- Solar and energy
- Solar inverter
039
Sungrow 039 on a solar inverter: low insulation resistance
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
The Sungrow inverter detects system insulation resistance that is too low and stops feeding in. Sungrow says to wait for it to return to normal. If it keeps coming back, note the weather and call your installer.
Safety first
Stop and check this first
- Do not unplug panel connectors or operate the rooftop isolator to test. Sungrow writes these checks for installers and requires measuring with strings safely disconnected.
- A 039 on its own does not prove the inverter is faulty. Sungrow links it to a possible earth fault in the PV array: insist on per-string measurements before accepting a replacement.
Safe checks you can perform
- Code and fault record in the app
- Link with the weather
- Insulation measurement per string
Leave these tasks to a qualified professional
- Do not attempt this professional task: Insulation test of each string to earth
- Do not attempt this professional task: String test per MPPT input
- Do not attempt this professional task: ISO threshold and fault history check
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Insulation test of each string to earth
What they doUses an insulation tester positive to earth and negative to earth on each string, with the array disconnected from the inverter, per the local standard.
They must show youeach string's value against the minimum of the applied standard.
In writingvalues, test voltage and the faulty string.
String test per MPPT input
What they doConnects strings one at a time to each MPPT to see which one brings the fault back, as Sungrow describes.
They must show youwhich string makes 039 return.
In writingthe string identified and the repair done.
ISO threshold and fault history check
What they doChecks the ISO protection value in the app or LCD against local regulations and reviews the fault history.
They must show youthe configured value and the standard it follows.
In writingwhether the setting was changed and why.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Code and fault record in the app
- Who:
- you, from the iSolarCloud app or the display.
- How:
- open the app's fault alarm record and write down code 039 with each date and time.
- Result that matters:
- whether the fault clears on its own and how often it repeats.
- If it appears and clears by itself:
- this matches what Sungrow describes; keep watching.
- If it repeats often or does not clear:
- pass the record to the installer.
- Evidence to keep:
- screenshots of the fault record with dates and times.
Evidence: User Manual 1-Phase PV Grid-Connected Inverter SG2.0RS-S / SG2.5RS-S / SG3.0RS-S / SG3.0RS / SG3.6RS / SG4.0RS / SG5.0RS / SG6.0RS (SG2.0-6.0RS-UEN-Ver18-202211) · Low PV Insulation Resistance Issues (Error 039 / 302)
Link with the weather
- Who:
- you, without touching the equipment.
- How:
- note whether there was rain, dew or humidity on days with 039, and whether it stays in sunny weather.
- Result that matters:
- whether the fault recovers in good weather.
- If it only shows when damp and recovers once dry:
- fits the cause Sungrow names as most common.
- If it stays in dry weather or for a long time:
- Sungrow asks for each string's insulation to be measured.
- Evidence to keep:
- list of days with weather and inverter status.
Evidence: User Manual 1-Phase PV Grid-Connected Inverter SG2.0RS-S / SG2.5RS-S / SG3.0RS-S / SG3.0RS / SG3.6RS / SG4.0RS / SG5.0RS / SG6.0RS (SG2.0-6.0RS-UEN-Ver18-202211) · Low PV Insulation Resistance Issues (Error 039 / 302)
Insulation measurement per string
- Who:
- qualified PV installer.
- How:
- with strings disconnected from the inverter, use an insulation tester between positive and earth and negative and earth of each string.
- Result that matters:
- each string's resistance against the local standard's minimum; Sungrow cites 1 MΩ as the AS/NZS 5033 example for 120 to 500 V.
- If a string is below it:
- check the rooftop isolator, DC cable and terminals on that string.
- If everything passes:
- Sungrow says to shut the inverter down, turn it on after 10 minutes and test strings per MPPT.
- Evidence to keep:
- table of values per string and pole, with the test voltage.
Insulation testing and disconnecting strings is DC side work, with dangerous voltage whenever there is daylight. Only a qualified installer does it.
