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MCT-CM
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9.2.3.2 After Defrost Switching
After defrost operation provides equal run time switching based on the user defined alternating switch interval.
When OEM configured for
‘After Defrost’ and no redundancy alarm condition exists, a defrost cycle will be
forced at each system switch time when
‘timed defrost’ is configured. If defrost is configured for timed process,
a defrost cycle will be forced at system switch only if required based on the condition of the loop or monitor
point configured. Normal defrost cycles will take place based on timed or timed process settings since the
‘equal run time’ switching is typically longer than the system should run without a defrost cycle.
If a defrost cycle is running and the equal run time switching timer has elapsed, the defrost cycle will be
completed, then the primary systems will switch. If a redundancy alarm condition exists (process alarm, system
1/2 fail), defrost cycles will be performed under fail mode operation based on timed or timed process settings.
Pressing the demand Defrost button will perform a single defrost cycle based on defrost settings (during normal
switching or fail mode conditions). Primary systems and actual system digital outputs (1/2) will only switch
when no redundancy alarm conditions exist and concurrent operation is not active.
9.2.3.3 Low Rise Backup Defrost Switching
Low rise defrost provides the same operation as “After Defrost” switching with one exception. When defrost is
activated on the Primary system, the Backup system is automatically started in order to maintain temperature
while the Primary system is in defrost. Once defrost is complete on the Primary system, it will be restarted and
the Backup system will continue to operate for a minimum time (the defrost Fan delay time as entered on the
Defrost screen). Once the delay time has expired, the Backup system will automatically turn off. This cycle will
continue until the equal run time switching timer has elapsed.
If a defrost cycle is running and the equal run time switching timer has elapsed, the Primary system will be
turned off and the Backup system that was already running during defrost of the Primary system, will continue
operating and become the Primary system. If a System Failure condition exists, defrost cycles will be performed
under fail mode operation based on Timed or Timed process settings; however, the failed system will be unable
to operate during defrost of the operating system, so a rise in temperature can occur during the defrost period.
9.2.4 System 1/2 Failure
If a system 1/2 failure occurs on alternating or concurrent systems, the MCT will always operate in fail mode on
the system that does not have a system failure. If both 1 and 2 system failures occur, the digital outputs
configured for
‘Redundant System 1 or 2’ will be de-energized. During any system failure (1/2 or 1 and 2), the
alarm must be reset from the main alarm screen and the condition cleared that caused the system failure.
Once the
alarm condition is acknowledged and cleared, the “Reset” button on the Redundancy screen must be
pressed in order for normal system operation to resume, at which point the backup system will become the
primary system. The user can switch from
‘Auto’ to ‘Manual’ mode at any time to run system 1 or 2 in manual
operation while performing maintenance on the systems.
NOTE: System 1/2 failures are defined by the OEM configured digital input selections. When the assigned
alarm input(s) for the system is active, the system failure occurs. Consult your OEM configuration for
information on which inputs are configured for system 1/2 failure (if any).