Ah! Back when I worked in a coal-fired power plant we called these boiler trips and turbine trips respectively. There was a nuanced difference between turbine trips and generator trips as well...
So if I recall correctly:
Boiler trips: water level/flow issues, combustion control issues, etc.; depending on circumstances turbine and generator trips might not be initiated, specifically in instances where boiler purging and re-ignition could be accomplished in a timely manner and power generation resumed without encountering turbine rotor/casing differential expansion issues.
Turbine trips: issues with the mechanical side of the turbo-generator, i.e. overspeed, loss of lube oil pressure, high vibration, loss of vacuum / high back pressure, circulating water pump trips due to high trash screen differential, etc., etc; in these circumstances since there was no electrical fault involved "sequential tripping" was invoked to ensure steam flow was collapsed by way of limits switches on the main and reheat emergency stop valves confirming these were shut before the unit ring bus breakers would be sent a trip impulse.
Generator trips: electrical issues like loss of excitation/synchronism, for example due to AVR trips; split-phase or differential protection operation; main unit or unit station service transformer gas [pressure wave] trip; and so forth. Breaker tripping was without intentional time delay in such circumstances, involving a degree of hoping for the best as far as turbine overspeed was concerned, in other words, the ESV's and RESV's had better work, because there's no alternate choice [and the usually worked quite well].
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]