Solar panels are one of the safest technologies ever installed on UK homes. But DC arc faults from small installation errors are among the leading causes of rooftop fires — and most homeowners have never heard of them. This is the complete guide: the science, the UK context, and the products that close the gap.
AIZO Quality Heating · February 2026 · 12 min read
- Why solar panels can cause fires — the science
- The UK situation: what the data shows
- Viridian ArcBox — the connector enclosure that stops fires before they start
- Our full fire safety product range
- Microinverters and the safety architecture advantage
- Enphase microinverters — safety by design
- Frequently asked questions
The UK now has over 1.5 million solar installations and the number grows every year. The vast majority will never cause a problem. Solar PV is, in absolute terms, a very safe technology — far less likely to cause a fire than a conventional gas boiler or poorly maintained electrical installation.
But low probability is not zero probability. As the installed base grows, the number of fires grows proportionally. Research by the Building Research Establishment (BRE) and European fire safety bodies has now clearly identified the dominant cause: DC arc faults at connector joints. A tiny installation error — a poorly crimped connector, two brands of MC4 cross-mated, a connector assembled in the rain — can create a fault that builds silently over months or years before discharging in an arc at temperatures far exceeding the melting point of copper. By then, the combustible materials around it are already alight.
The good news is that several products now exist that either prevent the arc from becoming a fire, or eliminate the high-voltage DC that makes arcs so dangerous in the first place. This guide explains both approaches — and links you directly to the products we stock.
The Science
Why Solar Panels Can Cause Fires — And Why DC Is the Culprit
All solar panels generate direct current (DC) electricity. In a conventional string inverter system, this DC travels down cables from every panel on your roof to a single inverter, typically located in your loft or garage, before being converted to the AC electricity your home uses. This means the roof is covered in high-voltage DC cabling — up to 600V in a residential system, and over 1,000V in larger commercial installations.
Here is the critical problem with DC electricity: it does not naturally extinguish. When an AC circuit develops a fault, the current naturally crosses zero 50 times per second — giving the arc a natural opportunity to self-extinguish. DC has no such zero crossing. Once a DC arc starts, it is self-sustaining, generating plasma temperatures far in excess of the melting point of metals. It will continue until either the fuel is exhausted or external intervention occurs.
And unlike an AC circuit breaker, switching off a string inverter does not de-energise the DC wiring. As long as sunlight is hitting the panels, the DC cables between the array and inverter remain live. There is no way to turn them off.
Most arc faults originate at the DC connectors — the MC4 plug-and-socket joints used to link panels into strings. These connections are made by hand, often on rooftops, often in bad weather, and the error rate is non-trivial. Common causes of arc fault conditions include:
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Poorly crimped joints
The inner conductor must be correctly crimped inside the pin before the pin is inserted into the connector body. An under-crimped joint increases resistance over time, generating heat, degrading the contact, and eventually creating a gap across which an arc can jump.
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Cross-mating connectors from different manufacturers
MC4 is a generic connector standard, but tolerances vary between brands. Mixing Stäubli MC4 with a third-party equivalent may appear to latch correctly but leave micro-gaps in the contact interface that degrade under thermal cycling and vibration.
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Assembly in wet conditions
Water trapped inside the connector during assembly leads to galvanic corrosion of the contact surfaces. Over months, this creates increased resistance, heat, and eventually an arc pathway as the contact interface degrades.
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Incomplete insertion
MC4 connectors must be fully inserted until the locking mechanism audibly clicks into place. Partial insertion creates a high-resistance contact. On UK rooftops, where installers often work in awkward positions, this is a surprisingly common oversight.
The UK Context
The UK Situation: What the Data Shows
The Building Research Establishment (BRE) studied UK solar PV fires between 2015 and 2018 and identified connectors and cabling as the second most common cause of solar-related fires, behind inverter faults. A parallel study by Dutch research body TNO in 2020 found that in the Netherlands, poorly installed DC connectors were the number one cause of solar fires, responsible for more than 80% of incidents studied.
The Netherlands data is particularly instructive for the UK market because the installation ecosystem is comparable: large numbers of installers making many connections under time pressure, often outdoors, in a wide range of weather conditions. The Dutch findings prompted a wave of safety innovation — and the ArcBox product that emerged from that work is now available in the UK.
Solar fires present unique challenges for fire services. In a string inverter system, the DC wiring from roof to inverter cannot be de-energised while the sun is shining — even if the inverter is switched off. Firefighters tackling a roof fire may be cutting through live 600V DC cabling. DC electricity does not let go: unlike AC, where muscle contraction alternates, DC causes sustained tetanic grip. This is why the solar industry's shift toward safer architectures — and products that contain arc faults before they spread — matters not just for property protection, but for the safety of emergency responders.
The UK's regulatory response has been slower than that of some European neighbours, but the direction of travel is clear. More stringent installer training requirements, increased focus on MCS compliance, and growing insurer scrutiny of solar installations all point toward a tightening of the safety standards landscape. Products that go beyond minimum compliance — like the ArcBox — are increasingly being specified by quality-conscious installers and developers as a standard inclusion rather than an optional extra.
