Diesel Generator Trip on Earth Fault

[shark_rose]

A 415V, 700kW, 3 phase diesel generator. When the generator is started, there is a sign of flash between the alternator drive end and the engine and the generator trips on earth fault.
When the generator is tested unloaded, it is fine. Winding resistance values between phase-ground is above 10Mohms.
Any idea what is the problem?

 

[GoldDigger]

Are you measuring winding resistance to ground with a Megger or with a multimeter?
Is this 4 wire plus EGC?
If one of the phases is grounded in your system and there is a "neutral" to ground bond in the connection between the generator and your system it might cause the symptoms you report.
Or an ungrounded system may be developing a high voltage to ground which is causing an insulation breakdown.

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