ActronAir Indoor Evaporator EC Plug Fan and Impeller 400mm 3-Phase is a mid-to-upper-capacity EC variable-speed plug fan for commercial ducted systems where the 400mm impeller occupies the capacity band between the 355mm and 500mm plug fan assemblies. Variable-speed EC operation on this assembly enables the commercial indoor PCB to modulate airflow independently in response to zone load and BMS demand signals -- a fan stuck at a single speed or running erratically indicates a failed motor drive rather than a winding fault. Isolate power at mains before replacement -- EC motors and variable drives retain dangerous residual voltages for up to 5 minutes after power isolation.