Product Deep-Dive
Viridian ArcBox — The Connector Enclosure That Stops Fires Before They Start
In 2020, the engineering team at Viridian Solar — a Cambridge-based UK manufacturer of roof-integrated solar PV systems — became concerned that a spate of rooftop fires in the Netherlands, linked to DC connector faults in plastic in-roof mounting systems, was damaging confidence in roof-integrated solar. Their response was three years of intensive research into the physics of DC arc faults and how they could be contained.
The result was the ArcBox — launched in 2022 at the Solar Solutions International show, where it immediately won an innovation award. The concept is elegantly simple: instead of trying to detect an arc fault after it has started, the ArcBox physically encloses the DC connector joint in a high-temperature material that prevents the arc from spreading to combustible materials even if it occurs.
Patented clamshell design. Snap-fit. No tools required.
The ArcBox comprises two hinged halves that snap together around an MC4 DC connector. The connector is completely enclosed within a high-temperature refractory material — the kind previously found only in crucibles and furnaces. Built-in ventilation and drainage ports keep the connector within its operating temperature limits in normal use. Cable sealing grommets at each end provide weatherproofing. In the event of an arc, the energy is safely contained within the enclosure, unable to reach the combustible roofing materials around it.
"Solar installers are making millions of connections each year, often in difficult locations or adverse weather conditions. ArcBox gives extra peace of mind to solar installers and their customers."
KT Tan, Chief Technical Officer, Viridian SolarArcBox in Action — Flat Roof & BIPV Testing
These are not simulations. The videos below show actual DC arc fault fire tests, conducted at certified fire safety laboratories. The contrast between a connector without ArcBox and one enclosed within it is stark.
▶ ArcBox fire safety testing overview — flat roof and BIPV (roof-integrated) solar installations · via ArcBox / Viridian SolarIn-Roof Solar Fire Test — Extended Coverage
This second video takes a more detailed look at the BIPV (Building Integrated Photovoltaic) fire testing, showing how ArcBox performs when installed within a roof-integrated system with combustible materials including rigid insulation board and roofing membrane immediately behind the panels.
▶ ArcBox in-roof (BIPV) solar fire test — in-depth look at two roof-integrated solar systems · via ArcBox / Viridian SolarCrucially, the testing was conducted without any special preparation or protective treatment of the surrounding roofing materials. These were real-world build-ups: rigid insulation between rafters, roofing membrane, tile battens, and roof-integrated solar systems including one with large amounts of combustible plastic sheeting below the panels. Tests without ArcBox showed rapidly developing fires consuming the roof structure. Tests with ArcBox contained the arcing entirely.
Independently tested to NEN 6063 by KIWA fire safety laboratory with over 5 minutes of continuous arcing without spread of fire to surrounding roofing materials. Confirmed by Loughborough University Department of Engineering that connector temperature under normal current-carrying load remains within manufacturer guidelines — meaning the ArcBox doesn't create any thermal issues in standard operation.
Shop
Our Full Solar Fire Safety Product Range
We stock a curated range of solar fire safety and protection products, from the ArcBox connector enclosures to DC arc fault detection devices and rapid shutdown equipment. Whether you're specifying a new installation or adding protection to an existing system, these products close the gap between a compliant system and a genuinely resilient one.
The ArcBox is available individually (one per MC4 connector joint) and in multi-pack quantities for installers equipping full string runs. Viridian also produce a range of mounting brackets that integrate ArcBox with widely-used rail systems from major manufacturers, and a batten bracket for roof-integrated installations that holds the unit in the batten space behind the panels.
Viridian recommends prioritising ArcBox on the manually crimped connectors — typically two per string (the connections made on-site by the installer, rather than the factory-made connectors in the panel leads). These are the highest-risk joints. Beyond this, applications warranting ArcBox on every connector include:
- BIPV / in-roof solar — DC cabling immediately adjacent to combustible building materials, often including wood, insulation, and membrane
- Flat roof installations above asphalt or single-ply membrane — even connectors elevated 50mm above the deck can ignite the covering in arc fault conditions
- High-consequence buildings — hospitals, schools, care homes, warehouses, and factories where any period of downtime carries significant operational or human impact
- Retrofit safety upgrades — adding ArcBox to an existing system provides meaningful additional protection, particularly for systems 5+ years old where connector degradation is more likely
System Architecture
Microinverters and the Safety Architecture Advantage
ArcBox addresses the risk of arc faults in systems that use high-voltage DC cabling — which means all conventional string inverter systems. But there is a fundamentally different approach to solar system architecture that eliminates this class of risk entirely: microinverters.
A microinverter is a small inverter mounted directly on or behind each individual solar panel. Instead of sending DC power down long runs of roof cabling to a central inverter, the microinverter converts the panel's DC output to AC at the panel itself, before any cabling leaves the roof zone. The wiring that runs from the roof into the building is standard AC cable — exactly the same type as the rest of your home's electrical installation.
