Marine Engineering-Electrical-Fuses

Fuses
High current flow through a thin fuse wire will raise its temperature causing it to melt and break the circuit before the current excess reaches a level sufficient to damage other, more substantial, parts of the system. Melting temperature depends on the material used (tinned copper in rewireable fuses melts at 1080ºC, the silver in cartridge fuses at 960ºC). The wire is sized so that the normal current is carried without overheating, but due to the resistance of the relatively small wire, that excess current will produce heat sufficient to melt it. Current rating gives the normal current that may be carried: minimum fusing current is the smallest current that will cause melting.
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A fuse will melt much quicker with very large fault current than when the value of fault current is only just above the minimum fusing current. Time/current characteristics are found by testing six or more of the same type of fuse at different currents and plot ting the results. The bottom current for the test is not more than 1.05 X minimum fusing current, and the top current is one that will melt the wire in not more than 0.5 second. The other test currents are equally spaced between these. Fuses are rated for particular ac. and/or d.c. voltages
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Cartridge Fuses
High Rupture Capacity (HRC) fuses have silver wire enclosed in a quartz powder filled ceramic tube with metal end caps (Fig. 9.1). Arcing when this type of fuse blows is buried in the powder, fusion of which in the arc path helps to extinguish it.
HRC fuses can be used for very high fault levels: deterioration is negligible; they have accurate time/current characteristics and reliability for discrimination; they are safer if accidentally inserted on a fault: there is no issue of smoke or flame; cartridges are sized to ensure that the correct value fuse is fitted.
Semi-enclosed Fuses
The rewireable fuse has an insulated carrier for safe handling and containment of the wire in an asbestos lined tube
The wire is easily replaced after operation, but the des is open to abuse as too heavy a wire can be used which could mask a fault and also cause severe arcing if it did operate. Another fault is that of premature failure if the wire is made thinner by oxidation or con tact with air, or by being stretched when fitted (a problem with wire made of lead, tin or an alloy of the two).
Fuses in Service
Fuses may be used as the only protection in a steady load circuit, such as for lighting. An ac. motor with its high starting current and varying load has fuses in each of the supply conductors, but fitted as a backup for the other forms of protection and to break the circuit in the event of a short-circuit current greater than that which the ordinary contact breakers are designed to interrupt without damage. Very accurate time/current characteristics are needed for fuses used in conjunction with other safety devices, to ensure that the overload trip is allowed time to operate for moderate over- current but that the fuse blows first if there is very high short-circuit current.
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marine Engineering-Electrical-D.C. Generators

D.C. Generators

Operation of a d.c. generator relies (as with alternators) on the principle that when magnetic lines of force are cut by a conductor(Fig. 6.1) a voltage is induced in the conductor. Size of induced voltage and resulting current are dependent on magnetic field strength, length of conductor and speed of cutting.
The direction of current flow is dictated by the relationship between magnetic field and direction of movement of the conductor. Ii can be found from Fleming’s Right Hand Rule, which is applied to give direction of conventional current flow during generation,A simple generator can be constructed from a loop or coil of wire mounted on a spindle and arranged for rotation between opposite magnetic poles The field-cutting action of the straight sides will cause current flow as the result of induced voltage. Direction of flow is shown by the arrows (found from the Right Hand Rule) and can be seen to be continuous around the loop. The voltages generated are in series and therefore add to give twice the voltage produced in one side.
Direct current can be collected from the wire ends through the commutator which consists of two half-rings with brushes. Each brush takes current from one half-ring in turn, so that current flow is always in the same direction f or each collecting brush. The out put is not steady but has a wave form.
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Marine Engine Q & A -Electrical-Static Automatic Voltage Regulator

Static Automatic Voltage Regulator
The carbon pile regulator uses a magnetic coil powered from the alternator output Strength of the field vanes with alternator voltage and this strength is tested against springs which are the voltage reference. The moving contact regulator employs a similar matching of alternator output effect through a magnetic coil against springs.
The availability of a small transformed, rectified and smoothed power supply from the alternator output makes possible the matching or it directly against an electronic reference in the static automatic voltage regulator. The direct current derived from the alternator output is applied to a bridge which has fixed resistances on two arms and variable resistances (zener diode voltage references) on the other two. The zeners operate in the reverse breakdown mode having been manufactured with a zener breakdown voltage of very low value. As can be seen from the earlier description of zener diodes, voltage remains constant once breakdown has occurred despite change in current. This implies, however, that changes in applied voltage while not affecting voltage across the diode will cause a change in resistance which permits change in current. .As with a Wheatstone bridge, imbalance of the resistances changes the flow pattern and produces in the voltage measuring bridge an error signal
The error signal can be amplified and used to control alternator excitation in a number of different ways. Thus it can control the firing angle of Thyristors (Fig. 3.14) through a triggering circuit to give the desired voltage in the brushless alternator described. It can be used in the statically excited alternator to correct small errors through a magnetic amplifier arrangement-The error signal has also been amplified through transistors in series, for Excitation control.
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Marine Engine Q & A- Electrical-Three-Phase Systems/