Evidence: User Manual 1-Phase PV Grid-Connected Inverter SG2.0RS-S / SG2.5RS-S / SG3.0RS-S / SG3.0RS / SG3.6RS / SG4.0RS / SG5.0RS / SG6.0RS (SG2.0-6.0RS-UEN-Ver18-202211) · Low PV Insulation Resistance Issues (Error 039 / 302)
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Solar and energy
- Brand
- Sungrow
- Product type
- Solar inverter
Known code variants
- Sungrow 039 aislamiento bajo
- Sungrow 039 insulation fault
- Sungrow error 039
- Sungrow fault 39
- Sungrow low system insulation resistance
What it means
Sungrow defines 039 as Low System Insulation Resistance on grid-connected inverters (302 on hybrids). It indicates there could be an earth fault in the PV array. The manual points to the configured ISO protection value, the resistance to earth of strings and DC cable and, with batteries, their cables and terminals. Sungrow adds that most cases come from wet weather and recover on their own. The code does not say which string or cable is at fault.
Warnings and stop conditions
- Higheditorial warning
Do not unplug panel connectors or operate the rooftop isolator to test. Sungrow writes these checks for installers and requires measuring with strings safely disconnected.
Evidence: User Manual 1-Phase PV Grid-Connected Inverter SG2.0RS-S / SG2.5RS-S / SG3.0RS-S / SG3.0RS / SG3.6RS / SG4.0RS / SG5.0RS / SG6.0RS (SG2.0-6.0RS-UEN-Ver18-202211) · Low PV Insulation Resistance Issues (Error 039 / 302)
- Mediumeditorial warning
A 039 on its own does not prove the inverter is faulty. Sungrow links it to a possible earth fault in the PV array: insist on per-string measurements before accepting a replacement.
Evidence: User Manual 1-Phase PV Grid-Connected Inverter SG2.0RS-S / SG2.5RS-S / SG3.0RS-S / SG3.0RS / SG3.6RS / SG4.0RS / SG5.0RS / SG6.0RS (SG2.0-6.0RS-UEN-Ver18-202211) · Low PV Insulation Resistance Issues (Error 039 / 302)
Symptoms and causes
Probable causes · in order
- Moisture from rain or wet weatherCommon
Sungrow states most cases are caused by wet weather and recover when weather improves. This fits if the code clears on dry days.
Evidence: User Manual 1-Phase PV Grid-Connected Inverter SG2.0RS-S / SG2.5RS-S / SG3.0RS-S / SG3.0RS / SG3.6RS / SG4.0RS / SG5.0RS / SG6.0RS (SG2.0-6.0RS-UEN-Ver18-202211) · Low PV Insulation Resistance Issues (Error 039 / 302)
- Earth fault in strings or DC cablePossible
If it persists in good weather, the installer must measure resistance to earth of each string and the DC cable. Fits damaged insulation or a short.
Evidence: User Manual 1-Phase PV Grid-Connected Inverter SG2.0RS-S / SG2.5RS-S / SG3.0RS-S / SG3.0RS / SG3.6RS / SG4.0RS / SG5.0RS / SG6.0RS (SG2.0-6.0RS-UEN-Ver18-202211) · Low PV Insulation Resistance Issues (Error 039 / 302)
- Moisture or damage in rooftop isolator or connectorsPossible
Sungrow's note asks to check devices on the DC cable, such as the rooftop isolator, and PV terminals for moisture or damage.
Evidence: Low PV Insulation Resistance Issues (Error 039 / 302)
- ISO protection value set too highPossible
The manual asks to check in the app or LCD that the ISO protection threshold is not excessive and meets local rules. A high setting triggers 039 without a real defect.
Evidence: User Manual 1-Phase PV Grid-Connected Inverter SG2.0RS-S / SG2.5RS-S / SG3.0RS-S / SG3.0RS / SG3.6RS / SG4.0RS / SG5.0RS / SG6.0RS (SG2.0-6.0RS-UEN-Ver18-202211)
- Damaged battery cables or terminalsLess common
If batteries are present, Sungrow asks to check for damaged battery cables or loose terminals. Only applies to systems with a battery.
Evidence: User Manual 1-Phase PV Grid-Connected Inverter SG2.0RS-S / SG2.5RS-S / SG3.0RS-S / SG3.0RS / SG3.6RS / SG4.0RS / SG5.0RS / SG6.0RS (SG2.0-6.0RS-UEN-Ver18-202211)
Virtual technician
Does 039 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Sungrow Power Supply Co., Ltd. · 2022 · manual
- 2
Low PV Insulation Resistance Issues (Error 039 / 302)
Sungrow Power Supply Co., Ltd. · 2021 · official_doc
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