Fronius Isolation Fault — State 475 & 502 Earth Fault Diagnosis
Your Fronius inverter has shut down because it detected a drop in DC insulation resistance — a leakage path to earth somewhere in the solar array wiring. This is the most common Fronius fault in the UK and is almost always caused by water ingress into connectors or the rooftop DC isolator. Do not repeatedly restart.
If the fault clears in dry weather and returns when it rains, the water entry point needs to be found. We identify the affected string remotely from Solar.web data before an engineer attends site.
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Safety warning. An isolation fault means there is a conductive path between the DC wiring and earth. Do not touch DC cables, MC4 connectors, or the inside of the inverter. DC voltage from the solar panels is present whenever there is daylight and cannot be switched off. Isolate the system at the consumer unit and do not restart until the fault has been investigated by a qualified engineer.
6-step isolation fault diagnosis
Most Fronius isolation faults are caused by water ingress. The diagnostic path below identifies whether that is the case and what needs to happen next.
Identify your isolation fault state code
Fronius uses two main state codes for isolation faults. State 475 (DC insulation fault) means the inverter has detected low insulation resistance on the DC side — a leakage current path to earth exists somewhere in the array wiring. State 502 (isolation fault DC+ or DC- to earth) is a more specific detection identifying a direct fault between a DC conductor and earth.
Both codes trigger an immediate safety shutdown. Check the Solar.web event log or the inverter display for the exact code and timestamp. If the fault appeared during or shortly after rain, moisture ingress is the most likely cause.
Do not repeatedly restart the inverter
An isolation fault is a genuine safety event — the electrical insulation between the DC wiring and earth has dropped below the safe threshold. Repeatedly restarting the inverter does not fix the fault and may make the underlying damage worse by driving current through a compromised connection. If the fault clears temporarily in dry weather and returns when it rains, this confirms moisture ingress. The fault path is still present and will deteriorate over time. Leave the system off and arrange an inspection.
Check the weather pattern against the fault log
Open Solar.web and compare the fault timestamps against recent weather. Isolation faults that appear during rain, heavy dew, or high humidity and clear when conditions dry out are almost always caused by water entering a connector, junction box, or DC isolator on the roof. This is the single most common cause of Fronius isolation faults in the UK. The corrosion at the water entry point is progressive — even if the system runs in dry weather, the underlying damage is getting worse.
Understand the common fault locations
In order of frequency, the most common fault locations on UK Fronius installations are:
Corroded connectors also introduce electrical resistance, causing them to heat up under load. This is a fire risk as well as an insulation fault.
Isolate the system safely
Turn off the AC isolator at the consumer unit first, then the DC isolator below the inverter. If you can safely access the rooftop DC isolator, turn that off too. Do not open the inverter enclosure, disconnect DC cables, or touch MC4 connectors while there is daylight — DC voltage from the panels cannot be switched off.
A visual inspection of accessible cable runs from ground level may reveal obvious damage — melted plastic, scorch marks, or water dripping from a connector — but the definitive test requires a qualified engineer with an insulation resistance tester on the roof.
Book a professional insulation resistance test
The fault location is identified using an insulation resistance (Riso) test on each PV string with the inverter disconnected. A healthy string reads above 1 megohm; a faulted string reads significantly lower. The engineer then isolates individual sections to narrow down whether the fault is in the cabling, a specific connector, the DC isolator, or a panel junction box.
STS can review your Solar.web data remotely to identify which string is affected and when the fault pattern started before an engineer attends site — reducing diagnosis time and cost.
Why isolation faults are the most common Fronius problem in the UK
Fronius Primo and Symo inverters have been installed in the UK since 2012, making many systems 10 years old or more. Over that time, the DC wiring on the roof is exposed to continuous UV radiation, temperature cycling, wind-driven rain, and in some cases rodent or bird damage. The weak points are always the connections — MC4 connectors, the rooftop DC isolator, and panel junction boxes.
The UK climate is particularly hard on these components. Persistent damp, frequent temperature swings around freezing, and salt air in coastal areas all accelerate seal degradation and corrosion. A connector that was perfectly sealed at installation can develop a moisture path after 5 to 8 years of weathering. Once water enters, it corrodes the contact surfaces, reducing insulation resistance and introducing electrical resistance that causes heating under load.
This is why Fronius isolation faults typically appear as an intermittent pattern that worsens over time — short faults during rain that eventually become permanent as corrosion progresses. Early diagnosis when the fault is still intermittent is significantly cheaper than waiting until a connector has failed completely or caused secondary damage to the inverter.
Isolation fault — common questions
The most common cause is water ingress into MC4 connectors or the rooftop DC isolator. State 475 and 502 both indicate the insulation resistance between DC wiring and earth has dropped below the safe threshold. Other causes include UV-degraded cable insulation, rodent damage, cracked panel junction boxes, and corroded connections. The fault typically appears during rain and may clear in dry weather — but the underlying damage is still present and worsening.
Water provides a conductive path between a DC conductor and earth. When the moisture evaporates, the resistance rises again and the inverter restarts. This pattern confirms moisture ingress — the entry point needs to be found and repaired. Leaving it causes progressive corrosion at the fault location, increasing the risk of a DC arc fault or fire. The connectors, DC isolator, and junction boxes should be inspected.
You can visually inspect accessible cable runs from ground level for obvious damage, melted connectors, or scorch marks. However, the definitive diagnosis requires an insulation resistance test on each PV string with the inverter disconnected — this must be done on the roof with DC-rated tools by a qualified solar engineer. Do not open the inverter, disconnect DC cables, or touch MC4 connectors while there is daylight.
State 475 is a general DC insulation fault — the inverter has detected low insulation resistance during normal operation. State 502 is more specific, indicating a direct fault path between DC positive or DC negative and earth. Both trigger an immediate safety shutdown. The diagnosis process is the same: identify the affected string, isolate the system, and perform an insulation resistance test to locate the fault.
A remote diagnostic session to identify the affected string and likely fault location starts from £75. If the fault is a single degraded MC4 connector or a failed DC isolator, the on-site repair is typically straightforward. If the fault is in the cable insulation or a panel junction box, more extensive work may be needed. STS reviews your Solar.web data first to narrow down the fault before an engineer visits — this avoids paying for exploratory time on the roof.
Isolation fault? Let us find the cause.
We review your Solar.web fault history and string data to identify the affected circuit and likely fault location — before anyone goes on the roof.