• In this system, the generator star point is connected directly to the ship’s hull.
• Any earth leakage current will therefore complete a circuit back to the generator.
• A Neutral Earthing Resistor (NER) is fitted to limit the earth fault current to about 5 Amps.
• This limited current is detected using a Current Transformer (CT), as illustrated in the sketch.
• The CT output is then connected to protection and alarm systems to indicate the fault.

(ii) Insulated Neutral System:

• An instrument is used which injects a DC voltage into the busbars through a resistor (R1) and a diode.
• No earth leakage condition:
  • No return path exists, hence no current flows through the circuit.
  • Voltage on both sides of R1 remains equal.
  • The Operational Amplifier (Op-Amp) detects no potential difference (PD), so the output remains zero.
• Earth leakage condition (resistance Re):
  • A return path is created through the ship’s hull.
  • Current now flows through R1, causing a voltage drop across it.
  • The Op-Amp detects a PD: one terminal sees full voltage while the other sees reduced voltage.
  • This imbalance causes the Op-Amp to send a signal to the meter/alarm system.
  • The magnitude of earth leakage determines the current flow and the PD across R1, allowing fault severity to be measured.