Critical What does this mean?

A safety-rated fault — there may be a risk of electric shock, fire, or equipment damage. Do not try to fix this yourself. Call a qualified electrician.

Fronius Inverter Code 240 — What an Arc Detection Warning Means

Code 240 means your Fronius inverter's arc detector found an electric arc in the DC wiring — a serious fire risk. The code shows for about 4 seconds, then 241 appears and the inverter disconnects from the grid for safety. Do not simply reset it: Fronius requires the entire affected PV system to be checked for damage before the inverter is reset. This one belongs to your installer, full stop.

What should I do first?

Two quick questions — we'll point you to the right starting point.

Is this code still showing on your inverter's display?

What this code means

An electric arc is what happens when current jumps across a gap in a damaged conductor — a cracked connector, a loose terminal, a wire chewed through or pinched during installation. The jump point can reach temperatures above 10,000 K, hot enough to melt metal and ignite roofing material. It's one of the leading causes of solar array fires, which is why inverters must watch for it.

Your Fronius does that watching with an arc detector that continuously analyzes the DC current for the distinctive electrical "noise" of arcing. Code 240 is the detector raising its hand: "Arc detected." It stays on the display for about 4 seconds — long enough to register, short enough that what follows matters more.

What follows is 241, displayed directly after 240, and 241 is the lockout: "the inverter disconnects from the grid for safety reasons." The inverter will not restart itself. An arc means something in your high-voltage DC wiring is physically damaged, and re-energizing it without finding the damage would invite the fire the detector just prevented.

The reset exists — you press the Enter key to clear 241, and then 242 appears as the inverter resumes grid feed-in — but Fronius puts a condition on it that changes everything: "If an arc has been detected, check the entire affected PV system for damage before resetting the inverter." The reset is the last step of a professional inspection, not the first step of getting your production back.

Two related codes complete the picture: 245 means the arc detector's self-test failed (the watchdog itself is broken — the inverter disconnects), and 247 means the arc detector's current sensor is defective (same response). If either persists, the manual sends you to a Fronius-trained service technician.

What is this downtime costing you?

Rough estimate while your system isn't producing. Adjust the numbers to match your setup.

System size (kW) Peak sun hours / day Electricity rate ($/kWh)

–per day

–per week

–per 30 days

≈ – kWh of lost production per day.

Estimate only — assumes ~80% real-world output vs. nameplate. Actual losses depend on weather, shading, panel direction, and your tariff.

Common causes

Damaged, pinched, or abraded DC conductors. Cables crushed under mounting rails, chafed against sharp roof edges, or nicked during installation eventually break down enough to arc.
Loose or separated connections. MC4 connectors that were never fully seated, or that worked loose over years of thermal cycling, are the classic arc source — the gap is tiny, but at hundreds of volts DC it's enough.
Cracked or corroded solder joints inside modules, and aging junction boxes. These develop silently over years.
Rodent and weather damage. Chewed insulation, hail-cracked backsheets, and UV-degraded cable jackets all expose conductors.
Installation errors that took years to surface. A connector from a mismatched brand pair, or a cable tie pulled too tight, can hold for years before the contact degrades into an arc.

Safe steps to try

Work top to bottom, ticking off each step as you complete it. 0 of 5 done.

1. Treat 240/241 as a fire-safety event, not a production nuisance. The inverter has already done the most important thing: it disconnected itself. Leave it in that safe state.
2. Do not press Enter to reset the code yourself. Fronius's own remedy requires the whole affected PV system to be checked for damage first — that means a visual and electrical inspection of modules, connectors, and DC cabling by someone qualified to work on live DC circuits.
3. Note the time the code appeared and take a photo of the display. If your monitoring shows which string or input was involved, record that too — it tells the technician where to start looking.
4. Don't cover the panels or poke around the array yourself. Damaged DC wiring can remain energized whenever there's light, and arc-damaged conductors are unpredictable.
5. If you smell burning or see any scorching, discoloration, or melted plastic around the inverter or array, keep clear and call emergency services, then your installer.

Is your inverter still under warranty?

Before paying for a repair, check — your installer may fix this free.

Brand Year installed

Based on typical standard warranty terms — always confirm with your warranty card or installer paperwork, as terms vary by model, region, and purchase date.

When to call an electrician

Every 240 requires a professional inspection before any reset — that is the manual's explicit condition, and there is no homeowner version of 'checking the entire affected PV system for damage.' Call your solar installer immediately; if 240/241 recurs after a reset, the fault wasn't found and the system must stay off until it is. If 245 (self-test failed) or 247 (sensor defective) persists, Fronius directs you to a Fronius-trained service technician rather than a general installer. Repeated arc detections across weeks mean the array needs a thorough rewire-level inspection, not another reset.

Frequently asked questions

What does an electric arc actually look like in a solar system?

Usually you never see it — it happens inside a connector or under a module. What you'd find afterward is the evidence: melted connector housings, blackened terminals, or pitted metal. That's exactly what the inspection is looking for.

Can I just press Enter to clear 241 and see if it comes back?

That's precisely what Fronius warns against. The manual says to check the entire affected PV system for damage before resetting. Resetting blind re-energizes a damaged high-voltage circuit.

What's the difference between 240, 241, and 242?

240 is the detection announcement (shown ~4 seconds). 241 is the safety lockout — the inverter disconnected from the grid and waits for inspection and a manual reset. 242 appears after 241 is reset, as the inverter resumes feeding in.

My system is only a few years old. Can arcs really happen already?

Yes. Most arc faults trace back to installation — a connector that never fully seated or a cable pinched on day one can take years of thermal cycling to degrade into an arc.

Does the arc detector ever false-trigger?

It's possible but uncommon, and you can't tell a false trigger from a real one from the display. Treat every 240 as real until a professional inspection says otherwise.

Need a professional?

This fault is a safety risk — do not attempt to fix it yourself. Contact a qualified solar electrician in your area.

Sources: meaning and troubleshooting verified against the manufacturer's documentation — Fronius Primo 208-240 Operating Instructions, doc 42,0410,2116 (official Fronius EN-US manual) — Status codes (Code 240: 'Arc detected', shown ~4s; 241 follows with inverter disconnecting from grid for safety, reset via Enter; 242 resumes feed-in after 241 reset; 245/247: arc detector self-test/sensor faults)