
14
Manual Redundancy Mode
The Manual Redundancy Mode is provided to allow the operator direct control of the
redundancy chain. With the Control Unit set to the Manual Redundancy Mode, the
operator has exclusive control of the transfer switch positions. This enables off-line
testing and maintenance to be performed.
Automatic Redundancy Mode
The Automatic Redundancy Mode provides the Control Unit with complete control of
switching operations. The Control Unit monitors the Primary Units, the Backup Unit and
the Switchover Modules. The Control Unit can back up only one Primary fault at a time.
In the event of more than one failure, the Control Unit will force the faulty unit with lowest
priority (highest numeric setting) back online while backing up a faulty unit of higher
priority (lowest numeric setting).
Placing the Control Unit into Automatic redundancy mode has the following
consequences:
•
All Primary Units are switched online.
•
The Backup Unit is switched to standby.
•
The NSU Control Unit screen switches to the Redundancy Chain Status display.
•
If a Primary Unit fails, it is switched into standby and the Backup Unit is switched
online in place of the failed Primary Unit (as long as there are no higher priority
units already in standby).
a) If the fault on the Primary Unit clears, the Primary Unit is never switched back
online unless another unit with higher priority (lowest numeric setting)
develops an active fault. In this case the unit with the active fault is switched
to standby, forcing the cleared unit back online.
The flowchart below illustrates the switching decisions performed by the Control Unit
when it is placed into the Automatic mode.