In a ship’s power system, why is the low voltage neutral not grounded while the high voltage neutral must be grounded, and what are the different earthing arrangements for HV systems?

flowchart TD
    A[IT earth system] -->|Single earth fault| B[No disconnection]
    A -->|Second earth fault| C[Protection operates]
    D[Earthed neutral HV] -->|Resistor| E[Limited fault current]
    D -->|Solid| F[Direct to hull]
    D -->|Divided| G[Neutral-to-hull per section]

• Low voltage earthing: Ships employ an IT earth system; a single earth fault does not disconnect supply, a second fault causes protection to operate (Rec 137 §2.1)
• High voltage earthing: Neutral of AC networks of 3000 V line-to-line and over may be earthed; HV systems are normally earthed via a resistor (SOLAS II-1 Reg 45 §4.1, TEAP Part B Appendix III/9.2)
• Fault current limit: Earth fault current not greater than full-load current of the largest generator, not less than three times the minimum earth-fault protection operating current; at least one neutral-to-ground connection must be available when energized (UR E11 §2.1.2)
• Divided systems: In systems with neutral earthed, a neutral-to-hull connection is to be provided for each section (UR E11 §2.1.5)
• Tankers: Earthed distribution systems shall not be used; HV neutral earthing is permitted only if fault current does not flow through dangerous spaces (SOLAS II-1 Reg 45 §4.1)
• Hull return: The ship’s hull is not to be used as a current-carrying conductor; systems are to be wired as insulated return systems (UK chap6rev3/4 §6.2.5)