E 2100-712 application rule for DC safety during firefighting, which many installers and fire services follow. National rules vary, so check them for the specific country.",{"text":117},"If your system predates rapid shutdown or arc-fault protection, or keeps reporting isolation or arc faults, we arrange an inspection with our licensed partner master electricians and provide a written estimate for any repairs or upgrades. Request [a solar system review](\u002Fservices\u002Fsolar) with photographs of the inverter label and the event codes shown in your monitoring app.","2026-10-25","solar",134,[122,126,130,134,138,142,146,150,153,156,160,164,168,172,175,178,181,184],{"title":123,"url":124,"publisher":125},"NEC Rapid Shutdown Requirements and UL 3741","https:\u002F\u002Fiaeimagazine.org\u002Felectrical-fundamentals\u002Fnec-rapid-shutdown-requirements-and-ul-3741\u002F","IAEI Magazine",{"title":127,"url":128,"publisher":129},"Tentative Interim Amendment NFPA 70, 2026 Edition, 690.12(C), TIA 26-2","https:\u002F\u002Fdocinfofiles.nfpa.org\u002Ffiles\u002FAboutTheCodes\u002F70\u002FTIA_70_26_2.pdf","National Fire Protection Association",{"title":131,"url":132,"publisher":133},"690.12 Rapid Shutdown of PV Systems on Buildings (2020 NEC)","https:\u002F\u002Fwww.mikeholt.com\u002Ffiles\u002FPDF\u002F20SOLAR_690.12.pdf","Mike Holt Enterprises",{"title":135,"url":136,"publisher":137},"SunSpec Communication Signal for Rapid Shutdown, Functional Specification","https:\u002F\u002Fsunspec.org\u002Fwp-content\u002Fuploads\u002F2025\u002F01\u002FSunSpec-Specification-Communication-Signal-For-Rapid-Shutdown-APPROVED-20210420.pdf","SunSpec Alliance",{"title":139,"url":140,"publisher":141},"PV Arc-Fault Circuit Interrupter: Direct Current Arc-Fault Detection in PV Systems","https:\u002F\u002Fwww.sma-america.com\u002Ffileadmin\u002Fcontent\u002Fwww.sma-america.com\u002Fdocs\u002FAFCI_White_Paper_130811_2_.pdf","SMA America",{"title":143,"url":144,"publisher":145},"IEC 63027:2023 Photovoltaic power systems, DC arc detection and interruption","https:\u002F\u002Fwebstore.iec.ch\u002Fpublication\u002F27362","International Electrotechnical Commission",{"title":147,"url":148,"publisher":149},"Resetting the Operation Inhibition after Detection of an Arc Fault","https:\u002F\u002Fmanuals.sma.de\u002FSBx1SPUS40\u002Fen-US\u002F832627595.html","SMA Solar Technology",{"title":151,"url":152,"publisher":149},"SMA ArcFix","https:\u002F\u002Fmanuals.sma.de\u002FSTPxx-80\u002Fen-US\u002F11618874123.html",{"title":154,"url":155,"publisher":149},"Residual-current monitoring unit (RCMU)","https:\u002F\u002Fmanuals.sma.de\u002FSBSExx-50\u002Fen-US\u002F13316496907.html",{"title":157,"url":158,"publisher":159},"EI Inverter Error Code 201 - Residual Current High Troubleshooting Guide","https:\u002F\u002Fsupport.tigoenergy.com\u002Fhc\u002Fen-us\u002Farticles\u002F4410273802387","Tigo Energy",{"title":161,"url":162,"publisher":163},"PV Isolation Protection","https:\u002F\u002Fsolisinverters.com\u002Fglobal\u002Fdocumentation\u002F38816.html","Solis",{"title":165,"url":166,"publisher":167},"VDE-AR-E 2100-712 Anwendungsregel:2018-12","https:\u002F\u002Fvde-verlag.de\u002Fstandards\u002F0100500\u002Fvde-ar-e-2100-712-anwendungsregel-2018-12.html","VDE VERLAG",{"title":169,"url":170,"publisher":171},"Power-line communication","https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FPower-line_communication","Wikipedia",{"title":173,"url":174,"publisher":171},"Solar micro-inverter","https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FSolar_micro-inverter",{"title":176,"url":177,"publisher":171},"Arc-fault circuit interrupter","https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FArc-fault_circuit_interrupter",{"title":179,"url":180,"publisher":171},"Insulation monitoring device","https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FInsulation_monitoring_device",{"title":182,"url":183,"publisher":171},"Residual-current device","https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FResidual-current_device",{"title":185,"url":186,"publisher":171},"Islanding","https:\u002F\u002Fen.wikipedia.org\u002Fwiki\u002FIslanding","inverter-rapid-shutdown-arc-fault-isolation-detection","en-US",[190,195,199,203],{"slug":191,"title":192,"description":193,"service":119,"date":194,"updated":118},"solar-for-homes","What to evaluate before installing solar on an NYC or Long Island home","Roof condition, shade, fire code setbacks, permits, utility interconnection and incentives: what decides whether solar makes sense for your property.","2026-10-24",{"slug":196,"title":197,"description":198,"service":119,"date":118,"updated":118},"long-island-solar-permits-by-town","Long Island Solar Permits, Town by Town","Long Island solar permits: who issues them, what the unified permit requires, how Islip, Laurel Hollow and East Hampton differ, and the PSEG steps.",{"slug":200,"title":201,"description":202,"service":119,"date":118,"updated":118},"ny-solar-heat-pump-ev-tax-credits-2026","New York Solar, Heat Pump and EV Charger Tax Credits in 2026","Which federal home energy credits ended in 2025 and 2026, and which New York State and NYC solar and storage tax benefits remain as of October 2026.",{"slug":204,"title":205,"description":206,"service":119,"date":118,"updated":118},"solar-fire-setbacks-rapid-shutdown-nyc","Rooftop Solar Setbacks and Rapid Shutdown in New York","FDNY clear paths, ridge setbacks in NYC and the rest of New York, rapid shutdown under the 2025 codes, and microinverters vs string inverters.",1792980726305]