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.

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



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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Ward Leonard motor speed control system for ships machinery operation

Motor speed control



Fig: Ward-Leonard speed control

As a very flexible, reliable means of motor speed control the Ward-Leonard system is unmatched.

The system is made up of a driving motor which runs at almost constant speed and powers a d.c. generator . The generator output is fed to a d.c. motor. By varying the generator field current its output voltage will change. The speed of the controlled motor can thus be varied smoothly from zero to full speed. Since control is achieved through the generator shunt field current, the control equipment required is only for small current values. A potentiometer or rheostat in the generator field circuit enables the variation of output voltage from zero to the full value and also in either direction. The controlled motor has a constant excitation: its speed and direction are thus determined by the generator output.

Depending upon the particular duties of the controlled motor, series windings may be incorporated in the field of the motor and also the generator. This may result in additional switching to reverse the controlled motor depending upon the compounding arrangements. The driving motor or prime motor for the Ward Leonard system can be a d.c. motor, an a.c. motor, a diesel engine, etc. Any form of constant or almost constant speed drive can be used, since its function is only to drive the generator.

Duties of Electrical Engineer Officer On Board Ship

All the machinery onboard ship is a combination of mechanical and electrical systems. The modern day shipping is more reliable on automations and electronics whose knowledge and maintenance can only be handled by an engineer expert in the electrical field. Marine electrical officer engineers are perfect for such jobs and that’s why they hold an important role on ship and in offshore industry.

Electrical engineer is one of the most vital positions in the technical hierarchy of a ship and engineer is responsible for his assigned work under the chief engineer’s instructions.
Some shipping companies do not carry electrical officers on their ship to cut down the manning cost and the electrical duties are carried by some one from the engineer’s side, normally third engineer. However, many companies realized that electrical and electronic system requires some extra attention and therefore require an expert to attend them.

As the technology is advancing, more and more automations and electronic circuit is replacing conventional and electrical systems. Hence the international Maritime Organisation (IMO) amended STCW 95 on 25thJune 2010 known as Manila amendment, to introduce a certified position of Electro-technical officer in place of electrical officer.

The general duties of electrical engineer or Electro-technical officer are:

• He is responsible for maintenance of all the electrical motors on ship i.e. in engine room and on deck.

• He is in charge of maintenance of all switchboard including main switchboard and emergency switchboard.

• He is responsible for maintenance of fire detectors and fire alarm system.

• He has to maintain all the ship’s alarm system.

• He is responsible for the electronic system fitted onboard ship.

• He is responsible for the ship’s navigational lights and other navigational equipments.

• He is responsible for all the batteries that are connected to machineries onboard. It includes:

-          Emergency batteries for alarm and lights
-          Lifeboat batteries
-          Batteries for emergency generator
-          Other batteries fitted onboard

• He is responsible for maintaining refrigeration unit in the engine room

• He has to take care of air conditioning unit of the vessel.

• Electrical officer is responsible for maintaining refrigerated containers carried on container ship.

• He is responsible for cargo and engine room cranes electrical system.

• He has to carry out routine maintenance for main engine alarms and trips along with the chief engineer.

• During the time of manoeuvring, he has to be present in the engine room along with other engineers to tackle any kind of electrical and other emergencies.

• Electrical officer can assist in watch keeping routines at desired time by the chief engineer.

• He has to assist ship’s engineer and deck officer in all kind of electrical problems.

Even the post of electrical officer is not a compulsion on a ship, but due to technicalities and complex knowledge requirement of the electrical and electronic system, they are extensively present in the shipping industry.