Three-Phase Systems
Outputs from the three sets of conductors in the alternator are delivered to three separate bus-bars in the switchboard. This is necessary because of the voltage and current disparity between them at any instant.
Three-phase, four systems use a single return wire which is connected to the neutral point of the star windings. Current in the turn wire is the sum of currents in the individual phases. If loads on each phase are balanced with voltages equal and at .1 20 apart, the three currents will sum to zero and the return wire will carry no current The fourth (return) wire will carry a small current it there is imbalance.
Three-phase, three-wire systems have no return wire. This is acceptable for snips where, direct from the main switchboard, three-phase motors make up much of the load and unless there is a fault they take current equally from the phases. Also some imbalance is acceptable with a three-phase, three-wire system provided load is connected in delta Supplies for lighting, heating, single-phase motors and other loads are taken through delta-star or delta-delta transformers.
The Neutral Point
The majority of British ships use three-phase, three-wire distribution with the neutral points of alternators insulated (Fig. Very little current will flow through an earth fault on one phase, because there is no easy path for it back to the electrical system. With such a system, an essential electric motor with an earth fault can be kept running until stoppage for repair is convenient This would be as soon as possible to avoid a full-phase fault that would result if an earth occurred on another phase as well.
Although fault current is negligible with an insulated/unearthed neutral point, over voltages are high. The transient likely is 2.5 X line voltage. Such a voltage surge is within the capability of the main insulation of marine electrical equipment which is tested to 2 X line voltage + 1000 volts.

A few British vessels have electrical distribution systems with an earthed neutral . This is a connection of the system, via the neutral point of the alternator, to the hull steel. The result of not isolating the electrical system from the hull is that current flow from an earth fault on any phase has a path through the hull steel and earthed neutral back to the system. The availability of the path encourages higher fault current flow than is the case where the neutral is insulated or connected by resistance. Equipment with an earth fault, where-the system is earthed, must be disconnected immediately if a fault develops. This can be effected automatically with an earthed neutral system because the level of fault current is ugh enough to operate a trip.
Earth fault current is high with earthed neutral systems, but over- voltages due specifically to earth faults are lower. The earthed system is chosen to limit over voltages and to give automatic earth fault location and disconnection.
Over voltages due to switching are not affected by choice of earthing or insulating the neutral. These high surges, and the possibility of others from failure of the voltage regulator, means that the same standard of equipment insulation is required for both arrangements.
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Marine Engine Q & A- Electrical-Alternator

Alternators

The operation of generators relies on the principle that whenever, there is mutual cutting between a conductor and a magnetic field, a voltage and resulting current will be induced in the, conductor. The flow of induced current is not random: it is governed: by the directions of cutting and of the field and can be found from Fleming’s Right Hand Rule (Fig.).

Magnitude of the induced voltage depends on the strength of ‘the magnetic field, rate of cutting and conductor.
Simple Alternator
The arrangement used in the majority of alternators to exploit the principle of generation is shown simply in the sketch. Mutual cutting between conductors and magnetic fields is produced by rotating poles, the magnetic’ fields of which move through fixed conductors.
The rotor shown has a pair of poles so that output is generated simultaneously in two conductors. Reference to the Fleming Right Hand Rule will confirm the instantaneous direction of conventional current indicated by the arrows. The two conductors (R and R1) are connected in series so that the voltages generated in them add together to deliver current to the switchboard. The rotating fields, although moving at constant speed, will cut the conductors at a changing rate because of the circular movement Voltage induced at any instant is proportional to the sine of the angle of the rotating vector. The pattern of build-up and decline, and also reversal in the voltage induced, is shown by the sine wave R.
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Voltage and current are generated in each of the pairs of conductors in turn—first in one direction and then in the other—to produce three-phase alternating current. The effect in conductors Y and B is also shown.

Marine Engine Q &A-Electrical Emergency Electrical power

Emergency Electrical Power Cobra CPI 1575 1500 Watt 12 Volt DC to 120 Volt AC Power Inverterfor Passenger and General cargo Vessel.

General cargo Vessel

In all Passenger and cargo vessels a number of essential services must be able to be maintained under emergency conditions. The requirements vary with type of ship and length of voyage. Self- contained emergency sources- of -electrical power must be installed in positions such that they are unlikely to be damaged or affected by any incident which has caused the loss of main power. The emergency generator with its Switchboard is thus located in a compartment which is outside of and away from main and auxiliary machinery spaces above the uppermost continuous deck and not forward of the collision bulkhead The same ruling a lies to batteries with the exception that accumulator batteries must not be fitted in the same space as any emergency Switchboard.

