lockout protection

<What will be done by operator in control room when this alarm appears?>

This is defined in the Control Philosophy document for the plant, which is an output from a design process that will probably be based upon a 'HazOp analysis' at well-run installations. The information is therefore installation-specific, and should be there 'higher upstream'.

In electrical protection signals lockout sometimes called (Ansi device number 86) refers to a manual reset only switch which operates when the electrical equipment protection relaying or instrumentation detects fault. This will require operator when receiving alarm to dispatch electrician to location to troubleshoot fault, and when fault has been found and repaired he will then manually reset the 86 lockout relay.

The low gas pressure lockout will be referring to either SF6 gas pressure or transformer nitrogen blanket, depending on which type of equipment you have in your plant. If the gas pressure hits low-low setpoint it will activate lockout relay and trip breaker isolating equipment in question.