Xylem HYDROVAR HV 4.150 / 4.185 / 4.220 - Manual (Page 11)

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To increase the operating reliability of such a system, also a second MASTER Inverter is
recommended:
Full-featured possibility: Each pump is equipped with a MASTER Inverter
In this mode it is possible to run all pumps in cascade serial mode and synchronous mode
as well.
This configuration allows each pump of the system to become the lead pump. This also
ensures a proper operation if one MASTER Inverter fails. In this case another HYDROVAR
takes control. This ensures that the operating hours of each pump will be the same to
ensure even wear of the pumps.