An emergency source of power should be capable of operating with a list of up to 22½º and a trim of up to 10°. The compartment should be accessible from the open deck

Passenger Vessels

Emergency generators for passenger vessels are now required to be automatically started and connected within 45 seconds. A set of automatically connected emergency batteries capable of carrying certain essential items for thirty minutes. is also required.  Alternatively, batteries are permitted as the main emergency source of power.

Regulations specify the supply of emergency power to essential services on passenger ships for a period of up to36 hours. A shorter period is allowed in vessels such as ferries. Some of the essential services may be .operated by other than electrical means (such as hydraulically controlled watertight doors), others may have their own electrical power. If the batteries are the only source of power the must sup the emergency load without recharging or excessive voltage drop (12% limit) or the required length of time. Because the specified period is up to 36 hours, batteries are used normally as a temporary power source with the emergency generator taking over essential supplies when it starts.Ship Passenger Lists: National and New England (1600-1825)

      Batteries fitted to provide temporary or transitional power supply emergency lights, navigation lights, and watertight door circuits including alarms and indicators, and internal communication systems. In addition they could Supply fire detection and alarm installations, manual fire alarms, fire door release, gear, internal signals, ships whistle and daylight signalling lamp. But some of these will have their own power or take it from a low-pressure D.C. system. Sequential watertight door closure by transitional batteries is acceptable.

The emergency generator when started supplies essential services through its own switchboard including the load taken initially by the transitional batteries. Additionally it would provide power for the-emergency bilge pump; fire pump, sprinkler pump steering gear .and other items if they were fed through the emergency switchboard-

Arrangements are required to enable lifts to be brought to deck level in an emergency. Also, emergency lighting from transitional batteries is required in all alleyways, stairs, exits, boat stations (deck and over side), control stations (bridge, radio room. engine control room, etc), machinery spaces and emergency machinery spaces.

Cargo Vessels

Emergency power for cargo ships is provided by accumulator battery or generator. Battery systems are automatically connected upon loss of the main supply, and in installations where the generator is not started and. connected within. 15 seconds automatically, are required as a transitional power source for at least 30 minutes.

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Power available for emergencies must be sufficient to operate certain essential services simultaneously for up to18 hours. These are :emergency lights , navigation lights, internal communication equipment, daylight signalling lamp, ship’s whistle fire detection alarm installations, manual fire alarms; other internal emergency signals, the emergency fire pump, steering gear, navigation aids and other equipment Some essential services have their own power or are supplied from a low-pressure d.c. system. .

Transitional batteries are required to supply for 30 minutes power for emergency lighting, general alarm, fire detection and alarm system; communication equipment and navigation lights.

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Marine Engine Q &A-Electrical Emergency Generator

Emergency Generator
There a number of ways in .which emergency powerBlack & Decker SS50B Storm Station With 50 Watt Power Inverter LED Flashlight And Digital AM/FM NOAA Weather RadioDuracell DPP-600HD Powerpack 600 Jump Starter & Emergency Power Source with RadioBlack & Decker SS50B Storm Station With 50 Watt Power Inverter LED Flashlight And Digital AM/FM NOAA Weather RadioDuracell DPP-600HD Powerpack 600 Jump Starter & Emergency Power Source with RadioWagan 400-Watt Power Dome EX Jumpstarter with Built-In Air Compressor can be supplied. The arrangement shown in Fig. 1.7 incorporates some common features.
The emergency switchboard has two sections, one operating at 440 volts and the other at 220 volts. The 440 volt supply, under circumstances, is taken from the main engine room switchboard through a circuit breaker A. Loss of main power this breaker to be tripped and the supply is taken over by the emergency generator when started through a An interlock prevents simultaneous closure of both breakers.
A special feeder is sometimes fitted so that in a dead- ship situation the emergency generator can be connected to the main switchboard. This special condition breaker would only be closed engine room board was cleared of all load. i.e. all distribution were open. Selected machinery within the capacity of the emergency generator could then be operated to restore power, at which stage the special breaker would be disconnected.
The essential services supplied from the 440 volt section of the emergency board depicted include the emergency bilge pump, the sprinkler pump and compressor, one of two steering gear circuits(the other being from the main board), and a 440/220 volt three-phase transformer through which the other section is fed.
Circuits supplied from the 220. volt section include those for navigation equipment, radio communication and the transformed and rectified supplies systems Separate sets of batteries are fitted for temporary emergency power and for a low pressure The former automatically supply emergency lights and other services not connected to the low-pressure tem Batteries for the radio are not shown.
The switchboard and generator for emergency power supply installed in one compartment which may be heated for starting in cold conditions. The independent and approved method of automatic starting (compressed air, batteries or hydraulic) should have the capacity for repeated attempts. And a secondary arrangement such that further attempts can be made within the 30 minute temporary battery lifetime.
The emergency generator is provided with an adequate and Independent supply of fuel with a flash point of not Less than 43ºC(110ºF).