Twice now I have experienced a negative AC input. I'm sure it is happening more frequently and has just gone unnoticed. I have two multiplus 24/3000 inverters in parallel. I have defined two AC inputs. Shore power is connected to the master AC input. When there is a load (eg, Air Conditioner, water heater), I'm seeing the AC input go negative, a significant drain on my batteries which appear to be supplying power for both the AC Output and the suspicious AC Input (feedback???). After the load drops, the system appears to return to normal and recharges the battery bank. I have no assistants loaded and have not defined a country code for feeding back to the grid. How can this be???