Ever since AC generation and distribution has been introduced on a large scale on ships around 1950, there has been debate about the type of distribution system. The main focus with the type of distribution system is the treatment of the systems neutral with respect to grounding.
When selecting the grounding method on ship the primary factor with the selection is the safety of people and secondly the safety of equipment. But loss of vital equipment can endanger a ship’s safety and this in turn can reduce the safety of the crew.
The main cause of faults on board of a ship are ground faults which occur when live conductors come into contact with the ” ground “.
The ” ground ” on a ship is basically the metal structure.
When an electrical system is “ungrounded ” this means that the neutral of the power supply is insulated from the ship’s metal structure.
In an “ungrounded” system a ground fault will be detected but not removed automatically on the first fault. This allows a service to remain in operation, which can be a big advantage for vital services such as those for DP operations.
Although “ungrounded ” there will still be a fault current flowing due to the capacitance of the cables and interference suppression capacitors fitted inside equipment. In large installations with many cables this fault current can be substantial.
To find a first ground fault in an “ungrounded” system can be some task as these are normally not self-revealing and would involve switching on and off circuits in distribution panels until the fault disappears. Only when a more sophisticated system is installed with core-balance current transformers in the distribution panels automated fault-finding can be obtained but this can be an expensive addition.
When an electrical system is “grounded” this means that the neutral of the power supply is connected to the ship’s metal structure. In a “grounded” system a ground fault will in most cases be removed by the automatic opening of a circuit breaker or the melting of a fuse in the faulty circuit.
A live conductor can touch the metal case of a piece of equipment which then would become a hazard to the crew.
Bonding all metallic enclosures of electrical equipment to the ship’s hull will ensure that these are on the same voltage level and will not cause electric shock. Furthermore the bonding of equipment will make paths available for fault currents to allow protection devices or detection devices to react. Bonding thus ads greatly to safety.
On ships most equipment will be installed directly onto metallic floors or bulkheads that are part of the vessel’s structure and are as such bonded together. When this is not the case, like for instance with equipment on skids with anti-vibration mounts, additional grounding arrangements must be in place. These arrangements must be suitably sized flexible ground wires connected to ground bosses welded to the ship’s structure.
In an “ungrounded“ system the voltage levels of the remaining phases will rise to 1.732(v’3) of the nominal value. When the fault is not solved this higher voltage level will cause the insulation of wires and cables to deteriorate. That is why most classification bureaus have set a limit to the total time per year that ground faults may occur in a system. When a wire is loose and re-strikes ground, which is likely to happen on a ship in service, this can cause transient over-voltages which may permanently damage equipment.
In general there is no single “best method” for grounding the electrical system on ship. It is to the engineers to select a system that is best fitted in relation to safety, cost and operation.
The result could be to use a number of restricted grounded systems for specific services such as domestic, hotel and galley via dedicated transformers.
Essential services, such as DP and propulsion related, could then be supplied from insulated systems. By splitting systems over different supplies and applying redundancy these systems can be further optimized.
There are generally three methods of grounding which are used:
– Insulated neutral (ungrounded)
– Solid and low impedance
– High impedance