- High-voltage DC wiring across entire roof (up to 600V residential / 1,000V+ commercial)
- DC cables remain live while sun shines, even with inverter switched off
- No inherent rapid shutdown — requires additional hardware to comply
- Single inverter failure can take down entire array
- Arc faults in DC cabling can be sustained indefinitely by solar generation
- Firefighters face live DC cabling during roof operations
- Maximum DC voltage ~80V — confined to within individual panel
- De-energises within 30 seconds of grid disconnection or emergency shutdown
- Rapid shutdown native to architecture — no additional hardware needed
- Panel-level independence — one fault doesn't affect rest of array
- AC wiring on roof: no arcing possible in AC circuits
- Firefighters encounter only standard AC cabling — same as rest of building
"As long as the sun is shining, the DC continues to flow, even when the inverters are switched off. If you come into contact with such a cable, this can result in a significant shock or even electrocution. With microinverter technology, it's a different story — there is no power left in the system within 30 seconds."
Martyn Berry, European Director of Technical Services, Enphase EnergyThis architectural difference has profound safety implications. In a microinverter system, the highest DC voltage anywhere on the installation is the voltage of a single panel — typically around 40–80V. This is well below the threshold at which DC electricity becomes life-threatening. In a string inverter system, the voltage of an entire string of panels is present at every connector, every junction, and every metre of DC cable on the roof.
The two approaches are complementary rather than competing. For existing string inverter systems, ArcBox provides meaningful protection. For new installations, specifying a microinverter architecture eliminates the DC high-voltage risk category entirely — making ArcBox-type protection unnecessary in the first place, because there are no high-voltage DC connectors to protect.
Microinverter Technology
Enphase Microinverters — Safety by Design, Not by Retrofit
Enphase Energy is the world's leading supplier of microinverter-based solar systems, with over 84 million microinverters shipped and active systems in more than 160 countries. Their IQ series microinverters represent the current generation of a technology platform that has been refined over 15+ years of real-world deployment.
Enphase microinverters are the most extensively safety-tested products in the solar industry. Every unit undergoes thousands of hours of internal testing before release, followed by rigorous third-party certification by organisations including Underwriters Laboratories (UL). The safety architecture built into their design — not a regulatory add-on — means they are inherently compliant with Rapid Shutdown requirements without additional hardware.
A January 2026 survey of firefighters who participated in Enphase's Solar and Fire Education (SAFE) training programme found that more than 98% would recommend microinverter-based solar systems to homeowners — specifically citing the absence of high-voltage DC roof cabling and the native rapid shutdown capability as the reasons for their preference.
Hands-on training with both string inverter and microinverter systems has led multiple fire departments to incorporate microinverter-specific safety learnings into their official operational guidelines. The all-AC roof architecture of Enphase systems creates clearer decision-making for first responders during residential incidents — and several departments are now working to add this to their Safety Policy and Guidelines manuals.
Common Questions
Frequently Asked Questions
Is my solar installation at risk?
Any solar PV installation using a string inverter — which includes the majority of UK residential and commercial systems — has high-voltage DC cabling on the roof. This does not mean it will cause a fire. The overwhelming majority of installations never have an incident. However, risk is not zero, particularly for systems where MC4 connectors were assembled manually, systems on buildings with combustible roof materials, and older systems where connector degradation may have begun. ArcBox provides a cost-effective, non-invasive way to reduce that residual risk.
Does ArcBox work with my existing system?
ArcBox is a retrofit product. The enclosure snaps around any standard MC4 DC connector without disassembling or modifying the connection itself. No tools are required. It can be added to an existing system at any time and is compatible with all major MC4 connector brands. Mounting brackets are available for the major rail systems, and a batten bracket accommodates roof-integrated installations.
Where should I prioritise ArcBox installation?
Viridian recommends using ArcBox on the manually crimped connectors first — ideally two per string. For BIPV in-roof systems, flat roof systems over combustible membranes, and any high-consequence building, every connector in the installation is worth protecting.
If I choose a microinverter system, do I still need ArcBox?
No. In a microinverter system such as Enphase IQ, there are no high-voltage DC connector joints outside of the individual panel. The DC voltage never leaves the immediate vicinity of the panel itself, and is far too low (around 40–80V) to sustain a propagating arc fault. ArcBox is not needed and not applicable.
Will my insurer require solar fire safety measures?
Insurance requirements vary by provider and policy, but there is a clear trend toward increased scrutiny of solar installations. Some insurers are now asking specific questions about solar system type and safety measures. Specifying ArcBox and/or microinverters demonstrates a proactive approach to risk management that may be viewed favourably in a claim situation.
Can these products be retro-fitted to an existing system?
Yes. ArcBox can be added to any string inverter system at any time, including systems installed years ago. Microinverters can also be used in a hybrid configuration in some cases, though for most existing systems the more practical route is ArcBox for immediate risk reduction, combined with a microinverter specification on the next installation or system upgrade.
Protect Every Connection. Specify Safe.
From ArcBox connector enclosures to Enphase microinverter systems — AIZO Quality Heating stocks the full range of solar safety products.