SMA 3501 fault codewhat it means and how to fix it
Insulation resistance too low. SMA are unusually direct: the error is not caused by the inverter, it is in the DC installation — and it can come and go with the weather.
Stop — this needs an engineer
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What 3501 means
Event 3501, which SMA also publish as event 35 and 3503 depending on where you read it, is an insulation failure. Their internal name for it is EvtLeakRis and their summary is check generator, meaning the inverter has detected a ground fault in the PV array or the insulation resistance is too low. What makes SMA's material valuable is a sentence most manufacturers would not write: the error is not caused by the inverter, it is in the DC installation. So the box in your loft is the messenger. The mechanism is a safety check run before the inverter connects to the grid.
SMA describe it as measuring the insulation resistance in the DC input circuit before grid connection, and if the values are below the specified threshold the grid connection is disabled to prevent ground faults. That is why a system with this event most often does not start, rather than stopping mid-generation — though SMA's technical information puts it both ways, the inverter disconnects from the utility grid for safety reasons and/or fails to connect during self-test mode. Their named causes are damaged PV connectors, DC cables or PV modules. Then comes the part that makes this genuinely hard to diagnose and that SMA are honest about: the value can vary depending on the weather, with humidity or moisture on the modules, and the event message can occur temporarily as it is influenced by ambient conditions such as rain or dew. They add that mechanical influences such as strong wind can cause live DC cables to come into contact with grounded system parts if the cable insulation is defective.
So an event that appears after rain, clears when things dry out, and returns the next wet week is not a glitch. It is a real defect in the insulation being exposed by moisture, and SMA say so: since insulation errors can occur sporadically due to climatic influences, it is not always easy to detect them, however a thorough system inspection remains essential. Their safety wording is the strongest in this whole category and it is worth quoting exactly. Danger to life due to electric shock. In the event of a ground fault, high voltages can be present. Do not touch the cables, the substructure, the frame or the modules of the PV array. Substructure and frame means the mounting rails and the metal edges of the panels.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Damaged PV connectors | SMA name these first. The plug connections between modules are outdoors for decades and are a common point of water ingress and degradation. | Common |
| Damaged DC cabling | Also named by SMA. Rodents, UV degradation and cable rubbing against a sharp edge or a rail all produce the same result over time. | Common |
| Moisture on the array bringing a defect to light | SMA publish that the value varies with humidity and moisture and that the event can occur temporarily due to rain or dew. The moisture reveals the defect rather than causing it. | Common |
| A damaged module | SMA list PV modules alongside connectors and cables. Physical damage, a cracked backsheet or a failed junction box will all lower insulation resistance. | Occasional |
| Wind bringing live cables into contact with grounded parts | SMA name this specifically. Where insulation is already defective, movement in strong wind can complete the path to the frame or rails. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£120–£600
Time on site
3 hours
An installer’s inspection with the right test equipment sits at the lower end. Tracing a fault across several strings, replacing connectors or a module, and roof access all push it up considerably.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related SMA fault codes
- 3701Stop — call an engineerResidual current. Unlike the insulation event this one trips a running system, and SMA publish the threshold: a sudden jump of 30 milliamps or more.Related: safety and isolation
- Inverter not connecting to the gridGet it checkedSMA disable the grid connection deliberately when a pre-start check fails. The event number in the Results menu is what tells you which check.Related: same brand
Own a SMA solar or battery system? Save it and its codes come first next time.
Related guides
Common questions about SMA 3501
It clears when the weather dries out. Can I ignore it?
No, and SMA address this directly. The value varies with humidity and the event can occur temporarily because of rain or dew, but their conclusion is the opposite of reassuring: since insulation errors can occur sporadically due to climatic influences, they are not always easy to detect, however a thorough system inspection remains essential. Moisture is revealing a defect, not creating one.
Can I check the insulation resistance myself?
SMA answer this in their own procedure and the answer is no. Their wording: the insulation resistance can only be measured with a suitable device for safe disconnection and short-circuiting of the PV modules, and if no suitable device is available, the insulation measurement must not be carried out. That is a manufacturer instruction rather than a caution.
Is the inverter faulty?
SMA say plainly that it is not: the error is not caused by the inverter, it is in the DC installation. What the inverter has done is measure the array before connecting and refuse to connect because the reading was below threshold. The fault is out on the roof or in the cabling.
Does leaving it cause damage?
It can. SMA warn that transformerless inverters can be damaged by sporadic ground faults in the module array if they occur during operation, and that consequential events in the 64xx and 77xx ranges can follow a ground fault. Getting it inspected is cheaper than the inverter it may take with it.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.