The main advantages low-impedance ship’s grounded systems are:
No special attention for equipment insulation required
– Automatic detection and immediate isolation of ground faults
– Ground fault current flows for a short period of time, restricting damage
– Avoiding arcing ground over-voltages
– Maintains phase voltages at a constant value to ground.
The main disadvantages low-impedance ship’s grounded systems are:
– Instant disconnection and loss of the service
– Fault currents can be large and can cause extensive damage and have the risk of explosion.
Most low-power, low-voltage systems in the range from 110-230V have a solid grounded neutral. This power is mostly supplied from a phase to neutral source like a transformer and is used to supply small power consumers and lighting. There are two basic types of distribution for solid or low impedance grounded systems:
- 3-phase 4-wire with earthed with hull return
- 3-phase 4-wire with neutral earthed without hull return (TN-S – system) for all voltages up to and including 500 V A.C.
The type without hull return (b) resembles installations commonly used on shore in houses and is used primarily in the accommodations of ships.
The additional advantage of such a system is that it will require the same skills for operation and maintenance as for onshore installations.
Labour legislation in various countries makes companies responsible for the safety of workers or crew on board of ships. Using this type of system would make it easier to comply as standards with respect to safety, training, operational authorisation, etc. would be the same.
Special consideration should be given to low-voltage supplies to for instance steering gear or pumps for essential services as these should not trip on a ground fault.
For these services it would probably be best to make a dedicated supply directly from the main power source. The diagram below shows the principle layout of a system with an ungrounded main power system but with a grounded low-voltage system.
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