DC Isolation Resistance Low

What does RISO Low mean?

The Fronius Primo GEN24 inverter detects a low DC isolation resistance (RISO) in the PV generator or associated wiring, leading to an error state and immediate shutdown of the system. This condition indicates a leakage current path from the DC conductors to ground, posing a safety risk and preventing normal operation. The threshold for this fault is an RISO value below 500 kOhm.

• Damage to the insulation of PV modules, DC cables, or connectors (e.g., due to mechanical stress, UV degradation, or animal damage).
• Moisture or water ingress into PV module junction boxes, combiner boxes, or cable conduits, reducing insulation resistance.
• Accumulation of dirt, dust, or debris on PV module surfaces, especially in high humidity, creating conductive paths.
• Faulty grounding of the PV array components.
• Incorrect wiring or faulty insulation during installation.

Click steps to track your progress.

1. 1

   Disconnect the PV array from the inverter to de-energize the system.

2. 2

   Using an insulation resistance tester (megohmmeter), measure the insulation resistance between the positive DC conductor (+PV), the negative DC conductor (-PV), and ground for each individual PV string. Verify that the measured insulation resistance for each string is above 500 kOhm.

3. 3

   Visually inspect all DC cables, connectors, and PV module frames for any signs of damage, wear, or moisture ingress. Pay close attention to areas prone to rubbing or sharp bends.

4. 4

   If a low resistance is found in a specific string, isolate it and systematically test individual modules and cable segments within that string to locate the precise point of insulation failure.

5. 5

   Ensure all PV module frames and mounting structures are correctly and securely grounded according to local electrical codes.