Solar PV Isolation Fault Troubleshooting in Ireland: DC Ground Fault (PV ISO-PRO)

Key Takeaways

• An isolation fault indicates that electrical resistance between DC solar array conductors and earth has dropped below the inverter's safety threshold.
• Solis specifies distinct ground sub-codes: code 01 indicates the negative wire is connected to ground, while code 02 indicates the positive wire is connected to ground.
• The ground fault will need to be resolved before the system can be energized or the inverter will be damaged by it.
• DC array voltages present deadly electric shock risks; always de-energise supplies, wait 5 minutes for discharge, and engage a Safe Electric Registered Electrical Contractor.

Why does my solar inverter show a PV isolation fault in Ireland?

A solar PV isolation fault—frequently displayed on Solis inverters as PV ISO-PRO—occurs when the inverter's internal insulation monitoring detects that electrical current is escaping from the direct current (DC) circuits to earth. In simple terms, this means the resistance in DC side solar is too low on one or more strings.

In Ireland's damp climate, this is one of the most common service calls. Moisture penetrates degraded rooftop connector seals, water enters rooftop DC isolator enclosures, or rodents gnaw through cable insulation under roof tiles. When morning dew or rain arrives, water bridges the conductor to the earthed mounting rail.

Manufacturer documentation warns sternly that the ground fault will need to be resolved before the system can be energized or the inverter will be damaged by it. Operating an inverter into a dead ground fault can destroy the internal DC sensing board or bridge rectifier.

For a full directory of related system error codes, consult our overview on /ie/guides/fixes/solis-inverter-troubleshooting-ireland.

Solis PV ISO-PRO sub-codes and diagnostic meanings

Solis inverters identify the polarity of the ground leak through detailed sub-codes:

• PV ISO-PRO01 (Negative Ground): This diagnostic sub-code confirms that the negative wire is connected to ground.
• PV ISO-PRO02 (Positive Ground): This diagnostic sub-code confirms that the positive wire is connected to ground.

Knowing whether the positive or negative conductor is leaking helps an electrician isolate whether the fault is situated near the beginning of a string, within a specific solar module bypass diode, or at the return line to the inverter.

Step-by-step string isolation procedure

Technicians can isolate which physical rooftop string is responsible for the ground alarm by using a systematic process of elimination at the inverter DC terminal block:

1. System De-energisation: Shut down the whole system for 5 minutes both DC and AC side to allow all internal capacitive charge to dissipate safely.
2. Disconnect Array Strings: Completely isolate the inverter. Remove all of the DC strings and AC wires, taking off all of the PV strings from their MC4 base terminals.
3. Connect Single Test String: Then input one of the PV strings to the inverter while leaving all other string pairs unplugged.
4. Perform Timed Test Run: Switch on the inverter with grid on for 5 minutes.
5. Evaluate Inverter Response: If there is no alarm that occurs, then shut off the whole system and pull the PV string off the inverter. This string is verified healthy.
6. Swap to Subsequent String: Then swap another PV string to the inverter and re-apply power.
7. Identify Faulty Array: Cycle this and check under which PV string the alarm will occur. The string that triggers the immediate PV ISO-PRO warning contains the physical ground fault.

If your inverter reports AC voltage disconnection rather than DC insulation faults, consult our guide on /ie/guides/fixes/solis-inverter-no-grid-alarm-ireland.

Common causes of DC array ground faults

Irish electrical standards and insulation verification

Under Irish regulations enforced by Safe Electric and the Sustainable Energy Authority of Ireland (SEAI):

• Insulation Resistance Testing: Periodic verification and commissioning follow clause 6 7 2 of I S EN 62446 1 extract below for systems where the array frame is not bonded to earth.
• Installation Wiring Standards: All equipment and array interconnecting wiring must comply with I S 10101 and maintain rated efficiency of 95 percent.
• Warning Placards: Every solar installation must carry the prominent warning label: "Inverter Isolate AC and DC before carrying out work".
• AC Disconnection: An AC isolator 2 pole switching live and neutral must be installed between the inverter and the consumer unit.

If testing reveals that your inverter trips due to high local line voltage under bright sunshine rather than ground faults, refer to /ie/guides/fixes/solar-inverter-grid-voltage-trip-ireland.

Safety: handling high-voltage DC arrays

Direct current from solar panel strings behaves very differently from household alternating current:

• DC circuits do not possess a natural zero-crossing point. Disconnecting DC cables under load creates a continuous, sustained electric arc that can cause fatal burns or ignite roofing felt.
• Always shut down the whole system for 5 minutes both DC and AC side before unclipping connectors.
• Never touch damaged DC conductors with bare hands. Even when disconnected from the inverter, a solar string in daylight remains fully energized.
• Insulation resistance testing must be conducted with specialized instruments rated for high DC operating voltages.

When to hire a Safe Electric Registered Electrical Contractor

While diagnosing which string causes the inverter alarm can be carried out at the ground-level inverter terminals, physical rectification requires specialised skills.

You must engage a Safe Electric Registered Electrical Contractor (REC) to:

• Access the roof to visually inspect, repair, or replace damaged DC solar cables.
• Re-terminate MC4 connectors using calibrated crimping tools and original manufacturer parts.
• Perform formal megohmmeter insulation resistance testing across conductors in accordance with I.S. EN 62446-1.
• Issue a formal completion certificate upon resolving the hazardous ground fault.