low lub oil pressure testing on wartsila 9l20

With the engine running isolate the pressure switch or transmitter, bleed it down or release the pressure, if your engine does not trip then your ESD trip is not functioning, it may have been bypassed by someone before you, if the engine does trip great, put the sensor back in service then hide from the operators.

Seriously, the best time to do this would be during a period of down-time for the equipment in that way you can test at your leisure without interrupting operations.