Safety-critical fault · Growatt

Growatt Isolation Fault — Insulation Resistance Failure

Your Growatt inverter has detected that the DC insulation resistance has dropped below the safe threshold. This is a genuine safety shutdown — the electrical barrier between the solar panels and earth has been compromised. The system will not restart until the fault is found and repaired. Most cases are caused by water ingress in MC4 connectors or UV-damaged cable insulation.

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Isolation fault — what to do now

Isolate the system at the consumer unit and DC isolator. Do not restart the inverter — it will detect the same fault and shut down again. Work through the diagnostic steps below, or book a remote session and we identify the cause from ShineServer data.

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Ronald tried to help solve a fault with communications between battery and inverter. Whilst in the end a hardware fault was diagnosed and not possible to fix remotely his attempt to fix and guide me through steps to fault find were appreciated. He was available at a time convenient to my availability. Great service for anyone requiring independant tech support for their solar or battery system.

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Written by Ronald · Independent Solar Engineer Last updated 28 October 2026

Insulation resistance — what the numbers mean

Above 1 MΩ — passes BS EN 62446-1

Below 1 MΩ — inverter trips, isolation fault triggered

Weather-dependent — drops when wet, recovers when dry

Progressive — intermittent faults become permanent

Diagnosis

6-step isolation fault diagnostic

Follow these steps to identify the source of the insulation resistance failure. The first step is a safety requirement — do not skip it.

Isolate the system at the AC and DC isolators

An isolation fault is a safety event. Switch off the AC isolator at the consumer unit and the DC string isolator near the panels. The inverter has detected that the electrical barrier between the DC circuit and earth has broken down.

Do not restart the inverter until the fault is diagnosed. Restarting does not fix insulation faults — the inverter runs its resistance check at startup and will shut down again immediately.

Check ShineServer fault logs for timing and pattern

Log into ShineServer or the ShinePhone app and check the fault history. Note when the isolation fault first appeared and whether it recurs at specific times.

A fault that appears in the morning during wet weather points to MC4 connector water ingress. A fault that appeared suddenly during dry weather suggests cable damage or a cracked panel junction box. Intermittent faults that clear and return are typically moisture-related and worsen over time.

Identify the affected DC string using ShineServer data

If the Growatt inverter has multiple MPPT inputs, ShineServer production data can help identify which string is affected. Compare the power output of each MPPT tracker in the days before the fault appeared.

A string showing reduced output before the isolation fault is likely the one with the insulation problem. This narrows the physical inspection to a specific set of panels and cables — saving time and site visit costs.

Inspect connectors, cables, and junction boxes

With the system safely isolated, a qualified person should inspect the DC wiring for the affected string. Check MC4 connectors for water ingress, corrosion, or physical damage. Inspect DC cables for cracked insulation from UV exposure or mechanical damage. Check panel junction boxes for cracks, separation, or discolouration.

Pre-2018 installations are most vulnerable to connector degradation. Connectors lose their weatherproof seal over time, particularly if they were not crimped correctly during installation.

Perform an insulation resistance test on each DC string

An insulation resistance (IR) test measures the electrical isolation between the DC conductors and earth. A qualified engineer uses a dedicated IR tester at 500V or 1000V DC. Each string must measure at least 1 megaohm to satisfy BS EN 62446-1, referenced through BS 7671 Section 712 (G98 is an AC grid-connection standard and sets no DC insulation limit).

Testing each string individually identifies which string has the fault. Testing individual panels within a string can isolate the exact component. This is the definitive test — visual inspection alone is not sufficient to confirm the fault source.

Repair and verify before recommissioning

Once the fault source is identified — damaged connector, degraded cable, or failed junction box — replace the component. After the repair, the engineer must repeat the IR test to confirm insulation resistance is above 1 megaohm on all strings before restarting the inverter.

Do not recommission without a documented IR test result. This protects the homeowner, records the BS EN 62446-1 result, and provides evidence for warranty claims if a panel or component was the cause.

Why Growatt inverters are sensitive to insulation faults

Growatt inverters include a built-in insulation monitoring device (IMD) that checks DC-to-earth resistance before the inverter connects to the grid each morning, and continuously during operation. Disconnecting on low insulation resistance is a product safety behaviour, not a G98 requirement: BS EN 62446-1 sets the 1 megaohm test minimum, while G98 and G99 govern the AC grid connection. If resistance drops below the threshold the inverter disconnects to prevent electric shock risk.

