Marine Electrical Systems 392i5e

Andrew Cooke

05ETOB

Marine Electrical Systems Marine Generation & Distribution Systems PART 1: Explain why marine electrical generation and distribution employs an insulated neutral: The main reason that marine electrical systems employ an insulated earth is that if an earth fault occurred on one line then the piece of equipment will not trip. Earth faults can be caused by insulation failure or a loose wire. If a second earth fault occurs on a different line a short circuit will result, causing the equipment to trip. Describe with the aid of a circuit diagram how earth faults are monitored using lamps: In 3 phase distribution 3 indicating lamps are used. They are connected in star, with the star point being earthed. If an earth fault occurs on one of the lines that lamp in that line will appear dim. The intensity of the light indicates the degree of the earth fault. Below is a circuit diagram of earth fault monitoring lamps:

Define High Voltage In Marine Systems: High voltage in a marine system is any voltage that is 1000v (1kv) or larger. Typical marine applications are 3.3Kv, 6.6Kv and more rarely 10Kv. Explain the basic idea of using a Neutral Earthing Resistor (NER) with marine generators: A neutral earth resistor is there to limit the maximum earth fault current that could be produced if the generator had an earth fault. It is connected between the star point of the generator and earth. The fault currents created could be massively larger than the generators designed full load current rating. If this current is not controlled then

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Andrew Cooke

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serious damage by overheating the winding insulation. The earth fault current is determined by the size of the NER. M3 Explain two reasons why it is important to locate and clear earth faults: • If there is an earth fault on one line then due to the insulated neutral the piece of equipment will not trip. If another earth fault occurs on another line then a short circuit is created which will cause the piece of equipment to trip, if this is an essential piece of equipment it could endanger the ship/machinery. This is one reason why it is important to locate and clear earth faults. • Secondly if another earth fault occurs in the same line then it becomes increasingly difficult to locate the earth faults. Explain the main advantage of using HV for marine installations: The main advantage of using HV for marine installations is it gives greater power for lower currents compared to LV applications. This results in being able to use smaller gauge cable due to the lower currents, and in turn the build cost of a ship would be greatly reduced. Investigate the IIE 6th Edition of the regulation for installation of electrical equipment on ships, and summarise the regulations regarding earthing of high voltage installations:

PART 2 Distribution Diagram on separate sheet Explain the likely connections for primary and secondary three phase transformer windings: The 3 phase transformers will most commonly comprise of 3 single phase transformers. If one of the single phase transformers fails then the 3 phase output will remain but at reduced power. The primary and secondary coils will normally be connected in delta-delta so if one single phase transformer is to fail then it becomes an open or ‘v’ delta-delta transformer. This is an advantage as the output is not be lost and will still supply machinery i.e. they will not stop. Also only one phase will need isolating to repair the transformer apposed to the solid 3 phase core type, where the whole transformer needs to be isolated to be repaired.

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Andrew Cooke

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Identify two examples of essential and two examples of non-essential services: Essential services are those that are required in the event of an emergency. Essential services: • Steering gear is classed as an essential service aboard a ship. In the event of power loss from the main switch board then the steering gear should still be able to function, when the emergency generator is supplying. Therefore one of the hydraulic oil pump motors will be supplied by the emergency switch board. • Fire prevention is classed as an essential service. If power is lost from the main board the emergency fire pump should still be able to operate. The emergency fire pump motor will be supplied by the emergency switch board. Non-Essential services: • Air conditioning is classed as a non-essential service. If power is lost from the main board then the air condition will stop, so is only supplied by the main board. • Sewage system would be classed as a non-essential service.

M3 Use your own symbol to show where earth lamps would be fitted:

Explain the equipment required at a shore connection point: The shore connection box must have suitable terminals to accept shore supply cable including an earth terminal to earth the ships hull. It must also have a circuit breaker or an isolator switch and fuses to protect the cable connecting the box to the main switch board. It should also contain a data plate giving details of the ship’s electrical system, showing the methods of connecting the cable. A volt meter and phasesequence indicator is fitted, to indicate the correct supply phase sequence. When the right lamp is bright and left one dark the sequence is correct. An alternative is a P.S.I. that consists of a rotary pointer driven by a small 3 phase induction motor, the direction the pointer turns shows if the phase sequence is correct. Below is a circuit diagram of the connection box:

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Andrew Cooke

05ETOB

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