Overview

In some situations, installers or service personnel may measure a DC voltage on the AC terminals of the inverter when the PV array is connected but the AC supply is disconnected. This behavior is normal and does not indicate an inverter fault.

Scenario

You may observe a DC voltage on the AC output terminals when:

• The PV strings are connected to the inverter.
• The inverter's AC connection to the grid is switched off or disconnected.
• A multimeter is used to measure voltage between AC terminals and Neutral (N) or Protective Earth (PE).

Why This Happens

The inverter contains an internal voltage sampling circuit which continuously monitors the AC grid voltage. This circuit uses high-impedance measuring resistors connected to the AC terminals.

When the PV array is connected, DC voltage is present internally on the inverter's DC bus. Even if the AC breaker is open, the voltage sampling circuit remains connected to the AC terminals. As a result, a DC voltage can be measured on the AC side.

Internal Sampling Circuit
Figure 1: Simplified representation of the inverter voltage sampling circuit.

The measured voltage originates through the high-impedance measuring resistors and does not represent a fault condition. Because the circuit impedance is very high, the available current is negligible.

What Happens When the Grid Is Connected?

When the AC side is connected to the utility grid, the grid presents a very low DC impedance. In practical terms, the AC conductors can be considered effectively shorted for DC voltage. Under these conditions, the DC voltage measured between L1/L2/L3 and N or PE will be approximately 0 V.

AC Connected to Grid

Figure 2: DC voltage is effectively removed when the inverter is connected to the grid.

Is This a Safety Concern?

The detected voltage is a result of the inverter's internal measurement circuitry and is associated with a very small current due to the high resistance of the sampling network. This behavior is expected and does not indicate damage or malfunction of the inverter.

Important Safety Notice

Always switch OFF both the DC isolator and the AC supply before performing maintenance or electrical work on the inverter. This ensures that both DC and AC circuits are safely isolated.

Frequently Asked Questions

Does measuring DC voltage on the AC terminals mean the inverter is faulty?

No. If the PV array is connected and the AC supply is disconnected, a DC voltage reading on the AC side can be normal due to the inverter's internal voltage sampling circuit.

Why does the voltage disappear when the grid is connected?

The utility grid provides a very low DC impedance path, which eliminates the measurable DC voltage on the AC terminals.

Should maintenance be performed if only the AC supply is disconnected?

No. Both the AC and DC supplies should be isolated before carrying out maintenance work.