Controller displays F15 – Earth fault / ground fault alarm

Safety first: Ground faults can be dangerous. Isolate the generator, lock out all sources, and use appropriate PPE.

• Visual inspection: Open the generator terminal box and inspect for moisture, dirt, or damaged insulation on cables and bus bars. Dry and clean as necessary.
• Insulation resistance test: With the generator isolated, perform a megger test between each phase and ground according to MTU guidelines. Low readings indicate insulation problems.
• Check cables to ATS/switchboard: Inspect the output cables along their route for mechanical damage or water ingress.
• Ground fault sensor: Verify the wiring and operation of the ground fault CT or sensor feeding the MTU controller. Replace if defective.
• Repair insulation faults: Repair or replace damaged cables or, if necessary, have the generator stator rewound by a qualified shop.
• Re-test: After repairs, clear the F15 alarm and repeat insulation tests before returning the generator to service.