Daikin AC Error U4: DC Isolation Fault Troubleshooting

Why your Daikin AC inverter shuts down during morning dew or heavy rain, and how to safely inspect the DC ground fault.

⚠️ HIGH VOLTAGE SAFETY WARNING: Error U4 means that high-voltage DC electricity (up to 400V–500V DC per string) is leaking from the solar panels or optimizers into the metal racking or building ground. Touching wet metal parts can cause lethal electrical shock.

1. What Exactly Is Error U4?

Under US electrical standards (NEC 690.41 and UL 1741), all grid-tied solar inverters must measure the DC electrical resistance between the positive/negative solar conductors and Earth ground prior to connecting to the utility grid. If this resistance falls below 600 kΩ (or 1 MΩ on commercial units), the inverter immediately halts power generation and locks out with Error U4 to prevent roof fires and equipment destruction.

2. Why Does It Usually Happen in the Morning or Rain?

Moisture is conductive. A micro-crack in a solar panel backsheet, an unsealed MC4 quick-connector, or a wire pinched beneath a mounting bracket will only trigger an isolation fault when morning condensation, dew, or rainfall creates a conductive bridge to the aluminum rail.

3. Diagnostic Protocol (What a Technician Does)

Isolate DC strings from the inverter disconnect.
Use a 1000V calibrated Megohmmeter (Megger) to test each individual DC string against roof ground.
Utilize the Daikin AC SetApp voltage division formula to pinpoint the exact optimizer position with insulation failure without lifting every module.

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