Mitsubishi Heavy Industries FDT28KXE6A Service Manual page 213

Backup operetion function is only for emergency purpose when one of compressors or one of units is damaged.
If backup operation is performed continuously for long period, it may cause the damage of good compressors.
Accordingly be sure to repair the damaged unit or to replace the damaged compressor and to cancel the
backup operation within 48 hours after starting backup operation.
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