Basics of Maintenance Override Switch - MOS

Emergency Shutdown System Maintenance Override Switch (MOS) is a built-in switch provided in the DCS to enable maintenance of trip transmitters/system so as to avoid inadvertent trip of the plant/unit.

Normally the overridden trips gets back to normal after a period of time say 8 hours.


Maintenance overrides are used to disable inputs or outputs from a safety related system for maintenance activities, to avoid trip actions caused by spurious signals or logic tests

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In general, we must put a lot of restrictions about using MOS. This could damage the plant while we are trying to maintain the availability.