Does anyone know what happens with the configuration described above if the NAS fails but the grid remains up? Is Gemini’s answer as follows correct?
For the specific setup you described using the S99 firmware (designed for external transfer switches / centralized NA protection), Victron Energy’s control logic dictates a clear behavior.
In the event of a grid disconnection caused by the centralized NA protection (NAS failure), the system does not trigger a critical fault. Instead, it continues to operate in Island Mode, provided a battery is connected.
Detailed System Behavior
- MultiPlus-II Behavior: As the contactors open, the voltage drops to zero at the MultiPlus AC terminals. For the MultiPlus units, this is treated as a standard grid failure. They immediately switch to inverter mode to supply power to the critical loads remaining on the islanded side.
- Cerbo GX & ESS Behavior: The Cerbo GX continues to receive data from the EM540 at the grid connection point, indicating that grid voltage is present and current is flowing (drawn by the heat pump and EV wallbox). However, since the MultiPlus units regulate independently based on their firmware and are physically disconnected from the grid, the ESS system ignores the grid meter’s regulation commands at this moment because the inverters have no physical grid connection.
- System Notifications: No critical system error (such as a hard fault or shutdown) is generated. The Cerbo GX will simply display an “Island Mode” status or a notification regarding the grid status discrepancy.
Crucial Technical Background on the S99 Firmware
The special S99 firmware was specifically developed to disable the internal transfer switch of the MultiPlus units and delegate grid connection control to external contactors.
- Mandatory Use of CT Clamps: For the S99 system to function correctly, Victron strictly requires the physical current transformer (CT) clamps to be wired directly to the MultiPlus units.
- Safety Verification: The MultiPlus-II units use these CT clamps to check for current flow before an external contactor is allowed to close. When the NA protection trips, the MultiPlus CT clamps detect 0 Watts. This tells the internal inverter logic instantly that there is no AC connection to the grid, completely independent of what the EM540 reports to the Cerbo GX.
Summary for Your Heavy Loads
The heat pump and EV wallbox will continue to be supplied normally by the public grid as long as the grid is live (since they are located upstream of the first contactor). If the public grid fails entirely, they will lose power. In either case, the Victron system will not backfeed into that upstream section because the contactors are open. The system continues to run stably as a backup island for the downstream loads.