The fault is almost never caused by the inverter itself. It originates in the external DC circuit — the panels, cables, connectors, and junction boxes that make up the solar array. Over time, UV radiation degrades cable insulation, weatherproofing seals on MC4 connectors break down, and junction box adhesive can crack. Water enters through these weak points, creating a conductive path between the DC circuit and the earthed metalwork of the array frame or roof mounting.

Growatt displays isolation faults as Error 04 on the inverter screen and logs them in ShineServer. Some models also display specific insulation resistance readings in the advanced diagnostics menu, which can help quantify how far below the threshold the system has fallen. A reading close to 1 megaohm suggests a single weak point; a reading well below suggests widespread degradation or a major fault.

Reference

Common causes of Growatt isolation faults

MC4 water ingress

Moisture inside MC4 connector housings

The most common cause. Connector seals degrade after five to seven years of UV exposure and thermal cycling. Water enters and creates a conductive path. Appears as a weather-dependent fault — trips when wet, clears when dry. Replace the affected connector pair.

DC cable damage

Cracked or split cable insulation

UV radiation causes fine cracks in the cable sheath over time. Physical damage during installation or from roof work also occurs. Damaged insulation allows moisture to reach the conductor. Inspect the full cable run from panels to inverter — pay attention to exposed sections and entry points through walls or roof tiles.

Junction box failure

Cracked or separated panel junction box

The junction box on the back of each panel contains the bypass diodes and string connections. Heat cycling and UV cause the adhesive to fail, allowing the box to separate from the panel. Water enters and compromises insulation. Requires panel replacement — junction boxes cannot be reliably resealed.

Rodent damage

Chewed cable insulation in loft or roof void

Squirrels, rats, and mice chew DC cable insulation where it runs through loft spaces or under panels. The exposed conductor contacts metalwork or damp surfaces, causing an insulation fault. Check any cable sections running through accessible roof voids. Rodent guards can prevent recurrence.

Panel backsheet crack

Micro-cracks in the panel rear laminate

Older or lower-quality panels can develop cracks in the rear backsheet that allow moisture to reach the cell layer. This is difficult to detect visually but shows as a persistent insulation fault on a single string. IR testing of individual panels identifies the affected unit. Usually covered under panel warranty if within the guarantee period.

Isolation fault — common questions

What causes an isolation fault on a Growatt inverter?

An isolation fault means the electrical barrier between the DC solar circuit and earth has broken down. The most common cause is water ingress into MC4 connectors, particularly on installations older than five years where the connector seals have degraded. Other causes include UV-damaged DC cable insulation, cracked panel junction boxes, and rodent damage to cables in loft spaces. The inverter detects the fault through its built-in insulation monitoring and shuts down as a safety measure.

Can I reset my Growatt inverter to clear an isolation fault?

No. Restarting the inverter does not fix an isolation fault. The inverter will run its insulation resistance check at startup and shut down again when it detects the same problem. Repeated restarts do not address the underlying cause and risk electric shock if the insulation has failed. The fault must be physically diagnosed and repaired before the system can safely operate.

How is an insulation resistance test performed on solar panels?

An insulation resistance test uses a dedicated IR tester to apply a test voltage (typically 500V or 1000V DC) between the DC conductors and earth. The tester measures the resistance of the insulation barrier. BS EN 62446-1, referenced through BS 7671 Section 712, sets the minimum at 1 megaohm. G98 is an AC grid-connection standard and sets no DC insulation limit. The test is performed on each DC string individually with the inverter and all isolators off. A qualified solar engineer or electrician should perform this test.

Why does my Growatt isolation fault only appear in wet weather?

Weather-dependent isolation faults are a classic sign of water ingress. When it rains or humidity is high, moisture enters degraded MC4 connectors or cracked junction boxes and creates a conductive path between the DC circuit and earth. When the weather dries out, the moisture evaporates and the insulation resistance recovers temporarily. This pattern worsens over time as the entry point degrades further. ShineServer fault logs correlated with weather data can confirm this pattern.

How much does it cost to fix a Growatt isolation fault?

Repair cost depends on the root cause, and the remote diagnosis to locate it is free. Typical repairs: MC4 connector replacement £30–50 in parts plus labour; DC cable £50–150 depending on run length and roof access; a failed panel junction box £200–400 for panel replacement. Because an isolation fault is physical it needs an on-site repair (from £295 plus parts) — we pin down the location remotely first and quote before any work.

How quickly can you find a Growatt isolation fault?

Usually the same day for the remote side. We read your ShineServer fault log, MPPT string data, and the weather pattern to pinpoint the likely string and component before anyone goes on the roof — then, if a site visit is needed for the repair, we book it with the right parts so it's a single trip. For other Growatt codes, see the fault code index.

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