• Reasons: as ship size and installed power increase (large container ships, cruise ships, electric propulsion), the current at low voltage becomes very large, requiring very heavy cables and switchgear. Using high voltage (e.g. 3.3 kV, 6.6 kV, 11 kV) reduces the current for the same power, allowing smaller cables, smaller switchgear, and lower losses.
• Advantages of HV:
• Lower current for the same power, so smaller and lighter cables and switchgear.
• Lower I^2 R losses in the cables, giving higher efficiency.
• Smaller generators and motors for the same power (better power-to-weight ratio).
• Easier to transmit power over long distances (e.g. to bow thrusters or propulsion motors).
• Reduced voltage drop over long cable runs.
• Disadvantages of HV:
• Higher cost of HV switchgear, transformers, and protection equipment.
• Requires specially trained personnel and strict safety procedures (HV is more dangerous).
• Higher insulation requirements and more complex maintenance.
• Requires step-down transformers for low-voltage consumers.
• Greater risk of electric shock and arc flash; more severe consequences of faults.

(i) Sustained low frequency tests (power frequency withstand test):

• A high voltage at power frequency (50/60 Hz) is applied between the conductor and earth (or between windings) for a specified time (e.g. 1 minute). It tests the insulation's ability to withstand the normal operating voltage plus a margin. The test voltage is gradually raised to the specified value, held for the time, then reduced. Any breakdown or excessive leakage current indicates insulation failure.

(ii) Constant DC/AC test:

• A d.c. or a.c. high voltage is applied and held constant for a period. The d.c. test (e.g. using a d.c. pressure test set) measures the leakage current and its decay; a steady, low leakage current indicates good insulation, while a rising leakage current indicates deterioration. The a.c. test applies a constant a.c. voltage and checks for breakdown. These tests detect moisture, contamination and insulation weakness.

(iii) High frequency test:

• A high-frequency a.c. voltage is applied to the insulation. High-frequency testing stresses the insulation more severely and can detect incipient faults, voids and partial discharge that may not show at power frequency. It is used to detect weaknesses in the insulation of cables and windings.

(iv) Surge or impulse test:

• A high-voltage impulse (lightning impulse or switching surge) of a specified waveform (e.g. 1.2/50 microsecond) is applied to the insulation. This simulates the effect of lightning or switching surges. The test checks the insulation's ability to withstand transient overvoltages without breakdown. The impulse is applied several times and the insulation must not flash over or break down.
• All HV tests must be carried out with the equipment isolated, earthed and discharged, and with strict safety precautions and trained personnel.